@article{du_mohan_sim_fang_roberts_2021, title={Study of spray structure under flash boiling conditions using 2phase-SLIPI}, volume={62}, ISSN={["1432-1114"]}, DOI={10.1007/s00348-020-03123-1}, abstractNote={In gasoline engines, including conventional gasoline direct injection (GDI) engines and newly developed gasoline compression ignition (GCI) engines, flash boiling of the spray occurs during throttling or low load operations. Superheated fuel that is injected into the cylinder, where the gas pressure is lower than the fuel’s saturation vapor pressure, experiences a fast phase change. Plume interaction and spray collapse can occur as a consequence of flash boiling. The structure of flashing spray has not been well elucidated experimentally because of strong multiple-scattering effects in conventional laser sheet imaging due to illumination of out-of-laser-plane droplets. Here, the structured laser illumination planar imaging (SLIPI) is implemented for the first time to study flash boiling sprays. Both front-view and side-view cross-sections are examined to reveal spray behaviors during collapsing events. A comparison of the reconstructed 3D spray volume by SLIPI and conventional laser sheet imaging clearly shows the advantage of SLIPI in resolving the inner structure of the collapsed spray. The near-nozzle region on the injector axis is found to be hollow, indicating that spray collapsing occurs a bit downstream of the nozzle instead of immediately at the nozzle. This observation could not be obtained by conventional laser sheet imaging nor by diffused back illumination (DBI) techniques. In this work, the central tip observed in the 2D DBI image at Rp = 0.1 case has been proven to be not a ’central jet on injector axis’ formed due to radial collapse, but a longer projection on the image caused by stronger adjacent plume circumferential interactions.}, number={1}, journal={EXPERIMENTS IN FLUIDS}, author={Du, Jianguo and Mohan, Balaji and Sim, Jaeheon and Fang, Tiegang and Roberts, William L.}, year={2021}, month={Jan} }
 @article{du_mohan_sim_fang_chang_roberts_2020, title={A comprehensive study of spray and combustion characteristics of a prototype injector for gasoline compression ignition (GCI) application}, volume={277}, ISSN={["1873-7153"]}, url={https://doi.org/10.1016/j.fuel.2020.118144}, DOI={10.1016/j.fuel.2020.118144}, abstractNote={In this study, the spray and combustion characteristics of high reactivity gasoline (HRG) fuel of RON 77 were tested and compared with E10 certification fuel under the gasoline compression ignition (GCI) engine conditions using a high-pressure multi-hole GCI engine injector. A comprehensive characterization in terms of the rate of injection, spray morphology under flash boiling conditions, penetration lengths under both nonevaporative and evaporative conditions, and ignition delay at reactive conditions was performed. It was found that both the high reactivity gasoline and E10 certification fuel exhibit very similar characteristics. The ignition delay times were found to be very similar between both the fuels tested under ambient temperatures higher than 800 K. This work further serves as an extensive database to validate and calibrate the spray models, combustion models and reaction mechanisms for computational fluid dynamics (CFD) driven development of GCI engines.}, journal={FUEL}, publisher={Elsevier BV}, author={Du, Jianguo and Mohan, Balaji and Sim, Jaeheon and Fang, Tiegang and Chang, Junseok and Roberts, William L.}, year={2020}, month={Oct} }
 @article{du_mohan_sim_fang_roberts_2020, title={Auto-ignition characteristics of high-reactivity gasoline fuel using a gasoline multi-hole injector}, volume={112}, ISSN={["1879-2286"]}, DOI={10.1016/j.expthermflusci.2019.109993}, abstractNote={Gasoline compression ignition (GCI) engines have proven to be a highly efficient engine technology with reduced emissions. The high efficiency and reduced emissions of GCI engines heavily rely on the stratification of the fuel being injected directly into the cylinder through multi-hole injectors. Therefore, it is critical to understand the fuel stratification and auto-ignition behaviors of the fuels used in GCI engines. Thus, in this work, the auto-ignition characteristics of high-reactivity and low carbon gasoline fuel (RON 77) were studied in an optically accessible constant volume chamber. A customized high-pressure gasoline multi-hole injector was used. Reactive tests were performed at two different ambient pressures (20 and 30 bar), three different ambient temperatures (800, 900, and 1000 K), three different oxygen concentrations (10, 15 and 21%) and three different injection pressures (100, 300 and 450 bar). The auto-ignition of fuel was achieved with varying ignition delay based on the experimental conditions tested. It was found that the operating conditions profoundly influences the diffusion and partially-premixed combustion mode. For high ambient pressures, temperatures, oxygen concentrations, injection pressure, and combinations, diffusion combustion mode was observed, and partially premixed combustion mode was observed at lower ambient pressures, temperatures, oxygen concentrations, injection pressures, and their combinations.}, journal={EXPERIMENTAL THERMAL AND FLUID SCIENCE}, author={Du, Jianguo and Mohan, Balaji and Sim, Jaeheon and Fang, Tiegang and Roberts, William L.}, year={2020}, month={Apr} }
 @article{du_mohan_sim_fang_chang_roberts_2020, title={INFLUENCE OF FLASH BOILING ON SPRAY MORPHOLOGY USING A PROTOTYPE INJECTOR FOR GASOLINE COMPRESSION IGNITION (GCI) APPLICATION}, volume={30}, ISSN={["1936-2684"]}, DOI={10.1615/AtomizSpr.2020034561}, abstractNote={Flash boiling occurs with gasoline direct injection spray at throttling, and low-load engine conditions leading to plume interactions and sprays collapse under low ambient densities. The change of fuel trajectory compared with the injector's initial design could leave an adverse effect on spray combustion quality, although flash boiling has the potential of achieving better atomization. Thus, studies on the plume to plume interactions and spray collapse processes are of high importance. Researches have mostly been carried out focusing on the plume interactions in the liquid phase, while in the flash boiling condition, the vapor phase of fuel is nonnegligible. This work focusses on the plume to plume interactions considering both the vapor and liquid phase of the fuel under specific throttling conditions in gasoline compression ignition (GCI) engines using a high-pressure wide spray angle prototype injector. The experiments were carried out at a wide range of pressure ratio (Rp) conditions (Rp = 0.05 to 1.4). Simultaneous front view and side view shadowgraph techniques were implemented to visualize the liquid & vapor phase of the fuel spray. Similarly, simultaneous front view Mie scattering and side view diffused backlit illumination (DBI) techniques were implemented to visualize the liquid phase of the fuel spray. Due to the line of sight plume overlapping at the side view, the difference in spray morphology obtained by DBI and shadowgraph is not apparent. However, the front view comparison shows that, in the transition regime, the plume to plume interactions in the vapor phase are more evident than that in the liquid phase. This work reveals that the front view techniques could be an excellent way to study multiplume interactions during flash boiling phenomena.}, number={5}, journal={ATOMIZATION AND SPRAYS}, author={Du, Jianguo and Mohan, Balaji and Sim, Jaeheon and Fang, Tiegang and Chang, Junseok and Roberts, William L.}, year={2020}, pages={351–369} }
 @article{wang_wu_badra_roberts_fang_2020, title={Soot characteristics of high-reactivity gasoline under compression-ignition conditions using a gasoline direct injection (GDI) piezoelectric fuel injector}, volume={265}, ISSN={["1873-7153"]}, DOI={10.1016/j.fuel.2019.116931}, abstractNote={Gasoline compression ignition (GCI) engine technology has become one of the promising alternative solutions to achieve better fuel economy and meet emission requirement. Higher reactivity gasoline-like fuels are more desirable in GCI engines. This study investigates the soot processes under autoignition combustion of high-reactivity gasoline (HRG) with an outwardly opening piezo gasoline direct injection (GDI) fuel injector. HRG fuels are mixtures of refinery streams with RON of 50–80 and they can potentially yield better fuel economy and emissions in GCI engines. Five ambient oxygen concentrations varying from 10% to 21% and three different ambient temperature combinations were selected to simulate various ambient environments. A two-color pyrometry was applied to measure flame temperature and soot concentration (i.e., KL factor). In general, HRG flame temperatures range from 1500 to 2400 K under selected conditions. HRG flames have relatively low KL factor for all selected experiment conditions. High KL factors are only observed at the flame periphery where flame temperatures are lower than 1800 K. Accumulated KL factor was calculated to evaluate overall soot amount. Flames at 800 K ambient temperature always have the highest accumulated KL factor. The soot and soot temperature trade-off were also discussed. The desired condition needs to approach a moderate soot temperature with a relative low integrated KL factor level. The conditions of 800 K with 15% O2, 1000 K with 10% O2 and 1000 K with 12% O2 shows better results. The findings can help facilitate the application of high reactivity gasoline fuels in next generation clean combustion engines.}, journal={FUEL}, author={Wang, Libing and Wu, Zengyang and Badra, Jihad A. and Roberts, William L. and Fang, Tiegang}, year={2020}, month={Apr} }
 @article{du_mohan_sim_fang_roberts_2020, title={Study of spray collapse phenomenon at flash boiling conditions using simultaneous front and side view imaging}, volume={147}, ISSN={["1879-2189"]}, DOI={10.1016/j.ijheatmasstransfer.2019.118824}, abstractNote={Flash boiling has become a topic of interest to researchers due to its potential of achieving good fuel atomization and negative influence on GDI engine emissions when spray collapses and spray-wall impingement exists. Under flash boiling conditions, the accompanying spray collapse phenomenon and plume interaction are not clearly elucidated. Simultaneous side view diffused back illumination (DBI) and front view Mie-scattering were implemented in this work to capture transient plume to plume interaction of iso-octane fuel spray from a 10 hole gasoline direct injection (GDI) injector at flash boiling conditions. Fuel temperature and ambient gas pressure were varied in a wide range to cover collapse, transitional and non-flashing regimes. Two new criteria named ‘spray collapse percentage’, defined based on the front view Mie-scattering technique and ‘optical thickness’ based on the side view DBI technique, were developed for classification of different spray regimes. These two criteria distinguish the collapsing and transitional regimes well from the non-collapsing regime compared to other criteria used in the literature.}, journal={INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER}, author={Du, Jianguo and Mohan, Balaji and Sim, Jaeheon and Fang, Tiegang and Roberts, William L.}, year={2020}, month={Feb} }
 @article{du_mohan_sim_fang_roberts_2019, title={Experimental and analytical study on liquid and vapor penetration of high-reactivity gasoline using a high-pressure gasoline multi-hole injector}, volume={163}, ISSN={["1359-4311"]}, DOI={10.1016/j.applthermaleng.2019.114187}, abstractNote={Spray penetration length is an important parameter which is of great interest to both experimentalists and modelers. As it affects engine efficiency and emissions, measurement and prediction of spray penetration can significantly benefit engine optimization under various operating conditions. In this study, penetration length was investigated in a pre-burn constant volume combustion chamber using a gasoline multi-hole injector with high reactivity gasoline-like fuel designed explicitly for gasoline compression ignition (GCI) engines. Diffused back illumination (DBI) and shadowgraph were implemented for liquid and vapor phase penetration measurements, respectively. Different pre-burn gas mixtures are compared to investigate the influence of ambient gas properties on gasoline spray penetration under evaporating conditions. The liquid penetration under the gas composition of higher molecular weight tends to be longer. However, the vapor penetration showed insignificant effect under different gas compositions. Ambient gas temperature and gas composition were found to be an essential parameter for liquid phase penetration. Pressure difference was found to affect the vapor penetration length while its influence on liquid phase steady state penetration length at high ambient gas temperature is marginal. Statistical analysis was performed for both liquid and vapor phase penetration lengths, and a prediction model was developed with good agreement to the data under all test conditions.}, journal={APPLIED THERMAL ENGINEERING}, author={Du, Jianguo and Mohan, Balaji and Sim, Jaeheon and Fang, Tiegang and Roberts, William L.}, year={2019}, month={Dec} }
 @article{du_mohan_sim_fang_roberts_2019, title={Macroscopic non-reacting spray characterization of gasoline compression ignition fuels in a constant volume chamber}, volume={255}, ISSN={["1873-7153"]}, url={https://doi.org/10.1016/j.fuel.2019.115818}, DOI={10.1016/j.fuel.2019.115818}, abstractNote={Recently, gasoline compression ignition (GCI) engines have become a topic of interest due to its benefits in high thermal efficiency and low emissions. Combustion in GCI engines is highly governed by the fuel stratification which is strongly associated with the spray characteristics like penetration length. Researchers have proposed both theoretical, and regression models for liquid penetration length of diesel and gasoline direct injection (GDI) spray at non-evaporating conditions. However, there are no models for gasoline sprays at elevated ambient gas temperatures, pressures, and injection pressures in literature. This research gap needs to be bridged, as it is crucial for GCI engine technology. In this study, penetration length was investigated using a high-pressure custom-made multi-hole gasoline injector. High reactivity low carbon fuel (RON 77), designed explicitly for GCI engines, and E10 certification fuel (RON 91) were used and compared. Diffused back illumination (DBI) and shadowgraph were implemented for liquid and vapor phase penetration measurement, respectively. It was found that the spray characteristics of high reactivity fuel were similar to E10 certification fuel under non-reacting conditions. Statistical analysis was performed for both liquid and vapor phase penetration lengths, and empirical models were developed with good agreements to the experimental data under high ambient gas pressure and temperature relevant to GCI engine operating conditions using GCI fuels at higher injection pressures. A ‘separation point’ was defined for the liquid phase after which it reaches a steady state, and it was demonstrated to be different from the ‘breakup time’ found in the literature.}, journal={FUEL}, publisher={Elsevier BV}, author={Du, Jianguo and Mohan, Balaji and Sim, Jaeheon and Fang, Tiegang and Roberts, William L.}, year={2019}, month={Nov} }
 @article{jing_wu_roberts_fang_2017, title={Effects of Fuel Quantity on Soot Formation Process for Biomass-Based Renewable Diesel Fuel Combustion}, volume={139}, ISSN={["1528-8919"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85020430438&partnerID=MN8TOARS}, DOI={10.1115/1.4036292}, abstractNote={Soot formation process was investigated for biomass-based renewable diesel fuel, such as biomass to liquid (BTL), and conventional diesel combustion under varied fuel quantities injected into a constant volume combustion chamber. Soot measurement was implemented by two-color pyrometry under quiescent type diesel engine conditions (1000 K and 21% O2 concentration). Different fuel quantities, which correspond to different injection widths from 0.5 ms to 2 ms under constant injection pressure (1000 bar), were used to simulate different loads in engines. For a given fuel, soot temperature and KL factor show a different trend at initial stage for different fuel quantities, where a higher soot temperature can be found in a small fuel quantity case. but a higher KL factor is observed in a large fuel quantity case generally. Another difference occurs at the end of combustion due to the termination of fuel injection. Additionally, BTL flame has a lower soot temperature, especially under a larger fuel quantity (2 ms injection width). Meanwhile, average soot level is lower for BTL flame, especially under a lower fuel quantity (0.5 ms injection width). BTL shows an overall low sooting behavior with low soot temperature compared to diesel; however, trade-off between soot level and soot temperature needs to be carefully selected when different loads are used.}, number={10}, journal={JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER-TRANSACTIONS OF THE ASME}, author={Jing, Wei and Wu, Zengyang and Roberts, William L. and Fang, Tiegang}, year={2017}, month={Oct} }
 @article{andersen_modak_winterrowd_lee_roberts_wendt_linak_2017, title={Soot, organics, and ultrafine ash from air-and oxy-fired coal combustion}, volume={36}, ISSN={["1873-2704"]}, DOI={10.1016/j.proci.2016.08.073}, abstractNote={Pulverized bituminous coal was burned in a 10 W externally heated entrained flow furnace under air-combustion and three oxy-combustion inlet oxygen conditions (28, 32, and 36%). Experiments were designed to produce flames with practically relevant stoichiometric ratios (SR = 1.2–1.4) and constant residence times (2.3 s). Size-classified fly ash samples were collected, and measurements focused on the soot, elemental carbon (EC), and organic carbon (OC) composition of the total and ultrafine (<0.6 µm) fly ash. Results indicate that although the total fly ash carbon, as measured by loss on ignition, was always acceptably low (<2%) with all three oxy-combustion conditions lower than air-combustion, the ultrafine fly ash for both air-fired and oxy-fired combustion conditions consists primarily of carbonaceous material (50–95%). Carbonaceous components on particles <0.6 µm measured by a thermal optical method showed that large fractions (52–93%) consisted of OC rather than EC, as expected. This observation was supported by thermogravimetric analysis indicating that for the air, 28% oxy, and 32% oxy conditions, 14–71% of this material may be OC volatilizing between 100 °C and 550 °C with the remaining 29–86% being EC/soot. However, for the 36% oxy condition, OC may comprise over 90% of the ultrafine carbon with a much smaller EC/soot contribution. These data were interpreted by considering the effects of oxy-combustion on flame attachment, ignition delay, and soot oxidation of a bituminous coal, and the effects of these processes on OC and EC emissions. Flame aerodynamics and inlet oxidant composition may influence emissions of organic hazardous air pollutants (HAPs) from a bituminous coal. During oxy-coal combustion, judicious control of inlet oxygen concentration and placement may be used to minimize organic HAP and soot emissions.}, number={3}, journal={PROCEEDINGS OF THE COMBUSTION INSTITUTE}, author={Andersen, Myrrha E. and Modak, Nabanita and Winterrowd, Christopher K. and Lee, Chun Wai and Roberts, William L. and Wendt, Jost O. L. and Linak, William P.}, year={2017}, pages={4029–4037} }
 @article{ogunkoya_roberts_fang_thapaliya_2015, title={Investigation of the effects of renewable diesel fuels on engine performance, combustion, and emissions}, volume={140}, ISSN={["1873-7153"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84908577588&partnerID=MN8TOARS}, DOI={10.1016/j.fuel.2014.09.061}, abstractNote={A study was undertaken to investigate renewable fuels in a compression-ignition internal combustion engine. The focus of this study was the effect of newly developed renewable fuels on engine performance, combustion, and emissions. Eight fuels were investigated, and they include diesel, jet fuel, a traditional biodiesel (fatty acid methyl ester: FAME), and five next generation biofuels. These five fuels were derived using a two-step process: hydrolysis of the oil into fatty acids (if necessary) and then a thermo-catalytic process to remove the oxygen via a decarboxylation reaction. The fuels included a fed batch deoxygenation of canola derived fatty acids (DCFA), a fed batch deoxygenation of canola derived fatty acids with varying amounts of H2 used during the deoxygenation process (DCFAH), a continuous deoxygenation of canola derived fatty acids (CDCFA), fed batch deoxygenation of lauric acid (DLA), and a third reaction to isomerize the products of the deoxygenated canola derived fatty acid alkanes (IPCF). Diesel, jet fuel, and biodiesel (FAME) have been used as benchmarks for comparing with the newer renewable fuels. The results of the experiments show slightly lower mechanical efficiency but better brake specific fuel consumption for the new renewable fuels. Results from combustion show shorter ignition delays for most of the renewable (deoxygenated) fuels with the exception of fed batch deoxygenation of lauric acid. Combustion results also show lower peak in-cylinder pressures, reduced rate of increase in cylinder pressure, and lower heat release rates for the renewable fuels. Emission results show an increase in hydrocarbon emissions for renewable deoxygenated fuels, but a general decrease in all other emissions including NOx, greenhouse gases, and soot. Results also demonstrate that isomers of the alkanes resulting from the deoxygenation of the canola derived fatty acids could be a potential replacement to conventional fossil diesel and biodiesel based on the experiments in this work.}, journal={FUEL}, author={Ogunkoya, Dolanimi and Roberts, William L. and Fang, Tiegang and Thapaliya, Nirajan}, year={2015}, month={Jan}, pages={541–554} }
 @article{yin_natelson_campos_kolar_roberts_2013, title={Aromatization of n-octane over Pd/C catalysts}, volume={103}, ISSN={["1873-7153"]}, DOI={10.1016/j.fuel.2012.06.095}, abstractNote={Abstract Gas-phase aromatization of n-octane was investigated using Pd/C catalyst. The objectives were to: (1) determine the effects of temperature (400–600 °C), weight hourly space velocity (WHSV) (0.8–∞), and hydrogen to hydrocarbon molar ratio (MR) (0–6) on conversion, selectivity, and yield (2) compare the activity of Pd/C with Pt/C and Pt/KL catalysts and (3) test the suitability of Pd/C for aromatization of different alkanes including n-hexane, n-heptane, and n-octane. Pd/C exhibited the best aromatization performance, including 54.4% conversion and 31.5% aromatics yield at 500 °C, WHSV = 2 h−1, and a MR of 2. The Pd/C catalyst had higher selectivity towards the preferred aromatics including ethylbenzene and xylenes, whereas Pt/KL had higher selectivity towards benzene and toluene. The results were somewhat consistent with adsorbed n-octane cyclization proceeding mainly through the six-membered ring closure mechanism. In addition, Pd/C was also capable of catalyzing aromatization of n-hexane and n-heptane.}, journal={FUEL}, author={Yin, Mengchen and Natelson, Robert H. and Campos, Andrew A. and Kolar, Praveen and Roberts, William L.}, year={2013}, month={Jan}, pages={408–413} }
 @article{steinmetz_herrington_winterrowd_roberts_wendt_linak_2013, title={Crude glycerol combustion: Particulate, acrolein, and other volatile organic emissions}, volume={34}, ISSN={["1873-2704"]}, DOI={10.1016/j.proci.2012.07.050}, abstractNote={Crude glycerol is an abundant by-product of biodiesel production. As volumes of this potential waste grow, there is increasing interest in developing new value added uses. One possible use, as a boiler fuel for process heating, offers added advantages of energy integration and fossil fuel substitution. However, challenges to the use of crude glycerol as a boiler fuel include its low energy density, high viscosity, and high autoignition temperature. We have previously shown that a refractory-lined, high swirl burner can overcome challenges related to flame ignition and stability. However, critical issues related to ash behavior and the possible formation of acrolein remained. The work presented here indicates that the presence of dissolved catalysts used during the esterification and transesterification processes results in extremely large amounts of inorganic species in the crude glycerol. For the fuels examined here, the result is a submicron fly ash comprised primarily of sodium carbonates, phosphates, and sulfates. These particles report to a well-developed accumulation mode (0.3–0.7 μm diameter), indicating extensive ash vaporization and particle formation via nucleation, condensation, and coagulation. Particle mass emissions were between 2 and 4 g/m3. These results indicate that glycerol containing soluble catalyst is not suitable as a boiler fuel. Fortunately, process improvements are currently addressing this issue. Additionally, acrolein is of concern due to its toxicity, and is known to be formed from the low temperature thermal decomposition of glycerol. Currently, there is no known reliable method for measuring acrolein in sources. Acrolein and emissions of other volatile organic compounds were characterized through the use of a SUMMA canister-based sampling method followed by GC–MS analysis designed for ambient measurements. Results indicate crude glycerol combustion produces relatively small amounts of acrolein (∼15 ppbv) and other volatile organic compounds, with emissions comparable to those from natural gas combustion.}, journal={PROCEEDINGS OF THE COMBUSTION INSTITUTE}, author={Steinmetz, Scott A. and Herrington, Jason S. and Winterrowd, Christopher K. and Roberts, William L. and Wendt, Jost O. L. and Linak, William P.}, year={2013}, pages={2749–2757} }
 @article{kailasanathan_yelverton_fang_roberts_2013, title={Effect of diluents on soot precursor formation and temperature in ethylene laminar diffusion flames}, volume={160}, ISSN={["1556-2921"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84872855275&partnerID=MN8TOARS}, DOI={10.1016/j.combustflame.2012.11.004}, abstractNote={Soot precursor species concentrations and flame temperature were measured in a diluted laminar co-flow jet diffusion flame at pressures up to eight atmospheres while varying diluent type. The objective of this study was to gain a better understanding of soot production and oxidation mechanisms, which could potentially lead to a reduction in soot emissions from practical combustion devices. Gaseous samples were extracted from the centerline of an ethylene–air laminar diffusion flame, which was diluted individually with four diluents (argon, helium, nitrogen, and carbon dioxide) to manipulate flame temperature and transport properties. The diluted fuel and co-flow exit velocities (top-hat profiles) were matched at all pressures to minimize shear-layer effects, and the mass fluxes were fixed over the pressure range to maintain constant Reynolds number. The flame temperature was measured using a fine gauge R-type thermocouple at pressures up to four atmospheres. Centerline concentration profiles of major non-fuel hydrocarbons collected via extractive sampling with a quartz microprobe and quantification using GC/MS + FID are reported within. The measured hydrocarbon species concentrations are vary dramatically with pressure and diluent, with the helium and carbon dioxide diluted flames yielding the largest and smallest concentrations of soot precursors, respectively. In the case of C2H2 and C6H6, two key soot precursors, helium diluted flames had concentrations more than three times higher compared with the carbon dioxide diluted flame. The peak flame temperature vary with diluents tested, as expected, with carbon dioxide diluted flame being the coolest, with a peak temperature of 1760 K at 1 atm, and the helium diluted flame being the hottest, with a peak temperature of 2140 K. At four atmospheres, the helium diluted flame increased to 2240 K, but the CO2 flame temperature increased more, decreasing the difference to approximately 250 K.}, number={3}, journal={COMBUSTION AND FLAME}, author={Kailasanathan, Ranjith Kumar Abhinavam and Yelverton, Tiffany L. B. and Fang, Tiegang and Roberts, William L.}, year={2013}, month={Mar}, pages={656–670} }
 @article{jing_roberts_fang_2013, title={Effects of Ambient Temperature and Oxygen Concentration on Diesel Spray Combustion Using a Single-Nozzle Injector in a Constant Volume Combustion Chamber}, volume={185}, ISSN={["0010-2202"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84882388541&partnerID=MN8TOARS}, DOI={10.1080/00102202.2013.798315}, abstractNote={This work investigates the effects of ambient conditions on diesel spray combustion in an optically accessible, constant volume chamber using a single-nozzle fuel injector. The ambient O2 concentration was varied between five discrete values from 10% to 21% and three different ambient temperatures (800 K, 1000 K, and 1200 K). These conditions simulate different exhaust gas recirculation (EGR) levels and ambient temperatures in diesel engines. Both conventional diesel combustion and low temperature combustion (LTC) modes were observed under these conditions. A transient analysis and a quasi-steady state analysis are employed in this article. The transient analysis focuses on the flame development from beginning to the end, illustrating how the flame structure changes during this process; the quasi-steady state analysis focuses on the stable flame structure. The transient analysis was conducted using high-speed imaging of both OH* chemiluminescence and natural luminosity (NL). In addition, three different images were acquired using an ICCD camera, corresponding to OH* chemiluminescence, narrow-band flame emission at 430 nm (Band A) and at 470 nm (Band B), and were used to investigate the quasi-steady state combustion process. From the transient analysis, it was found that the NL signal becomes stronger and confined to narrow regions when the temperature and O2 concentration increase during the development of flame. The OH* intensity is much lower for the 10% ambient O2 and 800 K conditions compared to the higher temperatures and O2 levels. This implies the occurrence of LTC under these conditions. Results from the quasi-steady combustion stage indicate that high-temperature reactions effectively oxidize the soot in the downstream locations where only OH* signal is observed. In addition, an area was calculated for each spectral region, and results show that the area of Band A and Band B emissions in these images is larger than the area of OH* emissions at the lower O2 concentrations while the area of OH* emission is larger than the area of Band A and Band B emissions at higher O2 concentrations, for a given ambient temperature. Moreover, the mixture stoichiometry was analyzed using a reformulated definition of excess air ratio for diluted combustion, and this shows that more mixing is required to achieve complete combustion for low ambient oxygen concentration conditions where longer and wider flames are observed. This observation is also verified by the flame length estimated from the NL images.}, number={9}, journal={COMBUSTION SCIENCE AND TECHNOLOGY}, author={Jing, Wei and Roberts, William L. and Fang, Tiegang}, year={2013}, month={Sep}, pages={1378–1399} }
 @article{nash_swanson_preston_yelverton_roberts_wendt_linak_2013, title={Environmental implications of iron fuel borne catalysts and their effects on diesel particulate formation and composition}, volume={58}, ISSN={["1879-1964"]}, DOI={10.1016/j.jaerosci.2013.01.001}, abstractNote={Metal fuel borne catalysts (FBCs) can be used with diesel fuels to effectively reduce engine out particle mass emissions. Internationally, metal FBCs are used for both on-road and off-road applications, although current regulations in the U.S. restrict their on-road use. Although metal FBCs are intended to be used with specialized diesel particulate filters (DPFs) where they are effectively trapped to oxidize additional soot and regenerate the filter, they are sometimes used without DPFs, and, under these conditions, contribute to ambient air emissions and potential health effects. This paper explores potential environmental and health aspects of FBC in diesel fuels, when burned in diesel engines without DPF's. However, rather than examine data from a range of diesel engines, the work focuses on the impact of systematic changes in the fuel composition on emissions from a single, small prototype diesel generator. Experiments using ferrocene as a diesel fuel additive, with varying fuel Fe concentrations from 0 to 200 ppm, indicate ∼30–40% decreases in particle mass, total particle volume, and black carbon emissions, and increases (approaching a factor of 5) in particle number concentrations associated with 10–30 nm Fe particles liberated during soot oxidation. Fe concentrations in overall particle emissions increase from 0.1% to 7.5% as the Fe catalyst is increased from 0 to 200 ppm. The Fe is emitted primarily in the elemental form. While polycyclic aromatic hydrocarbons (PAHs) are reduced with increasing Fe, emissions of alkanes and organic acids show no clear trend. These experimental results can be interpreted in the light of a mechanism whereby the Fe acts to oxidize soot-related PAH species, but does not affect organic compounds associated with unburned fuel and lubrication oil that avoid flame processes. Calculations performed to predict the evolution of the particle size distribution (PSD) associated with the emitted particles suggest that once diluted to simulate behavior along a highway, the Fe-rich nuclei mode is likely to persist for some time. This has health effect implications related to potential exposures associated with these particles.}, journal={JOURNAL OF AEROSOL SCIENCE}, author={Nash, David G. and Swanson, Nicholas B. and Preston, William T. and Yelverton, Tiffany L. B. and Roberts, William L. and Wendt, Jost O. L. and Linak, William P.}, year={2013}, month={Apr}, pages={50–61} }
 @article{kailasanathan_book_fang_roberts_2013, title={Hydrocarbon species concentrations in nitrogen diluted ethylene-air laminar jet diffusion flames at elevated pressures}, volume={34}, ISSN={["1873-2704"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84877712376&partnerID=MN8TOARS}, DOI={10.1016/j.proci.2012.06.148}, abstractNote={Hydrocarbon species concentrations are measured in a laminar jet diffusion flame at elevated pressures with the objective of better understanding soot production and oxidation mechanisms, which will ultimately lead to a reduction in soot emissions from practical combustion hardware. Samples were extracted from the centerline of an ethylene flame diluted with nitrogen. The diluted fuel and co-axial air top-hat exit velocities were matched and the mass fluxes were held constant at all pressures. This paper reports centerline concentration profiles of major non-fuel hydrocarbons and 5 different PAH species measured via extractive sampling with a quartz microprobe and quantification using GC/MS + FID. The peak concentration of acetylene decreased with increase in pressure, suggesting rapid conversion to heavier compounds, whereas the concentrations of the other major heavier non-fuel hydrocarbons increase with an increase in pressure. The measured peak species concentration as a function of pressure is seen to closely follow a power law function, Pn, where n varies from less than zero for acetylene, propane and diacetylene to greater than unity for the larger PAH species.}, number={1}, journal={PROCEEDINGS OF THE COMBUSTION INSTITUTE}, author={Kailasanathan, Ranjith Kumar Abhinavam and Book, Emily K. and Fang, Tiegang and Roberts, William L.}, year={2013}, pages={1035–1043} }
 @article{bohon_roberts_2013, title={NOx emissions from high swirl turbulent spray flames with highly oxygenated fuels}, volume={34}, ISSN={["1873-2704"]}, DOI={10.1016/j.proci.2012.07.064}, abstractNote={Combustion of fuels with fuel bound oxygen is of interest from both a practical and a fundamental viewpoint. While a great deal of work has been done studying the effect of oxygenated additives in diesel and gasoline engines, much less has been done examining combustion characteristics of fuels with extremely high mass fractions of fuel bound oxygen. This work presents an initial investigation into the very low NOx emissions resulting from the combustion of a model, high oxygen mass fraction fuel. Glycerol was chosen as a model fuel with a fuel bound oxygen mass fraction of 52%, and was compared with emissions measured from diesel combustion at similar conditions in a high swirl turbulent spray flame. This work has shown that high fuel bound oxygen mass fractions allow for combustion at low global equivalence ratios with comparable exhaust gas temperatures due to the significantly lower concentrations of diluting nitrogen. Despite similar exhaust gas temperatures, NOx emissions from glycerol combustion were up to an order of magnitude lower than those measured using diesel fuel. This is shown to be a result not of specific burner geometry, but rather is influenced by the presence of higher oxygen and lower nitrogen concentrations at the flame front inhibiting NOx production.}, journal={PROCEEDINGS OF THE COMBUSTION INSTITUTE}, author={Bohon, Myles D. and Roberts, William L.}, year={2013}, pages={1705–1712} }
 @article{wang_natelson_stikeleather_roberts_2013, title={Product sampling during transient continuous countercurrent hydrolysis of canola oil and development of a kinetic model}, volume={58}, ISSN={["1873-4375"]}, DOI={10.1016/j.compchemeng.2013.06.003}, abstractNote={A chemical kinetic model has been developed for the transient stage of the continuous countercurrent hydrolysis of triglycerides to free fatty acids and glycerol. Departure functions and group contribution methods were applied to determine the equilibrium constants of the four reversible reactions in the kinetic model. Continuous countercurrent hydrolysis of canola oil in subcritical water was conducted experimentally in a lab-scale reactor over a range of temperatures and the concentrations of all neutral components were quantified. Several of the rate constants in the model were obtained by modeling this experimental data, with the remaining determined from calculated equilibrium constants. Some reactions not included in the present, or previous, hydrolysis modeling efforts were identified from glycerolysis kinetic studies and may explain the slight discrepancy between model and experiment. The rate constants determined in this paper indicate that diglycerides in the feedstock accelerate the transition from “emulsive hydrolysis” to “rapid hydrolysis”.}, journal={COMPUTERS & CHEMICAL ENGINEERING}, author={Wang, Wei-Cheng and Natelson, Robert H. and Stikeleather, Larry F. and Roberts, William L.}, year={2013}, month={Nov}, pages={144–155} }
 @article{ford_thapaliya_kelly_roberts_lamb_2013, title={Semi-Batch Deoxygenation of Canola- and Lard-Derived Fatty Acids to Diesel-Range Hydrocarbons}, volume={27}, ISSN={["1520-5029"]}, DOI={10.1021/ef4016763}, abstractNote={Fatty acids (FAs) derived via thermal hydrolysis of food-grade lard and canola oil were deoxygenated in the liquid phase using a commercially available 5 wt % Pd/C catalyst. Online quadrupole mass spectrometry and gas chromatography were used to monitor the effluent gases from the semi-batch stirred autoclave reactors. Stearic, oleic, and palmitic acids were employed as model compounds. A catalyst lifetime exceeding 2200 turnovers for oleic acid deoxygenation was demonstrated at 300 °C and 15 atm under 10% H2. The initial decarboxylation rate of palmitic acid under 5% H2 decreases sharply with increasing initial concentration; in contrast, the initial decarbonylation rate increases linearly, indicative of first-order kinetics. Scale-up of diesel-range hydrocarbon production was investigated by increasing the reactor vessel size, initial FA concentration, and FA/catalyst mass ratio. Lower CO2 selectivity and batch productivity were observed at the larger scales (600 and 5000 mL), primarily because of the h...}, number={12}, journal={ENERGY & FUELS}, author={Ford, Jeffrey P. and Thapaliya, Nirajan and Kelly, M. Jason and Roberts, William L. and Lamb, H. Henry}, year={2013}, month={Dec}, pages={7489–7496} }
 @article{chen_wang_roberts_fang_2013, title={Spray and atomization of diesel fuel and its alternatives from a single-hole injector using a common rail fuel injection system}, volume={103}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84870517496&partnerID=MN8TOARS}, DOI={10.1016/j.fuel.2012.08.013}, abstractNote={Fuel spray and atomization characteristics play an important role in the performance of internal combustion engines. As the reserves of petroleum fuel are expected to be depleted within a few decades, finding alternative fuels that are economically viable and sustainable to replace the petroleum fuel has attracted much research attention. In this work, the spray and atomization characteristics were investigated for commercial No. 2 diesel fuel, biodiesel (FAME) derived from waste cooking oil (B100), 20% biodiesel blended diesel fuel (B20), renewable diesel fuel produced in house, and civil aircraft jet fuel (Jet-A). Droplet diameters and particle size distributions were measured by a laser diffraction particle analyzing system and the spray tip penetrations and cone angles were acquired using a high speed imaging technique. All experiments were conducted by employing a common-rail high-pressure fuel injection system with a single-hole nozzle under room temperature and pressure. The experimental results showed that biodiesel and jet fuel had different features compared with diesel. Longer spray tip penetration and larger droplet diameters were observed for B100. The smaller droplet size of the Jet-A were believed to be caused by its relatively lower viscosity and surface tension. B20 showed similar characteristics to diesel but with slightly larger droplet sizes and shorter tip penetration. Renewable diesel fuel showed closer droplet size and spray penetration to Jet-A with both smaller than diesel. As a result, optimizing the trade-off between spray volume and droplet size for different fuels remains a great challenge. However, high-pressure injection helps to optimize the trade-off of spray volume and droplet sizes. Furthermore, it was observed that the smallest droplets were within a region near the injector nozzle tip and grew larger along the axial and radial direction. The variation of droplet diameters became smaller with increasing injection pressure.}, journal={Fuel}, author={Chen, P.-C. and Wang, W.-C. and Roberts, W.L. and Fang, Tiegang}, year={2013}, pages={850–861} }
 @article{wang_roberts_stikeleather_2012, title={Hydrocarbon Fuels From Gas Phase Decarboxylation of Hydrolyzed Free Fatty Acid}, volume={134}, ISSN={["0195-0738"]}, DOI={10.1115/1.4006867}, abstractNote={Gas phase decarboxylation of hydrolyzed free fatty acid (FFA) from canola oil has been investigated in two fix-bed reactors by changing reaction parameters such as temperatures, FFA feed rates, and H2-to-FFA molar ratios. FFA, which contains mostly C18 as well as a few C16, C20, C22, and C24 FFA, was fed into the boiling zone, evaporated, carried by hydrogen flow at the rate of 0.5–20 ml/min, and reacted with the 5% Pd/C catalyst in the reactor. Reactions were conducted atmospherically at 380–450 °C and the products, qualified and quantified through gas chromatography-flame ionization detector (GC-FID), showed mostly n-heptadecane and a few portion of n-C15, n-C19, n-C21, n-C23 as well as some cracking species. Results showed that FFA conversion increased with increasing reaction temperatures but decreased with increasing FFA feed rates and H2-to-FFA molar ratios. The reaction rates were found to decrease with higher temperature and increase with higher H2 flow rates. Highly selective heptadecane was achieved by applying higher temperatures and higher H2-to-FFA molar ratios. From the results, as catalyst loading and FFA feed rate were fixed, an optimal reaction temperature of 415 °C as well as H2-to-FFA molar ratio of 4.16 were presented. These results provided good basis for studying the kinetics of decarboxylation process.}, number={3}, journal={JOURNAL OF ENERGY RESOURCES TECHNOLOGY-TRANSACTIONS OF THE ASME}, author={Wang, Wei-Cheng and Roberts, William L. and Stikeleather, Larry F.}, year={2012}, month={Sep} }
 @article{wang_thapaliya_campos_stikeleather_roberts_2012, title={Hydrocarbon fuels from vegetable oils via hydrolysis and thermo-catalytic decarboxylation}, volume={95}, ISSN={["1873-7153"]}, DOI={10.1016/j.fuel.2011.12.041}, abstractNote={Conversion of canola oil to normal alkane hydrocarbons was investigated using sequential reactions: continuous thermal hydrolysis and fed-batch thermo-catalytic decarboxylation. The free fatty acid (FFA) intermediate product from hydrolysis was quantified using GC–FID, which showed 99.7% conversion and the following components: palmitic, oleic, linoleic, linolenic, stearic, arachidic and behenic acids. The FFA was saturated then decarboxylated at an average rate of 15.5 mmoles/min using a 5% Pd/C catalyst at 300 °C. Approximately 90% decarboxylation conversion to n-alkanes was achieved within 5 h of the reaction. The resulting mixture of n-alkanes can be readily converted into renewable diesel using isomerization to improve the cold flow properties of the fuel.}, number={1}, journal={FUEL}, author={Wang, Wei-Cheng and Thapaliya, Nirajan and Campos, Andrew and Stikeleather, Larry F. and Roberts, William L.}, year={2012}, month={May}, pages={622–629} }
 @article{bohon_metzger_linak_king_roberts_2011, title={Glycerol combustion and emissions}, volume={33}, ISSN={["1540-7489"]}, DOI={10.1016/j.proci.2010.06.154}, abstractNote={With the growing capacity in biodiesel production and the resulting glut of the glycerol by-product, there is increasing interest in finding alternative uses for crude glycerol. One option may be to burn it locally for combined process heat and power, replacing fossil fuels and improving the economics of biodiesel production. However, due to its low energy density, high viscosity, and high auto-ignition temperature, glycerol is difficult to burn. Additionally, the composition of the glycerol by-product can change dramatically depending upon the biodiesel feedstock (e.g., vegetable oils or rendered animal fats), the catalyst used, and the degree of post-reaction cleanup (e.g., acidulation and demethylization). This paper reports the results of experiments to (1) develop a prototype high-swirl refractory burner designed for retrofit applications in commercial-scale fire-tube package boilers, and (2) provide an initial characterization of emissions generated during combustion of crude glycerol in a laboratory-scale moderate-swirl refractory-lined furnace. We report a range of emissions measurements, including nitrogen oxides, total hydrocarbons, and particle mass for two grades of crude glycerol (methylated and demethylated) and compare these to No. 2 fuel oil and propane. We also present preliminary data on the emissions of select carbonyls (by cartridge DNPH). Results indicate that a properly designed refractory burner can provide the thermal environment to effectively combust glycerol, but that high particulate emissions due to residual catalysts are likely to be an issue for crude glycerol combustion.}, journal={PROCEEDINGS OF THE COMBUSTION INSTITUTE}, author={Bohon, Myles D. and Metzger, Brian A. and Linak, William P. and King, Charly J. and Roberts, William L.}, year={2011}, pages={2717–2724} }
 @article{zheng_kuznetsov_roberts_paxson_2011, title={Influence of Geometry on Starting Vortex and Ejector Performance}, volume={133}, ISSN={["0098-2202"]}, DOI={10.1115/1.4004082}, abstractNote={For many propulsion devices, the thrust may be augmented considerably by adding a passive ejector, and these devices are especially attractive for unsteady propulsion systems such as pulse detonation engines and pulsejets. Starting vortices from these unsteady devices dominate the flowfield and control to a great extent the level of the thrust augmentation. Therefore, it is of fundamental interest to understand the geometric influences on the starting vortex and how these manifest themselves in augmenter/ejector performance. An unsteady Reynolds averaged Navier–Stokes calculation was used to study the physics of a starting vortex generated at the exit of a pulsed jet and its interaction with an ejector. A 50 cm long pulsejet (typical hobby scale, allowing comparison with experimental data) with a circular exit was modeled as the resonant driving source and used to suggest an optimal ejector geometry and relative position. Computed limit-cycle thrust augmentation values compared favorably to experimentally obtained values for the same ejector geometries. Results suggest that the optimal diameter of the ejector is related to its relative position, dictated by the trajectory of the vortex toroid. The effect of the length of the ejector (which determines the natural frequency of the ejector, related to the acoustic processes occurring in the ejector) on overall performance was also investigated and shown to be less important than the ejector diameter.}, number={5}, journal={JOURNAL OF FLUIDS ENGINEERING-TRANSACTIONS OF THE ASME}, author={Zheng, Fei and Kuznetsov, Andrey V. and Roberts, William L. and Paxson, Daniel E.}, year={2011}, month={May} }
 @article{geng_zheng_kuznetsov_roberts_paxson_2010, title={Comparison Between Numerically Simulated and Experimentally Measured Flowfield Quantities Behind a Pulsejet}, volume={84}, ISSN={["1573-1987"]}, DOI={10.1007/s10494-010-9247-6}, abstractNote={Pulsed combustion is receiving renewed interest as a potential route to higher performance in air breathing propulsion and ground based power generation systems. Pulsejets offer a simple experimental device with which to study unsteady combustion phenomena and validate simulations. Previous computational fluid dynamics (CFD) simulations focused primarily on pulsejet combustion and exhaust processes. This paper describes a new inlet sub-model which simulates the fluidic and mechanical operation of a valved pulsejet head. The governing equations for this sub-model are described. Sub-model validation is provided through comparisons of simulated and experimentally measured reed valve motion, and time averaged inlet mass flow rate. The updated pulsejet simulation, with the inlet sub-model implemented, is validated through comparison with experimentally measured combustion chamber pressure, inlet mass flow rate, operational frequency, and thrust. Additionally, the simulated pulsejet exhaust flowfield, which is dominated by a starting vortex ring, is compared with particle imaging velocimetry (PIV) measurements on the bases of velocity, vorticity, and vortex location. The results show good agreement between simulated and experimental data. The inlet sub-model is shown to be critical for the successful modeling of pulsejet operation. This sub-model correctly predicts both the inlet mass flow rate and its phase relationship with the combustion chamber pressure. As a result, the predicted pulsejet thrust agrees very well with experimental data.}, number={4}, journal={FLOW TURBULENCE AND COMBUSTION}, author={Geng, Tao and Zheng, Fei and Kuznetsov, Andrey V. and Roberts, William L. and Paxson, Daniel E.}, year={2010}, month={Jun}, pages={653–667} }
 @article{zheng_ordon_scharton_kuznetsov_roberts_2008, title={A new acoustic model for valveless pulsejets and its application to optimization thrust}, volume={130}, ISSN={["0742-4795"]}, DOI={10.1115/1.2900730}, abstractNote={Due to its simplicity, the valveless pulsejet may be an ideal low cost propulsion system. In this paper, a new acoustic model is described, which can accurately predict the operating frequency of a valveless pulsejet. Experimental and computational methods were used to investigate how the inlet and exhaust area and the freestream velocity affect the overall performance of a 50cm pulsejet. Pressure and temperature were measured at several axial locations for different fuel flow rates and different geometries. Computer simulations were performed for exactly the same geometries and fuel flow rates using a commercial CFD package (CFX) to develop further understanding of the factors that affect the performance of a valveless pulsejet. An acoustic model was developed to predict the frequency of these valveless pulsejets. The new model treats the valveless pulsejet engine as a combination of a Helmholtz resonator and a wave tube. This new model was shown to accurately predict geometries for maximum thrust. The model was further extended to account for the effect of freestream velocity. Evidence is provided that valveless pulsejet generates the highest thrust when the inherent inlet frequency matches the inherent exhaust frequency.}, number={4}, journal={JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER-TRANSACTIONS OF THE ASME}, author={Zheng, F. and Ordon, R. L. and Scharton, T. D. and Kuznetsov, A. V. and Roberts, W. L.}, year={2008}, month={Jul} }
 @article{kleinstreuer_zhang_li_roberts_rojas_2008, title={A new methodology for targeting drug-aerosols in the human respiratory system}, volume={51}, ISSN={["1879-2189"]}, DOI={10.1016/j.ijheatmasstransfer.2008.04.052}, abstractNote={Inhalation of medicine for the treatment of lung and other diseases is becoming more and more a preferred option when compared to injection or oral intake. Unfortunately, existing devices such as the popular pressurized metered dose inhalers and dry powder inhalers have rather low deposition efficiencies and their drug-aerosol deliveries are non-directional. This is acceptable when the medicine is inexpensive and does not cause systemic side effects, as it may be the case for patients with mild asthma. However, the delivery of aggressive chemicals, or expensive insulin, vaccines and genetic material embedded in porous particles or droplets requires optimal targeting of such inhaled drug-aerosols to predetermined lung areas. The new methodology introduces the idea of a controlled air-particle stream which provides maximum, patient-specific drug-aerosol deposition based on optimal particle diameter and density, inhalation waveform, and particle-release position. The efficacy of the new methodology is demonstrated with experimentally validated computer simulations of two-phase flow in a human oral airway model with two different sets of tracheobronchial airways. Physical insight to the dynamics of the controlled air-particle stream is provided as well.}, number={23-24}, journal={INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER}, author={Kleinstreuer, Clement and Zhang, Zhe and Li, Zheng and Roberts, William L. and Rojas, Carlye}, year={2008}, month={Nov}, pages={5578–5589} }
 @article{yelverton_roberts_2008, title={Effect of dilution, pressure, and velocity on smoke point in laminar jet flames}, volume={180}, ISSN={["1563-521X"]}, DOI={10.1080/00102200801931570}, abstractNote={Smoke point measurements of diluted methane and ethylene flames were made in a co-flowing laminar jet diffusion flame at pressures up to 8 atm. The smoke point corresponds to the fuel flow rate where the soot production is exactly offset by the soot oxidation, and as such is sensitive to changes in rates of production or oxidation. Flame height in these flames was measured as a function of pressure, diluent, and dilution level as well as both fuel exit velocity profile (i.e., plug or parabolic) and fuel/air velocity ratio. As pressure increases, the smoke point became less sensitive to diluent or dilution level. In addition to heat capacity and thermal diffusivity differences between CO2 and He for example, the large differences in kinematic viscosity was shown to play an important role in the diluent's ability to suppress the fuel's propensity to form soot.}, number={7}, journal={COMBUSTION SCIENCE AND TECHNOLOGY}, author={Yelverton, T. L. Berry and Roberts, W. L.}, year={2008}, pages={1334–1346} }
 @article{marley_danby_roberts_drake_fansler_2008, title={Quantification of transient stretch effects on kernel-vortex interactions in premixed methane-air flames}, volume={154}, ISSN={["0010-2180"]}, DOI={10.1016/j.combustflame.2008.02.003}, abstractNote={Relative flame speeds of time-dependent highly curved premixed methane–air flames (spark-ignited flame kernels) interacting with a laminar vortex have been quantified using high-speed chemiluminescence imaging, particle image velocimetry, and piezoelectric pressure measurements. The goals of this study are to improve fundamental understanding of transient stretch effects on highly curved premixed flames, to provide practical insight into the turbulent growth of spark-ignited flame kernels in internal combustion (IC) engines burning light hydrocarbon fuels, and to provide data for IC engine ignition and combustion model development. Lean and rich CH4–O2–N2 flames were tested (ϕ=0.64, 0.90, and 1.13, with nitrogen dilution to equalize the flame speeds (Sb) in the absence of vortex interaction). Transient stretch rates were varied using three different vortex strengths, and the size of the flame kernel at the start of the vortex interaction controlled by time delay between ignition and vortex generation. Vortex interactions with small (∼5 mm radius) flame kernels were found to increase burning rates for lean (ϕ=0.64) flame kernels substantially. Burning rates for rich (ϕ=1.13) flames were decreased, with total flame kernel extinction occurring in extreme cases. These small flame kernel–vortex interactions are dominated by transient stretch effects and thermodiffusive stability, in agreement with premixed flame theory. However, vortex interactions with larger methane–air flame kernels (∼30 mm radius) led to slight flame speed enhancements for both lean and rich flame kernels, with the flame–vortex process dominated by increased flamefront area generated by vortex-induced flame wrinkling.}, number={1-2}, journal={COMBUSTION AND FLAME}, author={Marley, S. K. and Danby, S. J. and Roberts, W. L. and Drake, M. C. and Fansler, T. D.}, year={2008}, month={Jul}, pages={296–309} }
 @article{yelverton_roberts_2008, title={Soot surface temperature measurements in pure and diluted flames at atmospheric and elevated pressures}, volume={33}, ISSN={["1879-2286"]}, DOI={10.1016/j.expthermflusci.2008.06.014}, abstractNote={Soot surface temperature was measured in laminar jet diffusion flames at atmospheric and elevated pressures. The soot surface temperature was measured in flames at one, two, four, and eight atmospheres with both pure and diluted (using helium, argon, nitrogen, or carbon dioxide individually) ethylene fuels with a calibrated two-color soot pyrometry technique. These two dimensional temperature profiles of the soot aid in the analysis and understanding of soot production, leading to possible methods for reducing soot emission. Each flame investigated was at its smoke point, i.e., at the fuel flow rate where the overall soot production and oxidation rates are equal. The smoke point was chosen because it was desirable to have similar soot loadings for each flame. A second set of measurements were also taken where the fuel flow rate was held constant to compare with earlier work. These measurements show that overall flame temperature decreases with increasing pressure, with increasing pressure the position of peak temperature shifts to the tip of the flame, and the temperatures measured were approximately 10% lower than those calculated assuming equilibrium and neglecting radiation.}, number={1}, journal={EXPERIMENTAL THERMAL AND FLUID SCIENCE}, author={Yelverton, T. L. Berry and Roberts, W. L.}, year={2008}, month={Oct}, pages={17–22} }
 @article{azzi_roberts_rabiei_2007, title={A study on pressure drop and heat transfer in open cell metal foams for jet engine applications}, volume={28}, ISSN={["0261-3069"]}, DOI={10.1016/j.matdes.2005.08.002}, abstractNote={This research investigated the feasibility of placing a ring of open cell metal foam between the combustor and the turbine section of a turbojet engine to homogenize the temperatures leaving the combustor to raise the overall efficiency of such engines. Heat transfer testing on Al foam samples was done using infrared imaging to test the foam's efficiency of mixing the hot and cool temperatures with very promising results (21% and 37% mixing factor in a 10 cm thick layer of 5 and 10 PPI foam, respectively). Pressure drop testing on Al foam was also done in the compressible region of air to investigate the loss of pressure through the foam. The tests concluded that when temperature effects where accounted for, the pressure drop was less than 34.5 KPa per 2.54 cm of foam.}, number={2}, journal={MATERIALS & DESIGN}, author={Azzi, W. and Roberts, W. L. and Rabiei, A.}, year={2007}, pages={569–574} }
 @article{geng_schoen_kuznetsov_roberts_2007, title={Combined numerical and experimental investigation of a 15-cm valveless pulsejet}, volume={78}, ISSN={["1573-1987"]}, DOI={10.1007/s10494-006-9032-8}, number={1}, journal={FLOW TURBULENCE AND COMBUSTION}, author={Geng, T. and Schoen, M. A. and Kuznetsov, A. V. and Roberts, W. L.}, year={2007}, month={Jan}, pages={17–33} }
 @article{geng_kiker_ordon_kuznetsov_zeng_roberts_2007, title={Combined numerical and experimental investigation of a hobby-scale pulsejet}, volume={23}, ISSN={["1533-3876"]}, DOI={10.2514/1.18593}, abstractNote={The pulsejet, due to its simplicity, may be an ideal micro propulsion system, but has received very little attention since the mid 1950’s. Here, modern computational and experimental tools are used to investigate the operation of a hobby scale (50 cm overall length) pulsejet. Gas dynamics, acoustics and chemical kinetics are all involved and are studied to gain an understanding of the various physical phenomena affecting pulsejet operation, scaleability and efficiency. A Bailey Machining Service (BMS) hobby pulsejet is instrumented to obtain pressure, temperature, thrust, and frequency. CH * chemiluminescence is utilized to determine combustion time and high speed imaging of the reed valve operation is undertaken to determine the valve duty cycle. Laser Doppler Velocimetry (LDV) has been used to measure the instantaneous exhaust velocity in these unsteady combustion devices. Numerical simulations are performed utilizing CFX to model the 3-D compressible vicious flow in the pulsejet using the integrated Westbrook-Dryer single step combustion model. The turbulent flow and reaction rate are modeled with the ke model and the Eddy Dissipation Model (EDM), respectively. Simulation results provide physical insight into the pulsejet cycle; comparisons with experimental data obtained in this research are carried out. The traditional view of a pulsejet as a 1/4 wave tube operating on the Humphrey cycle is modified with to account for valve operation and finite chemical kinetics.}, number={1}, journal={JOURNAL OF PROPULSION AND POWER}, author={Geng, T. and Kiker, A., Jr. and Ordon, R. and Kuznetsov, A. V. and Zeng, T. F. and Roberts, W. L.}, year={2007}, pages={186–193} }
 @article{geng_zheng_kiker_kuznetsov_roberts_2007, title={Experimental and numerical investigation of an 8-cm valveless pulsejet}, volume={31}, ISSN={["0894-1777"]}, DOI={10.1016/j.expthermflusci.2006.06.005}, abstractNote={This paper investigates the performance of a small scale pulsejet whose overall length is approximately 8 cm, the smallest pulsejet ever reported to the author's knowledge. Gas dynamics, acoustics and chemical kinetics were modeled to gain an understanding of various physical phenomena affecting pulsejet operation, scalability, and efficiency. Numerical simulations were performed utilizing CFX to model 3-D compressible vicious flow in the pulsejet using the integrated Westbrook–Dryer single step combustion model. The simulation results were validated with experimental data and provide physical insight into the pulsejet operation. The pulsejet was run in valveless mode on hydrogen fuel with either a forward-facing inlet or a pair of rearward-facing inlets. Pressure, temperature, thrust, and frequency were measured as a function of valveless inlet and exit lengths and different geometries. As expected, the rearward-facing inlet produced considerably more net thrust, although still not very efficient, with a TSFC of 0.02 kg/N-h. The operating frequency was found to scale with inlet length to the negative 0.22 power, in addition to the inverse of the overall length for valved pulsejet.}, number={7}, journal={EXPERIMENTAL THERMAL AND FLUID SCIENCE}, author={Geng, T. and Zheng, F. and Kiker, A. P. and Kuznetsov, An. and Roberts, W. L.}, year={2007}, month={Jul}, pages={641–647} }
 @article{berry_roberts_2006, title={Measurement of smoke point in velocity-matched coflow laminar diffusion flames with pure fuels at elevated pressures}, volume={145}, ISSN={["1556-2921"]}, DOI={10.1016/j.combustflame.2005.12.010}, abstractNote={Using a coflow burner, a quartz chimney, and a pressure vessel with good optical access, smoke points of pure fuels were measured in a laminar jet diffusion flame. The smoke point is a fundamental kinetic parameter, as this is the point where production of soot is exactly offset by its oxidation. Ethylene and methane, burning in a velocity-matched, overventilated coflow of air, were tested over a range of pressures from 1 to 16 atm. Fuel flow rate and air coflow rate were iteratively increased, keeping the exit velocity equal, until the smoke point was reached. The volumetric fuel flow and flame height were measured as a function of pressure to determine the functional relationship between these parameters and pressure. The volumetric fuel flow at the smoke point is observed to scale as a power law with pressure, while the smoke point height is best described by a log law with pressure. The residence time, based on flame height and exit velocity, was also calculated as a function of pressure and found to have a nonmonotonic behavior, with a peak at lower pressures.}, number={3}, journal={COMBUSTION AND FLAME}, author={Berry, T. L. and Roberts, W. L.}, year={2006}, month={May}, pages={571–578} }
 @article{li_roberts_brown_gord_2005, title={Acoustic damping rate measurements in binary mixtures of atomic species via transient-grating spectroscopy}, volume={39}, ISSN={["1432-1114"]}, DOI={10.1007/s00348-005-1012-6}, number={4}, journal={EXPERIMENTS IN FLUIDS}, author={Li, YY and Roberts, WL and Brown, MS and Gord, JR}, year={2005}, month={Oct}, pages={687–693} }
 @article{wan_roberts_kuznetsov_2005, title={Computational analysis of the feasibility of a micro-pulsejet}, volume={32}, ISSN={["1879-0178"]}, DOI={10.1016/j.icheatmasstransfer.2004.05.020}, abstractNote={This paper investigates the feasibility of a 2-cm micro-pulsejet by numerically simulating the inviscid gas dynamic phenomena within the exhaust tube and comparing them with those for a pulsejet on the order of 50 cm in length. After initial combustion, the pressure wave propagates towards the exit and reflects back as a rarefaction wave, which generates a minimum pressure in the combustion chamber. This low pressure must be sufficient to open the reed valves to allow fresh reactants to enter. It is shown that for both large and micro-pulsejets, the minimum pressure is low enough. The characteristic operating frequency is found to be approximately inversely proportional to the pulsejet length. Estimation of the boundary layer thickness in the pulsejet shows that viscosity plays a very significant role in the micro-pulsejet and cannot be neglected.}, number={1-2}, journal={INTERNATIONAL COMMUNICATIONS IN HEAT AND MASS TRANSFER}, author={Wan, Q and Roberts, WL and Kuznetsov, AV}, year={2005}, month={Jan}, pages={19–26} }
 @article{wan_wu_chastain_roberts_kuznetsov_ro_2005, title={Forced convective cooling via acoustic streaming in a narrow channel established by a vibrating piezoelectric bimorph}, volume={74}, ISSN={["1573-1987"]}, DOI={10.1007/s10494-005-4132-4}, abstractNote={Forced convection in a narrow channel is investigated both numerically and experimentally. The flow field is established through the mechanism of acoustic streaming. This is accomplished by high frequency vibration of one of the channel walls, which is composed of a piezoelectric bimorph. In the numerical computations, the Navier-Stokes equations are decomposed into the acoustic equations and the streaming equations by the perturbation method. The acoustic field is first numerically obtained, which provides the driving force for the streaming field. The streaming field and the associated temperature field are then obtained numerically. Heat losses from a heat source are measured to determine the efficiency of this as a cooling method. The air-flow patterns in the channel between the heat source and the bimorph actuator are visualized using the particle tracking velocimetry. The visualization clearly shows that vortical streaming (acoustic streaming) can be induced by bimorph vibration, which enhances heat transfer between the heat source and the surrounding air. The temperature decreases obtained computationally and experimentally are in good agreement.}, number={2}, journal={FLOW TURBULENCE AND COMBUSTION}, author={Wan, Q and Wu, T and Chastain, J and Roberts, WL and Kuznetsov, AV and Ro, PI}, year={2005}, month={Mar}, pages={195–206} }
 @article{chaos_chen_welle_roberts_2005, title={Fuel Lewis number effects in unsteady Burke-Schumann hydrogen flames}, volume={177}, ISSN={["1563-521X"]}, DOI={10.1080/00102200590883660}, abstractNote={ABSTRACT Flame response (as determined by temperature and flame thickness) to unsteady hydrodynamics has been measured in acoustically pulsed Burke–Schumann hydrogen flames at two different oscillation frequencies and amplitudes. The effect of fuel Lewis number (Le F) on flame dynamics is isolated by investigating steady and unsteady 40% H2/60% He (Le F > 1) and 40% H2/60% Ar (Le F < 1) flames. For a given flame with Le F < 1, local temperature was found to increase with stretch imparted on the reaction zone by the unsteady flow, whereas the opposite trend was observed for the Le F > 1 flame. Unsteadiness might qualitatively alter the effect of the fuel Lewis number. Notably, for Le F < 1 flames under oscillations of sufficiently high frequency and amplitude, the temperature at the flame tip is higher than that in the shoulder regions, and is different from the temperature field of both steady and low-frequency oscillation flames. This suggests that the effect of unsteady flame stretch may overwhelm that of the flame curvature for sufficiently high unsteadiness.}, number={1}, journal={COMBUSTION SCIENCE AND TECHNOLOGY}, author={Chaos, M and Chen, RH and Welle, EJ and Roberts, WL}, year={2005}, month={Jan}, pages={75-+} }
 @article{marley_roberts_2005, title={Measurements of laminar burning velocity and Markstein number using high-speed chemiluminescence imaging}, volume={141}, ISSN={["1556-2921"]}, DOI={10.1016/j.combustflame.2005.02.011}, abstractNote={A high-speed chemiluminescence imaging diagnostic system has been developed and characterized for measuring species-specific properties of spherically expanding flames, with the objective of obtaining synchronized measurements of multiple flame properties from a single flame experiment. The imaging system comprises a high-speed camera coupled to a high-speed image intensifier and optical filters to isolate emission from desired excited species. This study validates the diagnostic scheme specifically for measuring laminar flame speed and Markstein length from chemiluminescence at three wavelength bands corresponding to OH*, CH*, and total chemiluminescence, respectively. Mixtures of methane-air and methane/ethane-air were tested at initial conditions of atmospheric pressure and room temperature. An under-utilized methodology for computing unburned Markstein length from burned Markstein length was implemented. Flame speed modeling of each mixture was conducted in Chemkin using AramcoMech 2.0. No significant differences were observed between the results derived from the three emission wavelength bands for the flame speeds nor the Markstein lengths. For both of these properties, excellent agreement was observed between the experimental results and the values reported in the literature. Because laminar flame speed and Markstein length show no dependency on the imaging wavelength, the present results imply that multiple flame properties, such as laminar flame speed, Markstein length, flame thickness, and excited-species intensity profiles, can all be acquired simultaneously for the same flame using this method.}, number={4}, journal={COMBUSTION AND FLAME}, author={Marley, SK and Roberts, WL}, year={2005}, month={Jun}, pages={473–477} }
 @article{mccrain_roberts_2005, title={Measurements of the soot volume field in laminar diffusion flames at elevated pressures}, volume={140}, ISSN={["1556-2921"]}, DOI={10.1016/j.combustflame.2004.10.005}, abstractNote={Soot volume fraction (fv) is measured quantitatively in a laminar diffusion flame, with either methane or ethylene as fuel, at elevated pressures up to 2.5 MPa in order to gain a better understanding of the effects of pressure on the soot formation process. Soot continues to be of interest because it is a sensitive indicator of the interactions between combustion chemistry and fluid mechanics and known to be detrimental to human health. To examine the effects of increased pressure on soot production, laser-induced incandescence (LII) is used to obtain the desired spatially resolved measurements of fv as the pressure is incrementally increased to 2.5 MPa. The effects of pressure on the physical characteristics of the flame are also observed. Using a laser light extinction technique, the path-integrated soot volume fraction scales with pressure as p1.0 and p1.2 for the methane–air and ethylene–air flames, respectively, at 65% of the flame height. From the LII images, it is observed that the soot layer radius decreases with increasing pressure, scaling as approximately p−0.5 at 65% of the flame height, for both methane and ethylene flames. The local peak fv is found to scale with pressure as p1.2 for methane and p1.7 for ethylene flames, which is different than the path-integrated soot pressure dependence. The location of peak soot is observed to move from the edges toward the tip of the flame as the pressure is increased for both fuels.}, number={1-2}, journal={COMBUSTION AND FLAME}, author={McCrain, LL and Roberts, WL}, year={2005}, month={Jan}, pages={60–69} }
 @article{xiao_austin_roberts_2005, title={Relative polycyclic aromatic hydrocarbon concentrations in unsteady counterflow diffusion flames}, volume={177}, ISSN={["1563-521X"]}, DOI={10.1080/00102200590917239}, abstractNote={ABSTRACT Qualitative measurements of polycyclic aromatic hydrocarbon (PAH) concentrations made in counterflow diffusion flames subjected to both steady and oscillating strain rates are presented. Planar laser-induced fluorescences was used to make spatially and temporally resolved measurements of the relative concentrations of PAH. The measurements for each PAH size class were made as a function of strain rate, forcing frequency, and fuel type. The peak laser-induced fluorescence spectra from PAHs is known to be a function of the number of benzene rings that make up the particular PAH. It was found that, with increasing strain rate, the intensity of PAH fluorescence decreases dramatically, though unequally, among the three size classes measured. With an unsteady strain rate, the PAH intensity oscillates in phase for frequencies below 50 Hz. When nondimensionalized relative concentration is plotted, it is shown that the smaller PAHs continue to respond to the instantaneous strain rate at a higher frequency than larger PAHs.}, number={4}, journal={COMBUSTION SCIENCE AND TECHNOLOGY}, author={Xiao, J and Austin, E and Roberts, WL}, year={2005}, month={Apr}, pages={691–713} }
 @article{marley_welle_lyons_roberts_2004, title={Effects of leading edge entrainment on the double flame structure in lifted ethanol spray flames}, volume={29}, DOI={10.1016/j.ecpthermflusci.2004.01.009}, number={1}, journal={Experimental Thermal and Fluid Science}, author={Marley, S. K. and Welle, E. J. and Lyons, K. M. and Roberts, W. L.}, year={2004}, pages={23–31} }
 @article{brown_li_roberts_gord_2003, title={Analysis of transient-rating signals for reacting-flow applications}, volume={42}, ISSN={["2155-3165"]}, DOI={10.1364/AO.42.000566}, abstractNote={Single-shot transient-grating measurements for thermometry in pressurized reacting flows are examined in the context of rapid digital signal processing. Simple approaches are discussed for temperature determination and rejection of unwanted signals in real-time measurement applications. Examples of temperature data in pressurized postflame gases are presented in the form of probability-density functions (PDFs). Three contributions to the PDF half-widths are discussed. Analysis of phase-matching requirements indicates that beam steering as a result of density fluctuations affects the signal amplitude but not the grating period. Therefore, such stochastic beam deviations have little effect on the derived temperatures. Mode noise on the cw probe beam as well as linear light scattering are found to be insignificant in the frequency range of the observed transient-grating acoustic signature. Use of a single-mode laser for the pump beams is shown to enhance the signal intensity.}, number={3}, journal={APPLIED OPTICS}, author={Brown, MS and Li, YY and Roberts, WL and Gord, JR}, year={2003}, month={Jan}, pages={566–578} }
 @article{boothe_shih_kong_roberts_2003, title={Goniometric characteristics of optical fibres for temperature measurement in diesel engine exhaust filters}, volume={14}, ISSN={["1361-6501"]}, DOI={10.1088/0957-0233/14/5/305}, abstractNote={The accurate in situ, non-contact measurement of the temperature distribution within diesel after-treatment filters requires the employment of optical fibres with special tip geometry. The goniometric characteristics of optical fibres with flat, 45° angled and bent and polished tips are studied such that the specific radiation acceptance region can be determined. One 2 mm diameter fused silica and two 0.425 mm diameter sapphire optical fibres are examined. Detailed discussion of the relative intensity profiles observed for these fibres is presented. Of the three fibres evaluated, the 45° angled tip geometry provides the most precise response for measuring radiation emitted from the internal filter walls. Exploiting the characteristics of total internal reflection, the 45° angled tip fibre accepts the maximum quantity of incident radiation at an angle perpendicular to the optical axis.}, number={5}, journal={MEASUREMENT SCIENCE AND TECHNOLOGY}, author={Boothe, BJ and Shih, AJ and Kong, N and Roberts, WL}, year={2003}, month={May}, pages={563–572} }
 @article{welle_roberts_carter_donbar_2003, title={The response of a propane-air counter-flow diffusion flame subjected to a transient flow field}, volume={135}, ISSN={["0010-2180"]}, DOI={10.1016/S0010-2180(03)00167-6}, abstractNote={OH planar laser-induced fluorescence (PLIF) and particle image velocimetry have been used to study the frequency response of laminar C3H8-air counterflow diffusion flames to assess the adequacy of the steady-flamelet models. Particle image velocimetry was used to determine the flame strain rate, while OH PLIF was used both to measure temperature at the flame front, using the two-line PLIF technique, and the reaction-zone width. Both measurements demonstrate that the frequency response of flames subjected to a time-varying flow field is diffusion-limited. At the 30-Hz and 50-Hz forcing frequencies, the maximum reaction-zone temperature and width were found to respond quasi-steadily. However, at higher forcing frequencies-i.e., 100 and 200 Hz-transient behavior is evident from the phase relationship between the imposed sinusoidal strain rate and the resulting peak temperature and reaction-zone width. The measured values of the OH-field widths were well fit by an offset sine function. In all cases when the oscillation amplitude is normalized by the cycle mean strain rate and plotted against the non-dimensional flow field frequency, the results collapse onto a single line having a steep negative slope.}, number={3}, journal={COMBUSTION AND FLAME}, author={Welle, EJ and Roberts, WL and Carter, CD and Donbar, JM}, year={2003}, month={Nov}, pages={285–297} }
 @article{li_roberts_brown_herring_2003, title={Transient grating spectroscopy with delayed thermalization in non-combusting CO/air mixtures}, volume={226}, ISSN={["0030-4018"]}, DOI={10.1016/j.optcom.2003.08.036}, abstractNote={Transient grating spectroscopy is used to observe slowly forming thermal gratings in a mixture (non-combusting CO/air at ⩽0.7 MPa) of two gases that are individually transparent. Thermalization in the mixture implies a photochemical process at 532 nm that populates the metastable singlet level 1Δg in O2 through collision-assisted absorption in O2 pairs. The laser intensity dependence of this process is measured.}, number={1-6}, journal={OPTICS COMMUNICATIONS}, author={Li, YY and Roberts, WL and Brown, MS and Herring, GC}, year={2003}, month={Oct}, pages={255–258} }
 @article{kodal_watson_roberts_lyons_2003, title={Turbulence filter and POD analysis for velocity fields in lifted CH4-air diffusion flames}, volume={70}, ISSN={["1386-6184"]}, DOI={10.1023/B:APPL.0000004914.21646.c4}, number={1-4}, journal={FLOW TURBULENCE AND COMBUSTION}, author={Kodal, A and Watson, KA and Roberts, WL and Lyons, KM}, year={2003}, pages={21–41} }
 @article{li_roberts_brown_2002, title={Investigation of gaseous acoustic damping rates by transient grating spectroscopy}, volume={40}, ISSN={["0001-1452"]}, DOI={10.2514/2.1790}, abstractNote={An investigation of acoustic damping ratesin a pressurized gaseous medium by analyzing thetemporal behavior of laser-induced gratings is reported. Experiments were performed in various nonresonant gas samples as a function of pressure and grating spacing. Acoustic damping rates were determined through model e ts to the acquired signals. The results were compared with theoretical calculations using both classical acoustic damping rates and a more comprehensive model that includes rotational and vibrational energy transfer mechanisms. The relationships between the measured acoustic damping rate and molecular structure and pressure and grating spacing are discussed. The utility of exploiting the temporal signature from laser-induced gratings to determine acoustic damping rates in high-pressure gases is identie ed. Nomenclature cp = heat capacity at constant pressure crot = heat capacity of rotational modes cs = local speed of sound cv = heat capacity at constant volume cvib = heat capacity of vibrational modes Dth = thermal diffusivity f = acoustic frequency G = geometric factor I = signal intensity M = molecular weight P = pressure q = grating wave vector, 2 o=3 R = gas constant T = e uid temperature t = time Z = collision number ¯ = damping factor due to energy transfer 0 = acoustic damping coefe cient ° = specie c heat ratio 1n = local modulation in index of refraction ´ = scattering efe ciency of laser-induced grating µ = full angle between beams · = thermal conductivity 3 = grating spacing ¸ = laser wavelength π = dynamic viscosity Ω = e uid density ?rot = rotational relaxation time ?vib = vibrational relaxation time}, number={6}, journal={AIAA JOURNAL}, author={Li, YY and Roberts, WL and Brown, MS}, year={2002}, month={Jun}, pages={1071–1077} }
 @article{xiong_roberts_2002, title={Observations on the interaction between a premixed flame kernel and a vortex of different equivalence ratio}, volume={29}, ISSN={["1873-2704"]}, DOI={10.1016/S1540-7489(02)80207-3}, abstractNote={Charge stratification by direct local injection has been actively studied as a technology for spark-ignitedlean-burn engines. Local mixture enrichment by direct injection in the vicinity of the spark plug at the time of ignition can significantly affect flame kernel development and extend the lean flammability limit of a given fuel/air mixture. The interaction between a lean premixed spark-ignited flame kernel and a laminar vortex toroid containing a different fuel/air mixture was experimentally investigated to gain an understanding of the effect of “turbulent” bulk transport of material into the growing flame kernel. In this study, flame kernels were ignited in a lean premixed methane/air mixture with equivalence ratio of 0.6, while the equivalence ratio of the vortex core fluid was varied between zero and infinity. Chemiluminescence images from CH* of the kernel-vortex interaction were captured using either a single-shot ICCD camera or a high-speed intensified camera. Results of the flame kernel-vortex interactions for six test cases are presented and discussed. It was observed that vortices composed of fluid outside the traditional flammability limits were completely consumed inside the growing flame kernel. Due to entrainment of ambient fluid as the vortex propagates toward the kernel, the equivalence ratio of the vortex is not constant. The Lewis number of the vortex mixture determines if the vortex is consumed as a single pocket or breaks up into multiple pockets before being consumed. Pure fuel vortices were observed to have both a very diffuse reaction zone as well as a region at the upper rim of the kernel-vortex interface of intense chemiluminescence.}, journal={PROCEEDINGS OF THE COMBUSTION INSTITUTE}, author={Xiong, Y and Roberts, WL}, year={2002}, pages={1687–1693} }
 @article{xiong_roberts_drake_fansler_2001, title={Investigation of pre-mixed flame-kernel/vortex interactions via high-speed imaging}, volume={126}, ISSN={["1556-2921"]}, DOI={10.1016/S0010-2180(01)00293-0}, abstractNote={To reduce cycle-to-cycle variations in SI-IC engines, knowledge of early flame kernel growth in a turbulent flow field is required. Understanding the interaction between a flame kernel and a vortex is an important fundamental step toward this goal. This paper presents high-speed movies of combustion luminosity during the interaction of a laminar vortex with a spark-generated pre-mixed flame kernel in a quiescent combustion chamber. The resulting time evolution of the perturbed flame kernel shows that laminar vortices of various sizes and vortex strengths can increase the kernel growth rate by at least a factor of 3 and significantly increase combustion reaction rates by involving additional highly curved and stretched flame fronts.}, number={4}, journal={COMBUSTION AND FLAME}, author={Xiong, Y and Roberts, WL and Drake, MC and Fansler, TD}, year={2001}, month={Sep}, pages={1827–1844} }
 @article{santoianni_decroix_roberts_2001, title={Temperature imaging in an unsteady propane-air counterflow diffusion flame subjected to low frequency oscillations}, volume={66}, ISSN={["1573-1987"]}, DOI={10.1023/A:1011465203719}, number={1}, journal={FLOW TURBULENCE AND COMBUSTION}, author={Santoianni, DA and DeCroix, ME and Roberts, WL}, year={2001}, pages={23–36} }
 @article{chen_lattimer_roberts_2000, title={Extinction of laminar counterflow diffusion flames of CH4 and C3H8 fuels with inert jet impingement}, volume={160}, ISSN={["1563-521X"]}, DOI={10.1080/00102200008935798}, abstractNote={Experimental results of N2-diluted counterflow diffusion flames of CH4 and C3H18 vs, air with local extinction are reponed. The local extinction was caused by inert jet impingement on flames at selected locations either from the fuel or oxidizer side of the reaction zone. This was done to simulate how local extinction affects flame extinction over a larger flame area. The results are: (1) Local extinction of both CH4 and C3H8 flames occurs at a lower strain rate when the inert jet impingement originates from the airside. (2) The global extinction strain rate for CH4 flames is insensitive to the location and number (one vs. three) of local extinction sites. (3) For C3H8 flames, one single inert jet impinging from the airside along the centerline is more effective in causing global extinction than three inert jets impinging at regions away from the centerline. This suggests that flame extinction over a larger area may depend on strategically selected smaller local extinction sites. Furthermore, since similar results of (3) are not observed in this study for CH4 flames, fuel chemistry may also play a role in the effectiveness of local extinction transitioning to global extinction. Differences between flame stabilization mechanisms with and without local extinction are discussed and the implications for turbulent diffusion flames are outlined.}, journal={COMBUSTION SCIENCE AND TECHNOLOGY}, author={Chen, RH and Lattimer, C and Roberts, WL}, year={2000}, pages={103–118} }
 @article{welle_roberts_decroix_carter_donbar_2000, title={Simultaneous particle-imaging velocimetry and OH planar laser-induced fluorescence measurements in an unsteady counterflow propane/air diffusion flame}, volume={28}, ISSN={["1873-2704"]}, DOI={10.1016/s0082-0784(00)80609-8}, abstractNote={To study the transient response of a diffusion flame to an unsteady flowfied, quatitative measurements of velocity, using particle-imaging velocimetry, and OH measurements, using planar laser-induced fluorescence, were made simultaneously in an oscillating conterflow diffusion flame. These non-intrusive measurements were performed to spatially and tempoerally resolved flowrield and flame characteristics as a function of initial strain rate and forcing frequency. For the forcing frequencies considered in this study, the strain rate fluctuations were found to lag the velocity fluctuations, but the phase difference decresed with increasing forcing frequency. At lower forcing frequencies, the width of the OH field responded quasi-steadily, but as the forcing frequency increased, the OH field showed transient effects. The dilatation velocity, defined as the difference between the minimum velocity in the preheat zone and the maximum velocity in the reaction zone, was used as a flame temperature indicator. The dilatation velocity revealed that the phase difference between the velocity and the temperature increased with increasing forcing frequency, confirming the existence of a diffusion limited response. The resuls presented here help to illuminate the interconnecting relationships between the chemistry, fluid dynamics, and reactant transport times.}, journal={PROCEEDINGS OF THE COMBUSTION INSTITUTE}, author={Welle, EJ and Roberts, WL and Decroix, ME and Carter, CD and Donbar, JM}, year={2000}, pages={2021–2027} }
 @article{decroix_roberts_2000, title={Transient flow field effects on soot volume fraction in diffusion flames}, volume={160}, ISSN={["0010-2202"]}, DOI={10.1080/00102200008935801}, abstractNote={Quantitative measurements of soot concentration made in an oscillating propane-air counterflow diffusion flame are presented. The non-intrusive laser induced incandescence (LII) technique was used to make spatially and temporally resolved measurements of soot volume fraction in these transient flames as a function of initial steady strain rate, forcing frequency, and forcing amplitude of the strain rate fluctuation. The results of this study show that the soot formation process becomes insensitive to fluctuations in strain rate at high initial strain rates. At low initial strain rates, however, the maximum soot concentration is drastically reduced with high frequency, high amplitude fluctuations compared to the corresponding steady strain soot volume fraction. Low frequency oscillations are found to always increase the maximum soot concentration, by up to a factor of six for some conditions. These measurements provide important insight into the response of the chemistry control1ing the soot formation process in flamelets subject to unsteady rates of strain.}, journal={COMBUSTION SCIENCE AND TECHNOLOGY}, author={Decroix, ME and Roberts, WL}, year={2000}, pages={165–189} }
 @article{eichenberger_roberts_1999, title={Effect of unsteady stretch on spark-ignited flame kernel survival}, volume={118}, ISSN={["0010-2180"]}, DOI={10.1016/S0010-2180(98)00169-2}, abstractNote={The chemistry–turbulence interaction remains one of the most important topics in combustion research. The ignition of premixed reactants in a highly turbulent environment is fundamentally coupled to this chemistry–turbulence interaction. The spark-ignition (SI) internal combustion (IC) engine relies on the ability of the flame kernel to survive the high-strain-rate, unsteady environment of a turbulent flowfield and successfully transition into a fully developed flame to operate cleanly and efficiently. If certain length and velocity scales within the turbulence spectrum are found to promote flame kernel growth, then by tailoring the flow passages and aerodynamics of the intake valves, piston, and combustion chamber, it may be possible to increase the efficiency and reduce the emissions of SI IC engines. This paper describes a novel experimental investigation of a spark-generated flame kernel interacting with a single vortex toroid with well-defined length and velocity scales. This experiment measured the ability of a vortex to quench a growing kernel in a very lean methane–air mixture at atmospheric pressure. The absence of superequilibrium OH concentrations, qualitatively determined by planar laser-induced fluorescence (PLIF), was used as in indicator of quenching. It was found that larger eddies are more effective at globally quenching the flamefront, requiring a lower strength, when compared to vortices with a smaller characteristic length. At the globally quenching condition, the maturity of the kernel was then increased incrementally until the vortex was no longer able to completely strain out the kernel. The result of this was surprising in that the larger vortices had a much narrower range of kernel maturity for which the vortex could still quench the growing kernel.}, number={3}, journal={COMBUSTION AND FLAME}, author={Eichenberger, DA and Roberts, WL}, year={1999}, month={Aug}, pages={469–478} }
 @article{brown_roberts_1999, title={Single-point thermometry in high-pressure, sooting, premixed combustion environments}, volume={15}, ISSN={["0748-4658"]}, DOI={10.2514/2.5400}, abstractNote={We have performed nonintrusive thermometry in the burnt gases of rich, pressurized ethylene/air flames using a frequency measurement based on laser-induced gratings. Light from a continuous-wav e probe beam is coherently scattered from a thermal or electrostrictive grating induced by a pair of crossed, pulsed pump beams. The measured Doppler shift of the signal beam is a function of the local speed of sound from which a temperature can be extracted. At equivalence ratios of 1.6, the transient grating temperature agreed with a corrected thermocouple temperature. At higher soot loading, it is necessary to account for the change in local gas composition caused by soot particle vaporization. Soot particles, acting as blackbody absorbers, were observed to generate thermal gratings of diagnostic value.}, number={1}, journal={JOURNAL OF PROPULSION AND POWER}, author={Brown, MS and Roberts, WL}, year={1999}, pages={119–127} }
 @article{decroix_roberts_1999, title={Study of transient effects on the extinction limits of an unsteady counterflow diffusion flame}, volume={146}, ISSN={["0010-2202"]}, DOI={10.1080/00102209908924208}, abstractNote={Extinction strain rates in unsteady methane- and propane-air counterflow diffusion flames were experimentally measured as a function of initial strain rate, oscillation frequency, and amplitude of the imposed fluctuation. The maximum strain rate was found to occur at a temporal phase corresponding to the maximum velocity for the diluted methane flame. However, for the propane flame the maximum strain rate occurred when the imposed velocity fluctuation was zero and decreasing. Above an oscillation frequency of 100 Hz, the diluted methane flame was able to survive peak strain rates exceeding the steady extinction strain rate. The minimum air velocity in the pure methane and propane flames was negative for all cases studied, which is most likely responsible for flame extinction at low frequencies and initial strain rates. However, at high initial strain rates and forcing frequencies peak unsteady strain rates at extinction approached the steady extinction strain rate and flow reversal was much less significant.}, number={1-6}, journal={COMBUSTION SCIENCE AND TECHNOLOGY}, author={Decroix, ME and Roberts, WL}, year={1999}, pages={57–84} }
 @article{wang_jobin_allen_roberts_jaffe_1999, title={Suppression of NF-kappa B-Dependent proinflammatory gene expression in human RPE cells by a proteasome inhibitor}, volume={40}, number={2}, journal={Investigative Ophthalmology and Visual Science}, author={Wang, X. C. and Jobin, C. and Allen, J. B. and Roberts, W. L. and Jaffe, G. J.}, year={1999}, pages={477–486} }
 @article{pellett_isaac_humphreys_gartrell_roberts_dancey_northam_1998, title={Velocity and thermal structure, and strain-induced extinction of 14 to 100% hydrogen-air counterflow diffusion flames}, volume={112}, ISSN={["0010-2180"]}, DOI={10.1016/S0010-2180(97)00169-7}, abstractNote={Extensive results from axisymmetric convergent-nozzle and straight-tube opposed jet burners (OJBs) characterized strain-induced extinction of unanchored (free-floating), laminar H2/N2-air flames. Parameters included (a) plug-flow and parabolic input velocity profiles, (b) jet exit diameters ranging 2.7 to 7.2 mm for nozzles and 1.8 to 10 mm for tubes, (c) various relative jet gaps, and (d) 14 to 100% H2 in the fuel jet. Extinction, a sudden rupture (blowoff) of the mostly-airside disk flame, occurred as fuel and air flows were slowly increased and a critical radial strain rate was exceeded. The disk flame was restored at much lower flows, unique to H2 systems. Focusing schlieren, thermocouple, and airside LDV (and PIV) data confirmed the one-dimensional (I-D) character of nozzle-OJB flow fields; axial widths of velocity- and thermal-layers varied as (input strain rate)−1/2 for both nozzles and tubes. The global approximation of a I-D applied stress rate (ASR), using average air jet velocity divided by exit diameter, enabled high quality correlations of extinction data with varied H2 concentrations, for both nozzles and tubes. Pre-extinction ASRs for nozzles agreed closely with LDV-measured centerline input strain rates; for tubes, however, an empirical factor of 3 produced close agreement. For methane-air extinction, nozzle-OJB ASRs agreed within 4% of independent nozzle and Tsuji burner results. For extinction of 100% H2-air, an ASR of 5670 1/s compared with 7350, 8140, and 8060 from independent 1-D numerical evaluations using potential-flow inputs; for 50 to 14% H2 inputs, agreement was much closer. The nozzle-ASR/tube-ASR ratio for extinction was ≥3 for <50% H2 inputs, 2.74 ± 0.03 for 50 to 100% H2 inputs, and 2.83 for methane-air. Because these ratios exceeded 2.0, which "accounted" for centerline velocity inputs from parabolic profiles, an additional 3/2 radial strain component was apparent and was supported by the axial velocity gradient measurements.}, number={4}, journal={COMBUSTION AND FLAME}, author={Pellett, GL and Isaac, KM and Humphreys, WM and Gartrell, LR and Roberts, WL and Dancey, CL and Northam, GB}, year={1998}, month={Mar}, pages={575–592} }
 @article{herring_roberts_brown_debarber_1996, title={Temperature measurement by degenerate four-wave mixing with strong absorption of the excitation beams}, volume={35}, ISSN={["0003-6935"]}, DOI={10.1364/ao.35.006544}, abstractNote={We have made simultaneous temperature measurements by degenerate four-wave mixing (DFWM) and absorption spectroscopy of OH in a CH(4)-air, lifted-diffusion flame. After we corrected the DFWM data for laser beam absorption of as much as 60%, the DFWM-based temperatures were in good agreement with temperatures derived strictly from the absorption data, as well as a one-dimensional reacting flow simulation.}, number={33}, journal={APPLIED OPTICS}, author={Herring, GC and Roberts, WL and Brown, MS and DeBarber, PA}, year={1996}, month={Nov}, pages={6544–6547} }