@article{wang_vinod_fang_2021, title={Compression ignition and spark assisted ignition of direct injected PRF65 spray}, volume={291}, ISSN={["1873-7153"]}, DOI={10.1016/j.fuel.2020.120123}, abstractNote={In this study the spark assisted compression ignition combustion (SACI) developments were investigated using PRF65 (low octane fuel), a mixture of 65% isooctane (by volume) and 35% n-heptane (by volume) with a RON of 65. Characteristics like the cumulative heat release (CHR) and the peak heat release rates (HRR) were studied pressure data from experiments conducted in a constant volume combustion chamber (CVCC) for more precise control of the tested conditions. Spray flame images were also studied using high speed imaging systems to understand the effect of the conditions tested in the luminosity of the flame. Experiments were performed to understand the effects of oxygen concentration and ambient temperatures. Results show that the heat release rate increases initially and then decreases with the increase in the ambient temperature and the peak heat release rate appears around 650 K to 700 K. The peak heat release rate timing is advanced with the increase of the ambient temperature or oxygen level. Flame luminosity was also found to increases with the increase in ambient temperature. Under a low ambient temperature, the oxygen level plays a major role in affecting the peak heat release rate. Under lower oxygen levels, the flame becomes darker, the ignition delay becomes longer, and the combustion process takes more time to complete. A well timed spark timing was found to advance the peak HRR and shorten ignition delay, but this effect becomes minor when the temperature increases.}, journal={FUEL}, author={Wang, Libing and Vinod, Kaushik Nonavinakere and Fang, Tiegang}, year={2021}, month={May} }
 @article{wang_vinod_fang_2021, title={Spark effects on compression ignition of PRF95 direct injection spray in a constant volume combustion chamber}, volume={129}, ISSN={["1879-2286"]}, DOI={10.1016/j.expthermflusci.2021.110456}, abstractNote={In this study, spark assisted compression ignition (SACI) combustion was investigated using PRF95 (a low reactivity high octane reference fuel of common commercial gasoline), a mixture of 95% isooctane (by volume) and 5% n-heptane (by volume) with a RON of 95 and compared with pure compression-ignition without spark. Characteristics like the cumulative heat release (CHR) and the heat release rates (HRR) were studied using pressure data from experiments conducted in a constant volume combustion chamber (CVCC). The combustion process was visualized with a high speed imaging technique. Tests were conducted to understand the effects of ambient oxygen concentration and temperatures on the heat release and combustion flame developments while controlling other factors. From the experiments it is evident that the peak heat release rate increases initially and then decreases with the increase in the ambient temperature and the highest peak heat release rate appears around 650–700 K. The peak heat release rate timing decreases with the increase of the ambient temperature or oxygen level. Under a low ambient temperature, the oxygen level plays a major role in affecting the peak heat release rate. Under lower oxygen levels, the flame becomes weaker, the ignition delay becomes longer, and the combustion process takes more time to complete. Proper spark timing can help advance the peak HRR and shorten ignition delay, but this effect becomes minor when the ambient temperature increases. For the current high octane low reactivity fuel, auto-ignition can be achieved for all the investigated ambient temperature and oxygen levels. But it is noticed that a spark is necessary for low ambient oxygen and low ambient temperature to achieve proper combustion.}, journal={EXPERIMENTAL THERMAL AND FLUID SCIENCE}, author={Wang, Libing and Vinod, Kaushik Nonavinakere and Fang, Tiegang}, year={2021}, month={Nov} }
 @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{wang_vinod_fang_2019, title={Effects of fuels on flash boiling spray from a GDI hollow cone piezoelectric injector}, volume={257}, ISSN={["1873-7153"]}, url={https://doi.org/10.1016/j.fuel.2019.116080}, DOI={10.1016/j.fuel.2019.116080}, abstractNote={Flash-boiling of fuel sprays can have a significant effect on spray formation and its characteristics due to bubble nucleation, growth, and phase change, producing explosive-like atomization and complex spray structures. In this work, experiments were conducted to study the spray of both pure substance fuels (pure isooctane, pure ethanol) and multicomponent fuels (50/50 mixture of isooctane and ethanol, commercial gasoline), under flash boiling conditions and non-flash boiling conditions. Under different temperature and ambient pressure, different superheated degrees can be achieved for the fuels. Pure substances have a single vapor pressure curve, while mixtures do not have a single boiling point at a given pressure, and a two-phase region exists for multicomponent fuel. Under the same conditions, ethanol has higher superheated degree compared to isooctane, but the heat of vaporization for ethanol is also much higher. This contributes to the fact that less boiling is observed in the ethanol spray with longer penetration in several cases. Mixture 50/50 shows a good average of isooctane and ethanol for spray penetration and spray front plume ratio analysis. Gasoline, due to its low initial boiling point and wide range of components, has the widest plume ratio distribution and smallest gradient, as well as complex peak penetration velocity distribution. The results also imply that adding low boiling point (preferably with low heat of vaporization as well) additive or component to high boiling point fuel can facilitate flash boiling, fuel vaporization and mixing.}, journal={FUEL}, publisher={Elsevier BV}, author={Wang, Libing and Vinod, Kaushik Nonavinakere and Fang, Tiegang}, year={2019}, month={Dec} }
 @article{wang_wang_fang_2019, title={Flash boiling hollow cone spray from a GDI injector under different conditions}, volume={118}, ISSN={["1879-3533"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85066876915&partnerID=MN8TOARS}, DOI={10.1016/j.ijmultiphaseflow.2019.05.009}, abstractNote={Good spray atomization facilitates fuel evaporation in a gasoline engine, thus contributing to higher fuel efficiency and lower emissions. During certain operations of a gasoline direct injection (GDI) engine, the combination of increased fuel temperature and sub-atmospheric cylinder pressure during injection can lead to flash boiling condition, which promotes droplet breakup and evaporation. In this study, experiments were carried out to study the flash boiling and non-flash boiling spray of a hollow cone GDI piezoelectric injector. By the combination of different temperature and ambient pressure, different superheat degrees (Tf-Tb) and different ambient-to-saturation pressure ratios (Pa/Ps) can be achieved. For a hollow cone injector, the flash boiling spray can cause the cone shape spray to expand, both inwards and outwards. The axisymmetric inward expansion would converge together and form a fast developing plume shape, and the transition point for plume front to appear is around 0.5 for Pa/Ps ratio. When Pa/Ps is larger than 0.5, the spray development is dominated by the injection momentum and the effect of boiling is minor. When Pa/Ps is reduced to below 0.5, the flash boiling effect takes place and changed the spray dynamics. The peak penetration velocity starts increasing rapidly with the superheated degree only, and a good linear relationship exists between plume ratio and the log(Pa/Ps). The spray axial penetration result at a certain time frame shows three regimes: Pa/Ps > 0.5, 0.1 < Pa/Ps < 0.5 and Pa/Ps < 0.1. When Pa/Ps is less than 0.1, flare flash boiling happens and the original spray shape is hardly maintained due to the micro-explosion, meantime the spray axial penetration further increases at a reduced rate. While cases with similar Pa/Ps value can exhibit similar penetration character, cases with similar Tf-Tb value can show some difference.}, journal={INTERNATIONAL JOURNAL OF MULTIPHASE FLOW}, author={Wang, Libing and Wang, Fujun and Fang, Tiegang}, year={2019}, month={Sep}, pages={50–63} }
 @article{wang_lowrie_ngaile_fang_2019, title={High injection pressure diesel sprays from a piezoelectric fuel injector}, volume={152}, ISSN={["1359-4311"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85062213294&partnerID=MN8TOARS}, DOI={10.1016/j.applthermaleng.2019.02.095}, abstractNote={Increasing injection pressure can increase combustion efficiency in direct injection (DI) diesel engines attributing to enhanced atomization. In this paper, a high pressure experimental setup was built to generate ultra-high fuel pressure. An intensification unit was used to magnify the pressure by about 10 times. Preliminary testing of the high pressure system produced a peak pressure of about 8700 bar. Due to the pressure limitation of the commercially available diesel fuel system, the maximum pressure tested in a practical piezoelectric fuel injector was 2500 bar. A high-speed imaging technique was used to visualize the fuel injection events and spray images were taken by a high speed camera for quantitative analysis. A Schlieren technique was used to visualize the shock waves generated during spray penetration. The near nozzle early stage spray development was also studied using a long distance microscope and an intensified charge coupled device (ICCD) camera. Results show that the spray penetration velocity increases with the increase of the injection pressure, while a higher injection pressure leads to a later opening of the piezoelectric injector. The spray angle first has a large value, then remains relatively steady throughout the injection process. Schlieren results clearly demonstrate detached shock waves during the spray penetration. The near nozzle results show that during the very early stage the spray penetration is quite linear for different injection pressures and the spray angle also appears to be very large at the beginning, which is consistent with the high-speed imaging results. Both high-speed imaging and near nozzle results were compared with published empirical equations. The high-speed imaging result shows a good match with the linear stage of empirical equation, while near nozzle result shows lower penetration velocity, indicating that there exists a very short “acceleration stage” for spray development at the starting moment of fuel injection.}, journal={APPLIED THERMAL ENGINEERING}, author={Wang, Libing and Lowrie, James and Ngaile, Gracious and Fang, Tiegang}, year={2019}, month={Apr}, pages={807–824} }
 @article{wang_yang_wang_zhu_fang_2019, title={Maximum Spread of Droplet Impacting onto Solid Surfaces with Different Wettabilities: Adopting a Rim-Lamella Shape}, volume={35}, ISSN={["0743-7463"]}, url={https://doi.org/10.1021/acs.langmuir.8b03748}, DOI={10.1021/acs.langmuir.8b03748}, abstractNote={Experimental and theoretical investigations are presented for the maximum spread factor (βm) of an impacting droplet onto solid surfaces with contact angle hysteresis. Experiments were conducted with deionized water on six surfaces with different wettabilities. The examined Weber number ( We) falls between 10-1 and 103. A new energetic model adopting a rim-lamella shape is proposed to better represent the droplet shape at the maximum spread. The dynamic contact angle at the maximum spread (θβm) is introduced in the model to account for the curvature of the surrounding rim induced by surface wettabilities. A lamella-rim thickness ratio κ ≈ AWe- B ( A, B > 0) is utilized successfully to depict the droplet shape at different We in a unifying manner. Comprehensive evaluations of the model demonstrate that the theoretical prediction can well recover the features of the experimental observations. The L2-error analysis demonstrates the improvement of the proposed model in predicting βm for a wide range of We = 10-1 to 103: the calculated errors are smaller than 8% for all six surfaces. Moreover, the proposed model can also be applied to predict energy conversion/dissipation during the droplet spreading process and the effects of surface wettability on βm in a reasonable manner. The variation of the percentage of the surface energy and viscous dissipation is consistent with that in previous simulations. The weakness of the current model for predicting βm at extremely low Weber number ( We < 1) is also explained.}, number={8}, journal={LANGMUIR}, publisher={American Chemical Society (ACS)}, author={Wang, Fujun and Yang, Lei and Wang, Libing and Zhu, Yong and Fang, Tiegang}, year={2019}, month={Feb}, pages={3204–3214} }
 @article{wang_wu_ahmed_badra_sarathy_roberts_fang_2019, title={Auto-ignition of direct injection spray of light naphtha, primary reference fuels, gasoline and gasoline surrogate}, volume={170}, ISSN={["1873-6785"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85059141893&partnerID=MN8TOARS}, DOI={10.1016/j.energy.2018.12.144}, abstractNote={In this work, the spray and auto-ignition characteristics of light naphtha (LN), primary reference fuels (PRF65, PRF95), Haltermann gasoline (CARB LEVIII, 10 vol% ethanol), and a gasoline surrogate were studied in an optically accessible constant volume combustion chamber. An outwardly opening hollow cone piezoelectric gasoline direct injection fuel injector was used. Five ambient temperatures from 650 to 950 K with a 75 K step were selected with a fixed ambient density of 3.5 kg/m3, similar to the Spray G density defined by the engine combustion network (ECN). Fuel auto-ignition was achieved with varying ignition delays for the five investigated fuels depending on the selected experimental conditions. Results show that the auto-ignition locations are randomly distributed in the combustion chamber. Differences in ignition delay times among the five fuels are more significant when the ambient temperature is lower than 750 K. When the ambient temperature is lower than 750 K, PRF95 always has the longest ignition delay and LN has the shortest. Ignition delays of the five fuels are almost identical when the ambient temperature exceeds 750 K. Meanwhile, the five fuels have a similar spray front penetration length and spray angles before the occurrence of auto-ignition under all the investigated conditions.}, journal={ENERGY}, author={Wang, Libing and Wu, Zengyang and Ahmed, Ahfaz and Badra, Jihad A. and Sarathy, S. Mani and Roberts, William L. and Fang, Tiegang}, year={2019}, month={Mar}, pages={375–390} }
 @article{wang_badra_roberts_fang_2017, title={Characteristics of spray from a GDI fuel injector for naphtha and surrogate fuels}, volume={190}, ISSN={["1873-7153"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84996565995&partnerID=MN8TOARS}, DOI={10.1016/j.fuel.2016.11.015}, abstractNote={Characterization of the spray angle, penetration, and droplet size distribution is important to analyze the spray and atomization quality. In this paper, the spray structure development and atomization characterization of two naphtha fuels, namely light naphtha (LN) and whole naphtha (WN) and two reference fuel surrogates, i.e. toluene primary reference fuel (TPRF) and primary reference fuel (PRF) were investigated using a gasoline direct injection (GDI) fuel injector. The experimental setup included a fuel injection system, a high-speed imaging system, and a droplet size measurement system. Spray images were taken by using a high-speed camera for spray angle and penetration analysis. Sauter mean diameter, Dv(10), Dv(50), Dv(90), and particle size distribution were measured using a laser diffraction technique. Results show that the injection process is very consistent for different runs and the time averaged spray angles during the measuring period are 103.45°, 102.84°, 102.46° and 107.61° for LN, WN, TPRF and PRF, respectively. The spray front remains relatively flat during the early stage of the fuel injection process. The peak penetration velocities are 80 m/s, 75 m/s, 75 m/s and 79 m/s for LN, WN, TPRF and PRF, respectively. Then velocities decrease until the end of the injection and stay relatively stable. The transient particle size and the time-averaged particle size were also analyzed and discussed. The concentration weighted average value generally shows higher values than the arithmetic average results. The average data for WN is usually the second smallest except for Dv90, of which WN is the biggest. Generally the arithmetic average particle sizes of PRF are usually the smallest, and the sizes does not change much with the measuring locations. For droplet size distribution results, LN and WN show bimodal distributions for all the locations while TPRF and PRF shows both bimodal and single peak distribution patterns. The results imply that droplet size distribution is skewed to the larger side for locations close the axis and is skewed to the smaller side for distance away from the axis.}, journal={FUEL}, author={Wang, Libing and Badra, Jihad A. and Roberts, William L. and Fang, Tiegang}, year={2017}, month={Feb}, pages={113–128} }