@article{feng_zhao_kleinstreuer_wang_wang_wu_lin_2018, title={An in silico inter-subject variability study of extra-thoracic morphology effects on inhaled particle transport and deposition}, volume={123}, ISSN={["1879-1964"]}, DOI={10.1016/j.jaerosci.2018.05.010}, abstractNote={An understanding of human inter-subject variability is crucial for the implementation of personalized pulmonary drug delivery as well as exposure assessment of airborne hazardous materials. However, due to the lack of statistically robust data and subsequent comparisons, the influence of human respiratory morphology on inhaled nano-/micro-particle transport and deposition is still not fully known. Thus, focusing on identifying geometric parameters that significantly influence airflow and inhaled particle transport/deposition, an experimentally validated Computational Fluid-Particle Dynamics (CFPD) model based on the Euler-Lagrange method is developed. In analyzing deposition patterns to fill the knowledge gap, the particles are grouped into six diameter groups, i.e., 0.05, 0.1, 0.5, 2, 5, and 10 µm. To enhance the statistical robustness of the investigation, a virtual population group is created that contains seven distinct and widely used human upper-airway configurations, where the same tracheobronchial trees are extended to Generation 6 (G6). Numerical results and the inter-subject variability analysis indicate that the glottis constriction is the morphological parameter that significantly impacts the inhaled particle dynamics in the respiratory tract. For reasons of statistical robustness, anatomical features of the upper airways should be maintained to capture the personalized airflow and particle transport dynamics for particles smaller than 500 nm or larger than 2 µm. However, a single upper airway model, representing a basic subpopulation group, can be employed to evaluate the total deposition of particles in the diameter range of 500 nm < dp < 2 µm. The present study provides an in silico lung-aerosol dynamics framework with detailed particle-deposition results and new physical insight. It may serve as a guide for implementing optimal targeting of inhaled drug-aerosols as well as for the assessment of hazardous aerosol exposure in distinct populations.}, journal={JOURNAL OF AEROSOL SCIENCE}, author={Feng, Yu and Zhao, Jianan and Kleinstreuer, Clement and Wang, Qingsheng and Wang, Jun and Wu, Dee H. and Lin, Jiang}, year={2018}, month={Sep}, pages={185–207} }
 @article{feng_kleinstreuer_castro_rostami_2016, title={Computational transport, phase change and deposition analysis of inhaled multicomponent droplet-vapor mixtures in an idealized human upper lung model}, volume={96}, ISSN={["1879-1964"]}, DOI={10.1016/j.jaerosci.2016.03.001}, abstractNote={Numerous inhalable aerosols consist of multiple nano-to-microscale solid or liquid particles with dissolved or embedded compounds, as well as associated vapors. In general, of interest are the transport and conversion phenomena leading to local particle/droplet/vapor depositions. Selected examples include inhalation of aerosols from use of inhalers, cigarettes and electronic cigarettes. In this study the focus is on hygroscopic growth of nano-size multi-component droplets and droplet–vapor interactions during transport with subsequent deposition in a human upper lung-airway model. For that purpose a comprehensive and efficient computational fluid–particle dynamics model has been developed. It is capable of simultaneously analyzing multi-component droplet–vapor and airflow interactions with evaporation and condensation effects for different sets of inhalation conditions. Selecting inhaled electronic cigarette (EC) aerosols as an application, the simulation results include detailed transport, deposition and absorption data for different constituents (i.e., water, propylene glycol, glycerol and nicotine) in both vapor and liquid forms for an idealized human upper lung airway geometry, i.e., from mouth to generation 3. Results indicate that liquid–vapor phase change induces hygroscopic growth of droplets, which in turn impacts significantly the deposition concentrations of aerosols via inertial impaction, secondary flows, Brownian motion, and the vapor-specific absorption rates. Parametric sensitivity analyses were performed to evaluate the influence of different inhalation flow waveforms on EC-aerosol transport, interaction, and deposition.}, journal={JOURNAL OF AEROSOL SCIENCE}, author={Feng, Yu and Kleinstreuer, Clement and Castro, Nicolas and Rostami, Ali}, year={2016}, month={Jun}, pages={96–123} }
 @article{feng_kleinstreuer_rostami_2015, title={Evaporation and condensation of multicomponent electronic cigarette droplets and conventional cigarette smoke particles in an idealized G3-G6 triple bifurcating unit}, volume={80}, ISSN={["1879-1964"]}, DOI={10.1016/j.jaerosci.2014.11.002}, abstractNote={The present research study is intended to provide fundamental understanding of the dynamics and transport of aerosols from an e-cigarette in an idealized tubular G3–G6 respiratory tract model. A computational model has been developed that includes the effects of hygroscopic growth as well as evaporation from multicomponent aerosol droplets. The aerosols investigated usually contain carrier solvents such as propylene glycol (PG) and glycerol, along with water, nicotine, and flavors. An experimentally validated computational fluid-particle dynamics (CF-PD) model is presented, which for the first time is capable of simultaneously simulating interactive, multicomponent droplet-vapor dynamics with evaporation and/or condensation. As a first step to accomplish such complex numerical simulations, an idealized G3–G6 triple bifurcating unit (TBU) has been selected. The results are compared with the conventional smoke particles (CSPs) as well as solid particles. Parametric analysis and comparisons of the evaporation/condensation dynamics for EC-droplets vs. cigarette smoke particles were performed, including the effects of different droplet initial diameter, composition, temperature, and ambient relative humidity. The results indicate that EC-droplets, being more hygroscopic than cigarette smoke particles, tend to grow larger in maximum size in a typically highly humid environment. Additionally, a correlation for the growth ratio of EC-droplets in TBUs is proposed.}, journal={JOURNAL OF AEROSOL SCIENCE}, author={Feng, Y. and Kleinstreuer, C. and Rostami, A.}, year={2015}, month={Feb}, pages={58–74} }
 @article{feng_kleinstreuer_2014, title={Micron-particle transport, interactions and deposition in triple lung-airway bifurcations using a novel modeling approach}, volume={71}, ISSN={["1879-1964"]}, DOI={10.1016/j.jaerosci.2014.01.003}, abstractNote={Particulate suspensions inhaled by humans are typically dilute and hence interactions between particles can be neglected. In such cases conventional Euler–Lagrange or Euler–Euler methods are suitable to simulate micron- or nano-particle transport and deposition in human respiratory systems. However, when challenging conditions, such as large pressure differentials, high velocity gradients and/or intense particle collisions, exist, alternative approaches for numerical analysis are required to capture fluid–particle, particle–particle, and particle–wall interactions. In the present study, the dense discrete phase model (DDPM) in conjunction with the discrete element method (DEM) have been employed to simulate micron–particle transport, interaction and deposition dynamics in different triple bifurcations (i.e., G3–G6, G6–G9, and G9–G12), using ANSYS Fluent 14.0 enhanced by user-defined functions (UDFs). In light of the relatively high computational cost when employing DDPM–DEM for such simulations throughout the human respiratory system, it may be necessary to combine different computational fluid–particle dynamics (CF–PD) models based on the local intensity of particle–particle interactions. Thus, the validity and necessity of the DDPM–DEM approach for different lung airway generations were numerically investigated, considering new parametric criteria for the use of most suitable numerical models. Specifically, the relative intensities of three major particle deposition mechanisms (i.e., inertial impaction, secondary-flow effect, and particle–particle-interaction impact) in idealized lung-airway segments were investigated. As a result, a new criterion for CF–PD model combination in terms of a relationship between inlet-particle stacking-volume fraction, ϕ, and percentage-of-fate changing particles, Δβp, is proposed. Visualizations of the fluid–particle dynamics in bifurcating airways have been provided as well. Results of this study pave the way for accurate and cost-effective CF–PD simulations of lung-aerosol dynamics, aiming at the improvement of respiratory dose estimation for health risk assessment in case of toxic particles and for treatment options in case of therapeutic particles.}, journal={JOURNAL OF AEROSOL SCIENCE}, author={Feng, Y. and Kleinstreuer, C.}, year={2014}, month={May}, pages={1–15} }
 @article{feng_kleinstreuer_2013, title={Analysis of non-spherical particle transport in complex internal shear flows}, volume={25}, ISSN={["1089-7666"]}, DOI={10.1063/1.4821812}, abstractNote={Focusing on ellipsoidal particles of different aspect ratios, the motion characteristics, including critical angle and stable vs. unstable rotational periods, are computationally analyzed in developing and fully developed tubular flows. As an application of particle transport and deposition, the one-way coupled Euler-Lagrange method enhanced by Euler's rotation equations is then employed to simulate laminar-turbulent flow in a subject-specific lung-airway model. First, to gain some basic insight into the dynamics of non-spherical particles, tubular flow is considered where the trajectories of ellipsoidal fibers with randomly initialized incidence angles were released at different inlet-plane positions, computed and visualized. Local and overall particle deposition results are compared between spheres, ellipsoidal fibers, and sphere-equivalent particles for which a revised Stokes diameter was developed. Concerning non-spherical particle transport and deposition in a subject-specific respiratory system, the validated computer simulation model provides realistic and accurate particle-deposition results. Specifically, slender non-spherical particles (i.e., those with higher aspect ratios) are potentially more harmful than thicker ones due to their ability to penetrate into deeper lung regions when somewhat aligned with the major flow field. Furthermore, non-spherical particle deposition is enhanced as the breathing rate increases.}, number={9}, journal={PHYSICS OF FLUIDS}, author={Feng, Y. and Kleinstreuer, C.}, year={2013}, month={Sep} }
 @inproceedings{feng_kleinstreuer_2014, title={DDPM-DEM simulations of particulate flows in human tracheobronchial airways}, DOI={10.1115/imece2013-62307}, abstractNote={Dense particle-suspension flows in which particle-particle interactions are a dominant feature encompass a diverse range of industrial and geophysical contexts, e.g., slurry pipeline, fluidized beds, debris flows, sediment transport, etc. The one-way dispersed phase model (DPM), i.e., the conventional one-way coupling Euler-Lagrange method is not suitable for dense fluid-particle flows [1]. The reason is that such commercial CFD-software does not consider the contact between the fluid, particles and wall surfaces with respect to particle inertia and material properties. Hence, two-way coupling of the Dense Dispersed Phase Model (DDPM) combined with the Discrete Element Method (DEM) has been introduced into the commercial CFD software via in-house codes. As a result, more comprehensive and robust computational models based on the DDPM-DEM method have been developed, which can accurately predict the dynamics of dense particle suspensions.
               Focusing on the interaction forces between particles and the combination of discrete and continuum phases, inhaled aerosol transport and deposition in the idealized tracheobronchial airways [2] was simulated and analyzed, generating more physical insight. In addition, it allows for comparisons between different numerical methods, i.e., the classical one-way Euler-Lagrange method, two-way Euler-Lagrange method, EL-ER method [3], and the present DDPM-DEM method, considering micron- and nano-particle transport and deposition in human lungs.}, booktitle={Proceedings of the ASME International Mechanical Engineering Congress and Exposition, 2013, vol 3B}, author={Feng, Y. and Kleinstreuer, C.}, year={2014} }
 @inproceedings{feng_kleinstreuer_2013, title={DDPM-DEM simulations of particulate flows in human tracheobronchial airways}, booktitle={ASME 2013 International Mechanical Engineering Congress & Exposition}, author={Feng, Y. and Kleinstreuer, C.}, year={2013} }
 @misc{kleinstreuer_feng_2013, title={Lung Deposition Analyses of Inhaled Toxic Aerosols in Conventional and Less Harmful Cigarette Smoke: A Review}, volume={10}, ISSN={["1660-4601"]}, DOI={10.3390/ijerph10094454}, abstractNote={Inhaled toxic aerosols of conventional cigarette smoke may impact not only the health of smokers, but also those exposed to second-stream smoke, especially children. Thus, less harmful cigarettes (LHCs), also called potential reduced exposure products (PREPs), or modified risk tobacco products (MRTP) have been designed by tobacco manufacturers to focus on the reduction of the concentration of carcinogenic components and toxicants in tobacco. However, some studies have pointed out that the new cigarette products may be actually more harmful than the conventional ones due to variations in puffing or post-puffing behavior, different physical and chemical characteristics of inhaled toxic aerosols, and longer exposure conditions. In order to understand the toxicological impact of tobacco smoke, it is essential for scientists, engineers and manufacturers to develop experiments, clinical investigations, and predictive numerical models for tracking the intake and deposition of toxicants of both LHCs and conventional cigarettes. Furthermore, to link inhaled toxicants to lung and other diseases, it is necessary to determine the physical mechanisms and parameters that have significant impacts on droplet/vapor transport and deposition. Complex mechanisms include droplet coagulation, hygroscopic growth, condensation and evaporation, vapor formation and changes in composition. Of interest are also different puffing behavior, smoke inlet conditions, subject geometries, and mass transfer of deposited material into systemic regions. This review article is intended to serve as an overview of contributions mainly published between 2009 and 2013, focusing on the potential health risks of toxicants in cigarette smoke, progress made in different approaches of impact analyses for inhaled toxic aerosols, as well as challenges and future directions.}, number={9}, journal={INTERNATIONAL JOURNAL OF ENVIRONMENTAL RESEARCH AND PUBLIC HEALTH}, author={Kleinstreuer, Clement and Feng, Yu}, year={2013}, month={Sep}, pages={4454–4485} }
 @article{feng_2012, title={Comments on paper: "Transport and deposition on ellipsoidal fibers in low reynolds number flows" from L. Tian, G. Ahmadi, Z. Wang, PK Hopke, Journal of Aerosol Science, 45, (2012) 1-18}, volume={52}, journal={Journal of Aerosol Science}, author={Feng, Y.}, year={2012}, pages={127–128} }
 @misc{kleinstreuer_feng_2013, title={Computational Analysis of Non-Spherical Particle Transport and Deposition in Shear Flow With Application to Lung Aerosol Dynamics-A Review}, volume={135}, ISSN={["1528-8951"]}, DOI={10.1115/1.4023236}, abstractNote={All naturally occurring and most man-made solid particles are nonspherical. Examples include air-pollutants in the nano- to micro-meter range as well as blood constituents, drug particles, and industrial fluid-particle streams. Focusing on the modeling and simulation of inhaled aerosols, theories for both spherical and nonspherical particles are reviewed to analyze the contrasting transport and deposition phenomena of spheres and equivalent spheres versus ellipsoids and fibers.}, number={2}, journal={JOURNAL OF BIOMECHANICAL ENGINEERING-TRANSACTIONS OF THE ASME}, author={Kleinstreuer, Clement and Feng, Yu}, year={2013}, month={Feb} }
 @inproceedings{li_kleinstreuer_feng_2012, title={Computational analysis of thermal performance and entropy generation of nanofluid flow in microchannels}, booktitle={Proceedings of the ASME Micro/Nanoscale Heat and Mass Transfer International Conference, 2012}, author={Li, J. and Kleinstreuer, C. and Feng, Y.}, year={2012}, pages={135–144} }
 @article{feng_kleinstreuer_2012, title={Thermal Nanofluid Property Model With Application to Nanofluid Flow in a Parallel Disk System-Part II: Nanofluid Flow Between Parallel Disks}, volume={134}, ISSN={["1528-8943"]}, DOI={10.1115/1.4005633}, abstractNote={This is the second part of a two-part paper which proposes a new theory explaining the experimentally observed enhancement of the thermal conductivity, knf, of nanofluids (Part I) and discusses simulation results of nanofluid flow in an axisymmetric jet-impingement cooling system using different knf-models (Part II). Specifically, Part II provides numerical simulations of convective nanofluid heat transfer in terms of velocity profiles, friction factor, temperature distributions, and Nusselt numbers, employing the new knf-model. Flow structures and the effects of nanoparticle addition on heat transfer and entropy generation are discussed as well. Analytical expressions for velocity profiles and friction factors, assuming quasi-fully-developed flow between parallel disks, have been derived and validated for nanofluids as well. Based on the numerical simulation results for both alumina-water nanofluids and pure water, it can be concluded that nanofluids show better heat transfer performance than convectional coolants with no great penalty in pumping power. Furthermore, the system’s entropy generation rate is lower for nanofluids than for pure water.}, number={5}, journal={JOURNAL OF HEAT TRANSFER-TRANSACTIONS OF THE ASME}, author={Feng, Yu and Kleinstreuer, Clement}, year={2012}, month={May} }
 @article{kleinstreuer_feng_2012, title={Thermal Nanofluid Property Model With Application to Nanofluid Flow in a Parallel-Disk System-Part I: A New Thermal Conductivity Model for Nanofluid Flow}, volume={134}, ISSN={["1528-8943"]}, DOI={10.1115/1.4005632}, abstractNote={This is a two-part paper, which proposes a new theory explaining the experimentally observed enhancement of the thermal conductivity, knf, of nanofluids (Part I) and discusses simulation results of nanofluid flow in a radial parallel-plate channel using different knf-models (Part II). Specifically, Part I provides the derivation of the new model as well as comparisons with benchmark experimental data sets and other theories, focusing mainly on aluminum and copper oxide nanoparticles in water. The new thermal conductivity expression consists of a base-fluid static part, kbf, and a new “micromixing” part, kmm, i.e., knf = kbf + kmm. While kbf relies on Maxwell’s theory, kmm encapsulates nanoparticle characteristics and liquid properties as well as Brownian-motion induced nanoparticle fluctuations, nanoparticle volume fractions, mixture-temperature changes, particle–particle interactions, and random temperature fluctuations causing liquid-particle interactions. Thus, fundamental physics principles include the Brownian-motion effect, an extended Langevin equation with scaled interaction forces, and a turbulence-inspired heat transfer equation. The new model predicts experimental data for several types of metal-oxide nanoparticles (20 < dp < 50 nm) in water with volume fractions up to 5% and mixture temperatures below 350 K. While the three competitive theories considered match selectively experimental data, their needs for curve-fitted functions and arbitrary parameters make these models not generally applicable. The new theory can be readily extended to accommodate other types of nanoparticle-liquid pairings and to include nonspherical nanomaterial.}, number={5}, journal={JOURNAL OF HEAT TRANSFER-TRANSACTIONS OF THE ASME}, author={Kleinstreuer, Clement and Feng, Yu}, year={2012}, month={May} }
 @article{zhang_kleinstreuer_feng_2012, title={Vapor deposition during cigarette smoke inhalation in a subject-specific human airway model}, volume={53}, ISSN={["1879-1964"]}, DOI={10.1016/j.jaerosci.2012.05.008}, abstractNote={Validated computer simulation results of vapor deposition from inhaled cigarette smoke are helpful to assess potential health effects of conventional and so-called less-harmful tobacco products. In this paper, the depositions of four critical tobacco-smoke vapors, i.e., acrolein, 1,3-butadiene, acetaldehyde and CO, in a subject-specific human airway model from mouth to generation G9 under different inhalation conditions have been simulated. The results show that vapor deposition is strongly influenced by its property values as well as inhalation waveform, i.e., puffing behavior. As almost insoluble species in the mucus layer, the deposition of butadiene vapor and CO is very low in the upper airways. The remaining vapors may penetrate further and deposit in deeper lung regions. As medium-to-high soluble vapors, acrolein and acetaldehyde have very high deposition values in the human upper airways from mouth to generation G9. The deposition result for steady matching flow (where the average flow rate between mean and peak values was taken) is a good conservative estimate of the actual deposition fraction under transient normal inhalation condition. However, the vapor transport delay may largely reduce the vapor deposition for transient puffing. A correction factor has been proposed, considering vapor transport delay, in order to calculate efficiently actual deposition fraction values under transient puffing condition. Furthermore, the effect of different puffing waveforms for conventional cigarettes and PREPs (potential reduced exposure products) on smoke vapor deposition has been discussed as well. Finally, a set of deposition correlations have been developed to estimate the deposition of acrolein and acetaldehyde vapors in different segments of the human upper airways under both puffing and normal inhalation (i.e., post-puffing) conditions.}, journal={JOURNAL OF AEROSOL SCIENCE}, author={Zhang, Zhe and Kleinstreuer, Clement and Feng, Yu}, year={2012}, month={Nov}, pages={40–60} }
 @misc{kleinstreuer_feng_2011, title={Experimental and theoretical studies of nanofluid thermal conductivity enhancement: a review}, volume={6}, ISSN={["1556-276X"]}, DOI={10.1186/1556-276x-6-229}, abstractNote={AbstractNanofluids,i.e., well-dispersed (metallic) nanoparticles at low- volume fractions in liquids, may enhance the mixture's thermal conductivity,knf, over the base-fluid values. Thus, they are potentially useful for advanced cooling of micro-systems. Focusing mainly on dilute suspensions of well-dispersed spherical nanoparticles in water or ethylene glycol, recent experimental observations, associated measurement techniques, and new theories as well as useful correlations have been reviewed.It is evident that key questions still linger concerning the best nanoparticle-and-liquid pairing and conditioning, reliable measurements of achievableknfvalues, and easy-to-use, physically sound computer models which fully describe the particle dynamics and heat transfer of nanofluids. At present, experimental data and measurement methods are lacking consistency. In fact, debates on whether the anomalous enhancement is real or not endure, as well as discussions on what are repeatable correlations betweenknfand temperature, nanoparticle size/shape, and aggregation state. Clearly, benchmark experiments are needed, using the same nanofluids subject to different measurement methods. Such outcomes would validate new, minimally intrusive techniques and verify the reproducibility of experimental results. Dynamicknfmodels, assuming non-interacting metallic nano-spheres, postulate an enhancement above the classical Maxwell theory and thereby provide potentially additional physical insight. Clearly, it will be necessary to consider not only one possible mechanism but combine several mechanisms and compare predictive results to new benchmark experimental data sets.}, journal={NANOSCALE RESEARCH LETTERS}, author={Kleinstreuer, Clement and Feng, Yu}, year={2011}, month={Mar} }
 @article{feng_kleinstreuer_2010, title={Nanofluid convective heat transfer in a parallel-disk system}, volume={53}, ISSN={["1879-2189"]}, DOI={10.1016/j.ijheatmasstransfer.2010.06.031}, abstractNote={Inherently low thermal conductivities of basic fluids form a primary limitation in high-performance cooling which is an essential requirement for numerous thermal systems and micro-devices. Nanofluids, i.e., dilute suspensions of, say, metal-oxide nanoparticles in a liquid, are a new type of coolants with better heat transfer performances than their pure base fluids alone. Using a new, experimentally validated model for the thermal conductivity of nanofluids, numerical simulations have been executed for alumina-water nanofluid flow with heat transfer between parallel disks. The results indicate that, indeed, nanofluids are promising new coolants when compared to pure water. Specifically, smoother mixture flow fields and temperature distributions can be achieved. More importantly, given a realistic thermal load, the Nusselt number increases with higher nanoparticle volume fraction, smaller nanoparticle diameter, reduced disk-spacing, and, of course, larger inlet Reynolds number, expressed in a novel form as Nu = Nu(Re and Br). Fully-developed flow can be assumed after a critical radial distance, expressed in a correlation Rcrit = fct(Re), has been reached and hence analytic solutions provide good approximations. Nanofluids reduce the system's total entropy generation rate while hardly increasing the required pumping power for any given Rein. Specifically, minimization of total entropy generation allows for operational and geometric system-optimization in terms of Sgen = fct (Re and δ).}, number={21-22}, journal={INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER}, author={Feng, Yu and Kleinstreuer, Clement}, year={2010}, month={Oct}, pages={4619–4628} }
 @article{wang_lin_feng_2010, title={The central oblique collision efficiency of spherical nanoparticles in the brownian coagulation}, volume={24}, number={14}, journal={Modern Physics Letters. B, Condensed Matter Physics, Statistical Physics, Applied Physics}, author={Wang, Y. M. and Lin, J. Z. and Feng, Y.}, year={2010}, pages={1523–1531} }
 @inbook{kleinstreuer_li_feng, title={Computational analysis of enhanced cooling performance and pressure drop for nanofluid flow
in microchannels}, ISBN={9781439861929}, booktitle={Advanced in numerical heat transfer}, publisher={Boca Raton: CRC Press/Taylor & Francis Group}, author={Kleinstreuer, C. and Li, J. and Feng, Y.}, editor={W.J. Minkowycz, E. M. and Sparrow and Abraham, J. P.Editors}, pages={250–273} }