Numerical simulation of micro-particle deposition in a realistic human upper respiratory tract model during transient breathing cycle

An more reliable human upper respiratory tract model that consisted of an oropharynx and four generations of asymmetric tracheo-bronchial (TB) airways has been constructed to investigate the micro-particle deposition pattern and mass distribution in five lobes under steady inspiratory condition in former work by Huang and Zhang (2011). In the present work, transient airflow patterns and particle deposition during both inspiratory and expiratory processes were numerically simulated in the realistic human upper respiratory tract model with 14 cartilaginous rings (CRs) in the tracheal tube. The present model was validated under steady inspiratory flow rates by comparing current results with the theoretical models and published experimental data. The transient deposition fraction was found to strongly depend on breathing flow rate and particle diameter but slightly on turbulence intensity. Particles were mainly distributed in the high axial speed zones and traveled basically following the secondary flow. “Hot spots” of deposition were found in the lower portion of mouth cavity and posterior wall of pharynx/larynx during inspiration, but transferred to upper portion of mouth and interior wall of pharynx/larynx during expiration. The deposition fraction in the trachea during expiration was found to be much higher than that during inspiration because of the stronger secondary flow.     

流固耦合作用下真实人体上呼吸道呼吸流涡结构演化的数值仿真

通过构建真实人体上呼吸道三维规范模型,运用大涡模拟数值方法,对考虑流固耦合作用的低强度循环呼吸模式下人体上呼吸道内的呼吸流进行了数值仿真,研究分析了人体口喉模型及气管支气管内的气流涡结构及其演化过程。结果表明,循环吸气过程中,气流在口腔中部以及舌苔上部形成多个涡管,在声门部位形成强烈的射流,在气管前壁出现马蹄涡,到气管中部大尺度涡结构逐步消失,支气管中只剩下一系列小尺度涡结构;循环呼气过程中,气流在气管底部产生较为复杂的涡结构,随着气流在气管内的融合,涡强度逐步减弱,在咽喉后壁形成拱状涡,气流进入口腔后,涡结构破裂,涡量扩散,没有较大的涡结构产生。     

Deposition of non-spherical microparticles in the human upper respiratory tract

We investigated the deposition pattern of microparticles with different particle diameters, shape factors, and initial flow conditions in a realistic human upper respiratory tract model. We identified a close relationship between the deposition fraction and the particle shape factor. The deposition fraction of the particles decreased sharply with increasing particle shape factor because of the decreasing drag force. We also found that the deposition varied at different positions in the upper respiratory tract. At low shape factors, the highest fraction of particles deposited at the mouth and pharynx. However, with increasing shape factor, the deposition fraction in the trachea and lungs increased. Moreover, for a given shape factor, larger particles deposited at the mouth and pharynx, which indicates that the deposition fraction of microparticles in the human upper respiratory tract is affected first and foremost by particle inertia as well as by the drag force.     

人体上呼吸道内气流运动特性的数值模拟分析

运用计算流体动力学(CFD)方法对人体上呼吸道内的气流运动特性进行了数值模拟,通过PIV实验对数值模拟结果进行了验证。研究结果表明:气流在咽部外壁、气管外壁发生分离现象,气流在气管内壁形成局部高速区,支气管内的气流在分叉处发生分离,靠近支气管内壁速度较高,并且在支气管边界层的外缘速度达到最大值。气管和支气管内的二次涡流运动和轴向速度的分布使得气管支气管内壁受到的剪应力较大,内壁粘膜更容易受到损伤。     

On the deposition of particles in liquid metals onto vertical ceramic walls

The deposition of non-metallic particles in liquid-metal flows is a serious industrial problem because the build-up of particles on ceramic walls clogs the flow path and interrupts the production, and this leads to large economic losses. This paper is an effort to extend the current state-of-the-art knowledge of particle deposition in air in order to predict particle deposition rates in liquid-metal flows using an improved Eulerian deposition model and considering Brownian and turbulent diffusion, turbophoresis and thermophoresis as transportation mechanisms. The model was used to predict the rate of deposition of particles in an air flow, and the predictions were compared to published measurements to demonstrate its performance. The model was then modified to take into account the differences in properties between air and liquid metals and thereafter applied to liquid-metal flows. Effects on the deposition rate of parameters such as steel flow rate, particle diameter, particle density, wall roughness and temperature gradient near the wall were investigated. It is shown that the steel flow rate has a very important influence on the rate of deposition of large particles, for which turbophoresis is the main deposition mechanism. For small particles, both wall roughness and thermophoresis have a significant influence on the particle deposition rate. Particle deposition rates under various conditions were successfully predicted.     

Modeling micro-particle deposition in human upper respiratory tract under steady inhalation

A representative human upper respiratory tract (URT) with idealized oral region and asymmetric tracheo-bronchial (TB) airway has been modeled, and laminar-to-turbulent airflow for typical inhalation modes as well as micro-particle transport and deposition has been simulated using CFX10.0 software from Ansys Inc. on a personal computer. The asymmetric TB airway could not be replaced by an extended straight tube as outlet of the oral region while investigating the tracheal airflow field and particle depositio...     

循环呼吸模式口喉模型内气流运动特性数值模拟

应用计算流体动力学方法对人体在循环呼吸模式下口喉模型内的气流运动特性进行数值模拟,分析了循环呼吸模式下口喉模型内的气流组织形式以及气流运动对呼吸道壁面以及气溶胶运动沉积的影响. 吸气阶段,在咽部外壁和声门下游气管上部外壁气流发生分离,形成分离区. 呼气阶段,分别在咽部外壁和喉部外壁形成高速区. 循环呼吸模式下,咽部、喉部与气管内的高轴向速度区和二次涡流运动均是在呼吸过程中间歇性的产生,形成的高剪应力区也是间歇性的. 壁面受到的剪应力周期性地改变方向,引起壁面劳损和组织损伤的可能性增大,同时在这些部位容易造成气溶胶的沉积.      

Lattice Boltzmann model for predicting the deposition of inertial particles transported by a turbulent flow

Deposition of inertial solid particles transported by turbulent flows is modelled in a framework of a statistical approach based on the particle velocity Probability Density Function (PDF). The particle-turbulence interaction term is closed in the kinetic equation by a model widely inspired from the famous BGK model of the kinetic theory of rarefied gases. A Gauss-Hermite Lattice Boltzmann model is used to solve the closed kinetic equation involving the turbulence effect. The Lattice Boltzmann model is used for the case of the deposition of inertial particles transported by a homogeneous isotropic turbulent flows. Even if the carrier phase is homogeneous and isotropic, the presence of the wall coupled with particle-turbulence interactions leads to inhomogeneous particle distribution and non-equilibrium particle fluctuating motion. Despite these complexities the predictions of the Lattice Boltzmann model are in very good accordance with random-walk simulations. More specifically the mean particle velocity, the r.m.s. particle velocity and the deposition rate are all well predicted by the proposed Lattice Boltzmann model.     

Numerical modeling of 3-D turbulent two-phase flow and coal combustion in a pulverized-coal combustor

In the present paper, a multifluid model of two-phase flows with pulverized-coal combustion, based on a continuum-trajectory model with reacting particle phase, is developed and employed to simulate the 3-D turbulent two-phase flows and combustion in a new type of pulverized-coal combustor with one primary-air jet placed along the wall of the combustor. The results show that: (1) this continuum-trajectory model with reacting particle phase can be used in practical engineering to qualitatively predict the flame stability, concentrations of gas species, possibilities of slag formation and soot deposition, etc.; (2) large recirculation zones can be created in the combustor, which is favorable to the ignition and flame stabilization. Sponsored by the National Key Projects of Fundamental Research of China.     

Effect of source location on particle dispersion in displacement ventilation rooms

A proposed computer model for predicting aerosol particle dispersion in indoor spaces was validated with experimental data found in the literature,and is then used to study the effect of the area and point source locations on particle dispersion in displacement ventilation (DV) rooms.The results show that aerosol source location has a strong impact on the spatial distribution and removal rate of indoor particles.Particle removal performance depends strongly on ventilation efficiency and particle deposition rate on indoor surfaces.Important consideration for both relative ventilation efficiency and deposition rate consists of the position of the aerosol source relative to the main airflow pattern and the occupied zone.     

Experimental and numerical study of pseudo-2D circulating fluidized beds

We present experimental investigations and numerical simulations of a pseudo-2D riser. Experiments were performed for various airflow rates, particle types/diameters, and particle size distributions. Pressure distributions along the wall of the riser were measured. Additional measurements from a smaller pseudo-2D riser (Kallio et al., 2009; Shah et al., 2012) were used to analyze horizontal solids volume fraction profiles. The experimental data were compared with simulation results carried out using an Euler–Euler approach. A mesh sensitivity study was conducted for numerical simulations and effects associated with simplifying real 3D geometry to a 2D model were examined. In addition, the effect of using an algebraic equation to represent the granular temperature versus a full partial differential equation also was examined for numerical simulations. Results showed small but significant near-wall sensitivity of the flow variables to mesh size. Substantial differences in mean pressure, solids distribution, and solid velocities were obtained, when 2D and 3D simulation results were compared. Finally, applying the simplified granular temperature equation for turbulent fluidization and for dilute-phase transport can lead to incorrect predictions in models.     

Characteristics of turbulent particle transport in human airways under steady and cyclic flows

Motion of monodispersed aerosol particles suspended in air flow has been studied on realistic transparent model of human airways using Phase Doppler Particle Analyser (P/DPA). Time-resolved velocity data for particles in size range 1–8 μm were processed using Fuzzy Slotting Technique to estimate the power spectral density (PSD) of velocity fluctuations. The optimum processing setup for our data was found and recommendations for future experiments to improve PSD quality were suggested. Typical PSD plots at mainstream positions of the trachea and the upper bronchi are documented and differences among (1) steady-flow regimes and equivalent cyclic breathing regimes, (2) inspiration and expiration breathing phase and (3) behaviour of particles of different sizes are described in several positions of the airway model. Systematically higher level of velocity fluctuations in the upper part of the frequency range (30–500 Hz) was found for cyclic flows in comparison with corresponding steady flows. Expiratory flows in both the steady and cyclic cases produce more high-frequency fluctuations compared to inspiratory flows. Negligible differences were found for flow of particles in the inspected size range 1–8 μm at frequencies below 500 Hz. This finding was explained by Stokes number analysis. Implied match of the air and particle flows thereby indicates turbulent diffusion as important deposition mechanism and confirms the capability to use the P/DPA data as the air flow velocity estimate.     

Effect of source location on particle dispersion in displacement ventilation rooms

A proposed computer model for predicting aerosol particle dispersion in indoor spaces was validated with experimental data found in the literature, and is then used to study the effect of the area and point source locations on particle dispersion in displacement ventilation （DV） rooms. The results show that aerosol source location has a strong impact on the spatial distribution and removal rate of indoor particles. Particle removal performance depends strongly on ventilation efficiency and particle deposition rate on indoor surfaces. Important consideration for both relative ventilation efficiency and deposition rate consists of the position of the aerosol source relative to the main airflow pattern and the occupied zone.     

Simulation of sudden-expansion and swirling gas-particle flows using a two-fluid particle-wall collision model with consideration of the wall roughness

A two-fluid particle-wall collision model with consideration of wall roughness is proposed. It takes into account the effects of the friction, restitution and in particular the wall roughness, and hence the redistribution of Reynolds stress in different directions, the absorption of turbulent energy from the mean motion and the attenuation of particle motion by the wall. The proposed model is used to simulate sudden-expansion and swirling gas-particle flows and is validated by comparing with expermental results. The results show that the proposed model gives better results than those obtained by the presently used zero-gradient condition. Hence, it is suggested that the proposed model should be used as the wall boundary condition for the particle phase in place of the presently used boundary condition. The project supported by the Special Funds for the Major State Basic Research, China (G-1999-0222-08)     

Effect of near-wall turbulence enhancement on the mechanisms of particle deposition

The modification of deposition mechanisms of small particles in wall turbulence due to enhanced near-wall fluctuations is presented. The direct numerical simulation database of turbulent air flow over a water surface populated by gravity-capillary waves of small wave slope was used to mimic the enhancement in fluctuation intensity. Lagrangian tracking of particles is performed under the assumption of one-way coupling between the particles and the flow. Two sets of particles have been considered with inertial response times of 5 and 15, respectively, normalized using the friction velocity at the air–water interface and the kinematic viscosity of air. Compared to wall-bounded flow, the particle deposition rates on the interface were found to be considerably higher; specifically for the low-inertia particles, an eightfold increase was observed. The deposition rate for particles of higher inertia increased by only 60%. The correlation characterizing particle deposition rates for wall-bounded flows, where the deposition rate is proportional to the square of the particle response time, was found to be invalid for the flow with enhanced near-wall turbulence. Comparison with experimental results on particle deposition onto rough walls showed better correlation. Depositing particles were divided into free-flight and diffusional deposition populations. Since the primary effect of the interfacial waves is to increase the turbulence intensity in the near-interface region with high particle concentration, a remarkable increase in diffusional deposition is observed. As in wall-bounded flows, diffusional deposition is seen to be the dominant mechanism of deposition. The free-flight mechanism, where particles acquire velocities high enough to travel directly to the interface, remains unaffected by enhanced near-wall velocity fluctuations.     

Effect of wall rougheners on cross-sectional flow characteristics for non-spherical particles in a horizontal rotating cylinder

Discrete-element-method （DEM） simulations have been performed to investigate the cross-sectional flow of non-spherical particles in horizontal rotating cylinders with and without wall rougheners. The non-spherical particles were modeled using the three-dimensional super-quadric equation. The influence of wall rougheners on flow behavior of grains was studied for increasing particle blockiness. Moreover, for approximately cubic particles （squareness parameters [555]）, the rotational speed, gravitational acceleration and particle size were altered to investigate the effect of wall rougheners under a range of operating conditions. For spherical and near-spherical particles （approximately up to the squareness parameters [344]）, wall rougheners are necessary to prevent slippage of the bed against the cylinder wall. For highly cubic particle geometries （squareness parameters larger than [3441）, wall rougheners resulted in a counter-intuitive decrease in the angle of repose of the bed. In addition, wall rougheners employed in this study were demonstrated to have a higher impact on bed dynamics at higher rotational speeds and lower gravitational accelerations. Nevertheless, using wall rougheners had a comparatively small influence on particle-flow characteristics for a bed composed of finer grains.     

Relations for particle deposition in turbulent gas-particle channel flows

Using the dimensional and similarity analysis in a combination with the method of matched asymptotic expansions, the problem of particle deposition on boundaries in high-Reynolds number turbulent channel flows is studied. Based on the conventional two-layer scheme of a turbulent near-wall flow, the relations for the distributions of the particle concentration and the velocity fluctuation moments in the wall region are obtained. Asymptotic laws are derived for the particle deposition rate for diffusion-impact and inertial deposition regimes.