湍流通道内微粒受力对其输运特性的影响

柴油机排气过程中的微粒运动和沉降对微粒测量、排气再循环系统的正常工作以及微粒的净化捕集等具有重要的影响.利用微粒受力模型和微粒随机轨道模型,对湍流通道中微粒的输运特性以及微粒受力对其输运特性的影响进行了分析.通过研究,给出了重力、热泳力和Saffman升力在不同条件下对湍流通道内微粒输运特性的影响规律.研究结果对于加深柴油机微粒在湍流通道中运动及沉降规律的理解具有重要的意义.     

平面湍流射流细颗粒物团聚的PBM-LES耦合模拟

本文选取平面湍流射流为研究对象,通过大涡模拟与颗粒群体平衡模型耦合模拟了亚微米级细颗粒物在平面湍流射流流场中的团聚情况,分析了细颗粒物在流场中的运动情况以及粒径的演变过程,研究了不同时间、不同入口体积分数以及不同St数对细颗粒物团聚的影响。结果表明,在射流发展过程中细颗粒物粒径经历了逐渐增大且数密度逐渐减小并趋于稳定的过程;入口体积分数增大加强了流体与颗粒相之间相互作用的程度,细颗粒物的团聚效果更加显著;St数增大影响了细颗粒物在流场中的运动情况,削弱了细颗粒物的团聚效果。     

宽筛分流化床气—固两相流动结构离散颗粒模型

建立了适合描述宽筛分流化床气 固两相流动结构的离散颗粒模型。颗粒的运动满足牛顿第二定律 ,流体相的运动规律由局部平均的纳维 斯托克斯方程求解 ,两相间的耦合由牛顿第三定律决定。对宽筛分流化床中气泡的形成、颗粒的流化过程进行了数值模拟 ,结果与实验现象相符合 ;模拟结果还发现单颗粒的运动速度表现出不可预测特性 ,颗粒的总体速度不完全满足正态分布。     

速度滑移条件下气体-微颗粒两相流动数值模拟研究

基于欧拉-拉格朗日方法构建了气体-微颗粒两相流动数值计算模型,采用考虑速度滑移的拖曳力系数关联式以研究微颗粒表面动量非平衡效应。在此基础上,分析速度滑移、斯托克斯数(St)对微颗粒在受限空间中运动轨迹的影响规律。研究结果表明:速度滑移对颗粒运动轨迹影响明显,其运动过程明显滞后于常规颗粒运动;St较小时,颗粒能及时响应流场变化,可较好地跟随流体运动,随St增大,颗粒运动受自身惯性影响愈加明显。     

不同颗粒物性对异径混合非球形颗粒分离特性影响的数值模拟

采用多元颗粒模型描述了3种非球形颗粒,即玉米形颗粒、椭球形颗粒和圆柱形颗粒,分析了其碰撞机理,建立了非球形颗粒的运动方程,通过实验验证了模型的可靠性,并研究了不同颗粒物性对异径混合颗粒分离特性的影响.结果表明:不同形状颗粒的分离特性有很大不同,其中椭球形颗粒的分离程度最大,球形颗粒的分离程度最小,且分离程度随着颗粒直径比的增大而增大,而颗粒密度比对颗粒的分离情况基本没有影响.     

带电颗粒物在柴油机颗粒物检测装置中的运动特性

为了研究带电颗粒物在柴油机颗粒物检测装置中的运动特性,建立了气相、电场及颗粒物相耦合模型,以COMSOL Multiphysics软件为工具对模型求解,得到了柴油机颗粒物检测装置内部带电颗粒物的运动轨迹、速度特性及受力情况。结果表明:在电场力的作用下带电颗粒物会向接地极板方向运动,在轨迹上出现运动偏移,在电离-荷电区施加高电压,静电捕集区施加方波电压时,可在测量区出现方波电流,实现颗粒物浓度转化;在电离-荷电区和静电捕集区附近颗粒物运动速度较慢且稳定,实现了在电离-荷电区颗粒物充分荷电,并保证在静电捕集区部分颗粒物可被捕集;在电离-荷电区和静电捕集区,颗粒物受电场力和拖拽力共同作用运动,拖拽力随入口流速增加而略有增大。     

纳米流体毛细弯液蒸发界面热质迁移特性分析

基于增广杨拉普拉斯方程的毛细弯液面薄膜蒸发区的传热传质模型,数值分析了过热度和纳米流体工质对毛细弯液面薄膜蒸发区热质迁移特性的影响。结果表明,过热度增大导致薄膜区范围减小,蒸发界面热流密度增大,薄膜区总换热量增大,但同时减弱了薄膜界面的稳定性。在传统流体工质中添加合适的纳米颗粒,纳米流体运动粘性系数随体积分率增大而减小,导热系数随体积分率增大而增大,影响其传热传质效果。较大体积分率的纳米流体,其薄膜厚度更小,薄膜区热流密度和蒸发质量流率更大,但同时蒸发界面的稳定性减弱。不同种类的纳米流体对毛细弯液蒸发界面的影响也较为明显,具有较低运动粘性系数和较高导热系数的纳米流体能够迁移更多的热量。     

柴油机DPF湍流通道内微粒形态对微粒输运特性的影响

利用建立的椭球形微粒的运动学方程和动力学方程以及湍流场中椭球形微粒的受力及力矩模型,对柴油机微粒捕集器(DPF)湍流通道中微粒的输运特性进行了研究,着重对微粒形态的变化对湍流通道中微粒的运动轨迹、运动方位以及沉降速度的影响进行了分析.结果表明:微粒在壁面附近存在富集现象,且这种现象随着微粒长短半轴比的增加愈发明显;随着气流速度的增加,微粒有向通道内扩散的趋势,且微粒形态对微粒运动轨迹的影响变得显著;在湍流场通道中,椭球形微粒的长轴倾向于与流体流动方向平行,微粒长轴在垂直于壁面方向和展向上的姿态角分布则比较均匀,距壁面量纲为1距离对微粒长轴取向的影响不大;微粒量纲为1沉降速度与微粒形态关系紧密,且随短半轴长的增长并非呈单调变化规律.     

静止流体中颗粒污垢沉积模型数值模拟

为了研究静止流体中颗粒污垢沉积规律,基于固液两相流理论以及扩散理论建立了颗粒沉积模型,采取数值模拟的办法,分析研究了颗粒沉积速度、浓度及沉积量随沉积过程的变化,得到了适用于粒径大于5μm的颗粒沉积规律:颗粒沉积速度、浓度以及沉积量均呈现渐进型趋势,颗粒最终以恒定的沉降速度沉底;越靠近底端,沉积时间越短;越靠近表面浓度变化越快。分析对比模拟值与实验值,在误差允许的范围内认为两者吻合较好。     

强声波作用下煤粉颗粒的振荡特性研究

考虑Stokes力为颗粒的主要受力,建立了强声波作用下煤粉颗粒的动力学控制方程,并给出了颗粒的夹带系数计算公式.采用数值计算方法研究了颗粒位置、烟气温度、颗粒密度、颗粒粒径、声波频率和声压级等参数对煤粉颗粒在冲击波形成之前运动特征和夹带系数的影响.结果表明:强声波场中距声源不同位置处的颗粒具有不同的振荡周期和速度振幅,...     

Molecular Dynamics Simulation on Thermodynamic Properties and Transport Coefficients

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A three-dimensional mixed-domain PEM fuel cell model with fully-coupled transport phenomena

A three-dimensional mixed-domain PEM fuel cell model with fully-coupled transport phenomena has been developed in this paper. In this model, after fully justified simplifications, only one set of interfacial boundary conditions is required to connect the water content equation inside the membrane and the equation of the water mass fraction in the other regions. All the other conservation equations are still solved in the single-domain framework. Numerical results indicate that although the fully-coupled transport phenomena produce only minor effects on the overall PEM fuel cell performance, i.e. average current density, they impose significant effects on current distribution, net water transfer coefficient, velocity and density variations, and species distributions. Intricate interactions of the mass transfer across the membrane, electrochemical kinetics, density and velocity variations, and species distributions dictate the detailed cell performances. Therefore, for accurate PEM fuel cell modeling and simulation, the effects of the fully-coupled transport phenomena could not be neglected.     

Fundamentals of turbulent gas-solid flows applied to circulating fluidized bed combustion

A summary is made of the present state of knowledge of turbulent gas-solid flow modeling and in particular its application to circulating fluidized bed combustion chambers. Models are presented to close the set of equations describing isothermal non-reacting turbulent gas-particle flows applied to fluidization, and it is shown under which assumptions the models can be derived. With the kinetic theory of granular flow, transport equations for the velocity moments and closure laws for the stress tensor and the energy flux are derived for the particle phase. Closure equations for the drift velocity and for the fluid-particle velocity correlation tensor are presented, first based on algebraic models and, second, based on transport equations with the fluid-particle joint probability density function. An alternative derivation of the fluid-particle velocity covariance transport equation is compared to the formulation based on the fluid-particle joint probability density function. Two-way coupling is discussed, and a transport equation for the second-order velocity moments is used to derive a two-equation model accounting for the modulation of gas phase turbulence by particles. Boundary conditions for the set of equations describing a turbulent gas-solid flow are discussed. Provided that the domain of applicability of the models is known, a discussion on the usefulness of the models is given, as well as an application to fluidization and especially to circulating fluidized bed combustors. Prospects for improvement of the existing models are presented.     

Effect of diffusion on slowing the velocity of a bell-shaped wave in slow axonal transport

This paper models transport of organelles by slow axonal transport utilizing the stop-and-go hypothesis, which postulates that in slow axonal transport the motion of organelles does not occur continuously; instead, organelles move along microtubules (MTs) alternating between short periods of rapid movement, short on-track pauses, and prolonged off-track pauses, when they temporarily disengage from MTs. The model considers six kinetic states of organelles: anterogradely moving state, retrogradely moving state, anterogradely pausing state, retrogradely pausing state, off-track anterograde state, and off-track retrograde state. The paper extends the existing model of slow axonal transport by accounting for the diffusivity of off-track organelles and investigates how the diffusivity of these organelles affects the amplitude, velocity, and rate of change of the variance of the bell-shaped wave which describes the probability density function (PDF) corresponding to the ratio of the chance of finding an organelle within an infinitesimal interval in the axon to the length of this interval. The velocity of this wave characterizes the average effective velocity (calculated including pauses) of an organelle in slow axonal transport while the rate of change of the variance characterizes the rate of spread of the initial packet of organelles transported in the axon. The goal of this research is not only to develop a more accurate transport model, but also to understand fundamentally the effects of diffusion on slow axonal transport. It is demonstrated that diffusion decreases the amplitude of the wave and increases the rate of its spread but does not affect wave's velocity.     

Influence of the induced magnetic field and heat transfer on peristaltic transport of a micropolar fluid in a tapered asymmetric channel

In this paper, we investigate the peristaltic transport of a micropolar fluid in a tapered asymmetric channel with heat transfer and induced magnetic field effect. The flow is analyzed by long wavelength and low Reynolds number approximations. The reduced equations have been solved by using Adomian decomposition method and the expressions for velocity, stream function, microrotation component, magnetic‐force function, pressure gradient, axial induced magnetic field, and current density distribution across the channel have been computed. Expressions for shear stresses are also obtained. The effect of pertinent parameters is illustrated graphically.     

Interfacial transport characteristics between heterogeneous porous composite media for effective mass transfer in fuel cells

In this study, a comprehensive computational model based on a full statistical approach was developed to investigate the heterogeneous mass transport properties in the metal foam channels, gas diffusion layers (GDLs), and microporous layers (MPLs) of polymer electrolyte fuel cells (PEFCs) at the 95% confidence level. A series of channels, GDLs, and MPLs were, respectively, generated to reflect the random heterogeneous structures and transport characteristics. The critical hydrophobic pore radius in the mixed wettability GDLs was computed by applying a modified Leverett function. Furthermore, the gas transport phenomenon through a sufficient number of porous transport media was simulated using a D3Q19 (ie, three‐dimensional, 19 velocities) lattice Boltzmann method, and the corresponding mass transport characteristics were mathematically presented as a function of the porosity. The permeabilities in the channels, GDLs, and MPLs were derived from the pressure gradient and the simulated velocity distribution. It was found that the effective mass diffusion coefficient in the GDLs is mainly influenced by molecular diffusion. Nevertheless, Knudsen diffusion is the dominant mass transfer mechanism in the MPLs, because of small pore diameters. In addition, critical hydrophobic pore radius was derived using a modified Leverett function, which enables to estimate the fraction of pores larger than the critical pore radius in GDLs for effective water transport. Moreover, the interfacial areal contact ratio between two adjacent porous media (ie, channel/GDL and GDL/MPL) was calculated. The calculations indicated that the variation in the local porosity of the porous media has a significant influence on the interfacial connections. The proposed model is expected to improve the prediction performance of porous heterogeneous transport media in electrochemical energy systems and the optimization of porous media structures.     

Micropolar fluid flow with temperature‐dependent transport properties

In the present article, the heat transfer rate and the fluid flow of a micropolar fluid along with temperature‐dependent transport properties are scrutinized in the presence of heat generation. The variability in transport properties leads to a rise in the heat transfer and decreases the skin friction. Furthermore, Fourier's heat flux model is implemented in the analysis of heat transfer, employing a suitable transformation to convert the flow model into nonlinear ordinary differential equations. Numerical solutions are obtained by using the shooting method/bvp4c technique. Physical quantities of interest, such as local skin friction and Nusselt number, are discussed and computed. Skin friction decreases with the micropolar parameter but the Nusselt number shows the opposite behavior for the micropolar parameter.     

Nanoscale energy transport of inclined magnetized 3D hybrid nanofluid with Lobatto IIIA scheme

Key developments in the field of nanotechnology have drawn the attention of many scholars toward the interaction of nanoparticles due to their capturing applications in solar energy systems and thermal engineering. Larger consumption of energy posed a challenge for thermal science, so thermal engineering is trying to solve this issue by increasing the thermal conductivity of the fluid. The thermal conductivity of conventional fluid is increased by incorporating the nanoparticles in the base fluid. Keeping this in mind, the present research project addresses the utilization of nanoparticles in a steady three-dimensional rotating flow of magnetohydrodynamic water-based hybrid fluid over an extending sheet. Nanoparticles of aluminum oxide (Al₂O₃) and silver (Ag) are being used with water (H₂O) as base fluid. The velocity of nanoparticles is being captured under the influence of an inclined magnetic field and the transport of heat is scrutinized through thermal radiation. The physical model generates partial differential equations and then transported into an equivalent set of a nonlinear ordinary differential equations. The purpose of numerical computation is made by the Lobatto IIIA method, which is a type of Matlab scheme bvp4c and based on the finite difference method. Geometry of velocity profile is explained with different parameters in presence and absence of magnetic field and energy of hybrid nanofluid is explained under the influence of the inclined and perpendicular magnetic field. Gradual increment in ϑ both f and g profiles because strengthen the magnetic field results lower velocity. An increment in nanoparticle concentration of Al₂O₃ and Ag gives a larger magnitude of velocity. The rotation parameter shows the rotation of nanoparticles; due to these rotations both linear and angular components of velocity increase in the presence and absence of a magnetic effect.     

A statistical model for predicting the fluid displaced/added mass and displaced heat capacity effects on transport and heat transfer of arbitrary-density particles in turbulent flows

The objective of the paper is twofold: (i) to present a new statistical model for predicting the transport and heat transfer of arbitrary-density particles suspended in turbulent flows and (ii) to examine the performance of this model in an isotropic velocity flow field without and with a mean temperature gradient as well as in a near-wall turbulent flow. The model presented is based on a kinetic equation for the probability density function (PDF) of velocity and temperature distributions and coves the entire range of the particle-to-fluid density ratio (from heavy particles in a gas to bubbles in a liquid).     

Effects of heat and mass transfer on MHD nonlinear free convection non-Newtonian fluids flow embedded in a thermally stratified porous medium

This communication examines heat alongside mass transport in a nonlinear free convection magnetohydrodynamics (MHD) non-Newtonian fluid flow with thermal radiation and heat generation deep-rooted in a thermally stratified penetrable medium. The Casson and Williamson fluid considered in this communication flos simultaneously across the boundary layer and are mixed together. The model of heat alongside mass transport is set up with chemical reaction and thermal radiation alongside heat generation to form a system of partial differential equations (PDEs). Appropriate similarity variables are used to simplify the PDEs to obtain systems of coupled ordinary differential equations. An efficiently developed numerical approach called the spectral homotopy analysis method was used in providing solutions to the transformed equations. A large value of Casson term is observed to degenerate the velocity plot while the Williamson parameter enhances the velocity profile. The parameter of thermal stratification is found to enhance the rate of heat transport within the boundary layer. An incremental value of the magnetic parameter declines the velocity of the fluid and the entire boundary layer thickness. The present result was compared with previous studies and was seen to be in good agreement.