双流体颗粒-壁面碰撞模型在突扩两相湍流模拟中的应用

由于壁面粗糙度的双流体颗粒—壁面碰撞模型中，没有考虑颗粒—壁面碰撞前后颗粒速度相对于平均速度的差别，因此，修正了由此引起的颗粒雷诺应力的变化，并应用其对轴对称突扩流动进行了数值模拟，着重探讨了不同颗粒相边界条件的影响．结果表明，由于考虑了各方向雷诺应力之间的相互转化、雷诺应力从平均运动中得到能量以及壁面对颗粒运动的衰减作用等因素，包括摩擦系数、恢复系数和壁面粗糙度等影响因素的颗粒—壁面碰撞模型和由此得到的简化模型，给出的结果与实验值符合较好，而通常使用的零梯度颗粒壁面边界条件则给出失真的模拟结果。     

惯性分离器内气固两相流雷诺应力数值模拟

对循环流化床中气田惯性分离过程进行了详细的数值研究。湍流模型采用雷诺应力模型,研究对象为U型分离器内的气一固两相流动。为了真实描述固体颗粒与分离器壁面之间的碰撞过程,固体颗粒模拟采用颗粒轨道模型,引入壁面粗糙度的影响,同时考虑了固体颗粒在湍流中的扩散作用和颗粒之间的相互碰撞。模拟计算了不同入口速度、分离器挡板数目对颗粒分离效率和流体压降的影响,计算结果不仅给出了分离器内的气-固两相流动结构特性,而且给出了分离器效率、压降与入口主流速度和分离器结构参数的关系。     

旋流雾化水膜除尘器内尘粒运动特性及分离效率

基于气固多相流复杂的运动特征，结合旋流雾化水膜除尘器的特点，同时考虑尘粒碰壁面黏附与反弹条件，颗粒间的碰撞与并聚、喷水加湿对颗粒间并聚及碰撞、颗粒运动特性的影响，并考虑颗粒对气相的作用，构建了描述旋流雾化水膜除尘器内湍流气固多相流的三维时均方程组，模型封闭采用k-ε／RNG模型，数值模拟了旋流雾化水膜除尘器内尘粒运动特性及分离效率。结果表明：在设置水膜除尘器尘粒遇壁面条件时，应设置尘粒碰壁为黏附；除尘器内干段、湿段比例、尘粒粒径、尘粒入口位置等对除尘效率高低均存在影响；壁面水雾膜的黏附及喷水对尘粒并聚影响较大，尘粒在除尘器内运动位移小于尘粒碰壁面反弹及不考虑尘粒并聚的情况，越有利于尘粒分离；除尘器入口速度越大及喉部喷水速度越大，越有利于尘粒的除去。     

一个柴油喷雾与壁面碰撞的数学模型

提出一个柴油喷雾与壁面碰撞的数学模型，可以计算垂直碰壁和倾斜碰壁时壁面射流的性能，适合于在准维燃烧模型中使用。建立了新的壁面射流贯穿距离计算公式，直观地反映了影响壁面射流传播的主要因素。与试验数据对比证实了这个模型的精度是令人满意的。在参数研究中，预测了喷孔直径、从喷孔至壁面的距离和喷射压力对壁面射流贯穿距离和区域空燃比的影响。     

循环流化床的传热机理

本文论述了循环流化床的传热机理－－颗粒絮团更新模型，对不同来源模型中的壁面被颗粒絮团覆盖的面积分率、颗粒絮团在壁面的滞留时间以及颗粒絮团与壁面间的气膜厚度等关键参数的计算和设定进行了分析。对比了各模型的预测精度。     

油雾碰撞高温壁面的油滴分裂及与热壁间换热研究

1前言现实中许多领域都有液滴碰撞壁面的现象,如高速直喷式发动机中喷雾油束的撞壁,喷雾冷却及喷墨打印机的油墨与纸张的碰撞等。液滴碰壁涉及到液滴形状变化、运动状态改变以及与壁面之间热传递等复杂的物理过程,其中起控制作用的参数有:液滴的速度、粘度、表面张力和壁面的温     

贴体坐标系下喷动床内气固两相流体动力特性的数值模拟

采用稠密固相动力-摩擦应力模型,建立喷动床内气固两相双流体模型．应用贴体坐标系使得计算网格与喷动床的倒锥体边界相一致．通过数值模拟获得喷动床内喷射区、环隙区和喷泉区内颗粒速度及浓度分布．计算结果表明,喷泉区具有强烈的气固两相质量和动量交换．当倒锥体倾斜角度达到60°,在射流入口处形成一瓶颈．研究表明颗粒间滑动-滚动摩擦应力对环隙区颗粒流动具有明显的影响．     

单液滴在多孔介质内碰壁过程的数值模拟

应用数值模拟方法研究不同尺寸单一液滴碰撞常温多孔介质内壁面时的运动与变形行为.界面跟踪采用基于VOF的体跟踪模型,简化的多孔介质结构应用孔隙网络模型.主要考查了初始液滴碰撞动能、表面能及液滴与多孔介质内壁面相对尺寸大小对液滴碰壁现象的影响,分析了液滴在多孔内壁面上形成液膜、液膜延展碰撞喉道壁面及飞溅破碎等过程的动力学特...     

下降管反应器内颗粒流动升温过程数值研究

采用数值模拟的手段研究了下降管反应器内包含不同尺寸及密度冷热颗粒混合物的流动传热特性。双流体模型及离散单元法分别被用于描述颗粒混合物的流动过程并与实验结果进行了对比。反应器内气固相间传热,颗粒混合物间碰撞传热,以及壁面与气/固两相间的热量传递采用计算流体力学和离散单元法相耦合的方式进行了模拟,对颗粒到达反应器出口前影响温度变化趋势的因素展开了分析研究。模拟结果表明:在V型下降管反应器内,粒度较小的颗粒以沿壁面向下滑动为主;较大尺寸颗粒向下流动过程中在反应器截面上分布区域较广;当反应器壁面热边界条件发生变化时,颗粒升温过程变化明显,采用恒温壁面冷颗粒升温速率明显提高;同时热载体颗粒数目越多,冷颗粒在下降管反应器内升温越快。     

气固两相流动中的一种颗粒相湍流模型

颗粒湍流有其自身的产生与耗散。鉴于颗粒湍流模型对预测精度有很大影响,建立了考虑各向异性和两相相互作用的颗粒相湍流模型,采用代数雷诺应力模型封闭气固两相产生项及两相速度关联产生项。分别利用本模型和k_g-ε_g-k_p-ε_p-θ模型对比研究了轴对称旋流两相流动的颗粒相平均速度及脉动速度,结果显示,本模型的模拟结果比k_g-ε_g-k_p-ε_p-θ模型的更接近于实验数据,表明本模型更符合颗粒湍流的内在特性。     

Particle transport in turbulent flow using both Lagrangian and Eulerian formulations

Lagrangian and Eulerian models for particle transport by a turbulent fluid phase are presented. In both methods, particle distribution results from the action of applied forces (buoyancy, inertial, added mass and drag forces) and turbulent effects are shown. The carrier phase flow – which is solved by finite element method using a k–ε turbulence model – is assumed not to depend on the particles' motion. In the Lagrangian formulation the dynamic equation for the particles is solved. A discrete random walk model is used to account for the turbulent effects. In the Eulerian formulation, the particle concentration is calculated from a convection–diffusion equation using the terminal particles' velocity and turbulent diffusivity. Both models are compared to experimental measurements and analytical results; a good agreement is observed.     

SIMULATION OF EROSION BY THE DILUTE PARTICULATE FLOW IMPACT

In this study, the effects of turbulence intensity, temperature, particle sizes, and impinging velocity on erosion by particle impact are demonstrated numerically. Underlying turbulent flow on an Eulerian frame is described by the Reynolds averaged Navier - Stokes equations with an Renormalization Group Theory (RNG) k-epsilon turbulence model. The particle trajectories and particle - wall interactions are evaluated by a Lagrangian approach. An erosion model considering material weight removal from surfaces is used to predict erosive wear. Computational validation against measured data is performed on a one-phase and two-phase impinging jet. Numerical comparisons reveal that the current study provides better predictive capability for erosion than the previous works.     

Coagulation kernel of particles in a turbulent gas flow

On the base of modern probability density functions approach turbulent coagulation of particles in gravitational field is investigated. Spectral presentation of second velocity moments of gas phase is used for calculation of intensity of particles relative chaotic motion. Closed diffusion equation for two-particle distribution in space is obtained. Boundary condition taking into account coefficients of new particle formation and momentum restitution during two particles collision is found. Formula for calculation of turbulent coagulation kernel of particles in gravity field is gain. Influence of cloud turbulence and turbulence in a pipe flow on intensity of droplets coagulation is studied. Strong effects of relative turbulent diffusion between droplets, droplets inertia and droplets gravitational settling on intensity of coagulation are found out. Connection between internal structure of turbulence type and coagulation rate is illustrated. Obtained results are right for polydisperse additions in turbulent flows.     

Predicting turbulence and heat transfer in 3-D curved ducts by near-wall second moment closures

This paper presents discussions on predicting turbulence and heat transfer in two types of square sectioned U-bend duct flows with mild and strong curvature by recent second moment closures. Batten et al.'s [AIAA J. 37 (1999) 785] modified version of Craft and Launder's [Int. J. Heat Fluid Flow 17 (1996) 245] two-component-limit (TCL) turbulence model and Shima's [Int. J. Heat Fluid Flow 19 (1998) 549] wall-reflection free model are presently focused on. They are low-Reynolds-number models totally free from geometrical parameters. The former model is realizable and called the TCL model. For turbulent heat flux, a higher order version of the generalized gradient diffusion hypothesis by Suga and Abe [Int. J. Heat Fluid Flow 21 (2000) 37] is applied along with the TCL model. The results suggest that although both second moment closures are generally good enough for predicting flow and heat transfer in the case of mild curvature, only the realizable TCL model is reliable in the strong curvature case.     

Population balance model of heat transfer in gas–solid particulate systems

A population balance model is derived for heat transfer processes in gas–solid systems with intensive motion of particles in order to describe the temperature distribution of particulate phase. The model involves collisional particle–particle and particle–wall heat transfers, and continuous gas–particle, gas–wall and wall–liquid environment heat transfer processes. Collisional heat transfers are characterised by collision frequencies and random heat exchange parameters with general probability distributions with support [0, 1], describing the heat transfer efficiency between the colliding solid bodies. An infinite hierarchy of moment equations, describing the time evolution of moments of the temperature of particle population is derived from the population balance equation, which can be closed at any order of moments. The properties of the model and the effects of parameters are examined by numerical experiments using the second order moment equation model of a spatially homogeneous fluidized bed.     

On the modeling of turbulent evaporating sprays: Eulerian versus lagrangian approach

Recently developed mathematical models for turbulent evaporating sprays are described. The relative merits of Eulerian and Lagrangian approaches in handling the dispersed phase are discussed. These two approaches are evaluated vs reported data for evaporating dilute sprays (in the region z/D 50), produced by an air-atomizing injector in a still environment. Results show that both approaches are successful in predicting the main features of this type of flow; however, the Eulerian approach performs better. The ignorance of the turbulence effects on the droplet motion is found to lead to significant errors when the mean relative velocity becomes comparable to the carrier phase r.m.s. velocity fluctuation. The one-size Eulerian treatment yields very comparable results to that of the multi-size treatment when the droplet size range is not very wide. Finally, the cost analysis for the two approaches demonstrates that the use of the Monte Carlo technique in simulating droplet dispersion is even more expensive than the multisize Eulerian treatment.     

NO_x生成湍流反应率的二阶矩-PDF数值模拟

将对 NOx 生成湍流反应率模拟的二阶矩模型和设定 PDF模型结合起来 ,提出一种氮氧化物生成湍流反应率的二阶矩 - PDF模型 ,以兼顾解决工程问题的合理性和经济性 ,并对甲烷空气燃烧中的 NOx 生成进行了数值模拟     

Impact of wall roughness and turbulence level on the performance of a horizontal axis wind turbine with the U‐RANS solver THETA

The presented work investigates the impact of different sheared velocity profiles in the atmospheric boundary layer on the characteristics of a wind turbine by modifying the wall roughness coefficients in the logarithmic velocity profile. Moreover, the rotor and wake characteristics in dependence of the turbulence boundary conditions are investigated. In variant I, the turbulence boundary conditions are defined in accordance to the logarithmic velocity profile with different wall roughness lengths. In variant II, the turbulent kinetic energy and turbulent viscosity remain independent of the velocity profile and represent the free‐stream turbulence level. With an increase of the shear in the velocity profile, the amplitudes in the 3/rev characteristics of rotor thrust and rotor torque, induction factors, and effective angles of attack are increased. In variant I, the overall levels of thrust coefficient are hardly affected by the velocity profiles resulting from different wall roughness length values. The power coefficient is reduced about 1%. Conversely, compared with variant II, a difference of 2% in the power coefficient has been detected. Moreover, the wake recovery process strongly depends on the turbulence boundary condition. Simulations are carried out on an industrial 900‐kW wind turbine with the incompressible U‐RANS solver THETA.     

Turbulent forced convection of Cu–water nanofluid: CFD model comparison

This study compares the predictions of five types of computational fluid dynamics (CFD) models, including two single-phase models (i.e. Newtonian and non-Newtonian) and three two-phase models (Eulerian–Eulerian, mixture and Eulerian–Lagrangian), to investigate turbulent forced convection of Cu–water nanofluid in a tube with a constant heat flux on the tube wall. The Reynolds (Re) number of the flow is taken between 10,000 and 25,000, while the volume fraction of Cu particles used is in the range of 0% to 2%. The results from the CFD models are compared with results of experimental investigations from literature. According to the results of this study, the non-Newtonian single-phase model, in general, does not show a good agreement with Xuan and Li's correlation in the prediction of the Nu number. The Eulerian–Eulerian model gives inaccurate results except for φ = 0.5%. The mixture model gives a maximum error of 15%. The Newtonian single-phase model and Eulerian–Lagrangian model, overall, are the recommended models. This work can be used as a reference for selecting an appropriate model for future investigation. The study also gives a proper insight into the important factors such as the Brownian motion, fluid behavior parameters and effective nanoparticle conductivity which should be considered or changed by each model.