气固流化床的离散颗粒运动-碰撞解耦模型与模拟

基于分子动力学和气固两相流体动力学，建立流化床稠密气-固两相离散颗粒运动-碰撞解耦模型，采用硬球模拟方法处理颗粒与颗粒之间的碰撞，及大涡模拟方法处理气相湍流流动．单颗粒运动满足牛顿第二定律，颗粒相和气相相间相互作用的双向耦合由牛顿第三定律确定，数值模拟二维鼓泡流化床内稠密气-固两相流动，得到了气泡的形成、发展及颗粒的流化过程，计算结果表明颗粒弹性恢复系数影响气-固两相流动特性。     

纳米通道内气体流动特性的分子动力学研究

采用分子动力学的方法,对He气在纳米通道内的流动过程进行了模拟研究。研究发现,流体处在层流和紊流的流动状态时,原子在流动过程中速度和原子数密度沿流动方向近似呈线性变化。流体处在层流流动状态时,在壁面附近会产生一定厚度的吸附层。模拟结果与相关研究结果吻合。     

纳米流体强化内燃机冷却系统流动的基础研究

利用高导热率、传热性能好的传热工质(纳米流体)替代传统冷却介质应用于内燃机冷却系统中,通过纳米流体流动特性的基础研究,为其在内燃机冷却系统中的应用提供理论基础支持.因此,利用试验方法对纳米流体在波壁管内的流动进行可视化研究,以期对纳米流体的流动机理进行详细的探讨,从而推动纳米流体在内燃机冷却系统中的应用.研究发现:纳米流体的黏度增加值不大,且随着温度的升高,增加值降低;而相同入口速度状态下,纳米流体在波壁管内的流动比纯水更为活跃,漩涡数量增多,质量传递特性增强,且随纳米颗粒浓度的增加,流动湍流效应增大.通过分子动力学方法发现纳米颗粒在纳米流体流动过程中存在强烈的旋转作用,从而出现微湍流流动效应,进一步强化了纳米流体的湍流流动效果.     

入流条件对同轴射流旋流燃烧室内湍流流动模拟的影响

应用一种新的代数Reynolds应力模型对同轴射流旋流燃烧室内两股射流为同向旋转和反向旋转条件下的湍流旋流流动进行了数值模拟。为得到合理的流场分布结果，研究了入流条件对同轴射流旋流燃烧室内湍流流动模拟结果的影响。计算中对旋流燃烧室进口处两股旋转射流的轴向与切向速度采用了均匀分布和实验测量分布两种方式来给定。将两种进口速度分布条件下得到的燃烧室内气体轴向与切向速度分布计算结果与实验数据进行了比较。     

4气门直喷式汽油机缸内湍流场多周期循环变动的大涡模拟

应用大涡模拟方法对一台4气门直喷式汽油机(DISI)缸内冷态湍流流场进行了三维瞬态数值分析.通过连续13个工作循环的模拟计算,探索缸内湍流流动的循环变动特征与规律,并与PIV流场测试结果进行了对比.模拟相平均值及循环变动的均方值和试验值在总体上吻合得较好.计算结果表明:在进气过程前期缸内流场湍流脉动和循环变动都很强烈,两者强度为同一量级;但在后续过程中,湍流脉动不断衰减,其与循环变动的比值小于15%.大涡模拟方法不仅可以真实地反映内燃机循环过程中缸内气体流动的细节和规律,而且非常适合于研究内燃机的循环变动特性.     

环形进出口旋流燃烧室内有回流的湍流旋流流动的数值模拟

对环形进出口旋流燃烧室内有较强回流的湍流旋流流动进行了数值模拟。计算中分别采用了完整形式与简化形式的新的代数Reynolds应力模型及κ－ε模型。模拟结果与实验测量数据在气体轴向与切向时均速度和轴向、切向与径向湍流脉动速度均方根值分布上的对比表明，新的代数应力模型可预报出旋流燃烧室内范围较大的中心回流区，气体切向速度分布的Rankine涡结构，以及湍流脉动的各向异性。     

基于SIMPLE算法的湍流场数值模拟

k-ε模型及其双方程模式成功应用于湍流场的流动分析,并使湍流流动的质量、动量、能量基本方程组可以统一表达。SIMPLE算法是流体流场数值模拟的最重要方法。通过VC和MATLAB的混合编程用SIMPLE算法,实现了湍流场的数值模拟。基于SIMPLE算法对一个热流交汇扩散的问题做了数值求解,展示了内部湍流场的分布情况。     

流化床内非等密度双组分颗粒流动特性的研究

基于颗粒动力学和气固两相流体动力学,建立流化床稠密气一固两相非等密度双组分颗粒运动碰撞解耦模型,采用硬球模拟方法研究颗粒间碰撞,大涡模拟方法研究气相湍流流动。基于牛顿第二定律建立单颗粒运动方程,应用牛顿第三定律确定颗粒相和气相相间相互作用的双向耦合。数值模拟二维鼓泡流化床内非等密度双组分颗粒气一固两相流动,计算结果表明颗粒弹性恢复系数影响分层流动特性。     

基于数值模拟的燃惰气燃烧室的优化设计研究

惰性气体是一种高效、环保、经济的防、抑爆及消防灭火介质,惰气的来源制约了惰气安全防护技术的研究开发.以一种新型燃惰气燃烧室为研究对象,采用了RNGk-ε双方程模型描述湍流流动,随机颗粒轨道模型追踪燃油颗粒运动,考虑了化学反应动力学机制对燃烧的影响,采用了修正的EBU湍流燃烧模型计算燃烧速率,同时采用了离散坐标法表征辐射传热过程,建立了燃烧室三维气雾两相湍流燃烧模型,对燃烧室流动燃烧特性进行了深入数值模拟研究,模拟结果与实验结果的比较表明了数值模型及数值方法的合理性.在数值模拟的基础上对燃惰气燃烧室进行了优化设计,采用了多种强化燃烧技术.对优化设计燃烧室的数值模拟结果表明设计合理,满足设计要求.研究结果为惰气安全技术的开发设计奠定了基础.     

用于循环流化床的鳍片管束惯性分离器流动特性的研究

本文运用激光多普勒测速方法及数值模拟方法对于循环流化床的鳍片管束惯性分离器的流动动力特性进行了研究,得到了绕鳍片管流动的气流速度、湍流强度等参数的分布,并对试验和计算结果进行了分析讨论,为下一步对鳍片管束撞击式分离器的优化研究奠定了基础。     

SIMULATION OF SWIRLING TURBULENT FLOWS OF COAXIAL JETS IN A COMBUSTOR

A numerical simulation of swirling turbulent flows of coaxial jets in a combustor is presented in this paper. The new algebraic Reynolds stress model (ASM) is employed for the closure of the time-averaged governing equations for swirling turbulent flows. Two cases of the swirling flow, i.e., a coswirl jet flow and a counterswirl jet flow, are simulated. The calculated results of the axial and tangential velocities, static pressure, and turbulent fluctuating velocity are compared with the measured data for both cases. The results illustrate that the predictions by the new ASM are closer to the measurements than those obtained by the k-epsilon model.     

Molecular diffusion effects in LES of a piloted methane–air flame

Molecular diffusion effects in LES of a piloted methane–air (Sandia D) flame are investigated on a series of grids with progressively increased resolution. The reacting density, temperature and chemical composition are modeled based on the mixture fraction approach combined with a steady flamelet model. With a rationale to minimize interpolation uncertainties that are routinely introduced by a flamelet table look-up, quadratic splines relationships are employed to represent thermochemical variables. The role of molecular diffusivity in effecting spatial transport is studied by drawing a comparison with the turbulent diffusivity and analyzing their statistics conditioned on temperature. Statistical results demonstrate that the molecular diffusivity in the near-field almost always exceeds the turbulent diffusivity, except at low temperatures (less than 500 K). Thus, by altering the jet near-field, molecular transport plays an important role in the further downstream jet development. Molecular diffusivity continues to dominate in the centerline region throughout the flow field. Overall, the results suggest the strong necessity to represent molecular transport accurately in LES studies of turbulent reacting flows.     

Vortex Simulation of Axisymmetrical Flows in Cylindrical Geometries．PartII：Application to Pipes Incorporating an Orifice Plate

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Population balance modelling of polydispersed particles in reactive flows

Polydispersed particles in reactive flows is a wide subject area encompassing a range of dispersed flows with particles, droplets or bubbles that are created, transported and possibly interact within a reactive flow environment - typical examples include soot formation, aerosols, precipitation and spray combustion. One way to treat such problems is to employ as a starting point the Newtonian equations of motion written in a Lagrangian framework for each individual particle and either solve them directly or derive probabilistic equations for the particle positions (in the case of turbulent flow). Another way is inherently statistical and begins by postulating a distribution of particles over the distributed properties, as well as space and time, the transport equation for this distribution being the core of this approach. This transport equation, usually referred to as population balance equation (PBE) or general dynamic equation (GDE), was initially developed and investigated mainly in the context of spatially homogeneous systems. In the recent years, a growth of research activity has seen this approach being applied to a variety of flow problems such as sooting flames and turbulent precipitation, but significant issues regarding its appropriate coupling with CFD pertain, especially in the case of turbulent flow. The objective of this review is to examine this body of research from a unified perspective, the potential and limits of the PBE approach to flow problems, its links with Lagrangian and multi-fluid approaches and the numerical methods employed for its solution. Particular emphasis is given to turbulent flows, where the extension of the PBE approach is met with challenging issues. Finally, applications including reactive precipitation, soot formation, nanoparticle synthesis, sprays, bubbles and coal burning are being reviewed from the PBE perspective. It is shown that population balance methods have been applied to these fields in varying degrees of detail, and future prospects are discussed.     

A Simple Finite-Volume Formulation of the Lattice Boltzmann Method for Laminar and Turbulent Flows

A finite-volume formulation commonly employed in the well-known SIMPLE family algorithms is used to discretize the lattice Boltzmann equations on a cell-centered, non-uniform grid. The convection terms are treated by a higher-order bounded scheme to ensure accuracy and stability of solutions, especially in the simulation of turbulent flows. The source terms are linearized by a conventional method, and the resulting algebraic equations are solved by a strongly implicit procedure. A method is also presented to link the lattice Boltzmann equations and the macroscopic turbulence modeling equations in the frame of the finite-volume formulation. The method is applied to two different laminar flows and a turbulent flow. The predicted solutions are compared with the experimental data, benchmark solutions, and solutions by the conventional finite-volume method. The results of these numerical experiments for laminar flows show that the present formulation of the lattice Boltzmann method is slightly more diffusive than the finite-volume method when the same numerical grid and convection scheme are used. For a turbulent flow, the finite-volume lattice Boltzmann method slightly underpredicts the reattachment length in a separated flow. In general, the finite-volume lattice Boltzmann method is as accurate as the conventional finite-volume method in predicting the mean velocity and the pressure at the wall. These observations show that the present method is stable and accurate enough to be used in practical simulations of laminar and turbulent flows.     

Mean-field/PDF numerical approach for polydispersed turbulent two-phase flows

The purpose of this paper is to give an overview in the realm of numerical computations of polydispersed turbulent two-phase flows, using a mean-field/PDF approach. In this approach, the numerical solution is obtained by resorting to a hybrid method, where the mean fluid properties are computed by solving mean-field (RANS) equations with a classical finite volume procedure whereas the local instantaneous properties of the particles are determined by solving stochastic differential equations (SDEs). The fundamentals of the general formalism are recalled and particular attention is focused on a specific theoretical issue: the treatment of the multiscale character of the dynamics of the discrete particles, i.e. the consistency of the system of SDEs in asymptotic cases. Then, the main lines of the particle/mesh algorithm are given and some specific problems, related to the integration of the SDEs, are discussed, for example, issues related to the specificity of the treatment of the averaging and projection operators, the time integration of the SDEs (weak numerical schemes consistent with all asymptotic cases), and the computation of the source terms. Practical simulations, for three different flows, are performed in order to demonstrate the ability of both the models and the numericals to cope with the stringent specificities of polydispersed turbulent two-phase flows.     

Experimental study of turbulent jet induced by steam jet condensation through a hole in a water tank

A turbulent jet induced by steam jet condensation in a water pool was investigated experimentally. An experimental apparatus equipped with a steam boiler, a single-hole steam sparger, and a water pool, etc. was used. For the measurements, a pitot tube and thermocouples were used for turbulent flow velocity and temperatures, respectively. Overall flow shapes of the turbulent jet by the steam jet condensation are similar to those of axially symmetric turbulent jet flows. The angular coefficients of turbulent rays are quantitatively comparable between the traditional turbulent jet flows and the turbulent jet flows induced by the steam jet condensation in this work. Although the turbulent flows were induced by the steam jet condensation, general theory of turbulent jets was found to be applicable to the turbulent flows of this work.     

Numerical study on supersonic combustion of hydrogen and its mixture with Ethylene and methane with strut injection

In this paper, supersonic combustion and flow field of hydrogen and its mixture with ethylene and methane from strut injections in a Mach 2 supersonic flow are studied numerically. The fuel mixture of hydrogen, methane and ethylene represents the major products of pyrolysis of hydrocarbon fuels with large molecules such as kerosene as it acts as coolant and flows through cooling channels and absorbs heat. Detached Eddy Simulation with a reduced kinetic mechanism and steady flamelet model are applied to simulate turbulent combustion. The calculated temperature profiles of hydrogen are compared to the experimental results of DLR supersonic combustor for validation of the present numerical method. The supersonic combustion flows associated with shock waves, turbulent vortices and flame structures are studied. With addition of methane and ethylene, the flame zone moves further downstream of the strut and the maximum flow temperature at chamber exit decreases by 200 K. With analysis of total temperature ratios, it is found that combustion efficiency for hydrogen combustion is 0.91 and it decreases to 0.78 for the fuel mixture. The calculation of ignition delay time and flame speed reveals that fuel mixture of hydrogen and hydrocarbons has considerably larger delay time and smaller flame speed, that contributes to the weakened flame zone and lower combustion efficiency.     

Detailed modeling of soot formation in a partially stirred plug flow reactor

The purpose of this work is to propose a detailed model for the formation of soot in turbulent reacting flow and to use this model to study a carbon black furnace. The model is based on a combination of a detailed reaction mechanism to calculate the gas phase chemistry, a detailed kinetic soot model based on the method of moments, and the joint composition probability density function (PDF) of these scalar quantities.Two problems, which arise when modeling the formation of soot in turbulent flows using a PDF approach, are studied. A consistency study of the combined scalar-soot moment approach reveals that the molecular diffusion term in the PDF-equation can be closed by the IEM and Curl-type mixing models. An investigation of different kernels for the collision frequency of soot particles shows that the influence of turbulence on particle coagulation is negligible for typical flame conditions and the particle size range considered.The model is used as a simple tool to simulate a furnace black process, which is the most important industrial process for the production of carbon blacks. Despite the simplifications in the modeling of the turbulent flow reasonable agreement between the calculated soot yield and data measured in an industrial furnace black reactor is achieved although no adjustments were made to the kinetic parameters of the soot model. The effect of the mixing intensity on soot yield and different soot formation rates is investigated. In addition the influence of different operating conditions such as temperature and equivalence ratio in the primary zone of the reactor is studied.