障碍物和管壁导致火焰加速的三维数值模拟

基于K 模型和改进的EBU Arrehnius燃烧模型 ,考虑了障碍物对流动的附加作用 ,通过修改方程的源项 ,建立了湍流加速火焰现象的理论模型。选用Simple格式 ,壁面边界层采用壁面函数法处理 ,模拟了障碍物和摩擦管壁在三维空间中导致火焰加速的现象 ,结果表明障碍物和管壁对火焰都有明显的加速作用 ,且障碍物的加速作用更明显 ,最后分析了导致火焰加速的机理。     

RNGκ-ε模式在工程湍流数值计算中的应用

自Yakhot和Orszag提出RNGκ-ε湍流模式以来,很多学者对其进行了修正和发展,并将其应用于某些实际湍流问题的数值模拟,取得了一些与实验近似一致的结果.本文主要从工程湍流计算的角度出发,结合作者的部分研究工作,对近年来RNG κ-ε模式在湍流流动数值研究中的应用现状及进展情况进行了总结,指出了该模式存在的问题,并对其应用前景进行了展望.     

管内均相湍流燃烧加速的数值模拟

通过均相流体模型、湍流k 模型和EBU(EddyBreak Up) Arrhenius燃烧模型 ,选用Simple格式对管中戊烷和空气的燃烧实例进行了数值求解。其结果反映了燃烧导致的爆炸过程中管内流场各参数的变化规律 ,揭示了管内燃烧、流动、湍流之间的正反馈耦合关系 ,并与实验结果和相关数值结果基本相符。     

半封闭狭缝湍流冲击射流的数值模拟

将Yakhot和Orszag提出的RNGk-ε模型推广应用于半封闭狭缝冲击射流场的数值模拟，以评价该模型对这种复杂湍流的预测能力。将计算得到的流场平均速度分布、湍流强度分布和流函数分布与标准k—ε模型的预测结果以及相应的实验数据进行了比较，结果表明：RNGk—ε模型的预测结果总体上要好于标准k—ε模型，但与实验值相比，所有预测结果都还存在不同程度的误差，尤其是近壁区和滞止点较远下游处的湍流强度分布。说明RNG模型虽然已在某些湍流的预测中取得了一定的成功，但要定量准确地预测冲击射流场，还必须针对其流动特征对模型加以改进。     

湍流燃烧数值模拟研究的综述

对湍流燃烧数值模拟的研究进行了综合评述,其中涉及直接数值模拟（ＤＮＳ）、大涡模拟（ＬＥＳ）、随机涡模拟、概率密度函数输运方程模拟、条件矩模型、简化概率密度函数模型、关联矩模型以及基于简单物理概念的一些唯象模型等几个重要方面．对全面了解湍流燃烧数值模拟的研究现状及前景提出了看法．     

微重力下密闭腔体内火焰传播过程的数值模拟

建立了微重力条件下火焰沿固体可燃物表面蔓延的数学模型．在考虑固相传热和热解过程的基础上,研究气相的燃烧动力学过程,并且给出了体现微重力燃烧两相耦合作用的相界面处理方法．首次用数值模拟的方法研究了密闭腔体内火焰传播的三维非稳态过程,对于微重力大小分别为１０－４ｇ和１０－２ｇ时的计算结果进行了分析对比．数值模拟的结果基本合理．     

燃烧室中悬跨圆柱的火焰绕流

利用高速阴影照相系统,对方形管道内火焰流经悬吊圆柱时所产生的变化及其诱导的流场进行了实验研究,并利用高精度PPM格式对上述现象进行了数值模拟,其中,湍流采用大涡模拟(LES)方法,燃烧采用旋涡破碎(EBU)模型,圆柱边界采用沉浸边界法(IBM).实验结果与计算结果比较吻合,在一定程度上揭示了火焰阵面变形和加速的规律. 根据研究结果,对火焰与悬吊圆柱相互作用过程中火焰的三维形状、流场中涡量和湍流强度的分布、火焰与流场的相互影响以及火焰传播轨迹等进行了讨论.      

障碍物对丙烷-空气爆炸火焰加速的影响

研究了障碍物阻塞率、障碍物间距、障碍物空间位置对丙烷-空气爆炸过程及火焰加速效应的影响。采用雷诺平均（RANS）方程和湍流火焰封闭燃烧模型计算非稳态燃烧过程,主要分析障碍物周围复杂流场特性以及湍流涡与火焰面作用的详细机理。结果表明：阻塞率在0.5~0.7时,障碍物间距对火焰加速效果的影响较大,其中障碍物间距为一倍管径时火焰加速效应最大;而障碍物的空间位置对火焰传播的影响更为显著,当障碍物位于管道单侧时,湍流涡强度最大,火焰褶皱最明显,火焰传播速度最快。

     

瓦斯爆炸过程中火焰厚度测定及其温度场数值模拟分析

本文通过实验的方法,探讨了瓦斯爆炸过程中火焰厚度变化特性及其影响因素,并对瓦斯爆炸过程中的温度场变化进行了数值模拟。研究结果表明,障碍物存在时,瓦斯爆炸过程中产生的火焰厚度常常会小于无障碍物存在时所产生的火焰厚度;膜片所处位置对瓦斯爆炸过程中火焰厚度也有重要影响,膜片距离爆炸源较近时,火焰厚度明显增大,瓦斯爆炸后,火焰阵面着附近区域与管封闭端附近区域温度变化较为陡峭,而火焰阵面后一段区域的温度变化较平缓,且火焰阵面附近温度较高,在障碍物附近温度很快上升到最大值,然后温度开始下降。研究结果对指导现场防治瓦斯爆炸,减轻瓦斯爆炸灾害具有一定的指导意义。     

W型火焰锅炉炉内三维速度场数值模拟研究

W型火焰锅炉采用了煤粉浓缩、长火焰及分级送风等技术,有利于燃料的着火、火焰的稳定以及燃料的燃尽,在煤种适应性、低负荷稳燃能力、飞灰燃尽率等方面有很大的技术优势.本文通过数值模拟研究了W型火焰锅炉的三维速度场分布,得到了不同工况下炉内W型气流的流场图.通过比较数值结果和试验结果的流场图,结果表明:数值结果和试验结果基本吻合,W型火焰锅炉内的空气动力场取决于拱顶风与前后墙风的动量比,炉内气流形成两个明显的漩涡,延长了煤粉气流在下炉膛的停留时间,同时也增加了炉内气流充满度,有利于煤粉的完全燃烧.加大喉口风速,漩涡运动更加明显,在炉内形成γ型气流,煤粉气流在下炉膛的停留时间更长,炉内的一、二次风混合更加均匀.     

Numerical simulation of 3-D turbulent flows over dredged trenches

A 3-D free surface flow in open channels based on the Reynolds equations with thek-ε turbulence closure model is presented in this paper. Insted of the “rigid lid” approximation, the solution of the free surface equation is implemented in the velocity—pressure iterative procedure on the basis of the conventional SIMPLE method. This model was used to compute the flow in rectangular channels with trenches dredged across the bottom. The velocity, eddy viscosity coefficient, turbulent shear stress, turbulent kinetic energy and elevation of the free surface can be obtained. The computed results are in good agreement with previous experimental data.     

气固两相流中颗粒弥散的拉格朗日模拟

本文提出了一种对于均匀,稳定及各向同性气固两相紊流场中圆形固体颗粒弥散的拉格朗日模拟计算方法,应用该方法对带有网栅的垂直与水平管道中均匀,稳定的气固两相流模拟计算结果与Snyder及Wells等人所做的相同情况下的试验结果进行了比较,以证明该模拟计算方法的有效性,。     

Direct numerical simulation of compressible turbulent flows

This paper reviews the authors＇ recent studies on compressible turbulence by using direct numerical simulation （DNS）,including DNS of isotropic（decaying） turbulence, turbulent mixing-layer,turbulent boundary-layer and shock/boundary-layer interaction.Turbulence statistics, compressibility effects,turbulent kinetic energy budget and coherent structures are studied based on the DNS data.The mechanism of sound source in turbulent flows is also analyzed. It shows that DNS is a powerful tool for the mechanistic study of compressible turbulence.     

Direct numerical simulation of particle behaviour in the wall region of turbulent flows in horizontal channels

The motion of small particles in the wall region of turbulent channel flows has been investigated using direct numerical simulation. It is assumed that the particle concentration is low enough to allow the use of one-way coupling in the calculations, i.e. the fluid moves the particles but there is no feedback from the particles on the fluid motion. The velocity of the fluid is calculated by using a pseudospectral, direct solution of the Navier-Stokes equations. The calculations indicate that particles tend to segregate into the low-speed regions of the fluid motion near the wall. The segregation tendency depends on the time constant of the particle made non-dimensional with the wall shear velocity and kinematic viscosity. For very small and very large time constants, the particles are distributed more uniformly. For intermediate time constants (of the order 3), the segregation into the low-speed fluid regions is the highest. The finding that segregation occurs for a range of particle time constants is supported by experimental results. The findings regarding the more uniform distributions, however, still remain to be verified against experimental data which is not yet available. For horizontal channel flows, it is also found that particles are resuspended by ejections (of portions of the low-speed streaks) from the wall and are, therefore, primarily associated with low-speed fluid. The smaller particles are flung further upwards and, as they fall back towards the wall, they tend to be accelerated close to the fluid velocity. The larger particles have greater inertia and, consequently, accelerate to lower velocities giving higher relative velocities. This velocity difference, as a function of wall-normal distance, follows the same trend as in experiments but is always somewhat smaller in the calculations. This appears to be due to the Reynolds number for the numerical simulation being smaller than that in the experiment. It is concluded that the average particle velocity depends not only on the wall variables for scaling, but also on outer variables associated with the mean fluid velocity and fluid depth in the channel. This is because fluid depth in combination with the wall shear velocity determines how much time a particle, of a given size and density, spends in the outer flow and, hence, how close it gets to the local fluid velocity.     

Reynolds stress and vorticity in turbulent wall flows

Experimental measurements in a boundary layer and a large-eddy simulation of plane channel flow have been used to study the dynamics of vorticity and mass transport in the nearwall region. It was found that Reynolds stress generation occurs in the vicinity of quasi-streamwise vortices, and that smoke particles tend to be ejected from the wall near these vortical structures.     

Molecular transfer in turbulent flows

For modeling the molecular transfer of a passive scalar in a known turbulent field, the equations for the average scalar value and the correlation function for the scalar field are written in a form which makes it possible to examine the effect of molecular transfer on turbulent transfer and scalar dissipation. For the closure of the equation for the correlation function, the Prandtl hypothesis is used. The statistical reliability of this closure is demonstrated. The system proposed makes it possible to predict the dynamics of a decaying uniform scalar field and to explain why the effect of the real value of the molecular-transfer coefficient on the decaying scalar field is weak. Specific features of the transport process in a plane layer with prescribed scalar values on the layer boundaries are considered.     

Calculation of Reynolds stresses in turbulent supersonic flows

The baseline numerical procedure of interest in this study combines flux vector splitting, flux difference splitting and an explicit treatment of the diffusion terms of the flow equations. The viscous terms are treated explicitly to preserve the wave propagation properties of the Euler fluxes and permit splitting. The experience with this scheme has been limited to laminar or, at best, ‘eddy viscosity’ flows. In this paper the applicability of the scheme is extended to include the calculation of turbulent Reynolds stresses in supersonic flows. The schemes and our implementation are discussed. Both laminar and turbulence subsets of the Reynolds/Favre-averaged equations are tested, with a discussion of relative performance. The test problem for turbulence consists of a zero-pressure-gradient supersonic boundary layer as well as a supersonic boundary layer experiencing the combined effects of adverse pressure gradient, bulk compression and a concave streamline curvature. Excellent agreement with experimental measurements is observed for most of the quantities compared, which suggests that the numerical procedures presented in this paper are potentially very useful.     

Schlieren measurement of turbulent structure in a diffusion flame

The regular and random mixing structures in a turbulent diffusion flame were investigated using the quantitative, dynamic crossed-beam schlieren method. Evidence was found close to the nozzle relating to the vortexlike structure of eddies surrounding the central fuel jet flow. The observations also make possible resolution of turbulent intensity, scales, convection, and spectra within the diffusion flame without the use of seeding or intrusion of measuring probes. It is found that length scales and other turbulence parameters in the diffusion flame progressively revert to values similar to those expected and observed in scalar passive mixing as the combustion reaction intensity reduces with axial distance from the nozzle system.