针对射流火焰的条件矩封闭模型研究

针对较低Reynolds数下的射流火焰,使用直接数值模拟(DNS)数据,研究单条件矩和双条件矩封闭模型应用于此类火焰计算的特点和可行性.结果表明,单条件矩封闭模型在较小标量耗散率下能够较好地预测火焰结构,但在较大标量耗散率下则过高估计了火焰温度.双条件矩封闭模型可以改善预测精度,其模型系数α0、β0和γ0具有稳态特点.     

基于混合物分数和反应进度变量的二维火焰面模型

基于混合物分数Z和反应进度变量(Y)的二维层流火焰面模型,数值研究了混合物分数标量耗散率、反应进度标量耗散率等对Z-(Y)区域边界以及部分预混火焰结构的影响.结果表明,混合物分数标量耗散率对火焰结构有较大的影响,在混合物分数标量耗散率较小时产生了双火焰的结构;而反应进度标量耗散率对当量混合附近的结果有明显的影响,尤其是当混合物分数标量耗散率较小时;在稳定燃烧情况下,稳态一维火焰面模型只是二维火焰面模型的一个边界,表明二维火焰面模型方程的解给出的化学热力学参数表包含更多的信息,可用于多机制共存的火焰数值模拟.     

变密度射流混合的稀疏粒子拉格朗日模拟及粒子数密度研究

大涡模拟结合稀疏拉格朗日粒子方法的多维条件映射封闭(MMC)模型可以大大降低LES-FDF方法的计算量。基于等效焓方法,实现稀疏拉格朗日粒子场和LES欧拉场计算中的密度耦合,开展了同轴射流变密度混合问题的LES-MMC模拟,分析了稀疏拉格朗日粒子模拟中粒子数密度对模拟结果的影响,提出了通过改变进口粒子质量来改善粒子分布的方法并进行了验证。结果表明等效焓方法和提出的粒子数密度空间分布能够较好地给出标量的一阶矩和二阶矩。     

湍流燃烧场中高阶统计矩的PDF模拟

在非结构网格中,发展有限体积法和Monte Carlo法相结合的混合算法来模拟湍流燃烧,求解相互耦合并且相容的平均密度、动量、能量方程组和脉动速度-标量-频率的联合概率密度函数方程,采用稳态层流火焰面模型计算半详细的化学反应机理.对轴对称钝体火焰驻定器后的扩散燃烧进行模拟,比较了速度和标量的平均量和二阶矩的计算结果和实验数据,并研究了速度的扭率(三阶矩)、扁平因子(四阶矩)及边缘概率密度函数的分布.计算表明,高阶统计矩包含的统计误差比低阶统计矩的大,扭率和扁平系数的峰值则集中在剪切层处.     

湍流状态下双燃料混合物自燃和火焰传播过程

采用直接数值模拟(DNS)方法,研究了低速机缸内热力学状态下甲烷/正庚烷混合物的着火及燃烧过程,分析了湍流状态下双燃料混合层中的着火特性及火焰发展过程．结果表明：第一阶段着火后,湍流作用下混合气偏浓区域生成冷焰;第二阶段着火后,甲烷/空气预混气侧生成多个高温膨胀核心．混合分数梯度平缓区域更易生成高温核心,而混合分数梯度较大时会增大标量耗散率、加强热量和活性基团的耗散,不利于燃烧反应的稳定发展．湍流作用下火焰前沿形成褶皱向两侧传播,热膨胀核心位置的火焰前沿传播较快．甲烷/空气预混气侧含氧量增加导致火焰前沿传播加快,前沿褶皱程度逐渐降低;正庚烷侧火焰前沿在传播下游存在冷焰反应区域,形成“双火焰”结构,随着反应进行,火焰前沿传播进入更浓的混合物中,双火焰面之间的距离逐渐缩短．     

均匀各向同性湍流中颗粒所见被动标量的数值研究

对均匀各向同性湍流中惯性颗粒所见被动标量的统计特性进行了数值模拟研究,探讨了颗粒惯性的差异对统计结果的影响.结果表明,颗粒所见流体标量的自相关特性随颗粒惯性的增加而单调减少;颗粒所见流体标量脉动和耗散率随颗粒惯性的变化呈现出复杂的变化行为,在St=1.0附近分别达到最小和最大值,其原因是St=1.0的临界颗粒聚集于低涡...     

室内火灾区域模拟烟气羽流模型的适用性

结合不同的烟气羽流模型与门口溢流模型,对室内火灾过程进行建模,利用区域模拟方法计算室内热烟气层厚度及温度随时间的变化情况.利用ISO 9705全尺寸标准火灾实验系统进行实验研究,通过计算结果与实验结果的比较,分析不同羽流模型的适用性.结果表明:Thomas模型与MeCaffrey模型预测的羽流流量值偏大,导致烟气温度计算值比实验值偏低,不适用于大功率火源的情况;Zukoski模型预测的羽流卷吸量偏低,其温度计算值比实验值偏高,适合于小功率火源的情况;Heskestad模型预测的羽流流量值较为稳定,区域模拟结果与实验符合得较好,可用于大功率火灾羽流的预测.     

几何对称性处理对自由平面湍射流大涡模拟的影响

研究了几何对称性处理自由平面湍射流大涡模拟的影响，以Re数为113000的平面不可压缩湍射流流动为例，采用Chorin的分步投影法求解大尺度涡运动的Navier-Stokes方程，小尺度涡采用标准Smagorinsky亚格子模式模拟。初始条件采用平面射流无粘流动解，出流速度边界使用Sommerfeld辐射开边界条件处理，计算域的横向外边界使用自由裹入边界条件，对计算域的横向对称中心平面分别采用对称性条件和直接求解两种方法。模拟结果显示，采用对称性条件处理，会抑制自由平面湍射流中拟序结构的生长，阻碍大尺度涡从中心平面的穿透、长时间的统计平均不能给出合理的湍流低阶矩的时均结果。相反，对中心平面进行直接求解的做法能真实再现自由平面射流中涡的合并与破碎过程，得到合理的模拟结果。     

溢流反弧紊流边界层数值分析

本文在极坐标系下建立了一个模拟溢流反弧水流平均运动特性的数学模型.用Κ—ε紊流模式使基本方程封闭,用控制体积法对计算区域进行离散,最后编制大型计算机通用程序对四种不同反弧半径的紊流流场进行了求解.通过四个算例获得速度分布、边界层厚度、紊动动能变化、紊动能耗散率变化、反弧壁面切应力变化等资料.除紊动资料和反弧壁面切应力资料无实测资料验证外,其余计算均与实验结果吻合良好.     

涡模型对风力机气动特性的影响研究

比较了4种涡模型的诱导速度分布特征,包括两种层流涡模型和两种湍流涡模型。分别将4种涡模型应用至自由涡尾迹方法的尾涡诱导速度计算中,分析涡模型对风力机低速轴扭矩和尾流的影响。研究表明,在大风速下,湍流涡模型更能真实地反映流动状态;各个模型均能较好地捕捉流场结构和叶尖涡,层流涡模型的尾流涡量更集中,但耗散更快,湍流涡模型的涡量分布均匀,且耗散慢;涡模型对风力机近尾迹区域的尾流风速影响,比对远尾迹区域尾流风速的影响大。     

Modeling extinction and reignition in turbulent flames

The conditional moment closure method (CMC) has been extended to improve reactive species predictions in flames with significant local extinction and reignition. Simple first-order closure of the conditionally averaged reaction rate term does not give satisfactory results due to large fluctuations around the conditional mean and an alternative closure is suggested here. The new closure is based on a precomputed parameterized reference field that maps reactive species mass fractions as functions of mixture fraction and sensible enthalpy. During the computations, the reference field is continuously adjusted to ensure consistency with the CMC solution and doubly conditioned chemical source terms that are functions of time, space, mixture fraction, and sensible enthalpy can thus be obtained. Integration over sensible enthalpy space yields the improved singly conditioned chemical source term that can be used for the solution of the CMC equations. Full closure can be achieved by assuming a β-PDF for the probability distribution in sensible enthalpy space and an additional conditional variance equation needs to be solved. The overall agreement between the measured and the computed variance is satisfactory and the extended CMC model is applied to Sandia Flames D, E, and F. Excellent predictions of temperature, major species, intermediates, and NO are obtained in Flames D and E while temperature predictions can be significantly improved in Sandia Flame F.     

Large-eddy simulation of a dispersed particle-laden turbulent round jet

The numerical results obtained by large-eddy simulation (LES) of a particle-laden axisymmetric turbulent jet are compared with the available experimental data. The results indicate that with a new stochastic subgrid-scale (SGS) closure, the effects of the particles on the carrier gas and those of the carrier gas on the particles are correctly captured by the LES. Additional numerical experiments are conducted and used to investigate the effects of particle size, mass-loading ratio, and other flow/particle parameters on the statistics of both the carrier gas phase and the particle dispersed phase.     

Conditional reaction rate in a lifted turbulent H2/N2 flame using direct numerical simulation

In this study, reaction rate sub-models are investigated in the framework of conditional moment closure (CMC) using the direct numerical simulation (DNS) database of a lifted turbulent H₂/N₂ flame. The DNS code solves the fully compressible Navier–Stokes equation system. A 9 species and 19-step mechanism for hydrogen combustion is adopted. The comparison of the DNS results and the measurements shows that, in spite of the under predicted lift-off height, the predictions of the conditional means are satisfactory. Two improved models for the conditionally averaged reaction rate are investigated a-priori. The doubly conditioned reaction rate accounts for the fluctuations with two conditioning variables while the second-order closure is based on the Taylor expansion. It is shown that both of the models give promising results.     

用二阶矩亚网格尺度燃烧模型对丙烷-空气旋流扩散燃烧的大涡模拟

用二阶矩(SOM)亚网格尺度燃烧模型对环缝进燃料的丙烷-空气旋流湍流扩散燃烧进行了大涡模拟(LES).模拟得到统计平均的热态3个方向的速度、湍流度、温度、丙烷、氧和CO2浓度分布,其值与实验数据符合很好.结果表明,二阶矩(SOM)亚网格尺度燃烧模型适用于大涡模拟.环缝进气使湍流脉动强度、各向异性程度和温度分布趋于均匀.     

湍流氢气扩散火焰中磷酸三甲酯的抑制效果

基于湍流氢气-空气扩散火焰,对磷酸三甲酯(TMP,(CH3O) 3PO)的火焰抑制效果进行了数值研究,使用一阶条件矩封闭模型作为湍流燃烧子模型.为了研究氢原子浓度减小的机理,对其相关的重要基元反应进行了分析.最后根据所获得的数值结果及火焰燃烧结构和抑制效果,对TMP的灭火特性进行了详细的讨论分析.     

Assessment of closure schemes in second-order conditional moment closure against DNS with extinction and ignition

The performance of second-order conditional moment closure (CMC) depends on models to evaluate conditional variances and covariances of temperature and species mass fractions. In this paper the closure schemes based on the steady laminar flamelet model (SLFM) are validated against direct numerical simulation (DNS) involving extinction and ignition. Scaling is performed to reproduce proper absolute magnitudes, irrespective of the origin of mismatch between local flamelet structures and scalar dissipation rates. DNS based on the pseudospectral method is carried out to study hydrogen-air combustion with a detailed kinetic mechanism, in homogeneous, isotropic, and decaying turbulent media. Lewis numbers are set equal to unity to avoid complication of differential diffusion. The SLFM-based closures for correlations among fluctuations of reaction rate, scalar dissipation rate, and species mass fractions show good comparison with DNS. The variance parameter in lognormal PDF and the constants in the dissipation term have been estimated from DNS results. Comparison is made for the resulting conditional profiles from DNS, first-order CMC, and second-order CMC with correction to the most critical reaction step according to sensitivity analysis. Overall good agreement ensures validity of the SLFM-based closures for modeling conditional variances and covariances in second-order CMC.     

贫燃预混旋流火焰动力学失稳过程分析

利用实验和数值模拟方法对贫燃预混旋流火焰的动力学失稳过程进行了分析,发现燃烧室入口非反应旋流入射过程所诱发的低频压力振荡将引起反应流热声振荡,两者之间具有倍频关系;旋流剪切边界层内较高的速度梯度诱导产生小尺度漩涡,三维螺旋结构的涡漩进动过程将造成火焰面内出现周期性的局部熄火和重新着火,从而使非平衡羟基等值面和中心回流区...     

湍流扩散火焰中对基于混合分数的湍流燃烧模型的数值研究

根据条件矩模型(CMC)和小火焰面模型在模型构建上的相似,针对具有不同大小雷诺数和湍流-化学相互作用特性的非预混湍流射流火焰,对这两种模型进行了数值研究和比较.湍流燃烧模型采用Lagrangian型非稳态小火焰模型(LFM)和径向加权积分的CMC模型,而在H2/N2火焰的数值研究中还考虑了稳态小火焰模型的数值模拟结果....     

Highly resolved numerical simulation of combustion in supersonic hydrogen–air coflowing jets

The present study is focused on the analysis of non-premixed combustion in high-velocity (supersonic) flows. The computations make use of a large eddy simulation (LES) model, which has been recently introduced to address combustion in high Reynolds number turbulent flows featuring moderate Damköhler values. We expect that the corresponding closure is able to account for the specificities encountered in high Mach number turbulent reactive flows featuring chemical reaction time scales with the same order of magnitude as flow time scales. The model takes finite-rate chemistry and micro-mixing effects into account within the framework of the partially stirred reactor (PaSR) concept, it is hereafter denoted by U-PaSR (unsteady partially stirred reactor). (i) In a first step of the present investigation, the capabilities of the U-PaSR closure hence proposed are evaluated through a detailed comparison performed between numerical results and the data obtained from an experimental study devoted to non-premixed combustion in supersonic co-flowing jets of hydrogen and vitiated air. The simulated test case corresponds to a well-documented experimental database that includes Raman scattering and laser-induced pre-dissociative fluorescence measurements. The comparisons performed between computational results and experimental data establish that the physical processes are well-described by the performed simulation. (ii) In a second step of this study, the flame structure and associated stabilization zone are analysed in the light of numerical simulation results. The post-processing to the computational results indeed confirms the importance of self-ignition processes, as well as the relevance of diagnostic tools recently introduced by Boivin et al. [1,2]. Considering the stabilization zone, it also emphasizes the essential importance of the pressure dynamics associated with the discharge of compressible coflowing jets into the atmosphere – an importance that was not so clearly evidenced from previous numerical simulations conducted on the same experimental benchmark.     

LES–ODT study of turbulent premixed interacting flames

The LES–ODT model is implemented for the study of twin turbulent premixed flames in decaying isotropic turbulence. The approach is based on the coupling of large-eddy simulation (LES) for mass and momentum with a fixed 3D lattice of 1D fine-grained solutions based on the one-dimensional turbulence (ODT) model. The ODT solutions for momentum and reactive scalars are designed to capture subgrid scale physics that is not captured by LES. The LES–ODT formulation is capable of capturing important fine-scale processes, such as flame–flame interactions, which play an important role in flame shortening in turbulent premixed flames, and the role of preferential diffusion on curved flames’ structures.