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....     

钝体燃烧的数值模拟

钝体燃烧模拟考虑湍流和燃烧相互耦合。在标准κ-ε两方程模型下分别采用非预混燃烧模型中化学平衡、稳态小火焰和瞬态小火焰模型,研究不同燃烧模型对组分、温度场以及流场分布的影响。数值模拟结果表明,上述燃烧模型模拟的结果与前人研究成果存在不同程度的差异,稳态小火焰模型优于其它模型,但模拟该燃烧器的燃烧模型尚需进一步完善。     

甲烷/空气湍流扩散燃烧的小火焰模拟

以甲烷/空气的湍流射流扩散燃烧为基础,对通用的反应标量方程在火焰面上进行坐标变换,建立二维稳态湍流扩散火焰的小火焰模型。利用湍流流动模型、甲烷/空气半详细化学反应机理和小火焰模型耦合求解,分别计算出过量空气系数为1.2和1.4的速度在燃烧室内的分布状况以及混合分数、温度和组分的径向分布,模拟结果表明小火焰模型能够用来描述燃烧室内燃烧机理。     

层流小火焰模型在柴油机湍流燃烧中的应用

将湍流燃烧的层流小火焰模型应用于典型的柴油机扩散燃烧过程.以混合分数为自变量,以标量耗散率为参数,建立相空间中的层流小火焰数据库.应用KIVA-3程序模拟内燃机缸内多维湍流流场,并补充求解混合分数的时均值和脉动均方值的湍流输运方程.将两部分结果通过Beta概率密度函数进行耦合积分,便可得到组分质量分数和温度等参数在柴油机工作过程中的时间、空间分布.对一台直喷式柴油机的湍流燃烧过程进行了模拟计算,所得结果符合实际.     

柴油/空气湍流扩散燃烧的一个小火焰模型

本文将小火焰（flamelet）理论应用于分析柴油/空气湍流扩散燃烧的小火焰结构,以正十二烷同空气的一步反应为基础,建立柴油机燃烧的Flamelet模型,利用数值方法求出了柴油机湍流扩散燃烧的Flamelet结构.并采用假定PDF的方法,选取截尾式高斯分布的概率密度分布函数,将其与Flamelet结构相结合,求得燃烧过程中各参数的时均值,分析得出湍流脉动和非平衡作用对燃烧过程的影响.     

基于动态增厚火焰模型三维全可压缩非预混燃烧的大涡摸拟

利用动态增厚火焰模型对斯坦福大学甲烷/空气燃烧器非预混火焰进行了三维全可压缩大涡模拟,其中湍流亚网格模型采用Smagorinsky-WALE 模型,反应机理采用甲烷四步简化机理.将计算结果与层流小火焰模型及实验值进行比较发现:在进口附近的区域,动态增厚火焰模型的预测结果与实验非常吻合,但在远离进口区域,预测的混合作用大于实验值;动态增厚火焰模型的预测效果与层流小火焰模型相当.     

喷嘴旋向对多喷嘴预混燃烧的影响

针对五喷嘴模型燃烧室采用层流小火焰模型进行数值模拟,讨论了中心喷嘴同向旋流和反向旋流对甲烷/空气多喷嘴预混燃烧的影响,模拟结果与实验结果吻合较好.数值模拟结果表明,多喷嘴预混火焰悬浮于喷嘴上方0.3,d处,多喷嘴气流相互掺混形成了高度湍流的掺混区,在掺混区内发生强烈的燃烧化学反应.中心喷嘴同向旋流布置时,主回流区顺时针扭曲,流动最后发展为与单喷嘴类似的旋流流场结构;对于中心喷嘴反向布置,气流整体旋流偏转较小,掺混后喷嘴气流仍然保持着各自的旋流流动结构.此外,中心喷嘴的同向/反向布置对多喷嘴预混燃烧反应进程的影响不大.     

直喷式柴油发动机主要污染物的仿真及实验

为计算直喷式柴油机的主要污染物——碳烟和氮氧化物，提出了RIF计算模型。此小火焰计算模型是通过引入理想混和比分数空间坐标系，将通常坐标系下的组元方程转化到混和比分数空间坐标系下，得出新的组元扩散方程。同时将小火焰计算模型与对流体动力学程序（KIVA-3V）相结合，增加并修改原有计算模块，将湍流喷雾运动和湍流燃烧运动有机分开，计算出直喷式柴油机湍流燃烧后生成的碳烟和氮氧化物，最后通过实验检验了该计算模型。     

湍流燃烧模型在燃烧室数值计算中的对比分析

对比分析了不同燃烧模型对某型回流式燃气轮机燃烧室流场的影响,建立了描述燃烧室流场的控制方程组,采用Realizablek-ε湍流模型,湍流燃烧模型分别为涡耗散模型(ED)、涡耗散概念模型(EDC)、简单概率密度模型(PDF)和稳态小火焰模型(SFM).对比分析了不同燃烧模型下燃烧室的温度场、速度分布以及NOx排放量,并...     

湍流预混燃烧的小火焰模型

由Level set方法确定湍流预混燃烧火焰面的位置,考虑CHEMKIN库详细化学反应机理,通过PDF方法建立湍流预混燃烧数学模型,计算组分浓度和温度在火焰内部分布。以矩形突扩燃烧室为例,模拟甲烷/空气预混燃烧的平均火焰位置和火焰内部温度、浓度分布,计算结果与实验结果吻合良好,表明此模型能较好模拟湍流预混燃烧。     

Capabilities and limitations of multi-regime flamelet combustion models

Flamelet combustion models typically assume that burning occurs in either a fully premixed or a fully non-premixed mode. These assumptions tend to limit the applicability of the models to single-regime combustors. Efforts aimed at reducing this limitation have introduced multi-regime approaches that account for different types of mixing and chemistry interactions. In this study a multi-regime model is applied to two laminar n-heptane flames in an effort to characterize the capabilities and limitations of the approach. Both a 2-D laminar triple flame and a 2-D laminar counter-flow diffusion flame are numerically simulated using the multi-regime model. Data for comparison is generated by additionally simulating the flames using finite rate chemistry, a purely premixed flamelet model, and a purely non-premixed flamelet model. Simulations demonstrate that the multi-regime approach functions as desired, and tends to access flamelets from the appropriate regime under both non-premixed and premixed conditions. Some important differences between the flamelet solutions and finite rate solution are observed, however. These differences are caused by the finite rate solution deviating away from the assumed flamelet manifolds, rather than by inadequate regime predictions. In the analyses of these simulations, an emphasis is placed on understanding the formation of the pollutant species NO. It is shown that even when the local combustion regime is correctly predicted, small deviations from an assumed flamelet manifold can lead to changes in the NO production rate. The simulation results confirm that multi-regime flamelet models are applicable to a wide variety of reacting flows, but the results also help to characterize the limitations of these models.     

Future progress in turbulent combustion research

Turbulent combustion research is projected to be an important area of research well into the twenty-first century. Issues of current interest in turbulent flame structure and computational prediction are outlined and forecasts are made for approaches that are likely to lead to significant advances. There is a mounting body of evidence that concepts and models derived from the laminar flamelet hypothesis are not valid over many of the conditions of practical interest for both premixed and non-premixed systems. Approaches such as Conditional Moment Closure and Monte–Carlo simulation of the transport equation for the probability density function are considered to have the most promise for pollutant prediction in non-premixed systems. Large Eddy Simulation may be necessary for non-stationary premixed problems and for bluff-body and swirling flows.     

A general flamelet transformation useful for distinguishing between premixed and non-premixed modes of combustion

The flame index was originally proposed by Yamashita et al. as a method of locally distinguishing between premixed and non-premixed combustion. Although this index has been applied both passively in the analysis of direct numerical simulation data, and actively using single step combustion models, certain limitations restrict its use in more detailed combustion models. In this work a general flamelet transformation that holds in the limits of both premixed and non-premixed combustion is developed. This transformation makes use of two statistically independent variables: a mixture fraction and a reaction progress parameter. The transformation is used to produce a model for distinguishing between premixed and non-premixed combustion regimes. The new model locally examines the term budget of the general flamelet transformation. The magnitudes of each of the terms in the budget are calculated and compared to the chemical source term. Determining whether a flame burns in a premixed or a non-premixed regime then amounts to determining which sets of these terms most significantly contribute to balancing the source term. The model is tested in a numerical simulation of a laminar triple flame, and is compared to a recent manifestation of the flame index approach. Additionally, the model is applied in a presumed probability density function (PDF) large eddy simulation (LES) of a lean premixed swirl burner. The model is used to locally select whether tabulated premixed or tabulated non-premixed chemistry should be referenced in the LES. Results from the LES are compared to experiments.     

Predictions of free jet fires from high pressure, sonic releases

A numerical model for predicting jet fires resulting from high pressure, sonic releases of natural gas is described. The model is based on solutions of the density-weighted forms of the fluid flow equations. It is capable of accurately resolving the near-field shock structure that occurs in these flows through the use of a compressibility corrected version of the k-? turbulence model, and also includes sub-models for the flame lift-off height and a prescribed probability density function/laminar flamelet model of the turbulent non-premixed combustion process. Radiation heat transfer is described using an adaptive version of the discrete transfer method, with solutions of the radiation heat transfer equation obtained using a statistical narrow band approach. The complete model is demonstrated to yield plausible predictions of the structure of both the near-field non-reacting and subsonic combusting zones within wind blown fires, and to provide realistic predictions of flame lift-off heights, mean temperatures, trajectories and the radiation fluxes received about a number of field-scale jet fires.     

Partially premixed flamelet modeling in a hydrogen-fueled supersonic combustor

In scramjet combustors, the combustion process is usually partially premixed, that is, both the non-premixed and the premixed regimes should be taken into account. Based on the multi-regime flamelet (MRF) model proposed for low Mach number flows, a modified MRF model that applies to supersonic flow conditions has been developed. Taken a hydrogen-fueled model combustor as test case, the good agreement between the calculation and experiments was obtained. The distribution of weighting coefficient, which is defined based on the concept of combustion regime index, shows that the flow field in the supersonic combustor is partially-premixed. The premixed regime distributes in the backflow region, the shear layer and the boundary layer. Comparisons between the results of steady laminar flamelet (SLF) model and the modified MRF model show that the latter one gives a more precise prediction of temperature profiles, indicating the modified MRF model has better versatility and accuracy.     

Self-ignition and combustion modeling of initially nonpremixed turbulent systems

A model to simulate numerically self-ignition and combustion of initially non-premixed turbulent systems is proposed. Its development is based on Direct Numerical Simulations (DNS) of turbulent mixing layers between cold fuel and a hot oxidizer. The direct numerical simulations are used to better understand the physical mechanisms controlling mixing, self-ignition, and establishment of combustion inside turbulent mixing layers. They are also used to define the mathematical formulation of the model, and to test its assumptions. The model has a component for self-ignition and an additional component for subsequent high-temperature combustion. Self-ignition is simulated using an approach based on presumed Probability Density Functions (PDFA model) that takes into account the effects of turbulence on mixing formation during and after self-ignition. The PDFA model describes the turbulent reacting flow using a mixture fraction variable and a generalized reaction progress variable. The high-temperature combustion, established after self-ignition has occurred, is computed using a flamelet approach (CHI model). The two model components are coupled by a function of the progress variable deduced from DNS results. The PDFA-CHI model is implemented in a Reynolds averaged Computational Fluid Dynamics (CFD) code, and is tested in one-dimensional (1D) and two-dimensional (2D) configurations. The computational results reproduce ignition phenomena in the turbulent field similar to the DNS calculations.     

Computational models and methods for continuous gaseous turbulent combustion

Computational methods used to simulate turbulent reacting flow in combustors of complex shape are presented. The physical sub-models include either chemically equilibrated or partially-equilibrated (radical pool) models of varying complexity and laminar flamelet models. Limitations of turbulence models are discussed. Some results in non-premixed jet flames are reviewed showing the effects of non-equilibrium phenomena on major and minor species, temperature and pollutants. An algorithm applicable to recirculating flow in bodies with arbitrarily contoured boundaries is developed. Issues governing the choice of grid systems, discretization operators, and numerical solution procedures for recirculating flows are emphasized. Numerical results illustrate these issues. The method is demonstrated by application to a modern annular gas-turbine combustor.     

采用考虑详细化学反应机理的火焰面模型模拟湍流扩散火焰

采用详细的甲烷氧化化学反应动力学机理(GRI-Mech3．0)对不同拉伸率条件下的拉伸层流扩散火焰面结构进行了数值计算，建立了一个包含一系列拉伸层流火焰面结构的火焰面数据库．将这些层流火焰面结构和美国Sandia国家实验室测得的湍流扩散火焰(FlameD)的平均火焰结构进行了对比，发现层流火焰面所覆盖的范围基本包含了所考虑的湍流火焰中不同位置的平均火焰结构，这表明火焰面模型是合理的．然后，采用火焰面模型对该湍流扩散火焰进行了数值模拟并和实验数据进行了比较，考察了火焰面模型的精确程度和模拟深度.