NUMERICAL SIMULATION OF METHANE-AIR TURBULENT JET FLAME USING A NEW SECOND-ORDER MOMENT MODEL

A new second-order moment model for turbulent combustion is applied in the simulation of methane-air turbulent jet flame. The predicted results are compared with the experimental results and with those predicted using the well-known EBU-Arrhenius model and the original second-order moment model. The comparison shows the advantage of the new model that it requires almost the same computational storage and time as that of the original second-order moment model, but its modeling results are in better agreement with experiments than those using other models. Hence, the new second-order moment model is promising in modeling turbulent combustion with NOx formation with finite reaction rate for engineering application.

Mathematical Model of Combustion in Blunt Annular Ceramic Burner

The computer simulation of the combustion process in blast furnace(BF) stove has been studied by using the k-ε-g turbulent diffusion flame model.The combustion process in blunt annular ceramic burner was calculated by using the software.The profiles of gas and air velocity,temperature of the combustion products,concentration of the components,and the shape and length of the flame during combustion have been researched.Compared with the original annular ceramic burner,the new design of the blunt one improves the mixing of the gas and the air significantly,and shortened the length of the flame.

Modeling and Numerical Simulation of Wings Effect on Turbulent Flow between two contra-rotating cylinders

Many industries in the world take part in the pollution of the environment. This pollution often comes from the reactions of combustion. To optimize these reactions and to minimize pollution, turbulence is a funda- mental tool. Several factors are at the origin of turbulence in the complex flows, among these factors, we can quote the effect of wings in the rotating flows. The interest of this work is to model and to simulate numeri- cally the effect of wings on the level of turbulence in the flow between two contra-rotating cylinders. We have fixed on these two cylinders eight wings uniformly distributed and we have varied the height of the wings to have six values from 2 mm to 20 mm by maintaining the same Reynolds number of rotation. The numerical tool is based on a statistical model in a point using the closing of the second order of the transport equations of the Reynolds stresses (Reynolds Stress Model: RSM). We have modelled wings effect on the flow by a source term added to the equation tangential speed. The results of the numerical simulation showed that all the average and fluctuating variables are affected the value of the kinetic energy of turbulence as those of Reynolds stresses increase with the height of the wings.

REYNOLDS-STRESS MODELLING OF TURBULENT ROTATING FLOWS

The present analysis shows that the EVM can not reflect the turbulence physics in non-inertial frame. The effects of Coriolis force on turbulence is embodied naturally in the Reynolds-stress transport equation. It is observed that the existing second-moment closure model with appropriate near-wall treatment can adequately predict flows in rotating channel and in axially rotating pipe for moderate rotation rate.

CFD study of non-premixed swirling burners: Effect of turbulence models

This research investigates a numerical simulation of swirling turbulent non-premixed combustion.The effects on the combustion characteristics are examined with three turbulence models:namely as the Reynolds stress model,spectral turbulence analysis and Re-Normalization Group.In addition,the P-1 and discrete ordinate(DO)models are used to simulate the radiative heat transfer in this model.The governing equations associated with the required boundary conditions are solved using the numerical model.The accuracy of this model is validated with the published experimental data and the comparison elucidates that there is a reasonable agreement between the obtained values from this model and the corresponding experimental quantities.Among different models proposed in this research,the Reynolds stress model with the Probability Density Function(PDF)approach is more accurate(nearly up to 50%)than other turbulent models for a swirling flow field.Regarding the effect of radiative heat transfer model,it is observed that the discrete ordinate model is more precise than the P-1 model in anticipating the experimental behavior.This model is able to simulate the subcritical nature of the isothermal flow as well as the size and shape of the internal recirculation induced by the swirl due to combustion.

A flamelet model for turbulent diffusion combustion in supersonic flow

In order to develop a turbulent diffusion combustion model for supersonic flow, the physical argument of the extension of the flamelet model to supersonic flow was presented, and the flow field of a hydrogen/air diffusion combustion generated by axisymmetric supersonic jets was numerically simulated by employing the flamelet model. Using the experimental data, value of the model coefficient of scalar dissipation in the flamelet model was revised specifically for supersonic flow. The computational results of the modified flamelet model were compared with the experimental results, and it was indicated that the precision of the modified flamelet model was satisfying. Based on the numerical results and flamelet theory, the influence mechanisms of turbulence fluctuation on the average state equation and chemical reaction rate were studied for the first time. It was found that the fluctuation correlation of species mass fractions and temperature has little effect on the averaged gas state equation; the temperature fluctuation decreases the product of H2O, but its effect is small; the fluctuation of species mass fractions increases the product of H2O in the region close to oxidizer while decreases the product of H2O in other regions; the fluctuation correlation of species mass fractions and temperature largely decreases the product of H2O.

Development of SOM combustion model for Reynolds-averaged and large-eddy simulation of turbulent combustion and its validation by DNS

The derivation and closure methods of the second-order moment (SOM) combus- tion model are proposed. The application of this model to Reynolds averaged (RANS) and large-eddy simulation (LES) of turbulent swirling diffusion combustion, jet diffusion combustion, and bluff-body stabilized premixed combustion is sum- marized. It is indicated that the SOM model is much better than the eddy-beak-up (EBU) and presumed PDF models widely used in commercial software and engi- neering. The SOM modeling results are close to those obtained using the most accurate but much more complex PDF equation model. Moreover, it can save much more computation time than the PDF equation model. Finally, the SOM model is validated by the direct numerical simulation (DNS) of turbulent reacting channel flows.

Theoretical analysis of flamelet model for supersonic turbulent combustion

Being physically intuitionistic and computationally efficient,the flamelet model has obtained increasing attention and becomes popular in numerical simulations of supersonic combustion.However,the flamelet model was firstly built for simulations of the low-speed flows.Therefore it is still unclear whether the assumption of the flamelet model is reasonable in supersonic combustion.This paper tries to identify the existence of the flamelet mode in supersonic combustion.Firstly,the interaction of the turbulent fluctuation and the flame is discussed,based on which the combustion regime is distinguished.Then the charac-teristic length scale of the turbulent flows and the combustion inside the scramjet combustor are calculated and compared,which are used to identify whether the assumption of the flamelet model is reasonable in supersonic flows.The results show that for premixed combustion,due to the low fluctuation velocity in the recirculation zone and the shear layer,the assumption of the flamelet model is established for all the flight Mach numbers,while for non-premixed combustion the assumption is also established for most of the flight Mach numbers except for very high Mach number under which the slow reaction mode dominates the combustion.In order to quantitatively examine the combustion mode,numerical calculations are performed to simulate the strut-injection supersonic combustion which has been experimentally investigated in German Aerospace Center.The results show that the supersonic combustion occurs in the fully-developed turbulent regions and the assumption of the flamelet model is established in the whole flow field.

Non-premixed turbulent combustion modeling based on the filtered turbulent flamelet equation

In turbulent combustion simulations, the flow structure at the unresolved scale level needs to be reasonably modeled. Following the idea of turbulent flamelet equation for the non-premixed flame case, which was derived based on the filtered governing equations(L. Wang, Combust. Flame 175, 259(2017)), the scalar dissipation term for tabulation can be directly computed from the resolved flowing quantities, instead of solving species transport equations. Therefore, the challenging source term closure for the scalar dissipation or any assumed probability density functions can be avoided;meanwhile the chemical sources are closed by scaling relations. The general principles are discussed in the context of large eddy simulation with case validation. The new model predictions of the bluff-body flame show sufficiently improved results, compared with these from the classic progress-variable approach.

Review on Large Eddy Simulation of Turbulent Premixed Combustion in Tubes

This paper reviews the existing knowledge on the large eddy simulation(LES) of turbulent premixed combustion in empty tubes and obstructed tubes. From the view of model development in LES, this review comprehensively analyzes the development history and applicability of the important Sub-Grid Scale(SGS) viscosity models and SGS combustion models. LES is also used to combine flow and combustion models to reproduce industrial explosion including deflagration and detonation and the transition from deflagration to detonation(DDT). The discussion about models and applications presented here leads readers to understand the progress of LES in the explosion of tube and reveals the deficiencies in this area.

TPDF-ASOM复合湍流燃烧模型及其检验

先进航空发动机燃烧室设计要求对湍流火焰精确控制,现有模拟方法需提高精度和效率。输运概率密度函数(TPDF)湍流燃烧模型精度高,代数二阶矩(ASOM)湍流燃烧模型计算成本低,类比离散涡模拟思想,基于Da数将湍流燃烧场区分“高精度”和“低成本”2个区域,在输运方程框架下采用随机场TPDF(高精度)和ASOM(低成本)方法重构TPDF-ASOM复合湍流燃烧模型,以提高模拟的整体精度和效率。在大涡模拟(LES)-TPDF程序平台创建ASOM并进一步实现TPDF-ASOM复合湍流燃烧模型,用Flame D实验数据检验所建模型和方法。结果表明:所建模型的预测结果与实验值接近,而且能够兼顾精度和计算效率。

随机场TPDF在非结构网格平台的开发及检验

为实现对燃烧室内部燃烧过程的高精度数值模拟,增强航空发动机燃烧室研究手段,采用开源的非结构网格平台Saturne,开发可以耦合详细化学反应机理,高精度解析湍流燃烧过程的概率密度函数输运方程(TPDF)湍流燃烧模型。采用随机场方法求解TPDF方程,在原有程序基础上研发了TPDF程序模块及配合的加速算法等模拟单元,发展了针对燃烧室燃烧过程模拟的软件功能。采用射流火焰Flame D和旋流火焰TECFLAM对新软件进行了测试,结果表明:模拟结果与试验数据趋势一致,精度尚可。对某型燃烧室性能模拟验证中,计算的出口温度分布与试验结果一致,平均温度误差小于3.8%,新的软件可进一步应用于燃烧室性能的研究。

火花辅助压燃准维燃烧模型的建立及应用

本文采用湍流火焰传播速度模型描述火花点燃(SI)燃烧,针对单个自燃点仍采用湍流火焰传播速度模型,将所有自燃点的当量湍流火焰速度集合作为总当量湍流火焰速度用来描述压燃燃烧,进而建立了火花辅助压燃(SACI)准维燃烧模型。基于该模型研究了空气、外部废气再循环(EGR)稀释对SACI的影响,仿真和实验匹配良好。计算表明:SACI的火焰传播速度高于SI,随点火提前而增大,外部EGR稀释的火焰传播速度低于空气稀释;点火推迟或者增加外部EGR都会导致火焰前锋面面积峰值升高,衰减速度减慢,燃烧等容度减弱;稀释率相当时,空气稀释的热效率更高,但外部EGR稀释的尾气后处理更容易。

基于曲线坐标系浸没边界方法的折流燃烧室模拟

航空发动机折流燃烧室几何结构复杂,其高保真数值模拟需要高效的网格与边界条件处理方法。采用曲线坐标系隐式浸没边界方法结合大涡模拟-概率密度函数输运方程湍流燃烧模型开发自研软件,并实现WP11中折流燃烧室的高保真模拟。流动模拟中准确解析了该燃烧室中的三股主要气流,且三股气流分别约占进口流量的75%,12.5%和12.5%。两相燃烧模拟中针对拉格朗日框架下的液滴运动和欧拉框架下的湍流燃烧采用不同网格标记,模拟得到的出口径向温度分布规律与实验一致,平均相对误差为17.95%,表明基于本方法开发的自研软件能准确模拟折流燃烧室中的两相湍流燃烧现象。

A dual timescale model for micro-mixing and its application in LES-TPDF simulations of turbulent nonpremixed flames

The numerical simulation of modern aero-engine combustion chamber needs accurate description of the interaction between turbulence and chemical reaction mechanism. The Large Eddy Simulation(LES) method with the Transported Probability Density Function(TPDF) turbulence combustion model is promising in engineering applications. In flame region, the impact of chemical reaction should be considered in TPDF molecular mixing model. Based on pioneer research, three new TPDF turbulence-chemistry dual time scale molecular mixing models were proposed tentatively by adding the chemistry time scale in molecular mixing model for nonpremixed flame. The Aero-Engine Combustor Simulation Code(AECSC) which is based on LES-TPDF method was combined with the three new models. Then the Sandia laboratory's methane-air jet flames: Flame D and Flame E were simulated. Transient simulation results show that all the three new models can predict the instantaneous combustion flow pattern of the jet flames. Furthermore,the average scalar statistical results were compared with the experimental data. The simulation result of the new TPDF arithmetic mean modification model is the closest to the experimental data:the average error in Flame D is 7.6% and 6.6% in Flame E. The extinction and re-ignition phenomena of the jet flames especially Flame E were captured. The turbulence time scale and the chemistry time scale are in different order in the whole flow field. The dual time scale TPDF combustion model has ability to deal with both the turbulence effect and the chemistry reaction effect, as well as their interaction more accurately for nonpremixed flames.

Effect of turbulence on NO formation in swirling combustion

Turbulence affects both combustion and NO formation. Fluctuation correlations are ideally used for quantitative analysis. From the instantaneous chemical reaction rate expression,ignoring the third-order correlation terms, the averaged reaction rate will have four terms, including the term of averaged-variable product, a concentration fluctuation correlation term, and temperature-concentration fluctuation correlation term. If the reaction-rate coefficient is denoted as K, the temperature fluctuation would be included in the K fluctuation. In order to quantitatively study the effect of turbulence on NO formation in methane-air swirling combustion, various turbulencechemistry models are tested. The magnitudes of various correlations and their effects on the time-averaged reaction rate are calculated and analyzed, and the simulation results are compared with the experimental measurement data. The results show that among various correlation moments, the correlation between the reaction-rate coefficient K fluctuation with the concentration fluctuation is most important and is a strong nonlinear term.

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．

大涡模拟及其在湍流燃烧中的应用

大涡模拟作为一种研究湍流流动和湍流燃烧的有效手段，在国际上已经得到广泛应用．本文在回顾了大涡模拟（ＬＥＳ）的基本思想及其实施方法的基础上着重介绍了前人在大涡模拟的亚格子湍流模式和亚格子燃烧模式中的研究成果，同时给出了采用不同亚格子模式的大涡模拟在湍流燃烧中的应用实例，指出了大涡模拟在湍流燃烧中的重要作用，为大涡模拟的进一步发展和应用提供参考。

现象学燃烧分析模型及其在柴油机燃烧研究中的应用

本文提出一个新的直喷式柴油机燃烧分析模型,该模型利用台架实验数据计算燃空混合率、放热率、NO_x生成率等燃烧过程参数,研究燃烧系统参数对燃烧过程的影响,分析充量间传热传质过程及其影响因素。本模型已用于天津动力机厂130柴油机燃烧系统改进的研究。结果表明,强化扩散燃烧阶段缸内空气与产物间紊流混合率可减少NO_x排放量,预计适当提高130柴油机空气涡流将会改善高、低转速工况的综合排放特性。

超燃冲压发动机多凹腔燃烧室混合与燃烧性能定量分析

为从定量上研究超燃冲压发动机多凹腔燃烧室对增强混合、燃烧的影响,用大涡模拟方法和火焰面模型对燃料当量比为0.062的流场进行数值模拟。比较了不同多凹腔构型对混合效率、燃烧效率和总压损失的影响,并结合壁面压力分布、数值纹影解释了其原因。结果表明,凹腔串联、凹腔并联均能增强混合,混合效率最大可提高20.95%和9.52%;凹腔串联、凹腔并联均能增强燃烧,燃烧效率最大可提高14%和16.94%;燃烧时凹腔串联总压损失最小,但凹腔并联燃烧放热最快,对缩短燃烧室长度有利。