环形燃烧室火焰简及旋流器流场计算

本文采用偏微分方程法生成贴体网格,在任意曲线坐标系下数值研究两种先进燃烧室火焰筒及其旋流器三维紊流流场。由于旋流器的形状复杂,本文采用型线定点法确定网格的边界。在非交错网格系下采用SIMPLE算法和混合差分格式对离散方程进行求解。计算结果表明计算方法合理,这计算程序进一步扩展,可用来预估环形燃烧室反应流流场。     

U-RANS／PDF方法计算钝体后漩涡脱落

本研究发展了U-RANS/PDF混合算法研究湍流和化学反应相互作用对燃烧稳定性的影响,采用有限体积和Monte Carlo相结合的方法在非结构网格中求解相容的U-RANS方程和脉动速度-湍流频率-标量的联合PDF方程.本文对钝体火焰驻定器后冷态流场进行了计算,结果表明此混合算法能够捕捉流场中非稳态的漩涡脱落现象.着重研究了湍流频率模型系数的改变对漩涡脱落频率以及拟序结构在动量输运中的作用的影响.     

Euler及N-S方程的二维四边形/叉树混合网格自适应算法

描述了一种基于直角叉树网格的Euler和N-S方程自适应算法.由于考虑了粘性的作用,提出并使用了四边形叉树混合网格的方法,在几何表面附近生成贴体的四边形网格,外流场使用直角叉树网格.采用中心有限体积法,对Euler及N-S方程进行数值求解,对N-S方程的计算中加入了B-L代数湍流模型.在流场中,运用了网格自适应算法,提高了数值计算对激波、流动分离等特性的捕捉和分辨能力.采用上述方法,数值分析了单段和多段翼型的绕流问题.     

气固两相各向同性湍流的自适应网格模拟

气固两相湍流是一个复杂的多尺度,多物理场的难题.为了捕捉流场的小尺度涡以及其对颗粒的作用,传统上采用了静态的高分辨率网格,消耗极大的计算资源.本文基于小波算法对网格进行加密或疏化,从而实现网格的自适应.基于自适应网格的框架上,建立颗粒与流场分块的映射,搭建了欧拉-拉格朗日的气固两相流模拟的并行平台.通过对均匀各向同性湍...     

用非正交曲线坐标系数值计算航空发动机加力燃烧室无化学反应的湍流流场

本文采用非正交曲线坐标系下非交错网格的SIMPLE方法对航空发动机加力燃烧室无化学反应的湍流流场进行了数值计算，湍流模型采用k-ε双方程模型．差分网格采用分区方法生成，计算时对整个流场进行分区迭代直至得到收敛结果．加力燃烧室湍流流场数值计算结果合理．     

基于FGM的旋流预混燃烧室超大涡模拟

针对航空发动机燃烧室等复杂工程中的预混燃烧问题发展高精度、高效的数值预测方法,本研究发展了火焰面生成流型(FGM)详细化学反应建表方法结合超大涡模拟方法(VLES),对工程中的GE LM6000预混旋流燃烧室燃烧开展了高精度数值研究,并与实验结果进行了比较。计算结果表明,VLES-FGM方法可以较准确地预测出旋流预混燃烧室内的流场及温度场分布。为了进一步模拟航空发动机真实的燃烧工况,对原始单头部燃烧室使用周期性边界条件来类比全环燃烧室。计算结果表明,VLES-FGM方法计算得到的周期性燃烧室流场回流区相比较固壁边界燃烧室较小,并且固壁边界燃烧室温度场具有明显的颈部结构,燃烧室下游的高温区分布更为均匀。本文计算结果表明基于FGM燃烧模型的自适应湍流模拟方法VLES对于模拟复杂航空发动机相关的旋流预混燃烧具有很大的应用潜力。     

湍流扩散火焰的非稳态火焰面模拟

采用稳态的和非稳态的火焰面模型同时对一个湍流甲烷射流扩散火焰进行了数值模拟,比较了两者对湍流平均火焰结构、活性自由基和污染物(氮氧化物)排放的模拟效果。速度场采用κ-ε模型计算,守恒标量混合物分数的分布通过其概率密度函数(PDF)输运方程的求解得到。稳态的火焰面结构由查询火焰面数据库得到,而非稳态的火焰面结构由火焰面方程和流场方程耦合求解来计算。采用详细的GRI—Mech 3.0机理描述甲烷的氧化和氮氧化物的形成。数值模拟结果和实验数据作了广泛的对比,验证了火焰面模型对湍流扩散燃烧的定量模拟能力。     

模型加力燃烧室燃烧流场的并行计算

本文采用基于MPI的并行算法,采用动态内存分配、分区算法和多点重合交错网格系统,在贴体网格下对带V形槽稳定器模型加力燃烧室素流化学反应流场进行数值模拟,湍流模型采用k方程亚网格尺度模型,燃烧模型采用亚网格EBU模型,采用热通量辐射模型估算辐射通量。计算结果表明并行计算对复杂形状的化学反应流动计算效率很高,是模拟大规模的燃烧问题的有力工具。     

KIVA程序网格生成算法的改进及雷诺应力模型计算

针对KIVA程序中网格生成不能适用于复杂区域的缺点,对其提出了一种改进算法,该算法采用Laplace椭圆方程,生成计算区域的网格,具有收敛快、适用于复杂区域的特点。该方法只要给定边界点,就可以自动生成内部网格,无需差值形成内部网格。针对几种部同的内燃机燃烧室采用雷诺应力(DSM)模型,计算了压缩、膨胀、进气和排气过程的内流场,计算结果正确给出了缸内的湍流各向异性的特点。     

一种基于预估-修正策略的非定常流网格自适应加密/稀疏方法

给出一种非定常流动数值模拟的网格自适应处理方法.在"求解流动方程-自适应调整网格"的流程中,引入预估-修正步.根据自适应周期内每个时间步上的流场预估解,计算单元上的事后误差估算值.建立考虑解演变的网格自适应指示器,并进行多层次单元加密-稀疏的动态网格自适应处理.在自适应网格上重新计算流场.每个自适应周期中,流动演变区域的网格获得加密;而前一个周期中的特征现象已离开区域的网格被稀疏.应用边界非协调的当地DFD(Domain-Free Discretization)方法求解流动方程.为验证网格自适应处理方法,针对静止圆柱和自推进游鱼的流动进行了数值实验.     

Response of non-premixed flames to bulk flow perturbations

This paper describes the dynamics of non-premixed flames responding to bulk velocity fluctuations, and compares the dynamics of the flame sheet position and spatially integrated heat release to that of a premixed flame. The space–time dynamics of the non-premixed flame sheet in the fast chemistry limit is described by the stoichiometric mixture fraction surface, extracted from the solution of the

-equation. This procedure has some analogies to premixed flames, where the premixed flame sheet location is extracted from the G = 0 surface of the solution of the G-equation. A key difference between the premixed and non-premixed flame dynamics, however, is the fact that the non-premixed flame sheet dynamics are a function of the disturbance field everywhere, and not just at the reaction sheet, as in the premixed flame problem. A second key difference is that the non-premixed flame does not propagate and so flame wrinkles are convected downstream at the axial flow velocity, while wrinkles in premixed flames convect downstream at a vector sum of the flame speed and axial velocity. With the exception of the flame wrinkle propagation speed, however, we show that that the solutions for the space–time dynamics of the premixed and non-premixed reaction sheets in high velocity axial flows are quite similar. In contrast, there are important differences in their spatially integrated unsteady heat release dynamics. Premixed flame heat release fluctuations are dominated by area fluctuations, while non-premixed flames are dominated by mass burning rate fluctuations. At low Strouhal numbers, the resultant sensitivity of both flames to flow disturbances is the same, but the non-premixed flame response rolls off slower with frequency. Hence, this analysis suggests that non-premixed flames are more sensitive to flow perturbations than premixed flames at O(1) Strouhal numbers.     

Large-eddy simulation of a bluff-body stabilised turbulent premixed flame using the transported flame surface density approach

A premixed propane–air flame stabilised on a triangular bluff body in a model jet-engine afterburner configuration is investigated using large-eddy simulation (LES). The reaction rate source term for turbulent premixed combustion is closed using the transported flame surface density (TFSD) model. In this approach, there is no need to assume local equilibrium between the generation and destruction of subgrid FSD, as commonly done in simple algebraic closure models. Instead, the key processes that create and destroy FSD are accounted for explicitly. This allows the model to capture large-scale unsteady flame propagation in the presence of combustion instabilities, or in situations where the flame encounters progressive wrinkling with time. In this study, comprehensive validation of the numerical method is carried out. For the non-reacting flow, good agreement for both the time-averaged and root-mean-square velocity fields are obtained, and the Karman type vortex shedding behaviour seen in the experiment is well represented. For the reacting flow, two mesh configurations are used to investigate the sensitivity of the LES results to the numerical resolution. Profiles for the velocity and temperature fields exhibit good agreement with the experimental data for both the coarse and dense mesh. This demonstrates the capability of LES coupled with the TFSD approach in representing the highly unsteady premixed combustion observed in this configuration. The instantaneous flow pattern and turbulent flame behaviour are discussed, and the differences between the non-reacting and reacting flow are described through visualisation of vortical structures and their interaction with the flame. Lastly, the generation and destruction of FSD are evaluated by examining the individual terms in the FSD transport equation. Localised regions where straining, curvature and propagation are each dominant are observed, highlighting the importance of non-equilibrium effects of FSD generation and destruction in the model afterburner.     

Comparative analysis of methods for heat losses in turbulent premixed flames using physically-derived reduced-order manifolds

Heat losses have the potential to substantially modify turbulent combustion processes, especially the formation of pollutants such as nitrogen oxides. The chemistry governing these species is strongly temperature sensitive, making heat losses critical for an accurate prediction. To account for the effects of heat loss in large eddy simulation (LES) using a precomputed reduced-order manifold approach, thermochemical states must be precomputed not only for adiabatic conditions but also over a range of reduced enthalpy states. However, there are a number of methods for producing reduced enthalpy states, which invoke different implicit assumptions. In this work, a set of a priori and a posteriori LES studies have been performed for turbulent premixed flames considering heat losses within a precomputed reduced-order manifold approach to determine the sensitivity to the method by which reduced enthalpy states are generated. Two general approaches are explored for generating these reduced enthalpy states and are compared in detail to assess any effects on turbulent flame structure and emissions. In the first approach, the enthalpy is reduced at the boundary of the one-dimensional (1D) premixed flame solution, resulting in a single enthalpy deficit for a single premixed flame solution. In the second approach, a variable heat loss source term is introduced into the 1D flame solutions by mimicking a real heat loss to reduce the post-flame enthalpy. The two approaches are compared in methane–air piloted turbulent premixed planar jet flames with different diluents that maintain a constant adiabatic flame temperature but experience different radiation heat losses. Both a priori and a posteriori results, as well as a chemical pathway analysis, indicate that the manner by which the heat loss is accounted for in the manifold is of secondary importance compared to other model uncertainties such as the chemical mechanism, except in situations where heat loss is unphysically fast compared to the flame time scale. A new theoretical framework to explain this insensitivity is also proposed, and its validity is briefly assessed.     

Framework for simulating stationary spherical flames

Understanding and quantifying the effects of flame stretch rate on the laminar flame speed and flame structure plays an important role from interpreting experimentally-measured laminar burning velocities to characterizing the impact of turbulence on premixed flames. Unfortunately, accounting for these effects often requires an unsteady reacting flow solver and may be computationally expensive. In this work, we propose a mathematical framework to perform simulations of stationary spherical flames. The objective is to maintain the flame at a constant radius (and hence a constant stretch rate) by performing a coordinate change. The governing equations in the new flame-attached frame of reference resemble the original equations for freely-propagating spherical flames. The only difference is the presence of additional source terms whose purpose is to drive the numerical solution to a steady state. These source terms involve one free parameter: the flame stretch rate, which may either be computed in real time or imposed by the user. This parameter controls ultimately the steady state flame radius and the steady state flame speed. That is why, at a given stretch rate, the results of the stationary spherical flame simulations match those of a freely-expanding spherical flame. As an illustration, the dependence of the laminar flame speed on the stretch rate is leveraged to extract Markstein lengths for hydrogen/air mixtures at different equivalence ratios, as well as for hydrocarbon/air mixtures (CH₄ and C₇H₁₆). Numerical predictions are in good agreement with experimental measurements (within experimental uncertainties). Finally, the proposed methodology is implemented in the chemical kinetic software FlameMaster. The use of a dedicated steady-state solver with a non-uniform optimized mesh leads to significant reductions in the computational cost, highlighting that the proposed methodology is ideally suited for other chemical kinetic software such as Chemkin/Premix and Cantera.     

Partially premixed flamelet in LES of acetone spray flames

An effective partially premixed flamelet model for large eddy simulation (LES) of turbulent spray combustion is formulated. Different flame regimes are identified with a flame index defined by budget terms in a 2-D multi-phase flamelet formulation, and the application in LES of partially pre-vaporized spray flames shows a favorable agreement with experiments. Simulations demonstrate that, compared to the conventional single-regime flamelets, the present partially premixed flamelet formulation shows its ability in capturing the subgrid regime transitions, yielding a well prediction of peak gas temperature and the downstream flame spreading. A propagating premixed flame front is found coupled with a trailing diffusion burning through the spray evaporation, and the spray effect on regime discrimination is manifested with transport budget analysis. A two-phase regime indicator is then proposed, by which the evaporation-dictated regime is properly described. Its intended use will rely on both gas and spray flamelet structures.     

On laminar premixed flame propagating into autoigniting mixtures under engine-relevant conditions

Usually premixed flame propagation and laminar burning velocity are studied for mixtures at normal or elevated temperatures and pressures, under which the ignition delay time of the premixture is much larger than the flame resistance time. However, in spark-ignition engines and spark-assisted compression ignition engines, the end-gas in the front of premixed flame is at the state that autoignition might happen before the mixture is consumed by the premixed flame. In this study, laminar premixed flames propagating into an autoigniting dimethyl ether/air mixture are simulated considering detailed chemistry and transport. The emphasis is on the laminar burning velocity of autoigniting mixtures under engine-relevant conditions. Two types of premixed flames are considered: one is the premixed planar flame propagating into an autoigniting DME/air without confinement; and the other is premixed spherical flame propagating inside a closed chamber, for which four stages are identified. Due to the confinement, the unburned mixture is compressed to high temperature and pressure close to or under engine-relevant conditions. The laminar burning velocity is determined from the constant-volume propagating spherical flame method as well as PREMIX. The laminar burning velocities of autoigniting DME/air mixture at different temperatures, pressures, and autoignition progresses are obtained. It is shown that the first-stage and second-stage autoignition can significantly accelerate the flame propagation and thereby greatly increase the laminar burning velocity. When the first-stage autoignition occurs in the unburned mixture, the isentropic compression assumption does not hold and thereby the traditional method cannot be used to calculate the laminar burning velocity. A modified method without using the isentropic compression assumption is proposed. It is shown to work well for autoigniting mixtures. Besides, a power law correlation is obtained based on all the laminar burning velocity data. It works well for mixtures before autoignition while improvement is still needed for mixtures after autoignition.     

A LES-CMC formulation for premixed flames including differential diffusion

A finite volume large eddy simulation–conditional moment closure (LES-CMC) numerical framework for premixed combustion developed in a previous studyhas been extended to account for differential diffusion. The non-unity Lewis number CMC transport equation has an additional convective term in sample space proportional to the conditional diffusion of the progress variable, that in turn accounts for diffusion normal to the flame front and curvature-induced effects. Planar laminar simulations are first performed using a spatially homogeneous non-unity Lewis number CMC formulation and validated against physical-space fully resolved reference solutions. The same CMC formulation is subsequently used to numerically investigate the effects of curvature for laminar flames having different effective Lewis numbers: a lean methane–air flame with Le_eff = 0.99 and a lean hydrogen–air flame with Le_eff = 0.33. Results suggest that curvature does not affect the conditional heat release if the effective Lewis number tends to unity, so that curvature-induced transport may be neglected. Finally, the effect of turbulence on the flame structure is qualitatively analysed using LES-CMC simulations with and without differential diffusion for a turbulent premixed bluff body methane–air flame exhibiting local extinction behaviour. Overall, both the unity and the non-unity computations predict the characteristic M-shaped flame observed experimentally, although some minor differences are identified. The findings suggest that for the high Karlovitz number (from 1 to 10) flame considered, turbulent mixing within the flame weakens the differential transport contribution by reducing the conditional scalar dissipation rate and accordingly the conditional diffusion of the progress variable.     

Numerical simulation of a mixed-mode reaction front in a PPC engine

The ignition process, mode of combustion and reaction front propagation in a partially premixed combustion (PPC) engine running with a primary reference fuel (87% iso-octane, 13% n-heptane by volume) is studied numerically in a large eddy simulation. Different combustion modes, ignition front propagation, premixed flame and non-premixed flame, are observed simultaneously. Displacement speed of CO iso-surface propagation describes the transition of premixed auto-ignition to non-premixed flame. High temporal resolution optical data of CH₂O and chemiluminescence are compared with simulated results. A high speed ignition front is seen to expand through fuel-rich mixture and stabilize around stoichiometry in a non-premixed flame while lean premixed combustion occurs in the spray wake at a much slower pace. A good qualitative agreement of the distribution of chemiluminescence and CH₂O formation and destruction shows that the simulation approach sufficiently captures the driving physics of mixed-mode combustion in PPC engines. The study shows that the transition from auto-ignition to flame occurs over a period of several crank angles and the reaction front propagation can be captured using the described model.     

Premixed turbulent combustion modeling using tabulated detailed chemistry and PDF

Tabulated chemistry and presumed probability density function (PDF) approaches are combined to perform RANS modeling of premixed turbulent combustion. The chemistry is tabulated from premixed flamelets with three independent parameters: the equivalence ratio of the mixture, the progress of reaction, and the specific enthalpy, to account for heat losses at walls. Mean quantities are estimated from presumed PDFs. This approach is used to numerically predict a turbulent premixed flame diluted by hot burnt products at an equivalence ratio that differs from the main stream of reactants. The investigated flame, subjected to high velocity fluctuations, has a thickened-wrinkled structure. A recently proposed closure for scalar dissipation rate that includes an estimation of the coupling between flame wrinkling and micromixing is retained. Comparisons of simulations with experimental measurements of mean velocity, temperature, and reactants are performed.