碳氢燃料超声速燃烧分区实验研究

高保真度空天发动机数值模拟通常基于快速化学反应火焰面假设,即超声速燃烧反应的特征尺度小于湍流Kolmgorov尺度,该模型方法对于氢气燃料仿真计算结果较好,但对于乙烯等碳氢燃料仍需进一步研究.受限于极端环境特种非接触测量技术,目前尚未见超声速燃烧火焰分区判别的实验研究,导致目前超声速燃烧火焰面模型适用性以及分区燃烧物理模型认识不清,进而也制约了数值发动机技术发展.本工作基于自主研发的MHz发动机内窥光纤传感器,针对单边扩张双模态冲压发动机超声速燃烧火焰分区开展实验研究,通过化学自发光信号的最小香农熵定义超声速燃烧的特征时间τsc,根据理论方法和来流工况估算了超声速燃烧的流动特征时间,结合分区燃烧理论分析了双模态超燃冲压发动机内碳氢燃料燃烧的分区情况.通过燃烧分区情况以及与泰勒尺度的比较结果,验证了碳氢燃料超燃冲压发动机典型飞行条件下燃烧室内超声速燃烧处于旋涡小火焰区域(Re?50 000; Da∈1.80～2.60, B区),多尺度湍流涡结构发挥重要作用,并随着相对于泰勒尺度的不同大小,分别对应了不同尺度的涡结构主导该过程.同时给出了当量比、通量比以及来流马赫数对燃烧特征时间的影响规律...     

甲烷/空气预混气体火焰的传播特征

利用高速纹影摄像等技术探讨了密闭管道内不同当量比的甲烷/空气预混气体火焰的传播特征。结果表明,当甲烷含量接近当量值时,预混气体火焰传播中会发生火焰阵面由向未燃区弯曲到向已燃区弯曲的转折过程,逐渐由层流燃烧转变成湍流燃烧,并形成Tulip火焰结构;当甲烷含量偏离当量值一定程度时,预混火焰呈现出典型的层流燃烧特征,不会发生火焰阵面由向未燃区弯曲到向已燃区弯曲的转折过程。Tulip火焰结构形成于火焰传播速度迅速降低的区间里,且只有当减速阶段的最大加速度的绝对值大于某一数值时才能形成;Tulip火焰结构是预混火焰由层流燃烧向湍流燃烧转变的一个中间过程。     

高马赫数下超声速燃烧的自点火查表方法

超燃冲压发动机燃烧室工作在高马赫数工况时, 入口来流空气的总焓非常高, 自点火在高焓条件下成为维持火焰稳定的重要物理化学过程. 本文借鉴火焰面/进度变量模型的降维思路, 发展了一种基于化学动力学的自点火建表方法. 通过定义混合分数和进度变量将复杂多维的化学反应降维, 并成功将数据库方法结合到现有的大涡模拟求解器中. 经过测试和验证, 该方法初步具备对超声速自点火燃烧进行仿真描述的能力. 针对自点火诱导的超声速燃烧问题开展数值模拟, 该方法通过查表的方式有效降低了化学反应求解过程中的计算量. 在采用详细化学反应机理时能够准确地再现自点火行为和火焰结构, 并且预测的温度和重要组分分布与实验吻合较好.      

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.      

油气爆炸过程火焰燃烧模式的实验估计

首先分析讨论了油气爆炸过程中火焰燃烧模式的估计方法，然后在激波管内进行了低、中、高3次不同初始油气浓度条件下的油气爆炸实验，通过实验数据分别计算出了低、中、高初始油气浓度条件下油气爆炸在初期、中期和后期的丹姆克尔数和湍流雷诺数，最后依靠丹姆克尔数-湍流雷诺数图对低、中、高初始油气浓度条件下油气爆炸初期、中期和后期的火焰燃烧模式进行了定量估计。结果表明:低、中、高初始油气浓度条件下激波管油气爆炸过程初期、中期和后期的火焰燃烧模式均为漩涡内小火焰模式。     

塑料导爆管在燃烧转爆轰过程中的火焰结构及爆轰波生成机理

本文系统地研究了塑料导爆管从点火起至稳定爆轰时各个阶段的管内的火焰结构及火焰传播速度,分析了爆轰波的生成过程。高速扫描摄影技术证实了塑料导爆管在电火花引爆下确实存在燃烧转爆轰(DDT)过程,并显示出火焰阵面及火焰内部结构的多种变化性。发现,在燃烧阶段有一个暗区存在于火焰之中,并且火焰只是集中在管中心。清楚地观察到管内有气、固二相流动。证实了铝粉的敏化作用。在实验基础上,本文提出了一个比较符合实际情况的DDT模型及爆轰波结构。     

微重力燃烧研究进展

认识燃烧过程是安全、高效、洁净地利用能源的基础. 但是, 常重力条件下的浮力对流和重力沉降使得燃烧现象变得复杂. 而微重力条件下这些影响几乎消失, 这简化了对燃烧的研究. 在加深对地面燃烧过程和载人航天器火灾安全问题的认识的推动下, 经过近半个世纪特别是最近10多年的发展, 微重力燃烧研究已经涵盖了预混气体、气体扩散、液滴、颗粒和粉尘燃烧、燃料表面的火焰传播等燃烧学科的各个领域. 研究中实现了球对称液滴燃烧、不受沉降影响的粉尘燃烧、静止或低速对流环境中的燃烧, 观察到了火球、自熄灭火焰等现象,阐明了碳黑形成中的热泳力效应、可燃极限与火焰稳定性等机理. 加深了对燃烧现象,特别是对辐射效应的理解: 在预混气体、气体扩散、液滴等多种火焰中, 除了停留时间过短引起的吹熄极限外, 还存在辐射热损失过大引起的冷熄极限, 后者只能在微重力条件下观测到. 部分研究成果已经进入教材. 而火焰在微重力下不同于常重力下的现象, 对载人航天器火灾安全具有重要意义. 考虑到我国的现实情况和国内外的研究现状, 建议将煤炭颗粒和粉尘的燃烧、与碳黑相关的机理、辐射效应、化学动力学等作为我国微重力燃烧的主要研究方向.      

微小空间内丙烷/空气火焰传播特性与加氢爆燃实验

在内径150 mm的圆盘狭缝微型燃烧室内,实验探讨了在常温常压下,不同当量比的丙烷/空气预混气以及掺氢的丙烷/空气混合气在电火花点火后向外传播的特性,通过高速摄影方法获得了在狭缝间距为2.0、2.5、3.0、5.0 mm时微燃烧室内的火焰传播形态。实验中观察到火焰传播存在光滑、皱褶和断裂三种火焰锋面形态。当量比的增加和狭缝间距的减小会使火焰更容易发生褶皱。随着火焰的传播,火焰半径逐渐增大,火焰传播速度整体呈下降趋势。火焰传播速度随着间距的减小先增大后减小,在间距3 mm时最大。因为壁面散热的影响,微尺度效应在降低火焰传播速度和增加火焰不稳定性方面具有重要作用。掺入氢气能提高预混气的火焰传播速度,在间距2.5 mm的微燃烧腔中还观察到了爆燃现象。     

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．     

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。     

Subgrid Modeling of Turbulent Premixed Flames in the Flamelet Regime

Large eddy simulation (LES) models for flamelet combustion are analyzed by simulating premixed flames in turbulent stagnation zones. ALES approach based on subgrid implementation of the linear eddy model(LEM) is compared with a more conventional approach based on the estimation of the turbulent burning rate. The effects of subgrid turbulence are modeled within the subgrid domain in the LEM-LES approach and the advection (transport between LES cells) of scalars is modeled using a volume-of-fluid (VOF) Lagrangian front tracking scheme. The ability of the VOF scheme to track the flame as a thin front on the LES grid is demonstrated. The combined LEM-LES methodology is shown to be well suited for modeling premixed flamelet combustion. The geometric characteristics of the flame surfaces, their effects on resolved fluid motion and flame-turbulence interactions are well predicted by the LEM-LES approach. It is established here that local laminar propagation of the flamelets needs to be resolved in addition to the accurate estimation of the turbulent reaction rate. Some key differences between LEM-LES and the conventional approach(es) are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Subgrid Scale Modeling in Large-Eddy Simulation of Turbulent Combustion Using Premixed Flamelet Chemistry

Large-eddy simulation (LES) of turbulent combustion with premixed flamelets is investigated in this paper. The approach solves the filtered Navier–Stokes equations supplemented with two transport equations, one for the mixture fraction and another for a progress variable. The LES premixed flamelet approach is tested for two flows: a premixed preheated Bunsen flame and a partially premixed diffusion flame (Sandia Flame D). In the first case, we compare the LES with a direct numerical simulation (DNS). Four non-trivial models for the chemical source term are considered for the Bunsen flame: the standard presumed beta-pdf model, and three new propositions (simpler than the beta-pdf model): the filtered flamelet model, the shift-filter model and the shift-inversion model. A priori and a posteriori tests are performed for these subgrid reaction models. In the present preheated Bunsen flame, the filtered flamelet model gives the best results in a priori tests. The LES tests for the Bunsen flame are limited to a case in which the filter width is only slightly larger than the flame thickness. According to the a posteriori tests the three models (beta-pdf, filtered flamelet and shift-inversion) show more or less the same results as the trivial model, in which subgrid reaction effects are ignored, while the shift-filter model leads to worse results. Since LES needs to resolve the large turbulent eddies, the LES filter width is bounded by a maximum. For the present Bunsen flame this means that the filter width should be of the order of the flame thickness or smaller. In this regime, the effects of subgrid reaction and subgrid flame wrinkling turn out to be quite modest. The LES-results of the second case (Sandia Flame D) are compared to experimental data. Satisfactory agreement is obtained for the main species. Comparison is made between different eddy-viscosity models for the subgrid turbulence, and the Smagorinsky eddy-viscosity is found to give worse results than eddy-viscosities that are not dominated by the mean shear. Paper presented on the Eccomas Thematic Conference Computational Combustion 2007, submitted for a special issue of Flow, Turbulence and Combustion.     

Assessing the Predictive Capabilities of Combustion LES as Applied to the Sydney Flame Series

Large Eddy Simulation (LES) and flamelet-based combustion models were applied to four bluff-body stabilized nonpremixed and partially premixed flames selected from the Sydney flame series, based on Masri’s bluff-body test rig (University of Sydney). Three related non-reacting flow cases were also investigated to assess the performance of the LES solver. Both un-swirled and swirled cases were studied exhibiting different flow features, such as recirculation, jet precessing and vortex breakdown. Due to various fuel compositions, flow rates and swirl numbers, the combustion characteristics of the flames varied greatly. On six meshes with different blocking structure and mesh sizes, good prediction of flow and scalar fields using LES/flamelet approaches and known fuel and oxidizer mass fluxes was achieved. The accuracy of predictions was strongly influenced by the combustion model used. All flames were calculated using at least two modeling strategies. Starting with calculations of isothermal flow cases, simple single flamelet based calculations were carried out for the corresponding reacting cases. The combustion models were then adjusted to fit the requirements of each flame. For all flame calculations good agreement of the main flow features with the measured data was achieved. For purely nonpremixed flames burning attached to the bluff-body’s outer edge, flamelet modeling including strain rate effects provided good results for the flow field and for most scalars. The prediction of a partially premixed swirl flame could only be achieved by applying a flamelet-based progress variable approach.     

Study of a Filtered Flamelet Formulation for Large Eddy Simulation of Premixed Turbulent Flames

Despite significant advances in the understanding and modelling of turbulent combustion, no general model has been proposed for simulating flames in industrial combustion devices. Recently, the increase in computational possibilities has raised the hope of directly solving the large turbulent scales using large eddy simulation (LES) and capturing the important time-dependant phenomena. However, the chemical reactions involved in combustion occur at very small scales and the modelling of turbulent combustion processes is still required within the LES framework. In the present paper, a recently presented model for the LES of turbulent premixed flames is presented, analysed and discussed. The flamelet hypothesis is used to derive a filtered source term for the filtered progress variable equation. The model ensures proper flame propagation. The effect of subgrid scale (SGS) turbulence on the flame is modelled through the flame-wrinkling factor. The present modelling of the source term is successfully tested against filtered direct numerical simulation (DNS) data of a V-shape flame. Further, a premixed turbulent flame, stabilised behind an expansion, is simulated. The predictions agree well with the available experimental data, showing the capabilities of the model for performing accurate simulations of unsteady premixed flames.     

Turbulence Statistics and Scalar Transport in Highly-Sheared Premixed Flames

This paper describes recent progress in the analysis of the nature of turbulent premixed flames stabilised behind an axisymmetric baffle which are of fundamental interest in the development of new and cleaner combustion systems. The work includes the use of laser-based diagnostics for velocity and temperature measurements, which are extended to the analysis of turbulence statistics, including the energy spectrum and typical length scales in a reacting shear layer. The results provided experimental evidence of the extension of the flamelet regime beyond the Klimov--Williams criterion. Arguments based on the shape of the weighted-joint-probability distributions of axial velocity and temperature fluctuations show that the counter-gradient nature of heat flux is associated with the preferential deceleration of products of combustion in relation to the cold reactants.     

Modelling of a Bluff-Body Nonpremixed Flame using a Coupled Radiation/Flamelet Combustion Model

A coupled radiation/flamelet combustion modelling technique is applied to the simulation of a bluff-body flame. Radiation heat transfer is incorporated into the laminar flamelet model for turbulent combustion through the enthalpy defect. A new method is developed for generating flamelet library with enthalpy defect. The radiation within the flame is modelled using a raytracing approach based on the discrete transfer method. The predicted results are compared with the reported experimental data. Comparison shows that the effects of radiative heat transferr on the temperature and major species are small for the flame considered. However, a significant improvement in the prediction of OH is achieved when radiation heat transfer is included. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Priori Evaluation of the Laminar Flamelet Decomposition Model for Turbulent Premixed Flames using DNS Data

Laminar flamelet decomposition (LFD) is a dynamic approach for modelling sub-filter scale turbulence-chemistry interactions in Large-Eddy Simulations using a stretched flamelet library. In this work, the performance of the LFD model – that was previously used only in non-premixed combustion—is investigated a priori for premixed combustion using positively-strained flamelets in the reactant-to-product configuration. For this purpose, a DNS database of methane-air premixed flames is utilized. The flames are propagating in a rectangular box under homogeneous isotropic turbulence conditions over a wide range of Karlovitz numbers. The results show that the LFD model can correctly account for the sub-filter scale turbulence-chemistry interactions to predict the filtered reaction rates and the filtered scalar field, provided that turbulent and laminar mixing are well predicted. The deviations from the DNS results are attributed to the shortcomings of the strained flamelet library and the non-flamelet effects. Finally, the LFD results are compared with a different sub-filter scale model using the same strained flamlelet library, and the relative performances of the two models are discussed.—

     

Flow Physics of a Bluff-Body Swirl Stabilized Flame and their Prediction by Means of a Joint Eulerian Stochastic Field and Tabulated Chemistry Approach

In the frame of this work a transported joint scalar probability density function (PDF) method is combined with the flamelet generated manifolds (FGM) tabulated chemistry approach for large eddy simulation (LES) modeling of a three-dimensional turbulent premixed swirl burner. This strategy accounts for the turbulence-chemistry interaction at reasonable computational costs. At the same time, it allows the usage of detailed chemistry mechanisms for the creation of the chemical database. The simulation results obtained are comparatively assessed along with complementary measurements. Furthermore, transient and time-averaged data are used to provide insight into the flow physics of the bluff-body swirl stabilized flame considered. The sensitivity of the results to different modeling approaches regarding the predicted flame shape and its dynamics is also investigated, where the implemented approach is compared with the well-established artificially thickened flame (ATF) combustion model. Consequently, the investigation conducted in this work aims to provide a complete picture on the ability of the proposed combustion model to reproduce the flow conditions within complex bluff-body swirl stabilized flames.     

Large eddy simulation of turbulent premixed and stratified combustion using flame surface density model coupled with tabulation method

Large eddy simulations (LESs) are performed to investigate the Cambridge premixed and stratified flames, SwB1 and SwB5, respectively. The flame surface density (FSD) model incorporated with two different wrinkling factor models, i.e., the Muppala and Charlette2 wrinkling factor models, is used to describe combustion/turbulence interaction, and the flamelet generated manifolds (FGM) method is employed to determine major scalars. This coupled sub-grid scale (SGS) combustion model is named as the FSD-FGM model. The FGM method can provide the detailed species in the flame which cannot be obtained from the origin FSD model. The LES results show that the FSD-FGM model has the ability of describing flame propagation, especially for stratified flames. The Charlette2 wrinkling factor model performs better than the Muppala wrinkling factor model in predicting the flame surface area change by the turbulence. The combustion characteristics are analyzed in detail by the flame index and probability distributions of the equivalence ratio and the orientation angle, which confirms that for the investigated stratified flame, the dominant combustion modes in the upstream and downstream regions are the premixed mode and the back-supported mode, respectively.