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。     

湍流预混火焰LES/FDF模拟中标量混合时间尺度建模

在实际应用中,精确模拟高湍流度预混火焰十分关键但极具挑战性。本文将被动标量混合时间尺度动态封闭方法与hybrid混合时间尺度模型结合,在湍流预混火焰LES/FDF模拟中提出一种新的标量混合时间尺度建模方法。该方法不需要人为选取混合模型参数C_M,且动态地考虑了由湍流和火焰结构分别引起的亚网格标量混合。对悉尼值班预混射流燃烧器高湍流度PM1-150火焰进行了LES/FDF模拟。结果表明,新模型显著改善了对整体燃烧进程的预测结果,正确地预测了局部熄火/再燃现象。进一步的结果显示,模型参数C_M明显大于在RANS框架下量级为1的常用取值,这表明该参数在不同框架下具有不同的物理内涵。     

湍流强旋流预混火焰大涡模拟研究

本文采用合成涡方法生成湍流进口条件,对非受限强旋流甲烷/空气贫燃预混火焰进行了大涡模拟研究。模拟结果和实验以及DNS数据符合良好。基于此模拟方法,本文对甲烷火焰掺混氢气和水蒸气,进行了比较研究。研究发现,掺混氢气提高了火焰抗拉伸灭火能力,火焰在外剪切层强湍流作用下形成较厚火焰刷。掺混水蒸气,火焰降温明显,流场轴向速度加速较小,形成较大的当地旋流数。因而,相比于掺混氢气,其回流区更强、更宽。相比于掺混水蒸气,流场轴向速度加速较大的掺氢火焰,其轴向脉动在更强的Kelvin-Helmholtz不稳定性作用下,下游剪切层双峰结构得到了保持并增强。     

湍流对多孔介质内预混火焰影响的初步研究

当气流速度较大时,多孔介质内预混燃烧的模拟需要考虑湍流的影响,本文利用简化的k-ε双方程湍流反应流模型对多孔介质内的预混火焰进行了数值模拟.结果表明,湍流大大加强了气流的组分和能世扩散,计算得到的火焰传播速度、CO及NO的排放量都与实验值符合得比较好,与层流模型相比,湍流模型能够改善计算结果.     

超音速氢气燃烧火焰结构特性的直接数值模拟

在课题组前期工作的基础上,对一马赫数为1.2的三维超音速氢气射流抬升火焰进行了直接数值模拟研究,其中空间离散采用波带优化的四阶WENO格式,时间积分采用带有TVD性质的三步三阶龙格库塔格式,边界条件采用了无反射特征边界条件,总的计算网格数达到9.75亿。结果表明:超音速射流氢气燃烧火焰可分为根部层流状的高温高热量释放率稳燃区、高度褶皱的湍流剧烈混合区和远场燃烧区。火焰自燃稳燃点出现在喷口附近的x/D=0.86处,对应着最易反应混合分数。在此下游,预混燃烧和扩散燃烧两种模式同时存在,其中在剧烈混合区和远场区火焰以扩散燃烧为主,但在火焰根部的局部区域预混燃烧热量释放率达到35%左右。     

非预混旋流火焰中部熄火及重新稳燃机理研究

本文利用5 kHz同步PIV/OH-PLIF实验装置,结合大涡模拟(LES)耦合PDF燃烧模型,系统研究燃料中心喷射下非预混旋流火焰中部熄火及再次稳燃机理,对比了两种旋流器出口结构的影响.结果 表明,LES-PDF模型可以准确地捕捉到旋流流场分布及火焰形态,包括中部熄火及火焰重新稳定.相比于直燃道的结构,扩张燃道的存在引导了更宽的回流区,改善了中部熄火现象,两种结构对其下游的主火焰再次稳燃高度影响不大.中央燃气射流带来的高标量耗散率使得热损失变大,从而引起中部熄火.同时进一步促进了CH4与空气的预先混合及部分反应前置物(CH2O),为非预混旋流火焰在下游重新稳燃提供了有利条件.通过对火焰再次稳燃处OH反应项及扩散项的分析,发现反应项占据主导地位,部分预混火焰传播为再次稳燃的主要机理.     

电场对稀燃预混钝体旋流火焰结构的影响

本文研究了开放空间内的直流电场对甲烷稀燃预混钝体旋流火焰结构的影响。施加电场后,火焰面向上游移动,火焰面之间的夹角增大。电场能够提高钝体旋流火焰中上部分的火焰面褶皱率,从而提高湍流火焰速度,但是对根部火焰面结构没有影响。电场对最大火焰面密度没有明显作用,而是通过提高火焰刷厚度提高湍流火焰速度。预混气当量比和燃烧器出口平均速度能够影响电场作用强度。当量比越低,出口平均速度越高,电场的作用越不明显。电场对钝体旋流火焰根部结构影响较弱,且在临近吹熄工况下电场流体动力学作用很弱,对钝体旋流火焰的稀燃吹熄极限没有明显影响。     

预混火焰拉伸和曲率效率的物理分析

湍流燃烧的基本火焰结构是拉伸的曲面涡管;拉伸流场中的管形火焰模拟了湍流燃烧的细微结构。本文对平面预混火焰、拉伸预混火焰和管形拉伸预混火焰进行了质量、能量和组分的守恒分析。通过对比这几种火焰,揭示了火焰拉伸效果是通过优先扩散改变火焰温度和熄火极限;而火焰曲率通过增强或削弱这种优先扩散效果来影响火焰温度,影响的大小和火焰厚度与火焰半径的比值呈正比。     

高强度湍流预混火焰NO生成特性的研究

本文采用直接数值模拟方法对不同湍流强度下预混CH_4/H_2/air平面火焰进行了计算,并对湍流火焰中NO的生成规律进行了研究。采用了28组分268步反应的详细化学反应机理。研究表明高Karlovitz数下,湍流微团能穿透火焰面并存在于内反应区中。在强拉伸作用下,局部燃烧热释率达到层流火焰峰值的1.8倍。湍流作用下湍流燃烧速率显著提高,而NO的总生成速率增加不明显。高湍流强度下由于快速型NO的生成率的显著减小,湍流火焰中NO的总生成速率降低。     

CH4/O2/N2层流火焰瞬态响应特性数值分析

用数值分析方法研究CH₄/O₂/N₂层流扩散火焰的瞬态响应特性.采用详细的GRI-Mech 3.0机理(包含53种组分,325个基元反应)描述CH₄氧化和NO_x生成.首先比较火焰面稳态结构的计算结果和实验数据,以验证数值方法的可靠性.用台阶跃变的火焰拉伸率来模拟瞬态流场对火焰面局部结构的影响,给出了火焰面结构(温度、组分浓度)的瞬态响应曲线,分析了火焰面的响应特性.着重探讨了不同拉伸率跃变幅度对响应特性的影响,发现火焰面的响应对于拉伸率正向跃变和负向跃变并不对称,而是相反,且在小的拉伸率跃变范围内火焰面响应时间和拉伸率跃变幅度近似成反比关系.另外,温度的平均响应时间远大于一个典型湍流燃烧场的流动时间尺度,说明火焰面非稳态效应对于湍流燃烧数值模拟有重要意义.     

A spray flamelet/progress variable approach combined with a transported joint PDF model for turbulent spray flames

A spray flamelet/progress variable approach is developed for use in spray combustion with partly pre-vaporised liquid fuel, where a laminar spray flamelet library accounts for evaporation within the laminar flame structures. For this purpose, the standard spray flamelet formulation for pure evaporating liquid fuel and oxidiser is extended by a chemical reaction progress variable in both the turbulent spray flame model and the laminar spray flame structures, in order to account for the effect of pre-vaporised liquid fuel for instance through use of a pilot flame. This new approach is combined with a transported joint probability density function (PDF) method for the simulation of a turbulent piloted ethanol/air spray flame, and the extension requires the formulation of a joint three-variate PDF depending on the gas phase mixture fraction, the chemical reaction progress variable, and gas enthalpy. The molecular mixing is modelled with the extended interaction-by-exchange-with-the-mean (IEM) model, where source terms account for spray evaporation and heat exchange due to evaporation as well as the chemical reaction rate for the chemical reaction progress variable. This is the first formulation using a spray flamelet model considering both evaporation and partly pre-vaporised liquid fuel within the laminar spray flamelets. Results with this new formulation show good agreement with the experimental data provided by A.R. Masri, Sydney, Australia. The analysis of the Lagrangian statistics of the gas temperature and the OH mass fraction indicates that partially premixed combustion prevails near the nozzle exit of the spray, whereas further downstream, the non-premixed flame is promoted towards the inner rich-side of the spray jet since the pilot flame heats up the premixed inner spray zone. In summary, the simulation with the new formulation considering the reaction progress variable shows good performance, greatly improving the standard formulation, and it provides new insight into the local structure of this complex spray flame.     

A three mixture fraction flamelet model for multi-stream laminar pulverized coal combustion

A three mixture fraction flamelet model is proposed for multi-stream laminar pulverized coal combustion. The technique of coordinate transformation is utilized to map the flamelet solutions from a unit pyramid space into a unit cubic space to improve the stability of the simulation. The validity of the three mixture fraction flamelet model was assessed on different configurations, including a laminar counterflow pulverized coal/methane flame and a laminar piloted pulverized coal jet flame. The flamelet predictions were compared to the reference results of the detailed chemistry solutions. For the counterflow flame, it was found that the flame temperature and major species mass fractions are correctly predicted by the three mixture fraction flamelet model. However, discrepancies are observed for combustion-mode-sensitive species such as CO and H₂ in the premixed combustion region. The thermo-chemical quantities in the char surface reaction zone cannot be correctly predicted if the mixing between the char off-gas stream and other streams is neglected. For the piloted jet flame, it was shown that the stable thermo-chemical variables can be correctly predicted at the upper and middle stream locations. However, at the downstream location, discrepancies can be observed in certain regions. Overall, the validity of the three mixture fraction flamelet model for multi-stream pulverized coal combustion is confirmed and its performance in turbulent pulverized coal combustion will be tested in future work.     

1-庚烯/空气的高压层流火焰传播研究

本文使用定容圆柱形燃烧弹,在初始温度373 K和初始压力1、2、5、10 atm的条件下,对当量比从0.7到1.5的1-庚烯/空气混合物的层流火焰传播进行了研究.利用记录的纹影图像处理得到层流火焰传播速度和马克斯坦长度.基于先前报道的1-己烯燃烧反应动力学模型,发展了1-庚烯的模型.该模型验证了本工作测量的1-庚烯层流火焰传播速度数据及文献中的1-庚烯着火延迟时间数据.通过开展敏感性分析和路径分析,帮助理解了1-庚烯在不同压力下的高温化学及其对层流火焰传播的影响.另外,比较了1-庚烯/空气和先前报道的正庚烷/空气的层流火焰传播.由于更强的放热性及反应活性,1-庚烯/空气的层流火焰传播速度在绝大多数条件下均快于正庚烷/空气的结果.     

钝体驻定湍流扩散火焰的数值研究——燃烧模型比较

分别采用标量联合的概率密度函数方法、稳态火焰面模型、Euler非稳态火焰面模型和基于有限体积/Monte Carlo混合算法的完备PDF模型对钝体驻定的Sydney湍流扩散火焰HM1进行数值模拟,以比较不同燃烧模型的性能,并比较标量联合的概率密度函数方法和Euler非稳态火焰面模型对氮氧化物排放预测的差异.计算结果和实验数据的比较表明,采用概率密度函数方法计算化学反应可以得到更好的结果但计算量较大,而用火焰面模型求解计算量较小,在接近完全燃烧的情形下,其计算结果比较合理.     

湍流分层燃烧的直接数值模拟和小火焰模型研究

湍流分层燃烧广泛应用于工业燃烧装置,但是目前还比较缺乏适用于湍流分层燃烧的高精度数值模型。本文利用直接数值模拟数据库,对高Karlovitz数分层射流火焰的小火焰模型表现进行了先验性评估。考虑了两种小火焰模型,一种是基于自由传播层流预混火焰的小火焰模型M1,另一种是基于分层对冲小火焰的小火焰模型M2。研究发现M1和M2在c-Z空间的结果与直接数值模拟在定性上是一致的。在物理空间,M2对过程变量反应速率脉动值的预测结果要优于M1.     

Prediction of NO in turbulent diffusion flames using Eulerian particle flamelet model

The combustion characteristics for the turbulent diffusion flames using the unsteady flamelet concept have been numerically investigated. The Favre-averaged Navier–Stokes equations are solved by a finite volume method of SIMPLE type that incorporates the laminar flamelet concept with a modified k ? ε turbulence model. The NO formation is estimated by solving the Eulerian particle transport equations in a postprocessing mode. Two test problems are considered: CH₄/H₂/N₂ jet flame and CH₄/H₂ stabilised bluff body flame. The temperature and species profiles are well captured by the flamelet model. Two different chemical mechanisms (GRI 2.11 and 3.0) give nearly identical results for temperature and species except NO. The GRI 3.0 gives significantly higher NO levels compared to the GRI 2.11. This is mainly attributed to the difference in NO formation by the prompt mechanism. The NO formation is sensitive to the number of flamelet particles. The NO levels for two test flames do not change when the flamelet particle number exceeds six.     

小火焰模型在贫燃预混火焰中的研究

由层流小火焰库引入详细化学反应机理,通过简化的PDF方法计算组分浓度、平均温度和密度等变量,以钝体火焰稳定燃烧室和某燃气轮机上的燃烧室为例,模拟甲烷/空气贫燃条件下预混燃烧的平均火焰位置和火焰厚度,计算结果与实验结果吻合良好,这表明此方法能够较好计算出平均湍流火焰的主要特征。     

An assessment of large eddy simulations of premixed flames propagating past repeated obstacles

This paper presents an assessment of Large Eddy Simulations (LES) in calculating the structure of turbulent premixed flames propagating past solid obstacles. One objective of the present study is to evaluate the LES simulations and identify the drawbacks in accounting the chemical reaction rate. Another objective is to analyse the flame structure and to calculate flame speed, generated overpressure at different time intervals following ignition of a stoichiometric propane/air mixture. The combustion chamber has built-in repeated solid obstructions to enhance the turbulence level and hence increase the flame propagating speed. Various numerical tests have also been carried out to determine the regimes of combustion at different stages of the flame propagation. These have been identified from the calculated results for the flow and flame characteristic parameters. It is found that the flame lies within the ‘thin reaction zone’ regime which supports the use of the laminar flamelet approach for modelling turbulent premixed flames. A submodel to calculate the model coefficient in the algebraic flame surface density model is implemented and examined. It is found that the LES predictions are slightly improved owing to the calculation of model coefficient by using submodel. Results are presented and discussed in this paper are for the flame structure, position, speed, generated pressure and the regimes of combustion during all stages of flame propagation from ignition to venting. The calculated results are validated against available experimental data.     

Premixed flames for arbitrary combinations of strain and curvature

Many modeling strategies for combustion rely on laminar flamelet concepts to determine structure and properties of multi-dimensional and turbulent flames. Using flamelet tabulation strategies, the user anticipates certain aspects of the combustion process prior to the simulation and selects a flamelet model which mimics local flame conditions in the more complex configuration. Flame stretch, which can be decomposed into contributions from strain and curvature, is one of the conditions influencing a flame’s properties, structure, and stability. The objective of this work is to study premixed flame structures in the strain-curvature space using a recently published composition space model (CSM) and three physical space models for canonical flame configurations (stagnation flame, spherical expanding flame and inwardly propagating flame). Flames with effective Lewis numbers both smaller and larger than unity are considered. For canonical laminar flames, the stretch components are inherently determined through boundary conditions and their specific flame configuration. Therefore, canonical flames can only represent a certain sub-set of stretch effects experienced by multi-dimensional and turbulent flames. On the contrary, the CSM allows arbitrary combinations of strain and curvature to be prescribed for premixed flames exceeding the conditions attainable with the canonical flame setups. Thereby, also influences of negative strain effects and large curvatures can be studied. A parameter variation with the CSM shows that flame structures still significantly change outside the region of the canonical flame configurations. Furthermore, limits in the strain-curvature space are discussed. The present paper highlights advantages of composition space modeling which is achieved by detaching the representation of the flame structure from a specific canonical flame configuration in physical space.