稳态湍流非预混燃烧的小火焰模拟

以甲烷/空气的湍流射流非预混燃烧为对象,建立二维稳态湍流非预混火焰的小火焰模型.利用湍流流动模型和小火焰模型耦合求解,计算出速度、混合分数、温度以及反应标量的摩尔分数在燃烧室内的分布,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.     

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.     

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....     

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.     

基于小火焰生成流型模型的喷雾燃烧数值计算

基于Open FOAM开发了小火焰生成流型模型,并针对ECN(enginecombustionnetwork)SprayH(正庚烷喷雾燃烧)进行了数值模拟,研究了该模型对喷雾燃烧数值模拟的适用性.结果表明,该模型能够很好地捕捉着火延迟等特征参数.同时对比了基于OH质量分数和温升两种火焰浮起长度定义,结果显示前者对取值更不敏感,且能与实验更好地吻合.此外,深入分析了着火位置和燃烧发展历程,结果表明,在氧体积分数8%和12%工况,着火点的当量比均在0.8左右.对于氧体积分数15%工况,反应进度变量集中生成的区域对应于温度峰值,燃烧最迅速区域的当量比略大于1.     

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...     

利用RIF模型对内燃机燃烧过程的模拟

代表性互动小火焰（RIF）模型在计算准确性和经济性上的优势使它非常适用于内燃机燃烧过程的模拟．论述了RIF模型的基本原理,并在KIVA-3V2平台上补充了层流小火焰求解程序和β函数形式的概率密度函数积分程序,开发出KIVA-RIF软件,从而实现了RIF模型的求解．应用正庚烷简化机理对一单缸Caterpillar3400...     

燃气轮机无焰燃烧技术的研究进展

燃气轮机无焰燃烧具有分布式火焰、低压力波动、低污染排放等特性,总结了氧化剂温度、氧浓度和烟气循环率对无焰燃烧效果的影响,以及无焰燃烧的多燃料适应性,给出了适合无焰燃烧数值模拟的燃烧模型,归纳了产物的停留时间和燃烧室尺寸对污染物排放的影响;对国内外出现的燃气轮机无焰燃烧室进行了总结和可行性分析,指出了下一步的研究重点是液体燃料无焰燃烧的基础研究和应用研究。     

液体燃料扩散小火焰的实验研究

采用内径分别为1.0、0.6和0.4 mm的陶瓷管燃烧器,以液态乙醇为燃料,对其扩散小火焰特性进行实验研究.采用体视显微镜配合数字摄像头对火焰图像进行了可视化测量,得到了火焰图像的无量纲特征尺寸随Re变化关系.分别采用热电偶和红外热像仪对火焰温度及陶瓷管外表面温度场进行了测量,分析了火焰淬熄时的温度特性,并定性地分析了浮力对火焰的影响.     

涡流室式柴油机准维相关火焰微元燃烧模型的研究

把涡流室式柴油机不同区域与不同时期的燃烧过程分开处理，将涡流室的燃烧过程分为５个时期，即：低温着火化学动力学反应期，向高温预混燃烧化学动力学反应过渡期，高温预混燃烧化学动力学反应期，向空气和燃料混合控制的扩散燃烧过渡期和火焰微元的扩散燃烧期。而主燃烧室的燃烧只有火焰微元的扩散燃烧期。用Ｓｈｅｌ着火模型、Ａｒｈｅｎｉｕｓ方程和相关火焰微元模型来分别模拟其中的低温着火、高温预混燃烧和扩散燃烧过程以建立准维燃烧模型。模型预测的示功图和燃烧放热率与实验值吻合良好。本文还研究了模型中拉伸因子和耗散因子对示功图的影响。     

Large-eddy simulation of turbulent autoigniting hydrogen lifted jet flame with a multi-regime flamelet approach

In this work, the combustion model is focused on to describe a multitude of reaction regimes that are deemed to affect the flame stabilization. For this purpose, an efficient flame indicator is formulated to differentiate the differing flame structures and make use of flamelet chemistry that accounts for autoignition and multi-regime reactions. The large eddy simulation with this methodology is carried out to compute a turbulent lifted hydrogen-nitrogen flame in vitiated coflow. The canonical flame models of a laminar premixed flame and an unsteady counterflowing flame have been used to simulate the flamelet structure at different regimes. Present model improves the prediction of mean and rms profiles for temperature and species mass fraction in the comparison with experiments and a reference simulation, adopting the single-regime flamelet. The computed results also demarcate the formation of a triple flame structure at the flame base, where combustion develops into the premixed reaction that extends to the fuel-lean and rich branches. The counterflow mixing mode with autoignition is identified as the major mechanism for stabilization and is responsible for the propagating premixed zone above the liftoff height. The developed multi-regime flamelet approach properly accounts for the interactive different modes of burning.     

Eulerian particle flamelet modeling of a bluff-body CH4/H2 flame

In this paper an axisymmetric RANS simulation of a bluff-body stabilized flame has been attempted using steady and unsteady flamelet models. The unsteady effects are considered in a postprocessing manner through the Eulerian particle flamelet model (EPFM). In this model the transient history of scalar dissipation rate, conditioned by stoichiometric mixture fraction, is required to generate unsteady flamelets and is obtained by tracing Eulerian particles. In this approach unsteady convective-diffusive transport equations are solved to consider the transport of Eulerian particles in the domain. Comparisons of the results of steady and unsteady calculations show that transient effects do not have much influence on major species, including OH, and the structure of the flame therefore can be successfully predicted by steady or unsteady approaches. However, it appears that slow processes such as NO formation can only be captured accurately if unsteady effects are taken into account, while steady simulations tend to overpredict NO. In this work turbulence has been modeled using the Reynolds stress model. Predictions of velocity, velocity rms, mean mixture fraction, and its rms show very good agreement with experiments. Performance of three detailed chemical mechanisms, the GRI Mech 2.11, the San Diego mechanism, and the GRI Mech 3.0, has also been evaluated in this study. All three mechanisms performed well with both steady and unsteady approaches and produced almost identical results for major species and OH. However, the difference between mechanisms and flamelet models becomes clearly apparent in the NO predictions. The unsteady model incorporating the GRI Mech 2.11 provided better predictions of NO than steady calculations and showed close agreement with experiments. The other two mechanisms showed overpredictions of NO with both unsteady and steady models. The level of overprediction is severe with the steady approach. GRI Mech 3.0 appears to overpredict NO by a factor of 2 compared to GRI Mech 2.11. The NO predictions by the San Diego mechanism fall between those of the two GRI mechanisms. The present study demonstrates the success of the EPFM model and when used with the GRI 2.11 mechanism predicts all flame properties and major and minor species very well, and most importantly the correct NO levels.     

超声速湍流燃烧G/Z方程模型验证

为了研究超声速条件下的部分预混燃烧,引入一种基于Level set重构方法和稳态火焰面数据库的 G／Z方程模型,并利用德国宇航中心的DLR支板算例对 G／Z方程模型进行了验证。结果显示,超声速湍流燃烧G／Z方程模型可以捕捉到部分预混燃烧现象,数值模拟结果与实验结果吻合较好,验证了 G／Z方程模型运用到超声速部分预混条件下湍流燃烧流场计算的可行性。同时,超声速湍流燃烧 G／Z方程模型依赖于运用到的火焰传播速度模型与火焰面模型,模型精确度的提高有待进一步探究。     

涡流室式柴油机相关火焰微元燃烧模型的三维数值模拟

把涡流室式柴油机不同区域与不同时期的燃烧过程分开处理,将涡流室的燃烧过程划分为３个阶段,即：低温着火化学动力学反应阶段、高温预混燃烧化学动力学反应阶段和相关火焰微元的扩散燃烧阶段,而认为主燃烧室的燃烧只有相关火焰微元的扩散燃烧阶段。用Ｓｈｅｌ着火模型、Ａｒｒｈｅｎｉｕｓ方程和相关火焰微元模型来分别模拟低温着火、高温预混燃烧和扩散燃烧过程。开发了三维数值模拟计算程序并对其进行计算,研究了涡流室中瞬态温度场的变化过程。模型预测的示功图和涡流室中的燃烧放热率与试验值吻合良好。     

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.     

Sensitivity analysis of skeletal reaction mechanisms for use in CFD simulation of oxygen enhanced combustion systems

Oxygen enhanced combustion (OEC) techniques are supposed to be a fuel saving alternative to conventional air-fired combustion, due to the reduction or removal of nitrogen from the combustion system, which causes a higher flame temperature and radiation intensity. Therefore, more heat is available in OEC for heating, melting and annealing processes, and subsequently, increases the process efficiency. The main aim of the present study is the numerical investigation of different reaction mechanisms under air-fuel and oxy-fuel conditions using 1D simulation of laminar counter-flow diffusion flames. The mechanisms are further used in 3D CFD simulation with the steady laminar flamelet model for the development of a time efficient numerical approach, applicable in air-fuel and OEC. Three skeletal reaction mechanisms were tested and compared to the GRI3.0 mechanism. The calculated temperatures and species concentrations revealed that a skeletal mechanism with 17 species and 25 reversible reactions predicts a faster fuel conversion into the reaction products under oxy-fuel conditions, which leads to higher temperatures in the flame compared to the GRI3.0. Sensitivity analysis showed that two reversible reactions are mainly responsible for the faster fuel conversion. Furthermore, the reaction mechanisms investigated, were used for 3D CFD simulation of a lab-scale furnace under different OEC conditions and air-fuel combustion. Up to concentrations of 30% O₂ in the O₂/N₂ mixture, all reaction mechanisms were able to predict the temperatures in the furnace with a close accordance to measured data. With higher oxygen enrichment levels, only the mentioned skeletal mechanism with 25 reactions calculated good results, whereas the GRI3.0 failed for oxy-fuel combustion.