|英文题名||Unsteady Numerical Study on Supersonic Ignition and Flame Properties Based on Reduced Mechanisms|
|关键词||碳氢燃料 超声速燃烧 大涡模拟 火焰面模型 简化反应机理|
(4) 对乙烯在带凹腔超声速燃烧室内的燃烧非定常过程进行了数值研究，为了弥补稳态火焰面的局限性，采用了基于火焰面生成流形（FGM）方法的部分预混火焰面模型，利用大涡模拟方法对燃烧室湍流流动进行模拟，并与实验结果进行了对比。为了模拟真实的燃烧室火花塞点火过程，计算加入了火花模型（Spark Model）在冷态流场稳定后开启。冷态流场计算的沿程压力分布曲线、纹影图与实验结果基本一致；火花模型开启后，燃料开始点火，计算得到了与实验拍摄的CH*发光图像定性一致的点火过程图像，燃烧稳定后的沿程压力分布也与实验吻合得较好。计算还展示了中心区和近壁区点火过程的显著差异，中心区火焰结构基本稳定，而壁面火焰结构演化比较复杂，出现了火焰沿边界层逆流传播的现象；随着燃烧趋于稳定，近壁区的这种燃烧现象逐渐消失；释热导致的前传激波会打碎相对有序的湍流涡结构，而燃烧释热会耗散掉小尺度涡旋结构，使流场呈现出大尺度结构。对燃烧室压力和组分浓度脉动进行了提取和分析，发现燃烧室存在较低频率的振荡，分析了这种频率出现的机制，认为是由燃烧室在亚声速燃烧区形成的声学-对流反馈循环导致的热声耦合振荡造成。加入氢、甲烷等燃料后，会使得燃烧的强度变弱，火焰前传距离也变短，同时火焰形态也变得更窄。
Scramjet is one of the most efficient power solutions for hypersonic space vehicles in the near-space. Therefore, research on scramjet engines has always been a hot topic at home and abroad. Supersonic combustion chambers are the core components of scramjet engines and are used to organize the mixing and combustion of hydrocarbon fuels with air. Researchers have conducted a lot of research work on the combustion process in supersonic combustion chambers. However, due to the limitations of current test facilities and measurement methods, it is difficult to carry out high-precision measurement of the combustion chamber flow field, especially for such large-scale full flow field, extremely short test time and strong non-steady-effect supersonic combustion chambers. It is difficult to obtain the three-dimensional information of the combustion flow field. Numerical calculation methods have become a powerful tool to study the turbulent combustion process of supersonic combustion chambers. The numerical simulation of turbulent combustion in supersonic combustor needs to solve the problems of the chemical reaction kinetics and interaction between turbulent and combustion. The previous research mainly used the overall reaction and focused on hydrogen with a simple mechanism. The works focus on the relative stable combustion and rarely concerned the unsteady process from ignition to flame stabilization. Therefore, it is necessary to carry out numerical simulations on the unsteady process of turbulent combustion in supersonic combustion chambers, taking into account the hydrocarbon fuels with more complex reaction mechanism, studying the unsteady ignition processes of hydrocarbon fuels, the interaction between combustion and turbulence, and the instability of the combustion. It is expected to provide a reference for engine design optimization.
In this paper, the turbulent combustion processes of hydrocarbon fuel in the supersonic combustion chamber is studied, and an unsteady simulation method for turbulent combustion is established based on the reduced reaction mechanism. The unsteady phenomena such as ignition and flame propagation of fuel is numerically studied with the method. The main contents are as follows:
(1) The detailed reaction mechanism of ethylene, n-dodecane, and kerosene was reduced using the error-propagation based directed relation graph and sensitivity analysis (DRGEPSA). Reduced reaction mechanisms with smaller scale and higher precision are gained, and verified by calculating ignition delay time and laminar flame speed. In addition, there is a critical value to the mechanism simplification. When the number of components and elementary reactions decrease to a certain number, further reduction will result in lower accuracy.
(2) Detached eddy simulation (DES) method is used to calculate the diffusion flame of ethylene/air jet. The eddy-dissipation concept (EDC) model is adopted to describe the turbulence/combustion interaction and test the applicability of the reduced mechanism. The numerical results are compared with the results of direct numerical simulation (DNS). The interaction between turbulent vortex and flame is studied. The results show that the reduced mechanism can be used to simulate the process of ethylene diffusion flame with DES method. The calculated temperature profiles and the flame lift-off height are in good agreement with the results of DNS. The results of unsteady calculation show that the turbulent vortex can affect the flame surface by stretching, bending, folding and so on.
(3) Based on the German DLR strut supersonic combustion chamber, the numerical study of the hydrogen diffusion flame is conducted with DES model and steady diffusion flamelet model. The numerical results are compared with the experimental results in order to verify the accuracy of flamelet model. The calculated results are in good agreement with the experimental results, and are better than the results obtained by RANS. The numerical calculation results give the process of flame propagation evolution. The vortex shedding fuel falls off from the bottom of separation zone near the strut base, forming a discrete reaction zone “island”. With the addition of hydrocarbon fuel (ethylene, methane), the combustion intensity decreases and the reaction zone becomes thinner. The average temperature is also reduced by about 200K compared to hydrogen fuel, and the combustion efficiency also decreases.
(4) A numerical study was conducted on the unsteady combustion process of ethylene in a supersonic combustion chamber with cavity. The partially premixed flamelet model based on the Flamelet-Generation Manifold (FGM) method was used. The Large Eddy Simulation (LES) is used to simulate the turbulent flow in the combustor. The calculated results are compares with experimental results to verify the accuracy. In order to simulate the real firing process of spark plug, a spark model was added and turned on after the cold flow being stable. The pressure curve along the chamber and the schlieren map of cold flow are basically the same as the experimental results. After the spark model is turned on, the fuel starts to ignite. The ignition process images are basically consistent with the experimental CH* luminescence images and the pressure curves along the chamber also agree well with the experiment. The calculations also show a significant difference in the ignition process in the center and near wall areas. The flame structure in the center area is basically relatively stable, and the flame structure near the wall surface is more complicated. The phenomenon that the flame propagates upstream along the boundary layer occurs. As the combustion tends to be stable, the phenomenon in the wall area gradually disappears. The forward shock caused by heat release will break the relatively ordered turbulent vortex structure, and the combustion will dissipate the small-scale vortex structure that the flow field presents a large-scale structure. There is a lower frequency oscillation about 00~200Hz in the combustion chamber after extraction and analysis of the flow field fluctuations information of pressure and species concentration. The mechanism of this frequency is analyzed and thought to be caused by the thermosacoustic coupled oscillation in the acoustic-convection feedback loops formed in the subsonic combustion zone. When hydrogen and methane are added, the intensity of combustion will be weakened, and the distance of flame propagating upstream will also be shortened. At the same time, the flame shapes will also change thinner and longer.
|马素刚. 基于简化机理的超声速点火与火焰特性非定常数值研究[D]. 北京. 中国科学院大学,2018.|