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超声速燃烧室火焰稳定模式与化学反应机理简化研究
英文题名Investigation on the flame stabilization modes and chemical mechanism reduction in supersonic combustion
吴坤
导师范学军
2018-05-20
学位授予单位中国科学院大学
学位授予地点北京
学位类别博士
学位专业流体力学
关键词超声速燃烧 支板稳焰 凹腔稳焰 简化机理 动态自适应化学
摘要

随着超燃冲压发动机工作边界的拓展以及燃烧效率的提高,对在超声速条件下实现可靠点火与稳定燃烧都提出了新的挑战。由于稳焰过程中涉及到复杂流动和与之紧密耦合的燃烧反应,实验测量不易且提供的信息也相当有限,因此高精度的数值模拟成为研究超声速燃烧室中稳焰问题的重要手段。然而,考虑到高雷诺数壁湍流、复杂化学反应以及湍流-化学反应交互作用等诸多因素,通过数值模拟准确预测火焰稳定模式仍具有相当大的难度,而且对其中的主要影响因素理解还不够深入。为此,本文以超声速燃烧室中的火焰稳定模式问题为背景,以化学反应动力学对稳焰模式预测的影响为主线,开展了以下四个方面的研究。

系统地研究了支板喷注燃烧室中的稳焰模式,首次发现DLR燃烧室存在多种不同的稳焰模式,并给出了稳焰模式与来流总温-当量比的关系图。通过比较不同化学反应机理并结合敏感性分析,揭示了化学动力学在不同稳焰模式中的差异。在低来流总温的情况下(607~879 K),稳焰模式I为一种包括诱导、过渡和剧烈反应的三段式稳焰模式。不同的稳焰区域受不同主导化学反应控制,其中自由基的生成与输运起着主导作用。在较高来流总温条件下(1696~2141 K)的稳焰模式II中,火焰远离支板形成抬升火焰,燃烧主要发生在超声速区,主要受自点火影响。

基于氢气详细反应机理成功复现了双模态凹腔燃烧室中的两种稳焰模式,并深入分析了两种稳焰模式的物理机制。研究表明,在凹腔稳焰模式中,射流尾迹中的涡脱落实现了燃料与空气的初步混合,并在凹腔前缘剪切层和反应区锋面位置处形成预混火焰,在此过程中自由基的产生和输运至关重要。发现射流尾迹稳焰模式中,反向旋转涡对间的预混火焰对火焰稳定的重要作用。反向旋转涡对间以及射流背风面的低速壁面边界层附近形成的预混燃烧区是射流尾迹稳焰的关键。此外,在国际上首次系统考察了化学反应机理对稳焰模式预测的影响。发现凹腔稳焰模式对化学反应机理的敏感性较强,而对射流尾迹稳焰模式而言,化学反应机理的影响相对较小。揭示了稳焰模式的数值模拟中化学反应机理影响重要性,指出了准确的化学反应机理从定性上决定稳焰模式的预测结果。

考虑到化学反应机理在稳焰模式预测中的本质影响,针对复杂碳氢燃料在超燃数值模拟中的应用,系统地提出了基于物理目标的三层次简化机理保真度评估体系,包括:化学动力学特征匹配,发动机整体效率评估以及考虑更加复杂的非定常燃烧过程或者极限现象。进一步发展了适用于超燃数值模拟的24组分/86步反应的乙烯骨架反应机理,在三层次简化机理保真度评估体系下作了系统的验证。发现全局简化机理在燃烧室总体性能与详细机理的计算结果误差很小,但是火焰稳定结构上的预测误差相对较大。此外,从工程实用角度出发,发展了一套28组分/93步反应的煤油骨架机理,并成功应用于全尺寸超燃室的模拟中,在燃烧室壁面静压分布预测上与实验结果吻合良好。

针对全局简化机理在稳焰模式预测上的不足,本文在国际上首次实现了基于动态自适应化学(DAC)的稳焰问题数值模拟。在实际超声速燃烧室的模拟中,全面比较了包括DAC、TDAC和骨架机理等多种方法在稳焰结构预测上的准确性与效率。结合反应路径分析,深入分析了DAC方法较之于骨架机理更为准确的物理机制,进一步确证了化学反应机理的准确性是稳焰模式预测的核心。此外,在超声速燃烧模拟中,系统地比较了不同机理简化方法在动态自适应化学框架下的性能。结果表明,L-DAC、DRG和PFA三种方法在准确性上都取得令人满意的结果,而DRG方法在计算效率上更有优势。

英文摘要

The demands for extending the limiting operation conditions and enhancing the combustion efficiency of scramjets raise new challenges to the research of reliable ignition and robust flame stabilization in supersonic flows. Owing to the complexity of the aerothermodynamics and chemical reactions, measurement of supersonic reacting flow has been extremely difficult and usually provides limited information. Thus, high fidelity numerical simulation has become an important tool in studying flame stabilization in a scramjet. However, accurate prediction of flame stabilization mode through numerical simulation is still intractable because of the coupling effects among the intricate physics, such as high-Reynolds-number wall turbulence, complex chemical mechanisms and turbulence-combustion interactions. Furthermore, the roles of these influential factors in flame stabilization have not been well understood for supersonic combustion. Motivated by the interest of stabilizing flame in supersonic combustion, this thesis aims to reveal the influences of chemical mechanisms on the numerical predictions of flame stabilization mode. The main contributions of this thesis are summarized below.

The flame stabilization in a strut-based scramjet was investigated systematically. The existence of multiple flame stabilization modes in a strut injection DLR combustor was confirmed via a nomograph showing the flame stabilization modes influenced by the stagnation temperature and the overall equivalence ratio. By comparing different chemical mechanisms and analyzing their sensitivities, the different flame stabilization modes were attributed to different controlling elementary reactions. It was found that flame stabilization mode I corresponds to a low range of inflow stagnation temperature (607~879 K), featuring three stages, namely the induction stage, the translational stage, and the intense combustion stage. The different stages are dominated by different elementary reactions in which the production and transportation of radicals play a crucial role. Mode II appears at high inflow temperatures (1696~2141 K), which embodies a lifted flame with the main reactions occurring in the supersonic regime.

By employing the detailed hydrogen mechanism, this thesis reproduced the two flame stabilization modes of a cavity-based dual-mode scramjet and systematically analyzed their physical mechanisms. In the cavity flame stabilization mode, the vortex shedding in the fuel jet wake was found to contribute to the initial mixing of the reactants. The reactant mixtures were then convected to the cavity leading shear layer and the leading edge of the reaction zone to form stabilized premixed flames, in which process the formation and transportation of active radicals are indispensable. In the jet-wake mode, it was revealed that the counter-rotating vortex pair is responsible for generating a premixed flame between the vortices and the low-speed boundary layer near the bottom wall is the key factor in flame stabilization. Furthermore, the influence of different chemistry mechanisms on flame stabilization was investigated, indicating that the cavity flame stabilization is more sensitive to chemistry mechanism than the jet wake mode. This demonstrates the importance of chemical mechanism in numerical prediction of flame stabilization modes.

To study the essential influence of the chemical mechanisms in supersonic combustion simulation, this thesis proposed a three-level object-oriented mechanism reduction and fidelity evaluation methodology, namely chemical properties matching, engine global performance evaluation, and flame stabilization prediction, for complex hydrocarbon fuels. A 24 species/86 reactions skeletal mechanism for ethylene was developed under relevant conditions for a scramjet. Following the proposed framework of fidelity evaluation, it was shown that the performance of the global skeletal ethylene mechanism is promising in global performance prediction but not satisfactory in flame stabilization simulation. For the purpose of engineering applications, a 28 species/93 reactions skeletal mechanism for kerosene combustion was developed and implemented in modelling a full-scale scramjet. The fidelity of the skeletal kerosene mechanism was further validated through matching the simulation with wall pressure measurements.

To overcome the insufficiency of the global skeletal mechanism in flame stabilization prediction, the first ever large eddy simulation of supersonic combustion based on dynamic adaptive chemistry (DAC) augmentation was carried out. The accuracy and efficiency of flame stabilization prediction was compared among DAC, tabulation of DAC, and skeletal mechanism methods in the simulation of a realistic scramjet engine. The results suggested that the DAC method is capable of accurately retaining the main reaction path in the detailed mechanism, which further demonstrates the essential role of chemical mechanism in flame stabilization prediction. Furthermore, three mechanism reduction methods (L-DAC, DRG, PFA) were also compared in predicting the flame stabilization mode in supersonic combustion simulations. All the three methods showed good agreement with the predictions by the detailed mechanism, whereas the DRG method is the most computationally efficient.

语种中文
文献类型学位论文
条目标识符http://dspace.imech.ac.cn/handle/311007/73127
专题高温气体动力学国家重点实验室
空天飞行科技创新研究中心(筹)
作者单位1.中国科学院研究所 高温气体动力学国家重点实验室
2.中国科学院大学
推荐引用方式
GB/T 7714
吴坤. 超声速燃烧室火焰稳定模式与化学反应机理简化研究[D]. 北京. 中国科学院大学,2018.
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