先进航天动力是体现一个国家科学技术水平、军事科研实力和综合国力的重要标志之一，也是促进国家相关领域科研水平和技术实力快速发展的不竭源泉。目前，我国航天发动机研发正处于跨越式发展时期，发动机燃烧室的工作参数不断提高。甲烷作为液体火箭发动机推进剂，有着比冲高、绿色环保、经济性好等众多的优势。然而在火箭发动机燃烧室的极端热力学条件下存在燃烧不稳定等问题。这些现象的发生涉及高温高压燃烧化学反应、复杂传热传质以及流体流动等过程。解决这些问题已成为发展先进的发动机必须重点突破的关键科学难题。

本项研究以甲烷/氧气高压下的燃烧不稳定性为研究目标，针对高压扩散火焰燃烧难于控制等问题，通过合理化设计和试验调试优化完善，建立了我国首套碳氢燃料高压扩散燃烧基础试验平台（最高压力高达10 MPa），将火焰发光强度作为特征信号对火焰不稳定燃烧现象进行分析，利用纹影法这一非接触光学测量系统，实现了火焰的流场演化过程捕捉。

本研究展开了高压（0.1-4.0 MPa）极端热力学条件下的甲烷/氧气层流扩散火焰结构和稳定性试验。利用短时傅里叶变换、相空间重构等方法对火焰发光强度进行处理，表征了火焰的三种燃烧状态：（1）稳定燃烧火焰信号在全时域和频域上没有表现出震荡特征，火焰信号的相空间重构图形具有不动点平庸吸引子运动模式；（2）间歇震荡燃烧火焰信号在时域中分立的时段上出现了基波频率相同的震荡燃烧的燃烧状态，火焰信号的相空间重构图形具有不动点和极限环两种平庸吸引子运动模式；（3）持续震荡燃烧火焰信号在全时域上均表现出震荡特征，火焰信号的相空间重构图形具有极限环两种平庸吸引子运动模式。

基于火焰发光强度信号相空间重构和短时傅里叶变换，以燃烧器喷口处弗劳德数Fr和相对燃烧压力p_r为参数，对试验中各流量参数下的火焰燃烧状态进行了分区，提出了三种燃烧状态的临界条件（间歇震荡状态弗劳德数Fr_min和Fr_max上下临界条件），当燃烧器出口处甲烷射流的弗劳德数Fr < Fr_min时，火焰处在稳定燃烧状态；当Fr > Fr_max时，火焰处在持续震荡燃烧状态；而当Fr_min < Fr < Fr_max时，火焰处在间歇震荡燃烧状态；建立了无量纲震荡频率St与燃烧器喷口处弗劳德数Fr之间的关系，

基于火焰流场纹影图像分析，开展了甲烷/氧同轴射流层流高压扩散燃烧三种燃烧状态火焰结构和流场变化特征研究，得到了火焰在高压下的火焰燃烧模式转化机制。研究发现，火焰反应区外侧的流动剪切层失稳是火焰震荡现象的主要成因；随着燃料射流的雷诺数增加，剪切层的稳定区长度迅速降低，剪切层中涡环生成的位置降低；当这一涡环降低到一定高度后，火焰反应区受到影响，火焰开始震荡，当扰动出现高度降低到火焰羽流附近时，火焰便会出现间歇震荡燃烧现象；涡环的扩散传质传热引起的冷却效果会使得火焰剪切层稳定性受到影响，当雷诺数进一步增加，剪切层涡环的冷却效果不足以使火焰重回稳定后，火焰进入了持续震荡燃烧状态。

研究中探索了适用于高压扩散火焰的计算模型，利用层流小火焰燃烧模型与甲烷详细化学反应机理耦合，开展了火焰震荡燃烧数值模拟分析。计算获得了火焰流场中各中间产物的分布特征，分析了震荡燃烧状态下火焰头部分离现象的成因。

总之，本研究开展了甲烷/氧气极端热力学条件下燃烧稳定特性研究，建立了高压扩散燃烧基础试验平台，通过试验和理论分析，提出了三种燃烧状态精确分区和燃烧状态临界条件，获得了高压条件下火焰燃烧模式转化机制，这为探寻碳氢燃料高压扩散燃烧不稳定性研究提供了大量基础试验数据和理论依据。

Development of advanced aerospace engines is an important task for our country. Nowadays, the operating pressure and temperature of these engines are ever increasing. The methane/oxygen liquid rocket is a promising competitor for the next generation space transportation system, since it has high specific impulse, low cost and no pollution. However, in the extreme combustion environment (high pressure and high temperature) of the engine chamber, combustion instability problems, which involve complex coupling effects between chemistry, heat and mass transfer, and flow turbulence, may become more serious. Combustion instability can result in serious failure of the engine, and the problem must be well solved in the development of advanced aerospace engine.

A fundamental experimental platform for high-pressure diffusive combustion of hydrocarbon fuels was established through rational design and optimization. The maximal combustion pressure can reach 10 MPa. The unstable flame characteristics were analyzed using flame luminous intensity. Schlieren images of the flame were captured to analyze the flow field evolution process.

The instability characteristics of methane/oxygen laminar co-flowing jet diffusion flame (0.1-4.0 MPa) were studied. The flame luminous intensity was analyzed by short-time Fourier transform and phase space reconstruction method. The three combustion states of flame were characterized: (1) for steady combustion state, the flame signal maintains steady on whole time domain and the frequency domain, and the phase space reconstruction pattern of the flame luminous intensity is fixed-point motion mode; (2) for transition combustion state, the oscillatory combustion with the same frequency appear at discrete time intervals, and the phase space reconstruction pattern of the flame luminous intensity has both the fixed-point and the limit-cycle motion modes; (3) for oscillatory combustion state, the flame luminous intensity exhibits oscillations with a constant frequency in the whole time-domain, and the phase space reconstruction pattern of the flame luminous intensity has a limit-cycle motion mode.

The ranges of combustion states were defined by Fr (Froude number of fuel jet flow), where Fr was described with p_r (relative combustion pressure) as a parameter. The critical parameters of three combustion states (upper and lower critical Froude number Fr_min and Fr_max in transition combustion state) were proposed: at Fr < Fr_min, the flame is in steady combustion state; at Fr > Fr_max, the flame is in oscillatory combustion state; at Fr_min < Fr < Fr_max, the flame is in transition combustion state.

Based on the Schlieren images of flame, the flame structure and flow field characteristics of three combustion states were studied, and the mode transformation mechanism under high pressure of the flame was provided. The instability of the flow shear layer outside the flame reaction zone induces flame pulsation phenomenon. With the increase of Reynolds number, the length of the stable zone of the shear layer decreases rapidly, and the height of the vortex rings in the shear layer decreases. When the height of vortex ring decreases to a certain value, the flame reaction zone is affected, and flame begins to flick. Vortex brings about cooling effect through mass diffusion and thermal diffusion, which stabilized the flame shear layer. With the further increase in Re, the cooling effect of the shear layer vortex ring is not capable to stabilize the flame, and the flame becomes the pulsation combustion state.

A computational model suitable for high-pressure diffusion flame was established. The PDF Flamelet combustion model was coupled with the detailed reaction mechanism of methane to simulate the flame pulsation combustion. The distributions of intermediate products in the flame flow field were obtained, and the mechanism of flame head separation under pulsation combustion state was analyzed.

In this dissertation, the instability characteristics of methane/oxygen laminar co-flowing jet diffusion flame were studied. A fundamental experimental platform for high-pressure diffusive combustion was established. Three combustion states of methane/oxygen flame were defined and precisely classified through experiments and theoretical analysis and the mode transformation mechanism under high pressure of the flame was provided. This study lays a foundation for further research on the instability of high-pressure diffusive combustion of hydrocarbon fuels and provide quantities of basic experimental data.