超燃冲压发动机是吸气式高超声速飞行器动力的首选，其燃烧室中火焰的传播和稳定过程，对发动机性能有很大的影响。发动机为实现更大的推力，需要扩大燃烧室的尺度，由此带来射流尺度的变化会影响燃料的混合和燃烧，因此有必要深入探究射流尺度对燃烧稳定影响的内在物理机制。影响射流状态的因素有很多，包括射流、来流的物理参数，如射流/来流动量通量比，来流马赫数等；以及燃烧室入口、射流喷孔的几何参数，如燃烧室入口面积、喷孔直径、喷孔间距等。本文定义射流尺度为射流/来流水力直径比，利用数值仿真和地面直连实验，对超声速燃烧室中射流尺度及火焰稳定的关系展开了系统研究，分析了不同射流尺度下稳焰模式、激波串及分离区等特征，揭示了射流尺度对燃烧释热及发动机性能的影响规律。

本文通过数值模拟分析了直径分别为50mm、100mm及200mm的圆形燃烧室的火焰稳定特性与燃烧流动特征，探究了当量比和来流马赫数对燃烧流场的影响，并比较了其点火过程的差异。模拟同一来流工况时，发现大尺度燃烧室在射流上游产生涡的尺度更大，大涡的卷带作用促进燃料与主流混合，从而增强释热。同时较大尺度燃烧室的燃料羽流更集中，相对喷注深度更大，增强掺混，更利于形成射流尾迹稳焰。另外，由于较大尺度燃烧室能容纳更多的释热，在点火过程中，激波串前移速度变缓。

通过对定工况的实验和数值研究，总结了不同射流尺度下的稳焰模式及振荡特征。根据火焰稳定的位置分为纯凹腔稳焰模式、剪切层稳焰模式和射流尾迹稳焰模式。纯凹腔稳焰模式主流无明显压升且均为超声速；剪切层稳焰模式火焰为预混燃烧，根据火焰强弱分为弱剪切层稳焰模式和强剪切层稳焰模式，其中弱剪切层稳焰模式主流均为超声速，在当量比增大后，剪切层火焰会变强，并且凹腔处的一维质量平均沿程马赫数小于1；射流尾迹稳焰模式火焰为部分预混燃烧，发动机此时处于亚燃工作模态。随着燃烧室内释热的增强，火焰振荡幅值增大，并引起较大的压力脉动幅值。燃烧室内存在两种压力振荡主频，一种由凹腔自激振荡产生，约为2000Hz，以上四种稳焰模式下均存在；另一种由热-声耦合振荡产生，约为430Hz，此时燃烧室主流需形成亚声速区，只有在强剪切层稳焰模式和射流尾迹稳焰模式下存在。

火焰位置不同是由于射流的混合区分层导致的，横向射流在发展到凹腔附近时会存在两种状态，一种是与剪切层密切接触，并且有燃料流至凹腔；另一种是射流与剪切层分离并跨过凹腔。当点火燃烧后，前一种状态会产生弱或强的剪切层稳焰模式，后一种跨过剪切层的射流状态会有两种发展趋势，一种是剪切层火焰抬升后并引燃主流，形成强剪切稳焰模式，或者主流熄火，仅有部分燃料进入凹腔，形成凹腔稳焰模式。当强剪切稳焰模式进一步增加当量比，火焰会向上游传播并在射流位置稳定下来，形成射流尾迹稳焰，这时凹腔起到作为持续释热区的作用。

当量比是影响稳焰模式切换的关键因素，一般随着当量比的逐渐提高，火焰首先从弱剪切层稳焰开始，进一步提高当量比会有分岔出现，一是出现强剪切层稳焰，或者出现凹腔稳焰；继续提高当量比后，火焰变为射流尾迹稳焰。

影响稳焰模式分岔的主要因素是射流尺度，主要体现在小射流尺度从弱剪切层稳焰到强剪切层稳焰会比大射流尺度提前出现，而且火焰转变一般是弱剪切层稳焰-强剪切层稳焰-射流尾迹模式；大射流尺度的剪切层稳焰在较高当量比下开始弱到强模式的转变，但会在比小射流尺度更低的当量比实现强剪切层稳焰到射流尾迹稳焰的转换。另外射流孔间距的增大不利于燃烧，若此时当量比较小，甚至会造成火焰熄灭，这可能是由于射流展向涡对对火焰稳定起着关键的作用。

The scramjet is the first choice for the hypersonic airbreathing propulsion. The flame propagation and stabilization process in the combustor have a great influence on the engine performance. In order to achieve greater thrust, it is necessary to expand the scale of the combustor. As a result, the change of jet scale will affect fuel mixing and combustion. Therefore, it is necessary to further explore the internal physical mechanism of jet scale's influence on combustion stabilization. There are many factors that affect jet, including the physical parameters of jet and inflow, such as momentum flux ratio of jet and inflow, inflow Mach number and so on, and the geometric parameters of the inlet and injector, such as the inlet area, injector diameter, injector spacing and so on. In this paper, the jet scale was defined as the hydraulic diameter ratio of jet and inflow. The relationship between jet scale and flame stabilization in supersonic combustor was systematically studied by using numerical simulation and ground direct-connected test. The characteristics of flame stabilization mode, shock train and separation zone in different jet scale were analyzed, and the law of jet scale's influence on heat release and engine performance was revealed.

Numerical simulation was used to analyze the flame stabilization and flow characteristics of circular combustors with inflow diameter of 50mm, 100mm and 200mm respectively. The effects of equivalence ratio and inflow Mach number on the combustion flow field were investigated, and the differences in the ignition process were compared. In the simulation of the same inflow condition, it is found that the vortex in the upstream of jet is larger in larger combustor, and the winding effect of large vortex promotes the mixing of fuel and main flow, thus enhancing the heat release rate. At the same time, the fuel plume in the larger combustor is more concentrated and the relative jet depth is larger, which enhances mixing and is more conducive to jet-wake flame stabilization mode. In addition, larger combustor can accommodate more heat release, so the forward velocity of shock train becomes slower during ignition.

Through the experimental and numerical study of certain conditions, the flame stabilization modes and oscillation characteristics under different jet scales were summarized. According to the position of flame stabilization, it can be divided into pure cavity flame stabilization mode, shear-layer flame stabilization mode and jet-wake flame stabilization mode. There is no obvious pressure rise in the main flow at pure cavity flame stabilization mode and the velocity is supersonic. The flame in the shear-layer flame stabilization mode is premixed combustion, which can be divided into the weak shear-layer flame stabilization mode and the strong shear-layer flame stabilization mode according to the intensity of the flame. The main stream of the weak shear-layer flame stabilization mode is supersonic. When the equivalence ratio increases, the flame in the shear layer becomes stronger, and the one-dimensional mass-weighted averaged Mach number along the cavity is less than 1. The flame in the jet-wake flame stabilization mode is partial premixed combustion, and the engine is in ramjet mode. With the increase of heat release rate in the combustor, the amplitude of flame oscillation increases and the pressure oscillation amplitude is larger. There are two main frequency of pressure oscillation in the combustor. One is generated by self-excited oscillation of the cavity, which is about 2000Hz. It exists in the above four flame stabilization modes. The other is generated by thermo-acoustic oscillation, which is about 430Hz. For the latter, there needs to form subsonic zone in the main flow, which only exists in the strong shear-layer flame stabilization mode and the jet-wake flame stabilization mode.

The different flame positions are caused by the stratification of the jet mixing zone. There are two states when the transverse jet develops near the cavity. One is that it is in close contact with the shear-layer and there is fuel flows into the cavity. The other is that the jet is separated from the shear-layer and crosses the cavity. After burning, the former state can be divided weak or strong shear-layer flame stabilization mode, the latter state has two development trends, one is that the shear-layer flame lifts, ignites the mainstream, and forms the strong shear-layer flame stabilization mode, or mainstream is blown off, only part of the fuel flows into cavity and forms the pure cavity flame stabilization mode. When the equivalence ratio increases in the strong shear-layer flame stabilization mode, the flame propagates to the upstream and stabilizes near the jet, forming the jet-wake flame stabilization mode. At this case, the cavity becomes a continuous heat release area.

Equivalence ratio is the key factor which affects the transition of flame stabilization mode. Generally, the flame starts from the weak shear-layer flame stabilization mode, there will be bifurcation when the equivalence ratio is further increased. One is the strong shear-layer flame stabilization mode, the other is the pure cavity flame stabilization mode. When the equivalence ratio continues to increase, the flame mode becomes the jet-wake flame stabilization mode.

The main factor affecting the bifurcation of the flame stabilization mode is the jet scale, which is mainly reflected in that the transition from the weak shear-layer flame stabilization mode to the strong shear-layer flame stabilization mode for small jet scale is earlier than large jet scale’s. And the flame transition is generally from the weak shear-layer to the strong shear-layer to the jet-wake flame stabilization mode. The flame stabilization mode of the shear-layer with larger jet scale starts to change from weak mode to strong mode at higher equivalence ratio, but change from the strong shear-layer to the jet-wake flame stabilization mode at lower equivalence ratio. In addition, the increase of distance between jet is not conducive to combustion. If the equivalence ratio is relatively small, the flame is even blown off. It may be that spreading vortex of jet plays a key role in the flame stabilization.