随着对吸气式超声速推进研究的深入，在超声速燃烧点火控制和火焰稳定方面的挑战也日益增加。在以超过5 马赫数飞行时，气流在燃烧室驻留时间太短，没有足够的时间使燃料点火并完全氧化。因此，通过提高燃料氧化速率和整体化学动力学来减少化学反应时间的新技术也引起了广泛关注，微波辅助燃烧（MAC）是这些新技术的其中之一，通过使用微波能量来增强燃烧，是一个新的研究领域。本文构建了微波辅助燃烧的数值模型并开展了超声速条件下微波辅助燃烧的实验研究，通过改变工况分析了影响微波辅助燃烧效果的因素。

本文使用详细的电子输运模型和甲烷详细的反应机理来模拟微波电场下，预混火焰的传播速度和温度等火焰参数。在微波电场之下，电子-中性粒子碰撞作用会加速燃烧反应过程，并进一步提高火焰温度和火焰传播速度。因此，在非击穿条件下（亚临界），电子的作用更加突出，不仅有热效应还有动力学效应。为了完善的表达微波电场与燃烧的耦合作用，本模型在广泛使用的欧姆热模型中添加了电子激发态组分的化学反应机理，同时使用电子能量分布函数（EEDF）来计算一些涉及电子的输运参数和反应速率常数，大大提高了关于电子的计算精度。在本模型与相关实验结果的对比中，模型可以更准确预测微波电场下火焰温度、速度以及反应组分浓度。另外，使用本模型对可能影响微波助燃效果的因素进行评估，发现电场强度与火焰速度提升成正比，火焰速度的增加值与当量比关系更复杂。

本文依托直连式超声速燃烧实验台，设计了可以工作在1GHz-6GHz 的微波耦合超燃燃烧室，以及搭建了配套的测量系统。使用网络分析仪测量了燃烧室的回波损耗，发现燃烧室存在多个较低回波损耗的“峰值”频率点，并将这些频率作为实验中候选频率。对比有无来流状态下的回波损耗，发现来流和火焰存在对“峰值”频率点的回波损耗有影响。实验中使用了各种先进的诊断手段，包括CH∗ 图像、宽域光谱、壁面压力和推力等数据，对不同频率和功率水平微波的辅助燃烧效果进行了定量化的分析。

实验研究发现小功率的微波同样能提高各种当量比的自由基浓度和发动机推力，平均推力提高了15-30% 之间，还发现微波耦合与燃烧状态有较大关系，同时微波具有空间的传播性，不仅能影响天线辐射的凹腔内火焰，也会影响下游的火焰，对这种情况有两种解释：一是在凹腔位置的火焰中产生激发态组分，其传播到下游成功点火并燃烧；二是微波能量直接传播到下游并辅助燃烧。另外，发现加入不同“峰值”频率的微波，其频率对微波助燃效果影响不大，因为“峰值”频率点的回波损耗都较低，微波耦合到火焰的功率都很接近，这说明1-6GHz 的微波与火焰的耦合主要是电场作用而不是频率共振。显然，微波功率越大，助燃效果会越明显，但真正在动力学起作用的微波能量比例还需要进一步的研究。

With the deepening of research on air-breathing supersonic propulsion, the challenges in supersonic combustion ignition control and flame stability are also increasing. When flying at speeds exceeding Mach 5, the airflow stays in the combustion chamber for too short a time to ignite and completely oxidize the fuel. Therefore, new technologies to reduce the chemical reaction time by improving the fuel oxidation rate and overall chemical kinetics have also attracted wide attention. Microwave assisted combustion (MAC) is one of these new technologies, which enhances combustion through the use of microwave energy and is a new research field. This article constructs a numerical model of microwave assisted combustion and conducts experimental research on microwave assisted combustion under supersonic conditions. The factors affecting the effect of microwave assisted combustion are analyzed by changing operating conditions.

This article uses a detailed electron transport model and a detailed reaction mechanism of methane to simulate flame parameters such as propagation speed and temperature of premixed flames under microwave electric fields. Under a microwave electric field, electron neutral particle collision accelerates the combustion reaction process and further increases the flame temperature and flame propagation speed. Therefore, under non breakdown conditions (subcritical), the role of electrons is more prominent, with not only thermal effects but also dynamic effects. In order to perfectly express the coupling effect of microwave electric field and combustion, the chemical reaction mechanism of electron excited state components is added to the widely used ohmic thermal model
in this model. At the same time, the electron energy distribution function (EEDF) is used to calculate some transport parameters and reaction rate constants involving electrons, which greatly improves the calculation accuracy of electrons. In the comparison between this model and relevant experimental results, the model can more accurately predict flame temperature, velocity, and reaction component concentration under microwave electric field. In addition, using this model to evaluate the factors that may affect the effectiveness of microwave assisted combustion, it was found that the electric field intensity is directly proportional to the increase in flame speed, and the relationship between the increase in flame speed and the equivalence ratio is more complex.

This article relies on a direct connected supersonic combustion experimental platform to design a microwave coupled supersonic combustion chamber that can work at 1GHz to 6GHz, and to build a matching measurement system. The return loss of the combustion chamber was measured by network analyzer. It was found that there were
several ”peak” frequencies with lower return loss in the combustion chamber, and these frequencies were used as candidate frequencies in the experiment. Comparing the return loss with and without incoming flow, it was found that the presence of incoming flow and flame has an impact on the return loss at the ”peak” frequency point. Various advanced diagnostic methods were used in the experiment, including CH∗ images,
wide-domain spectra, wall pressure and thrust data, to quantitatively analyze the auxiliary combustion effects of microwaves at different frequencies and power levels.

Experimental research has found that low-power microwaves can also increase the concentration of free radicals and engine thrust under various equivalence ratios, with an average thrust increase of 15-30%. It has also been found that there is a significant relationship between microwave coupling and combustion state. At the same time, microwaves
have spatial propagation, which can not only affect the concave cavity flame radiated by the antenna, but also affect the downstream flame, There are two explanations for this situation: one is that excited state components are generated in the flame at the cavity position, and they propagate to the downstream to ignite and burn successfully; The second is that microwave energy directly propagates downstream and assists in combustion. In addition, it was found that adding microwaves with different ”peak” frequencies has little effect on the assist combustion effect of microwaves, as the return loss at the ”peak” frequency points is relatively low and the power of microwave coupling to the flame is very close. This indicates that the coupling between 1-6GHz microwaves and the flame is mainly due to electric field interaction rather than frequency resonance. Obviously, the higher the microwave power, the more obvious
the assist combustion effect will be, but further research is needed on the proportion of microwave energy that truly affects kinetics.