部分覆盖型凹腔实现超声速气流中冷煤油直接点火的机理研究

Directly ignite the room temperature kerosene in supersonic flow by spark plug is a crucial thing in scramjet engine investigation.To solve the problem, a novel partly-covered cavity was proposed, and the ignition ability was proved by engine direct-connected test and free-jet test at the simulated condition of Mach 4-6, but the mechanism was unclear.The PLIF, NPLS, high speed Schlieren method and high precision numerical simulation were employed to clarify the mechanism. The effect of streamwise vortices which were induced by the serrate cover-plane, on the cavity flow field and the fuel distribution will be investigated. The formation and propagation of initial flame will be studied. Furthermore,the flame shape evolve, stabilize and blow-out process will be obtained on the condition of rich/lean fuel.The interaction mechanism between streamwise vortices and cavity recirculation region will be clarified. The enhancement of streamwise vortices on the propagation of initial flame will be obtained. Models which explain the stabilize and blow-out process of partly-covered cavity will be developed. Finally, the mechanism of partly-covered cavity on room temperature ignition will be clarified. These work may fill the technical investigation blank of application the partly-covered cavity for flame holding and ignition in scramjet engine. This project may also set up a theoretical foundation for scramjet engine ignition research.

超声速气流中冷煤油的点火问题一直是超燃冲压发动机研制的难点。我项目组发明了一种基于部分覆盖型凹腔的点火方法，并已通过模拟飞行马赫数4至6条件下模型发动机直连式和自由射流冷煤油火花塞直接点火验证，但其机理尚不清楚。本项目拟采用PLIF、NPLS、高速激光纹影等观测手段和高精度的大涡模拟方法，开展冷流条件下覆盖板流向涡对凹腔流场和燃料分布的影响规律研究，开展覆盖型凹腔内初始火团的形成、发展和火焰传播过程的研究，开展贫/富燃情况下火焰形态的演变、火焰稳定与火焰吹熄规律的研究，明晰锯齿开缝流向涡与凹腔回流区的耦合作用机理，阐明流向涡增强初始火焰扩散并引燃主流的规律，建立部分覆盖型凹腔火焰稳定与吹熄边界的模型，揭示部分覆盖型凹腔实现冷煤油直接点火的机理，填补部分覆盖型凹腔在混合增强、点火和火焰稳定方面的研究空白，为超声速气流中冷煤油点火研究奠定理论基础。

本项目首先开展了乙烯燃料的点火试验，研究了凹腔长深比对点火过程的影响。同时利用串联凹腔燃烧室构型，采用凹腔前壁面、后壁面喷注和凹腔底壁喷注煤油的方式进行了煤油燃料的火花塞直接点火试验，证明了串联凹腔构型增强煤油燃料点火性能的可靠性。其次利用数值模拟对部分覆盖型凹腔进行了研究，形成对部分覆盖型凹腔流场结构的初步认识。数值仿真的内容包括二维冷流、三维冷流的数值模拟和燃烧的数值模拟。最后对部分覆盖型凹腔的点火能力开展实验研究，并通过数值模拟的手段研究了部分覆盖型凹腔点火位置的优化方案。. 研究结果表明，凹腔长深比较小（L/D=5）时，由于燃烧空间的限制，形成的燃烧强度不够，在往下游输运的过程中，火焰容易被高速主流的强烈气流拉伸效应所猝熄。使用串联凹腔构型可以提高液态碳氢燃料点火性能，因为下游凹腔本身适宜点火，且利用火焰的逆流传播实现了对上游凹腔的能量反馈。上游凹腔水平顺流喷注、下游凹腔点火的喷注方案是一种点火性能较好，可靠性高的方案，其点火边界大，在较广的喷注压力下均能实现煤油燃料的点火和稳定燃烧。部分覆盖型凹腔中，凹腔未覆盖部分的长深比是影响凹腔与主流质量交换的主要因素。覆盖区内气流速率很低，可以提供较好的点火环境。但覆盖区内部的区域质量交换率过低，容易引起贫燃或者富燃。研究认为适宜的点火位置应该在覆盖区与未覆盖区的分界附近，位于锯齿开缝的下方。因为覆盖板锯齿开缝增强了凹腔覆盖区的质量交换，同时保持了覆盖区内较低的气流速率。