Abstract: Dump combustors exhibit undesirable combustion instabilities under certain operating conditions.Experimental and numerical investigations were implemented to study low-frequency combustion instabilities.The results show that the longitudinal mode is dominant in pressure oscillations of the dump combustor whose frequency is not completely same as acoustic frequency.Including multi-steps chemical mechanisms coupling with large eddy simulation (LES),numerical simulation methods for combustion instabilities analysis are established.Large eddy simulation of low-frequency combustion instabilities is carried out for experimental combustor.The detailed forming and evolution process of combustion instabilities are obtained.It is discovered that vortices shedding from dump plane,pulse of combustion area,local equivalent ratio and heat release fluctuations during vortices motion process in combustor are responsible for the low-frequency combustion instabilities.Pressure fluctuations result in flow velocity pulse.Furthermore,vortex shedding is formed.The motion of large scale in combustor will result in large altitude pulse of heat release,and will excite pressure oscillations.The low-frequency combustion instabilities appear to be controlled by the motion of both vortices and acoustic waves.

Key words: Low frequency combustion instabilities;Dump combustor;Large eddy simulation;Experiment;Ramjet

摘要： 突扩燃烧室在一定的工作条件下会出现燃烧不稳定现象。采用实验和数值模拟的方法对突扩燃烧室形成低频燃烧不稳定的机理进行了研究。通过实验研究发现突扩燃烧室压强振动过程中纵向振型占主导地位,但其振动频率并不与声学频率一致。建立了适合分析燃烧不稳定的多步化学反应动力学与大涡模拟耦合的数值分析方法,对实验发动机开展了非稳态数值模拟,获得了低频燃烧不稳定形成演化的详细过程和流场结构。实验和数值计算表明突扩截面形成的旋涡脱落,以及旋涡在燃烧室内的运动过程中引起燃烧面积、局部当量比和热释放率的脉动是激发低频压强振动的主要原因。压强振动引起上游速度脉动,进而形成旋涡脱落。大尺度旋涡在燃烧室内的运动又会引起热释放率的大幅度脉动,反过来又会促进压强振动。振动频率是由压强波和旋涡运动特征时间共同决定的。

关键词: 低频燃烧不稳定;突扩燃烧室;大涡模拟;实验;冲压发动机