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Spontaneous Oscillations in Lean Premixed Combustors: CFD Simulation

机译：稀薄预混燃烧器中的自发振荡：CFD模拟

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Lean premixed (LPM) combustors allow reduction of NOx emissions, but they often experience instabilities that are detrimental for engines structural integrity and performances. Most of the works dealing with LPM combustors oscillations address the occurrence of dynamic regimes to system instabilities, i.e., to the coupling between variations of heat release from the flame and acoustic modes of the combustion chamber. However, premixed flames may be prone to intrinsic instabilities: the flame itself oscillates, independently from the coupling with combustor acoustics. In the present article, RANS-based CFD simulations were performed to study the dynamic behavior of an LPM combustor by varying the length of the inlect duct, the inlet gas velocity and fuel equivalence ratio, and the combustor wall temperature. The model results show that the LPM combustor oscillations may be different in nature: they may originate from system instabilities or from the propagation to the whole combustor of the flame intrinsic oscillations due to heat losses (thermo-kinetic oscillations). Furthermore, it is found that whatever the driving mechanism (system or flame instability) a whole acoustic mode is excited and, accordingly, Rayleigh's criterion is always verified. Conversely, the widely used re-statement of Rayleigh's criterion based on the time-delay approach is verified only for the oscillations originated by system instability, while it fails with the thermo-kinetic oscillations. A way to discern between the different mechanisms exciting the oscillations in an LPM combustor is then suggested.
机译：稀薄的预混合（LPM）燃烧器可减少NOx排放，但它们经常遇到不稳定的情况，这对发动机的结构完整性和性能不利。处理LPM燃烧器振荡的大多数工作都针对系统不稳定的动态机制，即火焰释放的热量变化与燃烧室的声模之间的耦合问题。但是，预混火焰可能容易产生内在的不稳定性：火焰本身会振荡，而与燃烧器声学耦合无关。在本文中，进行了基于RANS的CFD仿真，以通过改变导风管的长度，进气速度和燃料当量比以及燃烧室壁温来研究LPM燃烧室的动态行为。模型结果表明，LPM燃烧器的振荡本质上可能有所不同：它们可能源自系统的不稳定性，也可能源于由于热损失（热动力学振荡）引起的火焰固有振荡向整个燃烧器的传播。此外，发现无论是哪种驱动机制（系统或火焰不稳定），整个声学模式都会被激发，因此，始终要验证瑞利准则。相反，基于时延方法的瑞利准则的重述仅针对系统不稳定引起的振荡进行了验证，而由于热动力学振荡而失败。因此，提出了一种在LPM燃烧室中激发振动的不同机制之间进行区分的方法。

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