Numerical Investigation of Potential Cause of Instabilities in a Hydrogen Micromix Injector Array

Xiaoxiao Sun, David Abbott, Abhay Vir Singh, P. Gauthier, Bobby Sethi
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引用次数: 1

Abstract

Hydrogen micromix combustion is a promising concept to reduce the environmental impact of both aero and land-based gas turbines by delivering carbon-free and ultra-low-NOx combustion. The high-reactivity and wide flammability limits of hydrogen in a micromix combustor can produce short and small diffusion flames at lean overall equivalence ratios. There is limited published information on the instabilities of such hydrogen micromix combustors. Diffusion flames are less prone to flashback and autoignition problems than premixed flames as well as combustion dynamics issues. However, with the high laminar flame speed of hydrogen, lean fuel air ratio (FAR) and very compact flames, the risk of combustion dynamics for micromix flames should not be neglected. In addition, the multi-segment array arrangement of the injectors could result in both potential causes and possible solutions to the instabilities within the combustor. This paper employs numerical simulations to investigate potential sources of instabilities in micromix flames by modelling an extended array of injectors, represented by either single or multiple injectors with appropriate boundary conditions at elevated pressure and temperature. Both RANS and LES simulations were performed and used to derive the Flame Transfer Function (FTF) of the micromix flames to inform lower order thermoacoustic modelling of micromix combustion. LES simulations indicate that the gain of the FTF is lower than predicted from the RANS simulations indicating a lower risk of high frequency thermoacoustic issues than suggested by RANS. When LES simulations are conducted for certain representative configurations it is observed that there are persistent high-frequency instabilities due to the interaction of the flames. This phenomenon is not observed when only a single injector is modelled. LES simulations for two injectors are conducted with various geometries and radial boundary conditions to identify the cause of the instabilities. It is concluded that the observed high-frequency instabilities are related to aerodynamic jet instabilities enhanced by both aerodynamic and acoustic feedback and key geometric features affecting the occurrence of the instabilities are identified. Only transient simulations such as LES are able to capture such effects and RANS simulations typically used in early stage design will not identify this issue.
微混合氢喷射器阵列不稳定潜在原因的数值研究
氢微混合燃烧是一个很有前途的概念,通过提供无碳和超低氮氧化物燃烧来减少航空和陆基燃气轮机对环境的影响。氢在微混合燃烧室中的高反应性和宽可燃性限制可以在低总等效比下产生短而小的扩散火焰。关于这种氢微混合燃烧器不稳定性的公开信息有限。扩散火焰比预混火焰更不容易出现闪回和自燃问题以及燃烧动力学问题。然而,由于氢的层流火焰速度快,燃料空气比低,火焰非常致密,微混合火焰的燃烧动力学风险不容忽视。此外,喷油器的多段排列既可能引起燃烧室内部的不稳定,也可能引起燃烧室内部的不稳定。本文采用数值模拟的方法研究微混合火焰中潜在的不稳定源,通过模拟一个扩展的喷射器阵列,由单个或多个喷射器代表,在高压和高温下具有适当的边界条件。同时进行了RANS和LES模拟,并利用它们推导了微混合火焰的火焰传递函数(FTF),为微混合燃烧的低阶热声建模提供了依据。LES模拟表明,FTF的增益比RANS模拟预测的要低,表明高频热声问题的风险比RANS预测的要低。当对某些代表性结构进行LES模拟时,可以观察到由于火焰的相互作用存在持续的高频不稳定性。当只模拟一个喷油器时,没有观察到这种现象。在不同的几何形状和径向边界条件下,对两个喷射器进行了LES模拟,以确定不稳定的原因。结果表明,高频不稳定性与气动和声学反馈增强的气动射流不稳定性有关,并确定了影响不稳定性发生的关键几何特征。只有像LES这样的瞬态模拟才能捕捉到这种效果,而早期设计中通常使用的RANS模拟无法识别这个问题。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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