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A hybrid approach for coupling of acoustic wave effects and incompressible LES of reacting flows

机译：声波效应与反应流不可压缩LES耦合的混合方法

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Lean premixed combustion systems, attractive for low NO_x performance are, however, susceptible to thermo-acoustic instabilities - the interaction between the unsteady heat release of combustion and acoustic waves. In the present work, a hybrid, coupled CFD ap proach is described, which aims to combine the computational efficiency of incompressible LES of reacting flows with acoustic wave effects captured via an acoustic network model. The well-documented ORACLES experimental test case is used for validation. Simulations of the inert flow agree well with experimental data, reproducing the measured amplitude and distribution of turbulent fluctuations as well as capturing the asymmetric mean flow features. Under reacting flow conditions the measured data exhibit a vigorous plane wave acoustic pulsation mode at a frequency of 50Hz. A new approach to sensitise the incom pressible LES CFD to acoustic wave influence is adopted. An acoustic network model of the experimental geometry is first analysed to predict the dimensional amplitude of the 50Hz mode at the start of the combustion zone. This knowledge is used to introduce a coherent plane wave contribution at the incompressible LES inlet plane at an appropriate amplitude. Numerical predictions of this forced flow show good agreement with measured velocity field data. Simulations with varying amplitude of inlet forcing allow assessment of the flame dy namics and heat release response to be carried out and Flame Transfer Functions (FTFs) are extracted. Rather than a single set of gain and time-delay parameters as normally used to characterise flame response, the non compact nature of the ORACLES flame requires an axial distribution of FTF parameters. The gain/time-delay characteristics were also evaluated for a range of excitation amplitudes. When fed into the acoustic network model this allowed prediction of a limit-cycle equilibrium condition between stable and unstable regimes. The predicted amplitude of this condition was in reasonable agreement with the self-sustained acoustic mode strength observed in the ORACLES experiment, providing encouraging confirmation of the usefulness of the coupled hybrid approach.
机译：但是，对于低NO_x性能具有吸引力的稀薄的预混燃烧系统易受热声不稳定性的影响-燃烧不稳定的热释放与声波之间的相互作用。在本工作中，描述了一种混合的，耦合的CFD ap方法，其目的是将不可压缩LES反应流的计算效率与通过声网络模型捕获的声波效应相结合。有据可查的ORACLES实验测试用例用于验证。惰性流动的仿真与实验数据非常吻合，可再现测得的湍流波动幅度和分布以及捕获非对称平均流动特征。在反应流动条件下，测得的数据在50Hz频率下呈现出剧烈的平面波声脉动模式。采用了一种使不可压缩的LES CFD对声波影响敏感的新方法。首先分析实验几何形状的声学网络模型，以预测燃烧区开始时50Hz模式的尺寸幅度。该知识用于以适当的幅度在不可压缩的LES入口平面处引入相干平面波贡献。该强迫流动的数值预测表明与测得的速度场数据吻合良好。通过改变进气压力幅度的模拟，可以评估火焰动力学和放热响应，并提取出火焰传递函数（FTF）。 ORACLES火焰的非紧凑性质不是通常用于表征火焰响应的一组增益和时间延迟参数，而是需要FTF参数的轴向分布。还针对一定范围的激励幅度评估了增益/延时特性。当输入到声学网络模型中时，这可以预测稳定和不稳定状态之间的极限循环平衡条件。这种情况的预测幅度与在ORACLES实验中观察到的自我维持的声模强度在合理范围内一致，这为耦合混合方法的有用性提供了令人鼓舞的确认。

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