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Modeling of thermo-acoustic instabilities in counterflow flames.

机译：逆流火焰中的热声不稳定性建模。

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Under certain operating conditions, many combustion systems in aerospace propulsion and land-based power generation exhibit large-amplitude pressure oscillations coupled with unsteadiness in the combustion processes. This unsteady multi-scale phenomenon is generally referred to as thermo-acoustic instability. Recently, model-based active control approaches are being pursued to suppress the potentially destructive instability and for this purpose a fundamental understanding of the coupling mechanisms between the dynamics of the flame and acoustics is critical.; In the present investigation, a numerical model is developed to study the interaction of longitudinal acoustic waves with planar flames in the simplified counterflow configuration. The mathematical formulation of quasi one-dimensional, fully unsteady, laminar counterflow flames is derived and the governing equations are integrated numerically based on a MacCormack predictor-corrector scheme, with the inclusion of detailed transport and finite-rate chemistry. In order to accurately represent perfect and partial reflection of acoustic waves at the boundaries, Navier-Stokes characteristic boundary conditions are implemented.; The focus of the investigation is on the linear regime of thermo-acoustic instabilities associated with planar flames. Specifically, coupling mechanisms intrinsic to the dynamics of the flame are addressed, such as flow compressibility effects, which may be responsible for the initial triggering of thermo-acoustic instabilities, and finite-rate chemistry effects. For well-resolved simulations, the occurrence of the self-sustained amplification of pressure fluctuations is analyzed in both non-premixed and premixed methane-air flames for a range of flow strain rates and flame locations, and employing different chemical kinetic models. A detailed analysis of the characteristic time-scales associated with convection, diffusion, chemistry and acoustics is performed together with an analysis of the heat release rate and of the effects of flame location in order to provide a better understanding of the fundamental coupling mechanisms driving the instability, namely chemical kinetic-acoustic coupling and acoustically-induced fluctuations in the mass flux of reactants into the flame.
机译：在某些工况下，航空航天推进和陆上发电中的许多燃烧系统都表现出大幅度的压力振荡，并伴随着燃烧过程的不稳定。这种不稳定的多尺度现象通常称为热声不稳定性。最近，人们正在寻求基于模型的主动控制方法来抑制潜在的破坏性不稳定性，为此，对火焰动力学和声学动力学之间的耦合机制的基本理解至关重要。在本研究中，建立了一个数值模型来研究纵向声波与平面火焰在简化的逆流配置中的相互作用。推导了准一维，完全不稳定的层流逆流火焰的数学公式，并基于MacCormack预测器-校正器方案对控制方程进行了数值积分，包括详细的传输和有限速率化学过程。为了准确地表示声波在边界处的完全和部分反射，实现了Navier-Stokes特征边界条件。研究的重点是与平面火焰相关的热声不稳定性的线性机制。具体而言，解决了火焰动力学固有的耦合机制，例如流动可压缩性效应，这可能是热声不稳定性的初始触发以及有限速率化学效应的原因。对于良好解析的模拟，分析了非预混和预混的甲烷-空气火焰在一定的流动应变速率和火焰位置范围内，并采用不同的化学动力学模型，分析了压力波动的自持放大现象。与对流，扩散，化学和声学有关的特征时间尺度的详细分析，以及放热速率和火焰定位的影响的分析，以便更好地理解驱动空气的基本耦合机制。不稳定性，即化学动力学耦合，以及反应物进入火焰的质量通量中的声学诱导波动。

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