Nonlinear thermoacoustics of ducted premixed flames: The influence of perturbation convection speed
Nonlinear thermoacoustics of ducted premixed flames: The influence of perturbation convection speed
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DOI：
10.1016/j.combustflame.2013.06.019
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发表时间：
2013-12
影响因子：
4.4
通讯作者：
K. Kashinath;S. Hemchandra;M. Juniper
中科院分区：
文献类型：
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作者：
K. Kashinath;S. Hemchandra;M. Juniper
当一个预混火焰被放置在一个管道中，声波在火焰的底部引起速度扰动。它们沿着火焰向下传播，扭曲其表面并调节其热量释放。这可以诱导自我持续的热声振荡。虽然通常假设这些扰动的相速度等于平均流速，但其他研究的实验和本研究的直接数值模拟（DNS）表明，它随声波频率而变化。在本文中，我们研究了这些变化如何影响非线性热声行为。我们用一个非线性运动学g方程来模拟热释放，其中速度扰动是根据DNS结果建模的。声学是由线性动量和能量方程控制的。我们计算了火焰描述函数（FDF）使用谐波强迫在几个频率和幅值。然后，我们计算了热声极限环，并通过检查FDF的增益和相位的振幅依赖性来解释它们的存在和稳定性。我们发现，当相速度等于平均流速时，系统只有一个稳定状态。然而，当相速度不等于平均流速时，系统支持多个极限环，因为FDF的相位随着振荡幅度的变化而显著变化。这表明速度扰动的相速度对管道预混火焰的非线性热声行为有很大的影响。
When a premixed flame is placed within a duct, acoustic waves induce velocity perturbations at the flame’s base. These travel down the flame, distorting its surface and modulating its heat release. This can induce self-sustained thermoacoustic oscillations. Although the phase speed of these perturbations is often assumed to equal the mean flow speed, experiments conducted in other studies and Direct Numerical Simulation (DNS) conducted in this study show that it varies with the acoustic frequency. In this paper, we examine how these variations affect the nonlinear thermoacoustic behaviour. We model the heat release with a nonlinear kinematicG-equation, in which the velocity perturbation is modelled on DNS results. The acoustics are governed by linearised momentum and energy equations. We calculate the flame describing function (FDF) using harmonic forcing at several frequencies and amplitudes. Then we calculate thermoacoustic limit cycles and explain their existence and stability by examining the amplitude-dependence of the gain and phase of the FDF. We find that, when the phase speed equals the mean flow speed, the system has only one stable state. When the phase speed does not equal the mean flow speed, however, the system supports multiple limit cycles because the phase of the FDF changes significantly with oscillation amplitude. This shows that the phase speed of velocity perturbations has a strong influence on the nonlinear thermoacoustic behaviour of ducted premixed flames.