Dynamics of harmonically excited, reacting bluff body wakes near the global hydrodynamic stability boundary
Dynamics of harmonically excited, reacting bluff body wakes near the global hydrodynamic stability boundary
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全局水动力稳定性边界附近的简谐激励、反应钝体尾流的动力学
DOI：
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发表时间：
2015
影响因子：
3.7
通讯作者：
T. Lieuwen
中科院分区：
文献类型：
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作者：
B. Emerson;T. Lieuwen
本文描述了强迫轴向声学振荡与尾迹整体模式之间的线性和非线性相互作用。这项工作的动机是燃烧不稳定性问题，其中与自然燃烧室模式相关的声学振荡会激发流动的流体动力不稳定性，进而导致热释放振荡。在高雷诺数下，具有密度分层的尾迹流动可能是全局稳定的，也可能是不稳定的，因此火焰中的密度变化对自然火焰和流动动力学有重要影响。测量是在火焰密度比、唇速和强迫频率独立变化的装置中进行的，使用5 kHz粒子图像测速仪和Mie散射测量。通过改变密度比，可以系统地改变流体动力全模增长率。此外，还进行了强迫频率相对于全局振型频率变化的测量和分析。虽然轴向强迫激发了剪切层的曲张反应，但弯曲模式是增长最快的。正如预期的那样，在接近尾流全局模式频率的频率处的强迫导致涡扰动幅度的快速增长，对称的涡旋在向下游对流时迅速交错，导致尾流和火焰的大范围、弯曲的拍打。线性的局部稳定性分析，加上非线性的稳定性分析，有助于阐明控制涡旋交错的物理原因。研究最后对热释放动力学进行了分析。值得注意的是，研究表明，热释放表现出与流体动力学完全不同的敏感性；例如，在其全局模式频率附近对流动的轴向强迫导致热释放振荡的减少。这是正确的，尽管这种强迫频率最大化了涡流引起的火焰扑动的局部程度。因此，这项研究的结果显示了一些与传统概念相矛盾的现象，即流体动力整体模式的频率与声学模式的频率对齐的条件可能会导致钝体燃烧室中强迫放热振荡的减弱。
This paper describes linear and nonlinear interactions between forced axial acoustic oscillations and the global mode of the reacting wake. This work is motivated by the problem of combustion instabilities, where acoustic oscillations associated with natural combustor modes excite hydrodynamic instabilities of the flow that, in turn, induce heat release oscillations. Wake flows with density stratification can be globally stable or unstable at high Reynolds numbers, and so the density change across the flame has significant influence on the natural flame and flow dynamics. Measurements were obtained in a facility in which flame density ratio, lip velocity and forcing frequency are independently varied using 5 kHz particle image velocimetry and Mie scattering measurements. By varying the density ratio, the hydrodynamic global mode growth rate can be systematically varied. In addition, measurements and analyses were performed where the forcing frequency is varied relative to the global mode frequency. While axial forcing excites a varicose response of the shear layers, the sinuous mode is the most rapidly growing. As expected, forcing at a frequency near the wake’s global mode frequency leads to rapid growth in vortical disturbance amplitude, and the symmetric vortices quickly stagger as they convect downstream leading to a large scale, sinuous flapping of the wake and flame. A linear, local stability analysis, together with a nonlinear analysis, help elucidate the physics that govern the vortex staggering. The study concludes with an analysis of the heat release dynamics. Significantly, the study shows that the heat release exhibits quite different sensitivities than the fluid dynamics; e.g. axial forcing of the flow near its global mode frequency leads to a reduction in heat release oscillations. This is true even though this forcing frequency maximizes the local degree of vortically induced flame flapping. Thus, the results of this study show some phenomena that contradict conventional notions, namely that conditions which align the frequency of a hydrodynamic global mode with that of an acoustic mode may lead to diminished forced heat release oscillations in bluff body combustors.