Absolute/Convective Instability Transition in a Backward Facing Step Combustor: Fundamental Mechanism and Influence of Density Gradient
Absolute/Convective Instability Transition in a Backward Facing Step Combustor: Fundamental Mechanism and Influence of Density Gradient
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DOI：
10.1115/1.4028206
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发表时间：
2014-06
影响因子：
1.5
通讯作者：
K. Manoharan;S. Hemchandra
中科院分区：
文献类型：
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作者：
K. Manoharan;S. Hemchandra
贫油预混燃气涡轮机燃烧室中的流场的流体动力学不稳定性可产生速度扰动，该速度扰动在小于火焰长度的长度尺度上使火焰片起皱和变形。由此产生的放热振荡可能导致燃烧不稳定。因此，为了了解燃烧室流场对燃烧不稳定特性的总体影响，了解燃烧室流场的流体动力不稳定特性是必要的。为此，本文阐明了从线性流体动力学稳定性分析的波动涡量生产的作用，作为关键机制，促进绝对/对流不稳定的剪切层发生在后面的一个面向步骤的流动中的过渡。这些结果是在无粘，不可压缩，局部时间和时空稳定性分析的框架内获得的。在这种极限下，涡度波动是由两种相互竞争的机制之间的相互作用引起的：（1）速度扰动和基流涡度梯度之间的相互作用产生的，（2）存在基流密度梯度时的斜压扭矩。当基流密度和速度梯度共位时，这种相互作用对流体动力学不稳定特性有显著影响。- 表征基本流速分布的参数空间中的区域，即，确定了对应于对流和绝对不稳定性的剪切层厚度和正向与反向流速之比。本研究结果的影响，了解以前的实验研究的燃烧不稳定性，在面向后的步骤燃烧室和流体动力学不稳定性，在其他流动，如加热射流和海崖体稳定火焰进行了讨论。
Hydrodynamic instabilities of the flow field in lean premixed gas turbine combustors can generate velocity perturbations that wrinkle and distort the flame sheet over length scales that are smaller than the flame length. The resultant heat release oscillations can then potentially result in combustion instability. Thus, it is essential to understand the hydrodynamic instability characteristics of the combustor flow field in order to understand its overall influence on combustion instability characteristics. To this end, this paper elucidates the role of fluctuating vorticity production from a linear hydrodynamic stability analysis as the key mechanism promoting absolute/convective instability transitions in shear layers occurring in the flow behind a backward facing step. These results are obtained within the framework of an inviscid, incompressible, local temporal and spatio-temporal stability analysis. Vorticity fluctuations in this limit result from interaction between two competing mechanisms-(1) production from interaction between velocity perturbations and the base flow vorticity gradient and (2) baroclinic torque in the presence of base flow density gradients. This interaction has a significant effect on hydrodynamic instability characteristics when the base flow density and velocity gradients are colocated. Regions in the space of parameters characterizing the base flow velocity profile, i.e., shear layer thickness and ratio of forward to reverse flow velocity, corresponding to convective and absolute instability are identified. The implications of the present results on understanding prior experimental studies of combustion instability in backward facing step combustors and hydrodynamic instability in other flows such as heated jets and bluff body stabilized flames is discussed.