Response of a swirl-stabilized flame to simultaneous perturbations in equivalence ratio and velocity at high oscillation amplitudes
Response of a swirl-stabilized flame to simultaneous perturbations in equivalence ratio and velocity at high oscillation amplitudes
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DOI：
10.1016/j.combustflame.2014.09.025
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发表时间：
2015-04-01
影响因子：
4.4
通讯作者：
Paschereit, Christian Oliver
中科院分区：
文献类型：
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作者：
Cosic, Bernhard;Terhaar, Steffen;Paschereit, Christian Oliver
对于热声不稳定性的极限环预测，火焰描述函数是一个有价值的工具，热声不稳定性是现代燃气轮机中经常观察到的。在这些应用中，火焰是稀薄的部分预混火焰，它们的非定常响应源于与速度和当量比摄动有关的多种机制的组合。这两种机制在高强迫强度下的相互干扰是本文研究的重点。分析了实际相关旋流火焰非混合度的精确变化，得到了火焰对当量比波动的非线性响应。在雷诺数约为35000的大气条件下，对稀薄预混旋流稳定火焰进行了实验研究。利用高速粒子图像测速仪和OH~*化学发光成像技术，研究了不同频率和不同激励幅值下，完全和部分预混条件下的流场和火焰动力学。在燃料和空气的不同混合程度下，应用多传声器方法获得了火焰的非线性响应。通过改变两个喷油位置之间的燃油分配，控制不混合度。分析了不同部分预混火焰及其对应的预混火焰的非线性火焰响应。应用分解方法强调了当量比扰动对火焰描述函数的贡献所引起的趋势。流场测量表明，所研究的预混火焰和部分预混火焰的流场动力学非常相似。火焰描述函数分解的结果支持在大多数研究频率下，在低和高强迫振幅下的当量比扰动和速度起伏效应叠加的假设。分解揭示了当量比对火焰响应的贡献在小声强迫振幅时已经饱和。借助简化的火焰模型，给出了一个概念推理，该模型解释了观测到的部分预混火焰响应的饱和。提出了一种新的饱和机制，即在高强迫幅值下增加混合。特别是在小的和中等的强迫幅值以及较高的频率下，湍流产生的增加和平均流场的变化导致当量比波动的衰减增加。实验观察到的湍流剪应力随声强迫幅值增加而增长的现象支持这一解释。(C)2014年，燃烧研究所。爱思唯尔公司出版，版权所有。
The flame describing function is a valuable tool for the limit-cycle prediction of thermoacoustic instabilities, which are frequently observed in modern gas turbines. In these applications, flames are of lean partially premixed type, and their unsteady response originates from a combination of mechanisms related to perturbations in velocity and equivalence ratio. The interference between these two mechanisms at high forcing amplitudes is the focus of the present study. Well-defined variations of the degree of unmixedness of a practically relevant swirl flame are analyzed to obtain the nonlinear response of the flame to equivalence ratio fluctuations. A lean premixed swirl-stabilized flame is investigated experimentally at atmospheric conditions for a Reynolds number of approximately 35,000. The flow field and the flame dynamics are investigated for perfectly and partially premixed conditions at various frequencies and excitation amplitudes using high-speed particle image velocimetry and OH*-chemiluminescence imaging. The multi microphone method is applied at different degrees of unmixedness of fuel and air to obtain the nonlinear flame response. By a change in the fuel split between two injection positions the unmixedness is controlled. The nonlinear flame response of the different partially premixed flames and the corresponding premixed flame are analyzed in this study. A decomposition approach is applied to emphasize the trends caused by the contribution of the equivalence ratio perturbations to the flame describing function. Flow field measurements indicate that the flow field dynamics are very similar for the investigated premixed and partially premixed flames. The results of the flame describing function decomposition support the assumption of a superposition of equivalence ratio perturbations and velocity fluctuations effects at low and high forcing amplitudes for most of the investigated frequencies. The decomposition reveals a saturation of the equivalence ratio contribution to the flame response already at small acoustic forcing amplitudes. A conceptual reasoning is presented with the help of a simplified flame model, which explains the observed saturation in the response of partially premixed flames. Increased mixing at high forcing amplitudes is proposed as a new saturation mechanism. Especially at small and intermediate forcing amplitudes as well as higher frequencies, increased turbulence production and changes in the mean flow field lead to an increase of the damping of equivalence ratio fluctuations. An experimentally observed growth of the turbulent shear stresses with increasing acoustic forcing amplitude supports this explanation. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.