声学激励下扩散火焰的动力学模态分析

Dynamic modal analysis of diffusion flame under acoustic excitation

针对扩散火焰燃烧不稳定性,使用高速摄像机获得火焰图像,通过动力学模态分解法(dynamic mode decomposition, DMD),研究钝体扰流扩散火焰,在外加声学激励下,不同频率处的空间分布特征。结果表明,无激励火焰以低于50 Hz的低频杂乱振荡为主,在15 Hz存在明显的纵向振荡模态。外加的声学激励会引起激励频率或者其半频与倍频处的纵向振荡,但不会改变其他频率的空间结构。当给予的声学激励频率或者其倍频与火焰固有的纵向模态频率相接近时会引起火焰该频率灰度值幅值的强烈响应,增强火焰的该频率下的纵向振荡模态,引起火焰的不稳定性。

To investigate the combustion instability of diffusion flame,high-speed image of the bluff body turbulnce diffusion flame were recorded under different acoustic frequencies excitation.And the temporal and spatial dynamic features of the flame could be extracted by using dynamic modal decomposition(DMD).The results show that the unexcited flame is mainly composed of low-frequency oscillations below 50 Hz,and there is an obvious longitudinal oscillation mode around 15 Hz.The external acoustic excitation will cause longitudinal oscillations at the excitation frequency,its half frequency or its double frequency,but will not change the spatial structure of other frequencies.When an acoustic excitation with a frequency close to the flame frequency(or its double frequency clsoe to the flame frequency)is applied,the longitudinal oscillation of the flame will be enhanced,causing a strong flame response,and cause flame instability.