Analysis of heat release characteristics of methane/air lean premixed bluff-body flames

甲烷/空气贫燃预混钝体火焰热释放特性分析

National Key Laboratory of Solid Rocket Propulsion, School of Astronautics, Northwestern Polytechnical University, Xi'an 710072, China

Received: 3 July 2025

Abstract

To investigate the heat release characteristics of methane/air lean premixed bluff-body flames, this study employs experimental methods based on CH^* chemiluminescence to examine the effects of equivalence ratio, inlet flow rate, and acoustic excitation with varying frequencies and amplitudes on flame oscillation behavior and heat release dynamics. Experimental results reveal four typical flame oscillation modes influenced by the equivalence ratio: local extinction oscillation, asymmetric structure oscillation, small-scale symmetric structure oscillation, and stable combustion. The heat release intensity increases approximately linearly with the equivalence ratio. For BVK (von Kármán vortex shedding) oscillating flames, the fluctuation in heat release rate initially increases and then decreases along the axial direction, peaking around x/d=5.5. In contrast, for small-scale symmetric oscillating flames, the heat release rate fluctuation shows a nearly linear increase with x/d. The influence of acoustic excitation on the flame's heat release behavior was also investigated. It was found that with increasing excitation amplitude, BVK-type oscillations gradually transition into large-scale symmetric structure oscillations. Overall, the amplitude of heat release rate fluctuations increases with the acoustic excitation amplitude; a similar trend is observed with respect to frequency. Under an excitation of 35 Hz and 1.0 V, the fluctuation amplitude of the heat release rate approaches 0.6.

摘要

为了研究甲烷/空气贫燃预混钝体火焰的热释放特性, 基于实验手段利用CH^*自由基化学发光方法研究了当量比、来流流量、频率及振幅声激励对火焰振荡特性和热释放特性的影响。实验结果表明火焰在当量比影响下存在4种典型模态: 局部熄灭振荡、非对称结构振荡、小尺度对称结构振荡和稳定燃烧。火焰的释热强度近似与当量比呈线性关系。其中, BVK振荡火焰的热释放速率波动随着轴向距离的增加呈现先增大后减小的趋势, 在x/d=5.5位置处达到最大值。而小尺度对称振荡火焰的热释放速率波动与x/d基本呈线性关系。研究了声激励对火焰热释放特性的影响, 发现BVK振荡火焰随着声激励振幅的增大而逐渐转变为大尺度对称结构振荡。火焰的热释放速率波动值总体上随着声激励振幅的增加而升高, 频率的影响具有相似的结果。在频率35 Hz、振幅1.0 V的声激励下, 火焰热释放速率的波动值接近0.6。