声激励下低旋流火焰动态响应特性研究

Investigation on Dynamic Response of a Low-Swirl Flame to Acoustic Excitation

针对声激励下甲烷/空气预混低旋流火焰动态响应特性开展实验研究。声激励频率为55∼295 Hz,相对幅值为0.1∼0.3。采用光电倍增管记录火焰自发OH∗荧光信号,采用双麦克风法测量入口气流速度脉动,采用高速相机捕获火焰结构动态响应过程。实验结果表明,小幅值激励下火焰时均结构变化不明显,大幅值低频激励下火焰长度明显增加,推举高度明显减小。低旋流火焰描述函数存在两个波峰和一个局部最小点,65 Hz时火焰增益最大,105 Hz时为局部增益最小点。随着激励幅值的增大火焰出现非线性响应特性,不过激励幅值对火焰描述函数相位基本没有影响。声激励下火焰长度和推举高度发生周期性脉动,低频激励下火焰主要发生轴向跳动,高频激励下火焰主要发生径向脉动。

This paper presents an experimental investigation on dynamic response of a methane/air premixed low-swirl flame to acoustic excitation.The frequency of acoustic excitation ranges from 55 Hz to 295 Hz while the forced amplitude varies from 0.1 to 0.3 of the bulk velocity.A photomultiplier is used to measure flame global OH∗chemiluminescence,velocity fluctuation upstream the swirler is measured with a two-microphone method and a high-speed ICCD camera is employed to capture unsteady flame behavior.Experimental results show the variation of mean flame structure is not apparent under small amplitude excitation.Flame length increases and flame liftoff height decreases significantly at large amplitude excitation with low frequency.The gain of flame describing function of the low-swirl flame has two peak values with a maximum value at 65 Hz and a local minimum at 105 Hz.The low-swirl flame starts to saturate as the amplitude increases to a certain level,showing nonlinear behavior.However,the driving amplitude has little effect on the phase of flame describing function.Flame length and liftoff height fluctuate periodically under external acoustic excitation.The low-swirl flame oscillates mainly in the axial direction at low excitation frequency while it turns into radially dominated direction at high frequency.