多孔陶瓷板预混火焰热声振荡多模态转换实验研究

Multi-Modal Transition of Thermoacoustic Oscillations in Premixed Flame over Porous Ceramic Plates

针对多孔陶瓷板甲烷-空气预混火焰,搭建了两端开口的Rijke管实验台,研究了当量比对多模态转换的影响,分析了不同自激振荡模态下火焰的不稳定特征.实验结果表明,当量比的变化会引起自激振荡模态转换.当燃烧功率固定为0.96 kW,增加空气流量,当量比从1.17降低到0.71时,管内压力振荡主频由163 Hz迁移至355 Hz.利用格林函数法计算Rijke管的声学特征频率,表明实验测试的两组自激振荡频率分别为Rijke管的一阶和二阶声学特征频率.不同当量比下火焰平均热释放率图像表明,高当量比火焰更易与系统低阶振荡频率耦合,这是因为此时火焰长度较高,且预混气体流速较慢,对应对流延迟时间较长.使用相平均和本征正交分解方法得到了不同振荡频率下的振荡模态,低频振荡以轴向振荡为主,高频振荡以频闪振荡为主.

In this paper,a Rijke tube experimental rig with openings at both ends was constructed to study the methane-air premixed flame over porous ceramic plates.The effect of equivalence ratio on multi-modal transition was investigated,and the instability characteristics of the flame in different self-excited oscillatory modes were analyzed.It was found that the change of equivalence ratio induces the transition of self-excited oscillation modes.When the thermal power was fixed at 0.96 kW,the air flow rate was increased,and the equivalence ratio decreased from 1.17 to 0.71,the dominant frequency of pressure oscillations in the tube hopped from 163 Hz to 355 Hz.The acoustic eigenfrequency of the Rijke pipe was determined using the Green’s function method.The dominant frequencies of the self-excited oscillation in the experimental tests corresponded to the first and second-order acoustic eigenfrequencies of the Rjike tube.The images of the average heat release rates of the flames at different equivalence ratios show that the flames at higher equivalence ratio are more likely to be coupled to the lower order oscillation frequencies of the system due to the greater flame length and the smaller premixed gas flow rate,which means a longer convective delay time.The flame oscillatory modes at different oscillation frequencies were obtained using phase-averaged and proper orthogonal decomposition methods.It was found that low-frequency flame oscillations are dominated by the longitude pulsation mode,while the high-frequency flame oscillations are dominated by the blinking mode.