基于旋流火焰根部熄火现象的FGM模型研究

Prototype Swirl Flame Using Strained Flamelet Generated Manifold

针对原尺寸的复杂几何预混旋流燃烧器开展了同步OH/CH2O-PLIF测量,并利用发展的带拉伸查表法(SFGM)耦合大涡模拟(LES)的方法模拟了相同雷诺数下(Re=10000)稳定火焰及临近吹熄下的旋流燃烧特性.发展的SFGM模型在化学反应源项中引入与拉伸率直接相关的着火因子,如此拉伸率则无需与进展变量耦合建表,极大地缩小了建表工作量.随后将模拟结果与实验结果对比,验证了LES耦合SFGM模型可以较为准确地捕捉复杂几何旋流预混燃烧的火焰形态(OH及CH2O×OH分布):如预测到预混管出口处的局部熄火位置,临近吹熄的火焰相比于稳燃火焰有更高的抬升高度.在强旋流下,湍流火焰的传播速度和湍流速度的脉动值成正相关,火焰前锋面在上游侧的边缘位置很大程度上由流场向下游扩张的轴向速度和中央回流区(CRZ)中的主体火焰向上游传播的速度共同决定.随着当量比降低到熄火极限,CRZ内的混合物被不断稀释,火焰逐渐破碎成孤立的火核;与此同时,燃烧温度下降导致火焰传播速度降低,而流场速度不变,火焰前锋面被吹向下游远离CRZ并最终被吹熄.

A series of experimental measurements on a prototype premixed swirl burner with complex geometry were carried out using CH2O/OH-PLIF.FGM method(SFGM)taking account of strain rate was developed to study two modes of premixed swirl flame,one stabilized and the other close to blown-off flame under the same Reynolds number(Re=10000).The SFGM model in this work is related to a reaction factor,which controls the source reaction rate directly without extra dimension extension of strain rate,and thus this method could save considerable calculation time.By comparing the experimental and numerical results(OH and CH2O×OH distributions),it is clearly found that SFGM coupled with LES can capture much better results than the FGM model without considering strain rate.Specifically,the SFGM model can predict the flame structure correctly,e.g.no attached flame survives near the premixed tube exit compared with the FGM model in which the flame still survives,and the close to blow-off flame has a greater lift-off height than the stabilized flame.In addition,turbulent flame speed is proportional to turbulent fluctuation speed.The edge of main flame front is revealed to be determined jointly by the flow velocity expanding downstream and the main turbulent flame(in the CRZ)propagation velocity propagating upstream.When the equivalence ratio is decreased to the flammability limit,the reactants in the CRZ are diluted by the leaner mixtures,leading to the breaking of contact flame into smaller flame kernels.Moreover,the turbulent flame speed declines with the decreasing flame temperature while the flow speed is kept fixed,resulting in the flame front propagating downstream away from CRZ and finally blowing off.