钝体射流火焰及其点火过程的大涡模拟

LARGE-EDDY SIMULATION OF A BLUFF-BODY FLAME AND THE FORCED IGNITION PROCESS

钝体燃烧器广泛应用于航空发动机、燃气轮机、锅炉等设备的燃烧室中.对其点火过程的了解和控制直接关系到设备的安全运行和污染物排放等重要问题.我们采用基于火焰面/过程变量燃烧模型的大涡模拟方法对湍流非预混钝体射流火焰及其点火过程进行了详细的数值模拟.以Sydney钝体燃烧器的无反应射流和有反应甲烷/氢气(CH_4/H_2)火焰为研究对象,首先通过统计平均的数值结果与实验测量及文献数据的对比,全面检验了所用数值方法和燃烧模型;随后,详细展示了钝体燃烧器点火和火焰发展的瞬态过程;最后,对钝体射流的点火过程进行了细致的分析和表征.根据温度峰值、羟基(OH)质量分数和甲醛(CH_2O)质量分数峰值随时间的变化表征了强制点火过程的4个阶段:点火源衰减、点火触发、点火核生成和点火成功.其中,点火核驻留的空间位置位于钝体燃烧器冷态流场外侧涡的尾部回流区域附近.

Bluff-body burners are widely used in combustion chambers of industrial equipment such as jet engines,gas-turbines and boilers.Safe operation and low pollutant emission in these equipment directly demands understanding and controlling of ignition process in bluff-body burners.In this paper,a large-eddy simulation based on steady flamelet/progress variable approach was applied in a turbulent non-premixed bluff-body flame and its ignition process.Simulating two cases including non-reacting bluff-body flow(NRBB) and Sydney methane/ hydrogen flame(HM1E),we comprehensively compared statistical results with experimental measures and previous numerical investigation.Then,we presented an instantaneous ignition process and flame development in Sydney bluff-body burner.Finally,the ignition process was analysed and characterized in detail,and a four-step process of flame ignition was proposed:decaying of ignition source,ignition initiation,generating of ignition kernel,and ignition success,which was identified by variation of peak values of temperature,OH and CH_2O mass fraction.In addition,the ignition kernel stayed at the stern of the outer vortex in the recirculation zone of the cold flow of the bluff-body burner.