液滴群在两段自着火条件下的内群燃烧及向外群燃烧的转变数值模拟分析

Internal Group Combustion and Transition to External Group Combustion of Droplet Clouds Under Two-Stage Autoignition Conditions

利用数值模拟研究了正庚烷和正十二烷液滴群在两段自着火条件下的内群燃烧及向外群燃烧转变的机理.首先分别分析了冷火焰和热火焰内群燃烧的火焰结构.进一步比较发现,虽然正庚烷与正十二烷的挥发性不同,但液滴群参数相同时其燃烧阶段一致,通过对时间尺度和蒸发进程的分析发现,该现象的原因为液滴群内燃烧是蒸发控制的物理过程,挥发性不同仅影响不同过程发生的时间.最后,对液滴群内燃烧向外燃烧过渡的算例展开分析,发现液滴群内燃烧火焰穿透整个液滴群后,液滴群内部氧气减少,形成了富燃的区域,而液滴群外部形成了传统的扩散火焰.

The mechanism of internal group combustion and transition to external group combustion of n-heptane and n-dodecane droplet clouds under two-stage autoignition conditions was studied using numerical simulation.Firstly,the flame structure of cool flame and hot flame was analyzed separately.Further comparison found that although the volatility of n-heptane and n-dodecane is different,when the parameters of droplet cloud are the same,their combustion regime is consistent.By analyzing the time scales and vaporization processes,it was found that the cause for this phenomenon is that the combustion within the droplet cloud is a physical process controlled by vaporization,and the difference in volatility only affects the time at which different processes occur.Finally,the transition of internal group combustion to external group combustion in droplet clouds was analyzed through a case study.It was found that after the internal flame penetrates the entire droplet cloud,the oxygen inside the droplet cloud decreases,forming a fuel-rich region,while a conventional diffusion flame is formed outside the cloud.