非预混旋流火焰中部熄火及重新稳燃机理研究

Study on the Local Extinction and Re-stabilization Mechanism of a Non-premixed Swirl Flame

本文利用5kHz同步PIV/OH-PLIF实验装置,结合大涡模拟(LES)耦合PDF燃烧模型,系统研究燃料中心喷射下非预混旋流火焰中部熄火及再次稳燃机理,对比了两种旋流器出口结构的影响。结果表明,LES-PDF模型可以准确地捕捉到旋流流场分布及火焰形态,包括中部熄火及火焰重新稳定。相比于直燃道的结构,扩张燃道的存在引导了更宽的回流区,改善了中部熄火现象,两种结构对其下游的主火焰再次稳燃高度影响不大。中央燃气射流带来的高标量耗散率使得热损失变大,从而引起中部熄火。同时进一步促进了CH_(4)与空气的预先混合及部分反应前置物(CH_(2)O),为非预混旋流火焰在下游重新稳燃提供了有利条件。通过对火焰再次稳燃处OH反应项及扩散项的分析,发现反应项占据主导地位,部分预混火焰传播为再次稳燃的主要机理。

The effects of quarl on the non-premixed swirling flames were studied by performing simultaneous OH-PLIF/PIV measurements experimentally and coupling LES with PDF combustion model numerically.The results showed that LES-PDF method can successfully capture the swirl flow fields and flame structures even including local extinction and re-stabilization.Compared to the no quarl case,the quarl plays a significant role on the expansion of the recirculation zone thus influencing the attached flame and the following extinction,,though it affects the re-stabilization height rarely.With the high scalar dissipation rate,local extinction occurs due to the strong heat loss caused by the central fuel jet.In addition,fuel and air are partially premixed accompanied with some partial oxidation reactions producing CH_(2)O,which provides preferential conditions for the stabilization of the non-premixed swirling flame.Moreover,the reaction rate term of OH is at least 10 times larger than its diffusion term,indicating that the reaction front dominates the re-ignition process for the flame re-stabilization downstream.It is thus concluded that the partially premixed flame propagation plays a dominant role in controlling the re-stabilization of this flame.