回流区稳定燃烧的近熄火特性理论分析

A THEORETICAL STUDY ON NEAR-BLOWOFF CHARACTERISTICS OF RECIRCULATION-ZONE STABILIZED COMBUSTION

基于火焰稳定在回流区剪切层中的假设,建立了预混火焰近贫燃熄火极限特性的理论分析模型.一旦火焰稳定在剪切层中,由于剪切层同时从两侧卷吸流体,所以进入火焰的流体将是自由流与回流区流体的混合物.如果是贫燃火焰,回流区流体由燃烧产物和多余的氧化剂组成而没有燃料,因此进入剪切层火焰的混合物当量比将低于自由流的当量比.如此一来,即使自由来流是可燃的,剪切层中混合物的当量比也可能超出可燃极限.分析表明,对于回流区稳定的贫燃火焰,进入剪切层火焰的混合物有效当量比将低于自由来流的当量比.根据理论分析建立了近贫燃熄火极限的回流区稳定燃烧模式图,其中涉及4个参数:自由来流当量比、剪切层卷吸比、吹熄极限和再点火极限.分析揭示出4种可能的燃烧模式:超稳定火焰、亚稳定火焰、振荡火焰和熄火.特别地,在参数空间发现了一个振荡区,它可能为近吹熄火焰引入一种新的固有不稳定性机制.在这种机制中,剪切层中的当量比振荡实际上是由燃烧过程本身驱动的,因为火焰的位置/振荡影响着进入回流区的流体成分进而影响剪切层中的当量比.因此,当量比振荡与燃烧过程之间的反馈循环由于周期性的火焰吹熄和闪回/再稳定而得以封闭.

A theoretical framework has been developed for recirculation-zone stabilized combustion near lean blowoff,based on the assumption that the flame is stabilized in the adjacent shear layer.Once the flame is stabilized in the shear layer,the fluid approaching the flame is actually a mixture of the free stream and the recirculating flow since the shear layer simultaneously entrains fluids from both sides.If the flame is lean,the recirculating flow would essentially consist of products and excess oxygen but no fuel,the mixture approaching the flame should thus be leaner than the free stream.Accordingly,even if the free stream is flammable,the equivalence ratio of the mixture in the shear layer may be outside of the flammability limits.Analyses show that,for a recirculation-zone stabilized lean flame,the effective equivalence ratio of the mixture approaching the flame stabilized in the shear layer is lower than that of the free stream due to the existence of the recirculation zone.A diagram of regimes for the recirculation-zone stabilized combustion near lean blowoff is then constructed according to the theoretical analyses,which involves four parameters:the freestream equivalence ratio,the shear-layer entrainment ratio,the blowoff limit and the reignition limit.In the diagram,four regimes have been identified:a super-stable flame,a sub-stable flame,an oscillating flame and blowoff.In particular,the identification of an oscillation region in the parameter space is impressive,which introduces an intrinsical instability mechanism that has not previously been noted for near-blowoff flames.In this mechanism,the equivalence ratio oscillations in the shear layer are indeed driven by the combustion process since the position/oscillation of the flame affects the fluid composition entering into the recirculation zone and thus the equivalence ratio in the shear layer.Therefore,the feedback loop between the equivalence ratio oscillations and the combustion process is closed accompanied with periodic blowoff and flashback/re-stabilization of the flame.