液体中心式气液同轴离心式喷嘴火焰振荡特性

Flame Oscillation Characteristics for Liquid-Centered Swirl Coaxial Injectors

关键词:  火焰振荡  自激振荡  缩进  流动模态  液体中心式气液同轴离心式喷嘴

Key words:  Flame oscillation  Self-pulsation  Recess  Flow pattern  Liquid-centered swirl coaxial injector

摘要:

气液同轴离心式喷嘴在特定的结构和工况下极易发生自激振荡，为了探究自激振荡对燃烧过程的影响，针对液体中心式气液同轴离心式喷嘴，开展了氧气和酒精的可视化燃烧试验研究。基于非接触光学观测方法同步获得了喷雾与火焰的动态结构，研究了缩进长度及喷注工况对火焰的动态特性、自激振荡特性以及燃烧效率的影响。研究发现，随着喷嘴缩进长度的增加，火焰从稳态转变为自激振荡状态。稳态燃烧时，火焰具有明显的锥形分布特征，火焰主要分布于锥形液膜表面、喷嘴出口回流区以及喷雾与燃烧室壁面的撞击区域。对于振荡火焰，当缩进长度较小时，火焰附着于喷注面板上且主要发生径向振荡；而当缩进长度增大到一定程度后，火焰周期性地附着并远离喷注面板且由纵向振荡主导。火焰振荡模式的转变是由自激振荡喷雾结构的变化引起的。基于已建立的理论分析模型，深入分析了火焰自激振荡与缩进室内部流动模态的关系。火焰振荡与喷雾自激振荡强弱同步，且当缩进室内部流动处于临界流动状态时最强。此外，研究发现，稳态燃烧时的燃烧效率大于振荡燃烧状态下的燃烧效率，喷嘴缩进可适当提高燃烧效率。

Abstract:

Gas-liquid swirl coaxial injectors are known to experience self-pulsation easily under certain operating conditions with some special geometrical parameters. Visual combustion experiments in a combustor burning gaseous oxygen （GOX） and liquid ethanol （LET） through a liquid-centered swirl coaxial （LCSC） injector were conducted to explore the effects of self-pulsation on combustion process. High-speed camera captured side-on images of spray and flame simultaneously through the non-contact optical observation method. The effects of both recess length and injection condition on flame dynamics， self-pulsation characteristics and combustion efficiency were analyzed and discussed. With the increase of recess length， flame transforms from stable behavior to self-pulsated behavior with periodic intensity variations in flame emission. Stable flame shows an obvious conical shape， and mainly distributes at the surface of the conical spray， the recirculation zone at the backward facing step of the injection faceplate and the impacting region of spray and combustor wall surface. For the injector with smaller recess length， the self-pulsated flame distributes azimuthally with almost an axisymmetric pattern. Furthermore， flame attaches to the injection faceplate and obviously oscillates radially. However， flame transforms to longitudinal oscillation and detaches from the faceplate when the recess length is large enough. The transition of flame oscillation modes is deemed to be caused by the variation of self-pulsated spray structures. Based on the existed theoretical analysis model， the relationship between flame self-pulsation and flow patterns in recess chamber is analyzed. Self-pulsation of flame and spray is found to be the strongest simultaneously when flow in recess chamber is around the critical mixing flow. Moreover， the efficiency of stable combustion is larger than that of self-pulsated combustion. Recess can improve the combustion efficiency for the LCSC injectors.