乙烯燃料超燃燃烧室流动特性与燃烧稳定性研究

INVESTIGATION OF FLOW CHARACTERISTICS AND FLAME STABILIZATION IN AN ETHYLENE-FUELED SCRAMJET COMBUSTOR

在低飞行马赫数条件下,乙烯燃料超燃冲压发动机为实现成功点火及稳定燃烧,常使用先锋氢引燃乙烯,本文通过试验研究了多种喷注方案下的超燃燃烧室流动特性、火焰传播特性及燃烧稳定性,喷注方案包括单先锋氢、单乙烯和组合喷注方式.超燃燃烧室入口马赫数为2.0,总温为953 K,总压为0.82 MPa.多种非接触光学测量手段被应用于超燃冲压发动机流场结构和火焰传播规律的诊断,包括纹影、CH自发光照相和OH-PLIF,并使用10 kHz的压力传感器来采集燃烧室上壁面中线处压力.结果表明:在无燃料喷注情况下,发动机内流场会以约450 Hz的主频振荡;在有燃料喷注情况下,凹腔上游喷注方式会抑制振荡,而凹腔台阶下游喷注方式对流场振荡影响较小.OH-PLIF图像结果表明:先锋火焰是不稳定的,当先锋氢在凹腔上游喷注时,先锋火焰主要集中于凹腔中后部,OH基在凹腔中部重复地集聚与扩散;当先锋氢在凹腔台阶下游喷注时,先锋火焰呈破碎状分布于剪切层内,且凹腔后斜坡处无燃烧.燃料组合喷注时,燃烧也是不稳定的.先锋氢关闭后,火焰从凹腔中部后移至凹腔后斜坡处,且火焰形态稳定,组合喷注时的燃烧不稳定现象源于先锋氢燃烧的不稳定性.

To achieve successful ignition and stable combustion in an ethylene-fueled scramjet,the hydrogen is applied as the pilot fuel to ignite ethylene at low flight Mach numbers.Flow characteristics,flame propagation characteristics and combustion stability have been investigated in a scramjet combustor via various strategies of fuel injections,such as fuel injection of single hydrogen,single ethylene and combinations of fuels.The inflow conditions are Mach number of 2.0,a total temperature of 953 K and a total pressure of 0.82 MPa at the entrance of scramjet combustor.Multiple non-contact optical measurements,including the schlieren,CH luminosity images and OH-PLIF,have been applied to detect flow structures and flame propagation along with the 10 k Hz pressure transducers monitoring the pressure of the centerline on the top wall of combustor.The results indicate that without fuel injection,the internal flow of scramjet would oscillate at a dominant frequency of approximately 450 Hz.With fuel injection,the oscillation is suppressed when the fuel is injected upstream of the cavity and there is no effect on the internal flow when the fuel is injected downstream of cavity step.OHPLIF images reveal that the flame of pilot hydrogen is unstable.The flame mainly locates in the middle and posterior of cavity when the pilot hydrogen is injected upstream of cavity and OH radicals repeatedly gather and disperse in the middle of cavity.The flame of hydrogen cracks in shear layer and there is no chemical reaction around cavity ramp,when the pilot hydrogen is injected downstream of cavity step.Meanwhile,the combustion is also unstable with combined injection strategy.When the pilot hydrogen is closed,the flame transfers from the middle and posterior of cavity to the cavity ramp,then stabilizes,indicating that the combustion instability of combined injection strategy derives from the combustion instability of pilot hydrogen.