超燃冲压发动机自点火条件下波系演化规律试验研究

Experimental Study on Evolution Law of Wave System in a Scramjet Combustor under Self-Ignition Condition

关键词:  超燃冲压发动机  氢燃料  自点火  流动特性  振荡

Key words:  Scramjet  Hydrogen fuel  Self-ignition  Flow characteristics  Oscillation

摘要:

为研究超燃冲压发动机起动点火过程中流场结构变化和火焰传播规律，在发动机入口马赫数2.5，氢气当量比约为0.31的条件下，通过纹影和PLIF（Planar Laser-Induced Fluorescence）两种非接触式光学测量手段，同步研究了流场结构中的瞬时波系演化过程和火焰分布规律。结果表明：无反应的冷流阶段流场结构呈周期性振荡，是本文研究条件下凹腔构型燃烧室的固有特性，实验测定振荡频率为143Hz。氢气喷注进入燃烧室后，流场中产生一道斜激波，经下壁面反射后进入凹腔流场内，该反射激波对冷流流场结构影响较小，不会改变流场结构和振荡频率。但会加强剪切层和低速回流区相互作用下形成的周期性出现的λ型激波，而氢燃料自点火非常依赖这道λ型激波。当氢燃料喷注进入燃烧室一段时间后，凹腔中后部氢燃料的局部当量比达到临界值时，伴随λ型激波的形成，其波后的高温高压将立即触发氢燃料自点火。初步分析表明，流场中的λ型激波可以用于准确预测自点火出现的时刻。

Abstract:

In order to investigate the evolution law of wave system and flame development in a scramjet combustor under the self-ignition condition， with the inflow condition of Mach number 2.5 and the equivalence ratio of hydrogen being about 0.31， by means of non-intrusion optical measurement methods such as schlieren and Planar laser-induced Fluorescence （PLIF）， the evolution process of wave system and flame distribution were studied simultaneously. The results show that the flow field structure in the non-reacting flow stage shows periodic oscillation， which are the inherent characteristics of the cavity-combustor under the studied condition in this paper， and the oscillation frequency measured in the experiment is 143Hz. After the hydrogen is injected into the scramjet combustor， an oblique shock wave generates in the flow field， which is reflected by the bottom wall and then enters into the cavity flow field. This reflected shock wave has little influence on the non-reacting flow structure and the oscillation frequency. But the periodic λ-type shock wave generated by the interaction between the shear layer and the low-velocity recirculation region is strengthened， which has a great effect on the hydrogen self-ignition. When hydrogen is injected into the scramjet combustor for a period of time， and the local equivalence ratio of hydrogen in the back of the cavity reaches the critical value， the periodic λ-type shock wave generates. Then， the self-ignition of hydrogen immediately occurs in the high temperature and pressure region behind the shock wave. Preliminary analysis shows that λ-type shock wave can be used to predict the time of self-ignition accurately in the flow field.