Ignition And Flame Stabilization In A Flush Wall Supersonic Combustor

Scramjet is the best propulsion device for the hypersonic vehicle within the atmosphere and the overall performance of its key component combustor is very important. A flush wall combustor is usually take the strut as the flame holder, the combustion start in the center of the combustor, avoid direct contact with the combustion zone, heat load of the wall is reduced. Meanwhile the flush wall is favorable to the cooling channel design. Due to the enhancement of the strut fuel injection, the burning length can be shortened which means a relative shorter aircraft length. All these advantages help to improve the scramjet performance. But most research related to the flush wall supersonic combustor take the hydrogen as the fuel, if using kerosene, the relative long reacting time will greatly increase the difficulty to the combustion organization. This paper has tried to solve this problem and the main contents are as follows:First, a Micro Hot Gas Generator (MHGG) is designed and mounted on the combustor wall and combustion is to be realized by this permanent flame generated by this MHGG. During the ignition process, air throttling is introduced for ignition auxiliary. By changing the throttling air pressure and acting time, different throttling intensity can be controlled and led to four ignition modes. Too intense of the throttling will cause a un-start phenomenon while too weak of the throttling cannot get stable ignition. The law of the ignition mode distribution under different ER and the stable ignition boundary are gotten via analyzing a large number experiment data. This research has proved the feasibility of stable combustion in a flush wall supersonic combustor by the permanent flame.Further research has combined the MHGG with the fuel injection strut forming a strut-jet structure, based on this structure, permanent flame is moved the strut back. Two kinds of jet flow are tested: on is high temperature burned gas and the other is pure oxygen. It is discovered that the pure-oxygen plume is more suitable for the global ignition in the flush wall combustor and an oxygen pilot combustion strategy is then developed. Based on this, an oxygen-pilot strut is proposed. With the help of oxygen injection, low speed zone at the strut back is enlarged and the reaction speed is increased, then stable local flame is realized at the back of the strut in the high speed cold air flow. Based on this oxygen pilot strut, local flame at the strut back is not mean global combustion in the flush wall supersonic combustor; global combustion cannot be triggered until the oxygen mass flow rate reaches a certain level. The paper has investigated the organization characteristics, based on this oxygen pilot strut, stable combustion can be realized in a wide range of ER in the flush wall supersonic combustor, even at a low ER=0.19. Mean while, the ignition process is much steadier compared with the air throttling way. With the increase of ER, thermal choking will occur which means subsonic combustion zone in the combustor. Base on the proportion of the subsonic zone combustion modes can be divided to three kinds: supersonic combustion mode, dual combustion mode and subsonic combustion mode. It is discovered that, under different combustion mode, the flame transition process is totally different. When thermal choking occurs in the combustor, the global combustion is developed from back to front, this counter flow flame transition phenomenon does not happen in the supersonic mode.When global combustion is founded, the oxygen mass flow rate need to sustain the combustion is much lower than needed during the ignition process; based on this discovery, the optimization program of oxygen supply during the combustion process is given: large oxygen flow rate during the ignition stage and low oxygen flow rate during the stable combustion stage. This can reduce the load of the aircraft during the real flight process. The paper also investigated the influence of the fuel injection concentration to the combustion process. It is discovered the main area affected by the fuel injection concentration is in the strut nearby area, not so obvious at the rear part of the combustor.Finally a strut/wall combined fuel injection strategy is proposed. Experiment results show that, combustion process in the flush wall supersonic combustor will be affected if the fuel from the strut is moved to the wall. However, by optimizing the fuel proportion between the strut and the wall, the wall temperature can be noticeable reduced without affecting the combustion process seriously. Meanwhile, this strut/wall combined fuel injection strategy can also meet the multi-points fuel injection requirement at the other flight Mach number. To guaranty the combustion performance, the wall injection should pay attention to the injection location and direction.