Combustion Organization Of Liquid Hydrocarbon Fueled Scramjet Based On Strut Injection Technology

The flow velocity in the combustor of scramjet is increasing with the increase of flight Mach number and flight altitude, and the residence time of fuel and air is decreasing. It brings challenge to the combustion organization and thrust output of scramjet engines. To meet the power requirement of hypersonic vehicle, mixing and combustion of fuel and air must be very quick in the very short residence time, and strut injection that injecting fuel directly into the corestream is a well scheme to enhance fuel spreading and mixing. The key issue is the optimization of the components in the combustor such as strut injector, flameholder, and then is the reasonable assemblage in order to obtain stable and effective combustion. Aiming at the combustion performance of scramjet combustor, investigations are carried out as follows:CFD model for the flowfield around the swept strut is established, and the effects of strut structural factors on the drag, losses and thermal environment are investigated. The results show that the drag and total pressure losses is related to the shock strength before the strut. The drag and losses are increasing with the increase of the leading wedge angle. The influence is little when the swept angle is small, but both the drag and losses decrease rapidly when the swept angle is larger than60degree to make the flow Mach number component perpendicular to the strut leading edge close to or lower than1. The thermal environment analysis of strut shows that the aerodynamic thermal load is highest at the leading surface, and it is related to the part of the flow stagnated by the strut. The thermal load is decreasing more than three-quarters when the swept angle is increased from0to75degrees, but it is always high at the junction of the top position of the strut leading surface and adjacent combustor wall. Bluntness is found beneficial to reducing thermal load on the strut leading surface, but the influence is lower when the bluntness radius is larger than0.5mm. Thus the characteristics of the optimal structure for a strut with low drag and thermal load are as follows: large back swept angle of70degree, with tip gap, small leading wedge angle under15degree, small bluntness radius of0.5mm.Air utilizing range of the strut injected fuel with effective combustion is investigated by liquid kerosene combustion experiments holding flame with hot gas transverse injection, and numerical simulation of the evaporation and mixing in the combustor. Mixing rate and local equivalence ratio distribution on the cross-sections is found strongly influential to the flame spreading to the upstream zone and the combustion performance in the main combustion zone. More struts injection (less fuel injected from each strut) or strut installation closer to the isolator entrance leads quicker mixing, easier flame spreading to the upstream zone, and then impel the pseudo-shock wave to the entrance and weaken the performance in the main combustion zone. It is easier to blow off with low fuel equivalence ratio. With different fuel flow rate, it is beneficial to obtaining effective combustion when the fuel allocation between the struts makes the local equivalence ratio in the main combustion zone near the downstream flameholder is about1. To improve combustion, the ratio of the air utilizing width of the parallel injected fuel from each strut to the mixing distance to downstream main combustion zone is recommended0.25~0.3.The more effective flameholder, strut fuel injection is applied to promote the combustion of liquid kerosene in the combustor holding flame by cavity. The strut-cavity combustion organization is found helpful to enhance combustion and increase the specific impulse of the combustor. The specific impulse in the conditions of strut injection is15%higher than the value in the wall injection conditions with the same fuel flow rate. With low injection pressure, the strut injection gives better mixing and combustion performance. Transverse injection is found better than parallel injection for strut injectors to obtain stable and effective combustion. When the strut is mounted at a farther location upstream of the cavity, the long distance between the strut and the cavity is beneficial to mixing, but it is easier to make the flame spread upstream to the isolator and push the pseudo-shock out of the isolator.A new fuel injection and flameholding integrated combustion organization approach by the combination of small struts and shallow cavity is presented to avoid the problem of excessively high thermal load at the wall downstream of the cavity. A separate path for s small portion of the incoming airstream is formed between two neighboring small struts with shallow cavities on their opposite surfaces. Subsonic combustion is organized in the path to establish a pilot flame in the corestream, and then to ignite the main fuel of the combustor. The configuration of the struts and the fuel injection schemes in the separate path is determined in the overall design, and CFD method is employed to help deciding the size and obtaining the mixing performance. Pilot flame is successfully formed in both the tests in the open and confined incoming airflow. The integrated combustion organization approach by the flameholding strut with internal cavities is demonstrated feasible and well combustion performance and wide range of stable combstion is obtained.