Effect of crossflow impinging type throttling mechanism on combustion characteristics of a strut-based scramjet combustor

Abstract

Efficient mixing of fuel-air, improved combustion performance, and flame stabilization are the critical events for the successful operation of scramjets that must be realized within the short residence time. One of the significant concerns is flame blow-off during the start-up stage even with the presence of the flame holders. To counter it, an innovative mechanism of air-throttle injection has been proposed and investigated through computational fluid dynamics in the present study. In the proposed mechanisms, a strut is used as a base flame holder, and a provision is made to inject the air in the transverse direction behind the strut's base. Two- and three-dimensional Reynolds-averaged Navier-Stokes (RANS) equations have been solved using the k-ω SST turbulence model for reacting and non-reacting flows. A finite-rate/eddy-dissipation combustion model is selected to include the effects of turbulence-chemistry interaction. It was observed from the simulations that the impingement of hydrogen jet and the transverse throttled jet generates the vortices and recirculation zones. Improved fluid-dynamics associated with the impinging action of the air-throttle has been studied by considering argon as a surrogate for air; also, the enriched oxygen (due to air throttling) around the hydrogen jet makes fuel-air mixture more flammable. These fluid dynamics and chemistry aspects are responsible for the superior performance of air-throttling mechanisms. The proposed mechanism showed 46 % reduction in combustor length to reach the combustion efficiency of more than 95 %. The maximum improvement of mixing efficiency is 48 % observed at downstream of the throttle base over the non-throttled condition. Further the effectiveness of the proposed throttling mechanism is demonstrated to show a promise for holding the flame even at a higher Mach number, i.e., M=4.5. The ignition is achieved at a shorter distance in the air-throttling case, resulting in 28 % reduction in ignition length. The air throttling within the two-shear layer may reduce the ignition delay associated, and the flame can be held immediately downstream of the injector. Additionally, a three-dimensional flow field was also analysed to study the flow characteristics around the transverse jet. In the 3-D flow field, the circulation profiles indicated that the stronger vorticity was generated during the air-throttling than without air-throttling.