Ignition transient in an ethylene fueled scramjet engine with air throttling.

摘要

This research focuses on the modeling and simulation of ignition transient and subsequent combustion dynamics in an ethylene fueled supersonic combustion ramjet (scramjet) engine. The primary objectives are: 1) to establish an efficient and accurate numerical framework for the treatment of unsteady flow and flame dynamics in scramjet propulsion systems; and 2) to investigate the effects of transverse air throttling on flow development and fuel-air mixing, and to identify its positive influence on ignition and flameholding in a scramjet combustor; and 3) to construct a detailed study investigating ignition transient to identify underlying essential mechanisms by means of air throttling implementation technique.;A comprehensive numerical study of the combustion dynamics in a scramjet combustor is performed. The analysis treats the conservation equations in three dimensions and takes into account finite-rate chemical reactions and variable thermophysical properties for a multi-component reacting flow. Menter's k-o SST twoequation turbulence model is implemented, as it performs well for shear-layer flows and wall turbulence effects. The governing equations and the associated boundary conditions are solved using a density-based finite-volume approach and four-stage Runge-Kutta scheme to utilize explicit time marching. The code is parallelized using the domain decomposition technique and message passing interface (MPI). The theoretical formulation and numerical scheme is first validated with two test cases including turbulent flow over a flat plat and a two-dimensional oblique shock wave, and then validated with engine test data.;The analysis is first employed to a detailed investigation into the flow development and fuel-air mixing in the scramjet engine for non-reacting flow at Mach 5 flight condition. As the air throttling is implemented to increase the combustor pressure, a series of subsequent oblique shock waves following the fuel injectors is generated to separate the wall boundary layer, and lead to a dramatic increase in the fuel/air mixing. The detailed investigation reveals enhanced fuel-air mixing primarily results from elevated vorticity over combustor and cavity, as well as from increased residence time.;Effort is then expended to study the ignition and subsequent reacting flow in the modeled combustor. The ignition transient and flame development are comprehensively studied to investigate the influence of air throttling implementation on ignition and flameholding. The time history of combustion indicates that the engine model can hardly offer the ignition under the given flight condition in the absence of air throttling, as the ignition of ethylene fuel flow on the cowl surface fails to be initiated. Calculation is then employed to demonstrate the significant flow accommodation induced by air throttling implementation, including subsequent decrease in flow velocity and increases in temperature and pressure in the combustor. Autoignition occurs on the cowl surface due to extended residence time and higher initiate temperature, and results in an intense combustion zone with rapid flame spreading in combustor. Results indicate that the pre-combustion shock train is generated as a result of the combustion-induced pressure rise begins just upstream of the combustor entrance. The pre-combustion shock train at this condition forms a large region of low-momentum/separated flow near the combustor sidewalls. This proved to be an additional source for flameholding with the recessed cavity as primary flameholding support. The predicted combustor performance and flow distributions agree well with experimental measurements.