Numerical Simulation of Scramjet Combustion in a Shock Tunnel

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Title: Numerical Simulation of Scramjet Combustion in a Shock Tunnel
Author: Star, Jason Blue
Advisors: Jack R. Edwards Jr., Committee Chair
Hassan A. Hassan, Committee Member
D. S. McRae, Committee Member
William L. Roberts, Committee Member
Abstract: Three-dimensional computational simulations of reactive flowfields within a hydrogen-fueled scramjet-like geometry experimentally tested in a free piston shock tunnel are presented. The experimental configuration (Odam and Paull, AIAA Paper 2003-5244) involves injection of hydrogen fuel into the scramjet inlet, followed by mixing, shock-induced ignition, and combustion. The predictions for both fuel-off and fuel-on conditions were observed to be sensitive to the choice of the wall temperature boundary conditions. The best comparison with experimental data were achieved through the implementation of an approach that involves a simplified conjugate heat transfer model that couples the heat conduction through the wall with the heat conduction of the fluid within the boundary layer. This approach is able to predict thermal loads on the walls of the scramjet model due to shock wave interactions and due to heat release. As such, it is able to more accurately represent the physical temperature response of the engine model. Also shown to produce very good agreement with the statistically-steady experimental data was the isothermal ghost-cell boundary condition, which is based on a simplification of the time-dependent conjugate heat transfer boundary condition. This simplified boundary condition assumes a linear temperature distribution within the wall based on the effective depth that an applied heat load would penetrate, thus, it also allows the actual wall temperature to vary in response to the applied heat load. Results for fuel-off simulations showed that the solution generated by a steady-state simulation implementing the isothermal ghost-cell wall boundary condition was very comparable with the statistically-steady solution obtained from a fully transient simulation with coupled heat conduction within the walls. When integrated in a fully time-accurate manner, the fuel-on simulations showed a striking sensitivity to the modeled rate of air ingestion into the engine. For experimental data that showed steady combustion, the transient simulations resulted in either a steady combusting solution or a progression toward engine unstart, depending on the modeled rate of air ingestion. Also, for experimental data that showed an unsteady thermal choking event leading to eventual unstart, the transient simulations were able to predict both unstart and steady combustion, once again depending on the air ingestion rate. In all cases, the modeled air ingestion process is an approximation of the actual experimental process, in that uniform conditions are imposed as linear functions of time over the inlet plane. The computational results also provide some support for a 'radical-farming' hypothesis, proposed to explain the ability of the hydrogen-air mixture to auto-ignite at relatively low inlet contraction ratios.
Date: 2005-12-09
Degree: PhD
Discipline: Aerospace Engineering

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