Extending airbreathing flight to hypersonic speeds has faced numerous technical challenges. As the combustor’s slow speed gets to supersonic, the flow residence time in the combustor decreases; and as a consequence, the problem of maintaining mixing and reaction rates high enough to allow for complete combustion before the flow exits the combustors emanates. The Reynolds number of the engine fluid flow increases with the Mach number leading to thick turbulent boundary layers that interact with shock waves. They also respond to heat release generated through combustion.
A thorough understanding of the complex reacting flow in ramjet and scramjet combustors is needed in order to successfully design and operate airbreathing hypersonic propulsion systems. However, many challenges are realized when trying to achieve this knowledge through experimentation, for instance, optical access limitations, test-time limitation, and inability of ground-based experiments to adequately match flight conditions.
Jesse Fulton at Sandia National Laboratories and Jack Edwards from North Carolina State University in collaboration with Andrew Cutler from George Washington University) and Jim McDaniel and Christopher Goyne both from University of Virginia investigated turbulence/ chemistry interactions in a ramp-stabilized supersonic hydrogen–air diffusion flame. They described some computational studies performed for the configuration experiments at scram-mode conditions, with an objective to evaluate aspects of the computational model through quantitative comparisons with some of the available experimental data. Their work is now published in peer-reviewed journal, Combustion and flame.
The University of Virginia’s Supersonic Combustion Facility, which is a small-scale direct-connect dual-mode experimental scramjet combustor apparatus whose operation is designed to simulate flight conditions, was adopted for this study. It is vertically mounted consisting of an inlet nozzle, constant-area isolator, combustor, and extender, from which the exhaust gases are vented.
Hydrogen is used as the fuel for the Supersonic combustion facility. This fuel is introduced into the airflow via a single, wall mounted, un-swept, raised compression injector. The injection nozzle is a converging-diverging conical design that accelerates the fuel towards the exit. At lower equivalence ratios, the facility operates in scramjet mode; as the equivalence ratio is increased, it transitions to dual mode operation.
The configuration C simulations which follow were carried out at an equivalence ratio at which the facility was experimentally observed to be operating in scram mode characterized by the absence of a shock train in the isolator. Each calculation was evolved for nearly seven combustor transit times, 2 of which were used to expel solution transients and 5 were used for gathering statistics. A combustor transit time is described as that needed for a fluid moving at the combustor inflow velocity to travel from the leading edge of the fuel injector to the exit of the combustor.
Large-eddy Reynolds averaged simulations of supersonic configuration C isolator/combustion rig have been described in this paper. The equivalence ratio was found to be 0.17, a value small enough to ensure near complete combustion but with heat release insufficient to force a shock train into the isolator.
Jesse A. Fulton, Jack R. Edwards, Andrew Cutler, Jim McDanield, Christopher Goyne. Turbulence/chemistry interactions in a ramp-stabilized supersonic hydrogen–air diffusion flame. Combustion and flame, volume 174(2016), pages 152–165.Go To Combustion and flame