For the Supersonic Combustion Ramjet (Scramjet) to be realized as a viable hypersonic propulsion option, significant testing must be performed in test flows which duplicate those that would be experienced during anticipated atmospheric flight. The expense associated with real flight tests necessitates that ground-based testing play a significant role in the experimental development of the scramjet. Based on current, or even near-term technologies, the expansion tube concept offers superior physical simulation of hypersonic flow. This study investigates the potential performance capabilities of a large-scale, high-performance free-piston driven expansion tube based on the RHYFL shock tunnel project. The predictions are focused specifically on the ability of the RHYFL-X expansion tube to generate sub-orbital scramjet testing conditions and are obtained through one-dimensional and axisymmetric simulations of this proposed facility.
To validate the simulation techniques and approximations used to model the flow in the RHYFL-X expansion tube, simulations of a currently operating expansion tube, X2, are presented. These one-dimensional and axisymmetric simulations initially assumed equilibrium chemistry. The driver gas conditions after the two-stage compression process were obtained via a combination of numerical and experimental analyses. An accurate knowledge of the driver length at the point of primary diaphragm rupture was required for sound agreement with experimental results. The inertial effect of the secondary diaphragm was also examined for different X2 operating conditions. Results show that a hold-time imposed on the secondary diaphragm improved agreement with experimental data.
The effects of finite-rate chemistry on the air test gas of two standard operating conditions in the X2 facility were also investigated. A 5 species, 17 reaction model of air was used in inviscid one-dimensional simulations of a 6.8km/s condition and a 9.7km/s condition. By comparison with equilibrium simulations of the same conditions it was seen that finite-rate chemistry effects can be significant in conditions where test gas dissociation occurs prior to the unsteady expansion. For the higher enthalpy condition, the significant dissociation of the air test gas caused by the faster primary shock, combined with the more severe unsteady expansion process, resulted in a test flow highly influenced by nonequilibrium phenomena. The low primary shock speeds required to produce atmospheric static temperatures in the final test flow of the proposed RHYFL-X expansion tube was shown to result in negligible, if any, dissociation of the test gas prior to expansion. Even assuming the worst case scenario of a completely frozen expansion, these minimal dissociation levels prior to expansion indicate that the three RHYFL-X operating conditions investigated would have test flows essentially dissociation free.
Results from one-dimensional and axisymmetric simulations of the RHYFL-X expansion tube indicate that this proposed facility would be capable of generating true Mach number testing conditions over the intended scramjet flight trajectory, while inheriting the short test times and limited core flow diameters associated with expansion tubes. Simulated pressures well in excess of those that would be experienced during flight indicate that this facility would offer the unique capability of duplicating freestream conditions required for accurate aerodynamic, heating and combustion testing of integrated scramjet models. The use of a nozzle was also investigated for increasing the diameter of the core test flow. While seeing moderate increases in core flow diameter, the viscous axisymmetric simulations also displayed the desirable characteristic of increasing the duration of steady flow.