Experimental and numerical investigations of freejet and direct-connect dual-mode scramjet

To understand scramjet operations, comparisons between direct-connect and freejet dual-mode scramjet tests were investigated in this paper via experiment and numerical simulation. Being in accord with the condition of flight Mach number 6.0, the inlet flow conditions of the isolator were achieved with Mach number 3.0, total temperature 1500 K and total pressure 2.1 MPa. Based on these parameters, two tests with equivalence ratio 0.6-600 and 1.0-604 were carried out. In the experiment, pressure at the centerline of the combustor down wall was measured. Simulation including the Mach number distribution at the combustion chamber, the shock wave structures in the isolator, and heat release of the combustor were proceeded and compared. Experimental results show that the measured pressures match the numerically calculated outcomes. Simulation results show that when the combustion case is 1.0-604, the combustor operates in the ram-mode for the freejet test while it is in the scram-mode for the direct-connect test. When the case is 0.6-600, the combustor in both freejet and direct-connect tests operates in the scram-mode. In addition, the specific thrust in the freeject test presents a prominent difference to that in the direct-connect test when the engine operates in the same combustion mode. For case 1.0-604, the thrust difference is 4.38% and for case 0.6-600, the discrepancy is 8.19%. Besides, influence of the heat release on the thrust performance was analyzed for the two tests.