Theoretical investigation of air vitiation effects on hydrogen fuelled scramjet performance

Abstract

The renewed interest in the next generation hypersonic vehicles for commercial and military applications requires combined efforts between multidisciplinary teams involved in computational and experimental work. A common feature of experimental tests, at least for studies on air breathing propulsion systems, is the relatively short test time, limited by the test flow conditions high dynamic pressure and total enthalpy, i.e. 10 MPa at Mach 8 flight conditions. In general, it can be stated that test times decrease as the flow Mach number increases, with changes of several orders of magnitude: a few tens of microseconds for shock tube based tunnels (Mach numbers above 10–15) to tens of seconds or even minutes for the high supersonic range (Mach 4–7.5). In many cases the test flow chemical composition differs from the standard air composition because of pollutants production by the different techniques adopted to increase the flow stagnation enthalpy (combustion products, ionized species, dust, …). The real flow conditions are thus not perfectly duplicated, but only partially simulated in terms of a few main parameters, such as velocity, pressure, and temperature. Few studies have been performed until now, therefore a deep investigation of the air vitiation effects on combustion is mandatory to extrapolate ground to flight correlations. Numerical simulations accounting for the effects of vitiation are also scarce and a characterization of the chemical and turbulence models is still lacking. In this context, CFD simulations of the LAPCAT II MR2 combustor configuration have been performed to provide useful information in terms of efficiency at various vitiation percentages. Theoretical laws and remedies have been proposed for the ground to flight data extrapolation.