The level of viscous drag present at hypersonic speeds is one of the major challenges to the successful development of scramjet engines. A large component of the total viscous drag can be attributed to supersonic combustors, due to the increased density of the flow in that region. Tangential injection of fuel is known to reduce viscous drag, however, it does not produce efficient mixing. Conversely, normal injection promotes mixing and combustion. This study investigates the performance of a scramjet combustor fuelled by combined normal and tangential injection, with the aim of minimising viscous drag.
Experiments were performed on a cylindrical scramjet combustor in the T4 free-piston shock tunnel. Viscous drag was measured using a single-component stress-wave force balance for combustor lengths of 220, 420 and 500 mm. Wall pressure measurements were made using a 500 mm long combustor. All of the combustors had an internal
diameter of 33.2 mm. A series of fuel injectors were used to provide different proportions of tangential to normal injection, with between 45 and 100 % of the fuel injected tangentially. Tests were conducted at equivalence ratios between 0 and 5. The nominal test condition corresponded to a free-stream Mach number of 4.5, a nozzle supply pressure of 36 MPa and a nozzle supply enthalpy of 5.8 MJ/kg. Tests were also conducted over a range of nozzle supply pressures (20-43 MPa) and nozzle supply enthalpies (3.9-7.0MJ/kg) to investigate the effects of changes in simulated flight altitude and simulated flight speed.
With tangential injection only, reductions in the viscous drag of up to 50 % were measured. However, pressure measurements indicated that little or no combustion took place as a result of inefficient mixing of the fuel and free-stream flows. In contrast, tests conducted with various proportions of normal injection produced combustion. However, the
combustor pressure was significantly less than that for normal injection only at a similar total equivalence ratio. It is proposed that this is due to only the normally injected fuel being involved in combustion.
Viscous drag reductions of up to 25 % were measured for combined normal and tangential injection. For a given total equivalence ratio, greater proportions of tangential injection produced greater reductions in the viscous drag. The greatest reductions in viscous drag occurred at the lowest total equivalence ratio tested for each injector. It is shown that the flow disturbances produced by normal fuel injection increase the level of viscous drag as the total equivalence ratio is increased.
An analysis of the overall performance of the scramjet combustor was made by calculating the change in specific impulse for each injection distribution and equivalence ratio tested. The largest increase in the specific impulse of 950 s was
produced by 100 % normal injection. In comparison, the largest change in specific impulse produced by combined normal and tangential injection was approximately 715 s. For combined normal and tangential injection, the change in specific impulse was similar for all injection distributions with the best results obtained at the lowest total equivalence ratios tested, where the reduction in viscous drag was greatest.
As a result, it is concluded that, for the experimental configuration investigated, there is no benefit in terms of combustor performance produced by the tangential injection of fuel to reduce viscous drag.