Upstream Wall Layer Effects on Drag Reduction with Boundary Layer Combustion

One of the major challenges of scramjet propulsion remains the generation of sufficient thrust to overcome the large drag of hypersonic vehicles. Since the viscous drag constitutes a large portion of the overall drag, its mitigation offers potential for performance improvement. Viscous drag is generated on all wetted surfaces of the vehicle but is largest in the scramjet combustion chamber, where the fluid not only has a high flow speed, but also a high density. Reduction of the skin friction drag in the combustor hence promises large improvements to the efficiency of the propulsive system. Stalker (2005) proposed a novel approach to skin friction reduction that is based on the combustion of hydrogen within the turbulent boundary layer of supersonic or hypersonic flow. An extension to the theory of van Driest II was developed that suggests that the effectiveness of this method is significantly superior to that of film cooling without combustion effects. In essence, the combustion heat release reduces the velocity gradient at the wall and the density in the boundary layer so that the momentum transfer to the wall is decreased. This work investigates the applicability of this skin friction reduction method to scramjet combustors that would operate at flight Mach numbers between 8 and 13 at altitudes between 34 and 39 km. The corresponding combustor Mach number is approximately 4.5 and the total enthalpies are between 3.6 and 7.8 MJ/kg. Experiments that directly measured the skin friction drag on the internal scramjet combustor surface were conducted in the T4 Stalker tube at The University of Queensland. A constant area, axisymmetric combustor was tested with a matching constant area, axisymmetric inlet that did not compress the oncoming flow. Therefore, the experiments were of a quasi-direct-connect nature where the inlet was used to condition the wall layer of the flow that enters the combustion chamber. The start of the combustor was formed by a step at the end of the inlet which contained an annular slot for the injection of the gaseous hydrogen fuel. Fuel was injected tangentially to the main stream flow into the circular combustor as a uniform layer underneath the established boundary layer from this annular slot. Combustion was monitored via the measurement of the axial pressure distribution in the combustor and viscous forces on the combustor were measured with a stress wave force balance. Two different inlet lengths were tested to assess the effect of the boundary layer state and thickness on the ignition and combustion of the injected hydrogen. The leading edge of the inlet was either sharp or blunt to investigate the effect of the hot gas that is contained in an entropy layer that is generated by a blunt leading edge. Finally, the diameter of the duct was varied to ensure that the experimental data was not subject to duct scaling effects. The effect on skin friction of the combustion of fuel in the boundary layer was assessed directly by measurement as well as analytically with several prediction methods. The experimental data show reductions of skin friction drag of up to 77% when stable combustion was established. A thick, turbulent boundary layer results in ignition for lower enthalpy conditions than a thin, laminar layer. The blunted leading edge configuration creates conditions that results in ignition of the injected fuel at all tested flow enthalpies and when a sharp leading edge configuration does not. Analytical predictions of the skin friction drag are in close agreement with the experimental data for fuel-off, film cooling and boundary layer combustion cases. It is demonstrated that the characteristics of boundary layer combustion do not change when the duct diameter is increased and the hydrogen mass flow rate per unit circumferential length is kept constant