Hypersonic Turbulent Boundary-Layer Fuel Injection and Combustion: Skin-Friction Reduction Mechanisms

Drag reduction is important to improving the performance of scramjet engines operating at high Mach numbers. One demonstrated method for reducing skin-friction drag on a surface exposed to hypersonic flow is the injection and combustion of hydrogen fuel in the boundary layer. However, there are other fuels of interest in scramjet applications, and the underlying mechanisms that drive the reduction of skin friction are not well understood. An existing analytical model for boundary-layer combustion of hydrogen is rederived for a general fueling condition and then extended to allow investigation of the underlying flow physics in this model. Applying this theory to ethylene fueling indicates that skin-friction reduction through boundary-layer combustion is possible with fuels other than hydrogen. Analysis of the modeled boundary-layer profiles demonstrates that skin-friction reduction is accomplished through several coupled mechanisms: a change in near-wall viscosity, density changes and combustion act to reduce Reynolds stresses, and the low-momentum fuel stream thickens the boundary layer and changes the wall-normal velocity gradient. Finally, the theory is used to estimate the maximum fraction of fuel that should be used for skin-friction reduction in a typical scramjet engine.