An experimental investigation of the effect of ejecting a coolant gas at the nose of a bluff body

Abstract

An experimental investigation has been made of the effect of ejecting nitrogen and helium coolant gases at the nose of a bluff body at a nominal Mach number of 5·8. The gases were ejected with ‘swirl’, to encourage them to flow tangentially to the model surface at ejection, and also straight out. Measurements were made of the pressure and temperature on the surface of the model at incidences of 0, 4, 8°, and for a range of coolant gas flows. From these measurements the flow patterns and distributions of heat flux were deduced.It was found that ejection with swirl did not in fact lead to an easing of the heating problem because the high tangential velocity with which the coolant was injected into the boundary layer increased the wall shear stress, and hence, by the Reynolds analogy, the heat flux, so that it predominated over the reduced driving temperature difference associated with the cooled boundary layer.With straight-out ejection it was found that the heat alleviation capabilities of the ejected coolant were reduced considerably if the flow rate was sufficiently large that the bow shock wave was bulged out. However, provided that the external flow is not disturbed, straight-out ejection provides an effective way of reducing the heat flux.