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 blunt body in the GALCIT 5 inch x 5 inch hype r sonic wind tunnel 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 pressure, temperature and heat flux on the surface of the model at incidences of 0, 4, 8 degrees, and for a range of coolant gas flows. It was found that ejection with swirl did not in fact lead to an easement of the heating problem, because the high tangential velocity with which the coolant was injected into the boundary layer so increased the wall shear stress, and hence by the Reynolds analogy, the heat flux, 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 momentum flow was sufficiently high that the bow shock wave was bulged out. For the size of ejection orifice in the present study it was possible to eject only nitrogen coolant without disturbing the external flow appreciably. The results suggest, however, that straight-out ejection could provide an effective way of reducing the heat flux provided that the external flow is not disturbed, and tests with a larger ejection orifice are indicated. A technique is proposed for making steady-state heat-flux measurements by measuring the temperature difference across a uniformly thin skin of uniform, low thermal conductivity.