Displacement effects and scaling of ducted, supersonic flames

The paper describes a study of the mechanisms for generating pressure increase within a ducted, supersonic hydrogen-air flame. The combustor consisted of a constant-area rectangular duct with a centrally located fuel injector strut that spanned the width of the duct. The free stream flows, with total enthalpies of 5.6, 6.5 and 8.9 MJ/kg, were provided by a free-piston shock tunnel and the fuel was injected from a Ludwieg tube. The wall pressure increase generated within the duct was successfully estimated by first reducing the effective core area of the duct by the combined displacement thickness of the mixing layers and wall boundary layers and then calculating the properties of the inviscid core flow assuming an isentropic compression. It was found that heat addition decreased the density of the mixing layer and subsequently increased its displacement thickness without greatly altering the layer's velocity profile. If pressure gradients and shock waves have only a minor effect on turbulence production then the displacement thickness of the mixing and boundary layers on be assumed to be independent of the height of the duct. If the duct height is changed, the resulting pressure distribution within the new duct can be predicted using the displacement thickness distribution from the original duct. The paper thus presents a form of scaling for supersonic combustion experiments. (C) 1998 by The Combustion Institute.