Heat Transfer by Convection from a Hot Gas Jet to a Plane Surface

Abstract

Measurements have been made of the heat transfer from air jets, with temperature differences of up to 400 deg. C. (750 deg. F.) and velocities of up to 250 ft. per sec., impinging on a plane surface at various angles. It has been found, that for flow parallel to the surface, the recommended mean formula Nu =0.036(Re)0.8(Pr)0.33 is still valid over this extended range of velocity and temperature. For flow perpendicular to the surface, the formula Nu =0.1810(Re)0.7(Pr)0.33 expresses the results if the velocity and temperature are calculated from free jet formulae. A series of multiplying factors has been obtained for this formula for angles of gas impingement down to 15 deg., at which angle the heat transfer is approximately half that at 90 deg. Where a gas jet impinges on a relatively large plane surface, it has been found that the heat-transfer contours for any one angle are identical for all jet sizes at all cross-sections situated more than ten nozzle diameters downstream. This result holds even where the temperature of the gas at the orifice exceeds that of the ambient air by more than 500 deg. C. (930 deg. F.). This extension of the jet similarity theory to impinging gas jets provides a simple method of calculating the total convective heat transfer from such a jet.