Design of Turbine Blades Suitable for Supersonic Relative Inlet Velocities and the Investigation of Their Performance in Cascades: Part II-Experiments, Results and Discussion

Abstract

This paper describes cascade tests which were initially done to see whether blades designed by conventional methods for subsonic flows would in fact accept supersonic velocities at inlet to the blades but it was found that they would not do so. An analytical method for the design of turbine blades to accept supersonic relative inlet velocities has been presented in Part I (1). A blade section was designed for isentropic flow and from tests run on a cascade of these, values for losses and allowable contraction ratios were obtained which were incorporated in a revised design method. The tests and results obtained from the first supersonic cascade are described and also the results from the modified design which was based on non-isentropic conditions. The supersonic cascades were tested over a range of incidences in a small wind tunnel fitted with interchangeable liners to produce Mach numbers of approximately 1.5 and 1.3, Preliminary tests were also run at subsonic velocities. At design conditions flow-visualization by means of a Schlieren system showed wave formations in the blade passages which corresponded to the theoretical design values thus proving the validity of the design method. At off-design conditions a somewhat unusual shock-wave boundary-layer interaction was observed which appears to be associated with blades of this type. This was analysed by means of total head traverses, static pressure measurements and by the method of characteristics, and the effect which the configuration had on the downstream flow and losses is discussed. Total head traverses were done at the exits from the blade passages and also downstream of the trailing edges and the velocity coefficients were deduced from these values and from static pressure measurements obtained from wall tappings. For the first cascade which had a flow turning angle of 128° the velocity coefficient was approximately 0.95 and for the last cascade which had a turning angle some 9° greater, the value was 0.94. These velocity coefficients are higher than those for subsonic blades at comparative turning angles. With blades designed for supersonic flows by the method outlined in Part I (1) it appears to be possible to turn the flow through large angles such as 140° and to obtain acceptably high efficiencies. Tests in steam turbines indicate that with these blades the maximum efficiency is at least equal to that of a turbine designed for subsonic flow but that the heat drop is some 12 per cent higher.