An experimental high-temperature turbine (no. 126). Part 1 the cooling performance of a set of extruded air-cooled turbine blades. Part 2 the effects of cooling on the aerodynamic performance

Abstract

Part I. Cooling characteristics are given in terms of the variation of average blade temperature at mid-span with cooling-flow rate, Reynolds number, gas/cooling-air absolute temperature ratio, and, for the rotor, gas-stream incidence. Chordwise and spanwise variations of blade temperature are also presented. Measured pressure-drop characteristics indicate that in a gas-turbine engine, with the cooling-air inlet pressure equal to the turbine gas inlet total pressure, cooling flows roughly equal to 1.8 per cent and 3.0 per cent of the main gas flow could be passed through the nozzle and rotor blades respectively, giving average relative temperatures at mid-span of approximately 0.6 in each instance. Comparisons with theoretical estimates of blade temperature show that the external heat-transfer rates to the nozzle blades agreed well with those measured on similar blades in cascade tests. For the rotor blades the external heat-transfer rates appear to have been between those predicted from cascade and earlier turbine test data. From the measured effects of Reynolds number on blade temperature it seems that approximately equal areas of laminar and turbulent boundary layer existed on both nozzle and rotor blades, and that as the turbine inlet temperature was raised, the extent of the turbulent layer tended to increase. Part II of this report describes tests conducted on the experimental high-temperature turbine at N.G.T.E. to measure the effects on aerodynamic performance of discharging cooling air into the main gas stream. The test results show the individual and combined effects on turbine isentropic efficiency of cooling-air discharge from each blade, from parts of the turbine annulus walls, from the rotor and nozzle shrouds and from the upstream face of the rotor disc. Analysis of the test results has shown that the tip clearance losses associated with the unshrouded rotor blades in this turbine can be reduced to some extent when cooling air is discharged from the tips of the blades. Predictions based on a theoretical method, which allows for the changes in flow Mach number and angle and for the losses of total pressure and temperature due to mixing cooling air with the main gas stream, are in reasonable agreement with the observed efficiencies.