Investigations on an experimental air-cooled turbine Parts III and IV

Abstract

Part III. The Effects of Cooling on the Overall Turbine Aerodynamic Performance and Initial Operation at an Inlet Gas Temperature of 1400 deg K. Further tests on an experimental air-cooled turbine have been carried out to examine the effect on the turbine aerodynamic performance of cooling-air discharge from the blade tips into the main gas stream. With the design quantities of cooling air applied, a measured loss of efficiency of 2 per cent was recorded. The effect of the nozzle-blade tip clearance of 0. 052 in. was also examined, and it is demonstrated that a gain in efficiency of 5 per cent can be realised if this clearance is eliminated. Initial operation of the turbine at an inlet gas temperature of 1400 deg K for 50 hours is also reported and is shown to be satisfactory. Part IV. Examination of Sintered Vitallium Air-Cooled Turbine Blades Following 100 Hours' Operation at a Turbine Inlet Mean Gas Temperature of 1400 deg K. A set of sintered vitallium air-cooled nozzle and rotor blades has been examined following a period of 100 hours' operation with a turbine inlet mean gas temperature of 1400 deg K. The blades, in particular the rotor blades, have distorted in a manner contrary to that which had been generally expected and cracks have formed in the sharp trailing edges and internally, around cooling passages in the region of the leading edge. Turbine tests and tests carried out in the laboratory suggest that the cracks in the trailing edges of the blades are due primarily to the repeated high thermal stresses set up by non-uniform heating and cooling, and the unexpected distortion in the rotor blades is the result of creep strain under a combination of steady thermal and centrifugal stresses. The distortion in the nozzle blades is associated with combined thermal and gas bending stresses. In the rotor blades internal cracking has been caused by excessive tensile creep strain in local regions in the blade, associated with the blade distortion, and it is possible that cooling passages in the vicinity have accelerated failure. Chordwise cracking in the blade trailing edges is accentuated by their razor-like form and their consequent weakness to thermal fatigue or thermal shock is to be expeCted. Fortunately, other factors which may influence future trends in cooled blade design result in a blade shape which is less susceptible to the distortion and cracking as exhibited by the present sintered vitallium blades. In principle these are (i) more rounded and slightly thicker trailing edges and (ii) no more than moderate degrees of blade camber.