Investigations on an experimental air-cooled turbine Part 1 general description of turbine and experimental technique Part 2 cooling characteristics of blades having a multiplicity of small diameter coolant passages

Abstract

Part 1. Tests on internally air-cooled turbine blades operating under realistic conditions are required to determine (a) the degree of cooling achievable and (b) the effects of cooling on the overall turbine performance. This requirement has led to the manufacture and installation of an experimental air-cooled turbine specially designed for testing internally air-cooled nozzle and rotor blades. This part of the report records a general description of the turbine, the first set of cooled blades to be tested, and the associated instrumentation. It also records some results of earlv tests made to check the cooling of the turbine structure (excluding blades). Part 2. A thorough examination in an experimental turbine has been made of the cooling characteristics of a set of internally air-cooled nozzle and rotor blades. Particular attention was paid to the rotor blade cooling characteristics and results show that a very substantial reduction in mean rotor blade temperature may be achieved with a cooling flow equal to 2 per cent of the main gas flow. The blades were not uniformly cooled, the hlade temperatures being very much higher near the leading and trailing edges than in the mid-chord sections. However, examination of the steady thermal stresses created by the non-uniformity in cooling, and the transient stresses likely to occur during 'thermal shock', suggest that the non-uniform cooling is not very detrimental. An increase in turbine inlet gas temperature of about 270 deg C above the permissible value for the same rotor blades and same life without cooling appears possible with a cooling flow ratio of 0.02. The results further indicate that when operating with a fixed cooling flow ratio the mean degree of cooling achieved on the present blades is not substantially influenced by changes in the gas flow Reynolds number or, within limits, by substantial changes in blade aerodynamic loading. The results also indicate that cascade tests to investigate cooling characteristics of hlades may not be wholly applicable to similar blades operating in an actual turbine stage. This may be due largely to the fact that the amount of heat transferred from the gas to a blade is influenced considerably by the location of the boundary-layer transition points and these in turn may be greatly influenced hy the degree and nature of the turhulence in the main gas stream.