Internal air-cooling for turbine blades a general design survey

Abstract

The degree of cooling that might be achieved in gas-turbine blades with simple internal air-cooling is surveyed with a view to pin-pointing the essential requirements for effective cooling with small quantities of cooling air. A shape parameter Z (defined as (SJc) *'2/(Ao/c~)) is derived which forms a useful figure of merit for comparing the relative efficiencies of various cooling passage configurations. To secure maximum economy in total cooling air in blades with a given cooling passage configuration, it is desirable that the turbine-blade rows should be designed with high pitch/chord ratios and low relative gas outlet angles (measured from axial direction). These requirements may run counter to those for high turbine-expansion efficiency and in practice some compromise must be sought to give optimum overall efficiency. It is shown that efficient blade cooling becomes progressively more difficult at lower values of turbine-flow Reynolds numbers, and cooling systems should be designed to give adequate cooling at the lowest operating Reynolds numbers since this represents the most onerous condition. The potentialities of blades with laminar-cooling and turbulent-cooling flow in the cooling passages are compared. Although laminar-cooling flow might enable better cooling at low Reynolds numbers to be achieved, turbulent cooling flow is generally to be preferred since this (a) permits more consistent cooling over a wide range of Reynolds number in a simple air-cooled engine and (b) presents a simpler blade manufacturing problem.