Improving Turbine Component Efficiency

The thermal efficiency of a gas turbine engine depends on the cycle pressure and temperature ratio and on the aerodynamic efficiencies of the gas path components. Maintaining and/or improving structural integrity and aerodynamic efficiency in this high pressure, high temperature environment is the preeminent problem of the turbine designer. High gas temperatures require at least some of the metal structure to be cooled, yet cooling air is a loss to the cycle and its consumption must be kept to a minimum. Research into cooling techniques and boundary layer behavior on airfoils and endwalls and into test procedures for obtaining heat transfer data are providing some of the answers the designer needs. Increased operating-pressures generate increased mechanical stresses. Finite element analyses and automated design procedures are proving to be powerful aids to the designer. Improving aerodynamic efficiency requires careful control of the flow in three dimensions, particularly in low aspect ratio machines. The first practical computation method for three-dimensional, viscous, transonic flows became available in late 1977 and has made this one of the most exciting areas of turbine technology. Additional gains in aerodynamic efficiency can be realized by controlling leakages, especially those over the rotor tip, by accounting for the transient interactions between rotor and stator; and by careful control of discharged coolant flow. This paper briefly describes the turbine cooling research conducted by the Air Force Aero Propulsion Laboratory and describes more extensively the AFAPL programs in turbine aerodynamics, including applications of three-dimensional flow analysis.