Axial Turbine Performance Evaluation. Part B—Optimization With and Without Constraints

Abstract

The maximum obtainable efficiency and associated geometry have been calculated based on the use of generalized loss correlations from Part A and are presented for full and partial admission turbines over a wide range of specific speeds. The calculated effects of varying values of Reynolds number, tip clearance, and trailing edge thickness on turbine performance are presented. Because of the anticipated difficulty in fabricating some of the optimum geometries calculated, the effects of using nonoptimum values of geometric parameters on attainable efficiency have also been investigated. The derating factor for machine Reynolds number is shown to be a strong function of specific speed, varying from 0.96 at a specific speed of 100, to 0.6 at a specific speed of 3, when Reynolds number is 105 compared to a reference value of 106. The derating factor for tip clearance is shown to be similar to what would be expected if the clearance area were considered as a leakage area. The use of blade heights, blade numbers, rotor exit angles, and degrees of reaction varying from the optimum by 25 percent produce maximum derating factors of 0.99, 0.98, 0.985, and 0.97, respectively, when compared to full optimum values.