The Effect of Specific Heat Ratio on the Performance of Compressible Flow Turbo-Machines

Abstract

The paper presents a method of determining the pressure ratio characteristic of a compressible flow turbo-machine when the specific heat ratio of the gas flowing through it is changed. It is shown that Mach number scaling in efficient machines running at low fractional increases in total temperature could tolerate limited variations in specific heat ratio. A method of finding the scaled operating conditions for the achievement of similar density ratios in scaled and prototype machines operating with gases with different specific heat ratios is presented. The method achieved very good accuracy in scaling the performance of a centrifugal impeller working with argon to its performance with carbon dioxide. The stage polytropic efficiencies were chosen to agree with measured data, although analysis indicated that polytropic stage efficiency should be independent of specific heat ratio. Further investigation and validation may lead to a standardised method of scaling turbo-machinery performance between gases with varying properties, which includes the effect of specific heat ratio.