The effect of the isentropic exponent on the performance map of supercritical compressors with carbon dioxide as an example

Authors

Ihab Abd El Hussein - University of Duisburg-Essen
Sebastian Schuster Dieter Brillert

Abstract

In this paper, a study is conducted to identify the effect of the isentropic exponent on the compressors performance and on the representation of performance maps (i.e. polytropic efficiency and reduced enthalpy change in terms of reduced mass flow rate). Similarity analysis requires that the isentropic exponent is kept constant. This is, however, often not the case especially for fluids at supercritical state, where its value can vary by a factor of more than ten. Carbon dioxide in the supercritical state (sCO2) is selected as an example fluid given the high attention it has drawn lately. Computational fluid dynamics (CFD) is utilized to compute the performance map of a single stage radial compressor for different isentropic exponents at the inlet. CFD predicts a reduction of the stage total polytropic efficiency with the increase of the isentropic exponent from 1.4 to 7.1. On the other hand, deviation of the reduced enthalpy are less pronounced with the results confined to a 1 percent band.  Based on the results, a proposition to correlate compressors performance with the exit flow coefficient instead of the reduced mass flow rate is made. The correlation provides improved accuracy in the performance predication with the reduction of the efficiency deviation to less than half a point