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TECHNICAL PAPERS: Gas Turbines: Cycle Innovations

# Effect of the Specific Heat Ratio on the Aerodynamic Performance of Turbomachinery

[+] Author and Article Information
Stephen K. Roberts

Department of Mechanical and Aerospace Engineering, Carleton University, 3135 MacKenzie Building, Ottawa, ON K1S 5B6, Canadasroberts@mae.carleton.ca

Steen A. Sjolander

Department of Mechanical and Aerospace Engineering, Carleton University, 3135 MacKenzie Building, Ottawa, ON K1S 5B6, Canadassjoland@mae.carleton.ca

J. Eng. Gas Turbines Power 127(4), 773-780 (Mar 01, 2002) (8 pages) doi:10.1115/1.1995767 History: Received December 01, 2001; Revised March 01, 2002

## Abstract

Many gases, including carbon dioxide and argon, have been considered as alternative working fluids to air in a number of design studies for closed and semi-closed gas turbine engines. In many of these studies, it has been assumed that if the gas constant $R$ and specific heat ratio $γ$ are included in the speed and flow parameters, the compressor map or turbine characteristic is applicable to other working fluids. However, similarity arguments show that the isentropic exponent itself is a criterion of similarity and that the turbomachinery characteristics, even when appropriately nondimensionalized, will, in principle, vary as the $γ$ of the working fluid varies. This paper examines the effect of $γ$ on turbomachinery characteristics, mainly in terms of compressors. The performance of a centrifugal compressor stage was measured using air $(γ=1.4)$, $CO2$$(γ=1.29)$, and argon $(γ=1.67)$. For the same values of the nondimensional speed, the pressure ratio, efficiency, and choking mass flow were found to be significantly different for the three test gases. The experimental results have been found to be consistent with a CFD analysis of the impeller. Finally, it is shown that the changes in performance can be predicted reasonably well with simple arguments based mainly on one-dimensional isentropic flow. These arguments form the basis for correction procedures that can be used to project compressor characteristics measured for one value of $γ$ to those for a gas with a different value.

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## Figures

Figure 1

Summary of centrifugal impeller geometry

Figure 2

Schematic of test rig

Figure 3

Comparison of measured pressure ratio with manufacturer’s data for air

Figure 4

Comparison of measured efficiency with manufacturer’s data for air

Figure 5

Measured stage pressure ratio as function of γ

Figure 6

CFD prediction of rotor pressure ratio and choking mass flow rate as function of γ

Figure 7

Predicted pressure ratios for argon and CO2 using correction Eq. 8

Figure 8

Measured stage isentropic efficiency for argon; comparison with results for air

Figure 9

Measured stage isentropic efficiency for CO2; comparison with results for air

Figure 10

Variation of measured peak stage efficiency with speed parameter (corrected for Reynolds number)

Figure 11

Impeller efficiencies for air, argon, and CO2 as predicted by CFD

Figure 12

Predicted impeller efficiencies using correction Eqs. 10,11

Figure 13

Predicted stage efficiencies using correction Eqs. 10,11

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