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

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.

Oxyfuel combustion for CO2 capture in power plants

“…Roberts and Sjolander (2005) experimentally investigated the influence of working fluids with different ratios of specific heats, namely air, CO 2 , and argon, on a single stage radial compressor. They showed a significant effect on the pressure ratio, the surge behaviour and the isentropic efficiency of the compressor.…”

Hybrid Fuel Cell Gas Turbine System Design and Optimization

“…Recent work extended previous capabilities with simulation of a variable speed compressor. This allowed for an additional control input for maintaining cathode inlet temperature that both lab models showed decreasing in the previous work [15,16]. The results presented in this study showed improvements in the controllability of the stack temperature.…”

Similarity and cascade flow characteristics of a highly loaded helium compressor

“…Calculations show that the temperature difference between inlet and outlet of the cascade in Case1 at the attack angle of 0 • is 10.7 K, but the difference is 20.85 K in Case3 at the same attack angle. The previous research has shown that the single stage temperature rise is small (about 5 K), so the effect of viscous dissipation term on energy equation can be neglected when determining the similarity criterion (Roberts and Sjolander, 2005). However, combined the results of the paper with energy equation, we can find that temperature is related to viscous and resistance and the effect of viscous dissipation on the flow become obvious when the temperature is high enough.…”

“…A very comprehensive work, based on experimental data has been performed by Roberts (2001) and Roberts and Sjolander (2005) on the subject of impact of the use of alternative gas with different adiabatic exponent on centrifugal compressor. The results showed that k was an important criterion of similarity in the performance of turbomachinery and for compressors, in particular, the choking mass flow rate, the isentropic efficiency and the pressure ratio were all related to the adiabatic exponent k. Methods were proposed for predicting the changes in performance of compressors when substitute working medium were used in the research.…”