60241 - The Effect of Compressibility Factor on Turbine Performance 

The compressibility factor Z is one of the most common properties that describes a fluid diversion from an ideal gas. Especially for the case of ORC turbines, Z can vary substantially between different working fluids. Still, its effect on turbine performance is not well known. The paper proposes a set of non-dimensional parameters, that will allow fixing the gas dynamic behaviour independent of Z and thus isolate the effect that Z has on turbine performance. A range of working fluids is found in the REFPROP library that reflects a variation in Z from 0.75 to 0.95. The performance of the different working fluids is compared for the case of stator vanes with a design Mach number of 1.30. The results indicate that, contrary to the common perception, low values of Z and hence a strong diversion from an ideal gas lead to a reduction in loss for supersonic operating conditions. The aerodynamic mechanisms responsible are a reduction in shock loss and a reduction in boundary layer loss. The change in the boundary layer, due to Z, directly increases the loss within the passage and also increases trailing edge loss due to the rise in effective trailing edge thickness. Within the given range of compressibility factor, a 10% change in Z results in a 3% change in total loss. The results form this paper are relevant for all future turbines operating with non-ideal working fluids.