Real gas effects in ORC turbines

Abstract

This thesis aims to study the effects of varying gas properties on turbine performance. Most prominently ORC turbines are operating with a multitude of different working fluids that have very strong variations in gas properties. Moreover, this also applies to conventional steam turbines or first stage HP turbines with very high temperatures, where the gas properties can divert significantly from air at ambient conditions. The isentropic exponent and compressibility factor are commonly used to describe a fluid's gas dynamic behaviour and its diversion form ideal gas conditions. Though, the effects of these parameters on turbine performance are not well known. This thesis discusses a series of experimental and computational studies to determine the impact of the isentropic exponent and compressibility factor on the flow field within a turbine vane and radial stage. A series of experiments are performed on a newly modified transient wind tunnel, that allows testing of vanes with various working fluids at a range of operating conditions. Detailed wall static pressure measurements of the cascade were obtained, as well as wake measurements in the supersonic flow downstream of the vane. For this thesis, experiments were conducted with argon, air, CO2 and R134a, which give a range of isentropic exponent from 1.67 to 1.08 and a range in compressibility factor from 0.88 to 1.0. The experimental results are used to validate the computational approach, that has revealed that over the given range of isentropic exponents, the loss can vary between 20% and 35%, depending on vane exit Mach number. These difference in aerodynamic loss are driven by a change in the mixing loss at the trailing edge loss and a shift in shock loss within the passage. These effects are a strong function of vane exit Mach number whereby they only occur at supersonic speeds and sensitivity is increasing with rising Mach numbers. Moreover, computational analysis on the effect of the compressibility factor has shown that low values of Z have a favourable effect on turbine performance through reduced shock loss and boundary layer loss. Within the given range of compressibility factor, a 10% change in Z results in a 3% change in total loss. Thus, the sensitivity of loss to the isentropic exponent as much higher than for the compressibility factor.