Numerical Simulation of a Micro Pelton Turbine Working in the Two-Phase Flow Regime

This paper deals with the numerical calculation of an energy-recovering unit for air-conditioning and heat pump systems. Currently, working fluids are expanded in expansion valves in air-conditionings or, in general, refrigeration systems. The aim of this work is to increase the efficiency of the air-conditioning systems by replacing the irreversible nozzle with an energy recovering working machine. An air-conditioning system with a high enthalpy difference between inlet and outlet combined with a small volume flow is in the focus of the investigation in the presented study. According to the well-known Cordier diagram, a Pelton turbine with one nozzle is the most efficient machine for this purpose. One part of the investigation is the nozzle and the other is the Pelton wheel. The inlet conditions for the nozzle are supercritical and the pressure ratio is 2.3; the working fluid is CO₂. During the expansion, the phase changes from supercritical to liquid and finally at the outlet two-phase flow is expected. The numerical calculations were executed with Ansys CFX 12. Two different numerical approaches — the homogeneous binary mixture and the Eulerian-Eulerian inhomogeneous multiphase model — are used. For the Eulerian-Eulerian calculation, the flow morphology is assumed to be polydispersed. Different bubble diameters are simulated with the MUSIG model. A comparison between the calculation with homogeneous binary mixture and Eulerian-Eulerian is presented as well. A parameter study about the nozzle, impeller and casing is performed which leads to the design point geometry. It will be shown that the optimal nozzle contour at design point conditions is converging - diverging although the flow is subsonic.