Unsteady Computational Fluid Dynamics Analysis of End-Sector Losses in a Two-Phase Axial Turbine With Partial Admission

Abstract Two-phase axial turbines offer a promising solution for improving the efficiency of energy conversion systems such as partial-evaporation organic Rankine cycles power systems, and refrigeration processes. However, their performance is limited by complex flow phenomena associated with partial admission and two-phase expansion, particularly the end-sector losses caused by unsteady filling and emptying of rotor blade passages. To date, these losses have not been quantified, as prior computational studies of two-phase turbines have relied on frozen rotor simulations that cannot resolve unsteady effects. This study presents the first unsteady computational fluid dynamics analysis of a two-phase axial turbine using a barotropic flow model, validated against experimental data from a water-nitrogen turbine. A detailed comparison between unsteady and frozen-rotor simulations is performed across various speeds using a barotropic homogenous equilibrium model. The results suggest that both modeling approaches predict nearly identical flow fields in the nozzle, but significant differences emerge in the rotor. Only the unsteady model captures reverse blade loading due to suction and pumping effects, reducing the time-averaged torque by 7.7% at design speed compared to the frozen-rotor approach. Validation against experimental data indicates excellent agreement in mass flowrate and, after correcting for nozzle velocity overprediction, torque predictions fall within ±3.5% across all rotational speeds. These results suggest that the barotropic model can effectively capture the unsteady flow physics in the rotor domain while highlighting the need for improved modeling of nonequilibrium effects in the nozzle.