Windage Loss and Flow Characteristics in Back Gap of Radial Inflow Impellers with Steam Medium

The impeller back gap of radial inflow turbines generates disk-type windage loss, and it is necessary to accurately predict and minimize the loss. In this paper, a steady-state numerical analysis for a two-stage radial inflow turbine with steam as working medium is carried out, and a modified model for predicting the windage loss is proposed and validated. Under the design condition, the windage loss affects obviously the turbine power output, the power of the Stage 1st and Stage 2nd reduced by 6% and 5%, respectively. The back gap makes the vortex structure of the impeller leading edge enhanced, and pressure distribution turns different at the blade inlet section. With the increase of rotational speed and mass flow rate, the vortex structure near the outlet expand and moves radially to the center of the gap. Finally, models from Daily and Coren indicate that Reynolds number and throughflow Reynolds number are important influencing parameters for windage loss in disk-type gap with throughflow. And the results show that the Daily model is more accurate. Therefore, based on the simulation results, a model is proposed to improve the accuracy of windage loss model. And the model containing these two parameters can accurately predict the windage loss with a maximum deviation of 10.6 %. Despite its preliminary character, the model has an important reference value for the design of radial inflow turbomachinery.