Skin-Friction Coefficient Model Verification and Flow Characteristics Analysis in Disk-Type Gap for Radial Turbomachinery

Under the conditions of high speed and density flow, the windage loss in the gap behind an impeller has an important influence on its thermal power conversion efficiency. Daily and Nece took the relative gap and Reynolds number as the characteristic parameters and divided the flow of a rotating disk in a closed chamber into four flow regions through laminar flow, turbulent flow and state of boundary layers. In this paper, the skin-friction coefficient model of Regio III and IV was verified by a numerical method under conditions corresponding to the typical Reynolds number range for a radial impeller. The results show that when the relative gap increases, the relative deviation between the numerical calculation and the model prediction results decreases, and the maximum deviation is −12.4%. With the increase of Reynolds number, the Region IV model is more accurate. Region III has the flow state of boundary layers merging and separation at the same time. Both models have good prediction accuracy with different mediums of air, water-liquid and critical CO₂. The deviation in Region III is larger and shows a decreasing trend with respect to Region IV, Based on the two models, a method for predicting the optimal gap is proposed. The method is verified to be reliable and can minimize windage loss.