Analytical and CFD Methods Investigating Shroud Blade Tip Leakage.

This study deals with the leakage flow over a shrouded turbine stage, its interaction with the main passage flow, and the associated losses. The study addressed these topics by providing an analytical correlation loss model and detailed CFD simulations. An analytical model of leakage flow loss over a shrouded turbine stage has been developed. The analytical model uses directly measurable flow quantities to predict the effect of some of the over-shroud design parameters on stage performance. The model displays good predictive ability for the mass leakage fraction and the mixing losses. The model resolves the negative incidence angle induced by mixing the leakage flow with the main stream and predicts the increment in the total mixing loss coefficient at increasing leakage jet injection angles. The main contributions of this model to the leakage jet models documented in the open literature are the effect of the leakage jet injection angle on the mixing loss and the accounting of the effect of the number of fins on the leakage mass fraction in an explicit way. The present model exhibits a good qualitative and quantitative agreement with comparative benchmark data. An in-house three-dimensional turbomachinery CFD code was developed and validated against six test cases, showing its ability to capture the salient flow features in each test case. This work makes an innovative use of Detached Eddy Simulation as an advanced Reynolds Averaged Navier-Stokes (RANS) model. A detailed leakage flow structure over the rotor shroud and its interaction with the main passage flow were modeled for seven test cases to investigate the effect of the number of fins, the clearance gap ratio, and the leakage jet injection angle on the flow. The results showed that reducing the injection 90° to 30° leads to a reduction in entropy mixing loss coefficient by up to 24.7% and gives a 0.2% increase in the rotor static to static efficiency and highlighted that reducing the leakage jet injection angle is a promising concept to control most of the adverse effects of the leakage flow.