(A) numerical study on flow and heat transfer and development of tip-leakage loss and broadband noise model for axial flow turbomachinery tip gaps

Abstract

Effect of tip geometry on the leakage flow through a tip gap is investigated by performing an experiment, and numerical calculations using the standard k-ε turbulence model. The blade tip geometries selected in this study are flat, pressure side cut, suction side cut, groove, and pressure side winglet. The three-dimensional flow field around the tip gap is simplified to that which consists of a sink and a source flow in the pressure and the suction side with appreciable axial flow in the suction side, respectively. Based on the experimental observation, the computed tip discharge coefficient and the total pressure loss, a prediction procedure for the relative sealing performance of the tip geometry is developed, and the loss generation mechanism is investigated in detail. In addition, the conduction and convection heat transfers are calculated to study the influence of the tip leakage flow on the temperature distribution in the blade tip region. The results indicate that the hot gas leakage flow through the tip gap causes more extensive heat transfer in the suction side than in the blade pressure side. In the present study, available recent experimental data in turbine cascades and three-dimensional numerical computations of the rotor tip clearance flow have been used to develop an improved model for tip clearance losses. The tip leakage discharge coefficient and the stage loading factor are taken as the modeling parameters. The model takes proper account of the loss created by the mixing of the clearance jet with the main passage flow. Earlier tip clearance loss models are reviewed and comparisons are made with the present work. The validity of the developed model in off-design conditions is proved by examining the tip leakage loss as a function of the incidence as well as the efficiency degradation depending on the relative tip gap height. In order to prevent the undesirable occurrences of broadband (non-rotational) noise of the rotor, we have to kno...