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 know exactly when and under what conditions it is generated. Its aeroacoustics source generated by tip leakage flow among the broadband noise sources has been numerically investigated.
First of all, the shear layer of the tip gap flow is represented by discrete rectilinear vortices. The numerical model employing the vortex method is used to investigate the separated flow around tip edges. A two-part calculation has been performed : the flow field is first handled by calculating time-dependent incompressible flow, and then the acoustic field is obtained by solving the wave equation. In this paper, this approach is applied to calculate the sound produced by viscous flow over an idealized two-dimensional rotor tip gap. The acoustic power generated by the flow is calculated using Howe's theory (1975, 1984) of aerodynamic sound. It is clearly shown that considerable reduction of broadband noise can be obtained by decreasing the tip gap. It is thus concluded that reducing the tip clearance leads to improvement of both machine performance and the noise characteristics.
