The incompressible flow in a labyrinth seal is computed using the ‘κ−ε’ turbulence model with a pressure-velocity computer code in order to explain leakage phenomena against the mean pressure gradient. The flow is axisymmetric between a rotating shaft and an enclosing cylinder at rest. The main stream in circumferential direction induces a secondary mean flow vortex pattern inside annular cavities on the surface of the shaft. The domain of interest is one such cavity of an enlarged model of a labyrinth seal, where the finite difference result of a computer program is compared with measurements obtained by a back-scattering laser-Doppler anemometer at a cavity Reynolds number of ∼ 3 × 104and a Taylor number of ∼ 1·2 × 104. The turbulent kinetic energy and the turbulence dissipation rate are verified experimentally for a comparison with the result of the turbulence model.

The purpose of this paper is to investigate numerically the tip leakage flow for different blade tip geometries in an axial compressor stage under design and off-design conditions. Using flat tips, suction and pressure side squealers in combination with knife tips, a comparison of the rotor performance in terms of pressure and efficiency gain is reported. Detailed flow characteristics within the tip clearance gap, interaction of the leakage flow with the main flow and resultant turning effects at the exit of the row have been investigated. The CFD method is based on a commercially available compressible Navier-Stokes solver (STAR-CD), using a turbulent compressible high Reynolds number k-ε model. Accurate numerical comparison of different blade tip geometries is achieved by using the same grid for the various shapes. The blocking strategy with O-grid structure is presented. The numerical results show clearly the beneficial effect of cutting away material from the pressure side. The higher surface curvature of the suction side squealer affects the pressure blade loading and increases the lift in the same way. This effect is increased by increasing the squealer height and results in a lower efficiency gain near the surge line. The best modification of the blade tip shows a maximum reduction of the tip discharge coefficient of 20 %. This leads to an improved total pressure ratio of 0.29% and an improved total polytropic efficiency of 0.40% under design condition. The influences of favourable squealer geometries on stage characteristics are described along an operating line. With a simulation of IGV-setting from Δα = −15° to Δα = +20° different operating points have been investigated in a swirl performance map. The beneficial effect of the suction side squealer found for the rotor row could assign to the stator row and results in an improved static pressure gain. Furthermore, design indications are presented which help to keep the efficiency gain under surge condition as high as possible.

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This work presents guidelines for designing a ring-type inlet duct of an intermediate pressure steam turbine. It looks at aerodynamic characteristics of the flow field in the analyzed ducts as well as the impact of the duct topology on the radial clearances influencing casing deformations in different load cases. To show the reliability of the CFD-program used, calculated results are compared to measured data. The numerical method reflects the main physical effects observed in the reference cases with high accuracy. The inlet duct shape influences the loss of total pressure within the duct and the following stator blades as well as the leakage losses in the whole turbine casing. To minimize these losses, geometric parameters such as area ratios within the duct and the shape of its flow channels, are considered and the influence of these parameters on the losses is quantified. The optimization of the inlet duct leads to a geometry with minimum losses from an aerodynamic and mechanical point of view.

In this paper numerical results of wake and secondary flow interaction in diffuser flow fields are discussed. The wake and secondary flow are generated by a rotating wheel equipped with 30 cylindrical spokes with a diameter of 10 mm as a first approach to the turbine exit flow environment. The apex angle of the diffuser is chosen such that the flow is strongly separated according to the well-known performance charts of Sovran and Klomp [1]. This configuration has been tested in an experimental test rig at the Leibniz University Hannover [2]. According to these experiments, the flow in the diffuser separates as free jet for low rotational speeds of the spoke-wheel as expected by theory. However, if the 30 spokes of the upstream wheel rotate beyond the value of 500 rpm the measurements indicate that the flow remains attached to the outer diffuser wall. It will be shown by the present numerical analysis with the commercial solver ANSYS CFX-10.0 that only an unsteady approach using the elaborate SAS-SST turbulence model is capable of predicting the stabilizing effect of the rotating wheel to the diffuser flow at larger rotational speeds. The favourable comparison with the experimental data suggests that the mixing effect of wakes and secondary flow pattern is responsible for the reattachment. As a result of our studies it can be stated that the considerably higher numerical costs associated with unsteady calculations must be accepted in order to increase the understanding of the physical flow phenomena in turbine exit flow and its interaction with the downstream diffuser.