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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A numerical method for calculating three-dimensional, steady or unsteady, incompressible, viscous flow is described. The conservation equations for mass and momentum and the equations of the k–ε turbulence model are solved with a finite volume method on nonorthogonal boundary-fitted grids. The method employs cell-centered variable arrangement and Cartesian velocity components. The SIMPLE algorithm is used to calculate the pressure and to enforce mass conservation. The computer code is vectorizable as far as possible to achieve an optimal performance on modern vector computers. Results of steady flow calculations in the guide vane, the pump rotor, and the turbine rotor and of the unsteady interaction simulation of the pump and the turbine of a one-stage one-phase non-automotive hydrodynamic torque converter are presented.

A systematic numerical study will be described considering a coupled arrangement of turbine and diffuser flow fields. For this purpose a multi-stage solver commonly used for industrial applications has been applied. The interaction with the turbine stage was investigated assuming two annular axis-symmetric diffuser configurations. One of these has been designed according to the performance charts by Sovran and Klomp [1] at optimum performance and the other close to stall conditions. In addition to a classical mixing plane approach, a frozen rotor solution has been considered as a first-order approach to the unsteady flow to study the effect of rotor wakes on the diffuser flow field. The predicted diffuser performance was strongly influenced by the rotor wakes, and their effect could only be obtained by an approximate frozen-rotor calculation. The results for the computed specific work and pressure recovery for both configurations showed an optimum distance of a/cR = 0.431 between rotor exit and diffuser inlet. Increase of radial rotor clearance gap has a beneficial effect on the diffuser characteristic for the built near the stall point. However, the larger tip leakage loss of the turbine rotor cannot be compensated by improved diffuser performance.

A numerical method for calculating threedimensional, steady or unsteady, incompressible, viscous flow is described. The conservation equations for mass and momentum and the equations of the kε-turbulence model are solved with a finite volume method on nonorthogonal boundary-fitted grids. The method employs cell-centered variable arrangement and Cartesian velocity components. The SIMPLE-algorithm is used to calculate the pressure and to enforce mass conservation. The computer code is vectorizable as far as possible to achieve an optimal performance on modern vector computers. Results of steady flow calculations in the guide vane, the pump rotor and the turbine rotor and of the unsteady interaction simulation of the pump and the turbine of a one-stage one-phase non-automotive hydrodynamic torque converter are presented.