A linear design system, already in use for the forward and inverse design of three-dimensional turbine aerofoils, has been extended for the design of their end walls. This paper shows how this method has been applied to the design of a nonaxisymmetric end wall for a turbine rotor blade in linear cascade. The calculations show that nonaxisymmetric end wall profiling is a powerful tool for reducing secondary flows, in particular the secondary kinetic energy and exit angle deviations. Simple end wall profiling is shown to be at least as beneficial aerodynamically as the now standard techniques of differentially skewing aerofoil sections up the span, and (compound) leaning of the aerofoil. A design is presented that combines a number of end wall features aimed at reducing secondary loss and flow deviation. The experimental study of this geometry, aimed at validating the design method, is the subject of the second part of this paper. The effects of end wall perturbations on the flow field are calculated using a three-dimensional pressure correction based Reynolds-averaged Navier–Stokes CFD code. These calculations are normally performed overnight on a cluster of work stations. The design system then calculates the relationships between perturbations in the end wall and resulting changes in the flow field. With these available, linear superposition theory is used to enable the designer to investigate quickly the effect on the flow field of many combinations of end wall shapes (a matter of minutes for each shape). [S0889-504X(00)00902-8]
The Nozzle Guide Vanes (NGV’s) of an axial gas turbine generate circumferential non-uniformities in static pressure after their trailing edges. This paper aims to show that these non-uniformities can be removed locally by re-shaping the hub endwall. The ultimate objective is to improve engine performance by cutting the leakage of coolant from the disc rim seal. The technique is demonstrated theoretically using 3D viscous CFD. The required endwall shape is non-axisymmetric and is generated parametrically by altering a calculation grid for an existing NGV. The results show a reduction of the static pressure non-uniformities by 70%. Other favourable effects are also predicted and are commented upon.
The Durham Linear Cascade has been redesigned with the nonaxisymmetric profiled end wall described in the first part of this paper, with the aim of reducing the effects of secondary flow. The design intent was to reduce the passage vortex strength and to produce a more uniform exit flow angle profile in the radial direction with less overturning at the wall. The new end wall has been tested in the linear cascade and a comprehensive set of measurements taken. These include traverses of the flow field at a number of axial planes and surface static pressure distributions on the end wall. Detailed comparisons have been made with the CFD design predictions, and also for the results with a planar end wall. In this way an improved understanding of the effects of end wall profiling has been obtained. The experimental results generally agree with the design predictions, showing a reduction in the strength of the secondary flow at the exit and a more uniform flow angle profile. In a turbine stage these effects would be expected to improve the performance of any downstream blade row. There is also a reduction in the overall loss, which was not given by the CFD design predictions. Areas where there are discrepancies between the CFD calculations and measurement are likely to be due to the turbulence model used. Conclusions for how the three-dimensional linear design system should be used to define end wall geometries for improved turbine performance are presented. [S0889-504X(00)01002-3]
Part I of this paper described how the HP turbine model rig of the Rolls-Royce Trent 500 was redesigned by applying non-axisymmetric end walls to both the vane and blade passages, whilst leaving the turbine operating point and overall flow conditions unaltered. This paper describes the results obtained from testing of the model rig and compares them with those obtained for the datum design (with conventional axisymmetric end walls). Measured improvements in the turbine efficiency are shown to be in line with those expected from the previous linear cascade research at Durham University, see Harvey et al. [1] and Hartland et al. [2]. These improvements are observed at both design and off-design conditions. Hot wire traverses taken at the exit of the rotor show, unexpectedly, that the end wall profiling has caused changes across the whole of the turbine flow field. This result is discussed making reference to a preliminary 3-D CFD analysis. It is concluded that the design methodology described in part I of this paper has been validated, and that non-axisymmetric end wall profiling is now a major new tool for the reduction of secondary loss in turbines (and potentially all axial flow turbomachinery). Further work, though, is needed to fully understand the stage (and multistage) effects of end wall profiling.
This paper describes how the Intermediate Pressure (IP) turbine model rig of the Rolls-Royce Trent 500 engine was redesigned by applying non-axisymmetric end walls to both the vane and blade passages. The blading aerofoil shapes, the turbine operating point and the overall flow conditions were unaltered from the original design. The results from testing of the model rig are presented and compared with those obtained previously for the datum design. A feature of this is that the IP turbine was tested in a “two-shaft” arrangement with the (upstream) Trent 500 High Pressure (HP) model turbine. Previously, non-axisymmetric end wall profiling had been shown to achieve a 0.59 ± 0.25% improvement in the stage efficiency of the Trent 500 HP model turbine when tested as a single stage, Rose et al. [1]. This had exceeded the design expectation of 0.4% improvement, Brennan et al. [4] — based on previous linear cascade research at Durham University, see Harvey et al. [2] and Hartland et al. [3]. The IP and HP turbines with profiled end walls were tested together, while for the datum test both model turbines had blading with axisymmetric end walls. The results have met expectations with an improvement in the IP turbine stage efficiency of 0.9 ± 0.4% at the design point. The turbine characteristics are shown to change significantly from the datum test.
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