J. D. Denton

The exploitation of three-dimensional flow in turbomachinery design

Abstract Many of the phenomena involved in turbomachinery flow can be understood and predicted on a two-dimensional (2D) or quasi-three-dimensional (Q3D) basis, but some aspects of the flow must be considered as fully three-dimensional (3D) and cannot be understood or predicted by the Q3D approach. Probably the best known of these fully 3D effects is secondary flow, which can only be predicted by a fully 3D calculation which includes the vorticity at inlet to the blade row. It has long been recognized that blade sweep and lean also produce fully 3D effects and approximate methods of calculating these have been developed. However, the advent of fully 3D flow field calculation methods has made predictions of these complex effects much more readily available and accurate so that they are now being exploited in design. This paper will attempt to describe and discuss fully 3D flow effects with particular reference to their use to improve turbomachine performance. Although the discussion is restricted to axial flow machines, many of the phenomena discussed are equally applicable to mixed and radial flow turbines and compressors.

The Effects of Lean and Sweep on Transonic Fan Performance

The aerodynamics of transonic fans is discussed with emphasis on the use of three-dimensional design techniques, such as blade sweep and lean, to improve their performance. In order to study the interaction of these 3D features with the shock pattern a series of five different designs is produced and analysed by CFD. It is found that the 3D features have remarkably little effect on the shock pattern near the tip where the shock must remain perpendicular to the casing. Lower down the blade significant shock sweep, and hence reduced shock loss, can be induced by 3D design but this is usually at the expense of reduced stall margin and increased loss elsewhere along the blade span. Overall, very little change in peak efficiency or pressure ratio is produced by blade sweep or lean. However, there are significant effects on stall margin with forwards sweep producing a better stall margin and maintaining a high efficiency over a wider range.Copyright

The Trailing Edge Loss of Transonic Turbine Blades

Trailing edge loss is one of the main sources of loss for transonic turbine blades, contributing typically 1/3 of their total loss. Transonic trailing edge flow is extremely complex, the basic flow pattern is understood byut methods of predicting the loss are currently based on empirical correlations for the base pressure. These correlations are of limited accuracy. Recent findings that the base pressure and loss can be reasonably well predicted by inviscid Euler calculations are justified and explained in this paper

Prediction of Low Engine Order Inlet Distortion Driven Resonance in a Low Aspect Ratio Fan

This paper presents a forced response prediction of 3 resonances in a low aspect ratio modern fan rotor and compares with other worker’s experimental data. The incoming disturbances are due to low engine-order inlet distortion from upstream screens. The resonances occur in the running range at 3 and 8 engine orders which cross low modes (flap, torsion and stripe) of the blade. The fan was tested with on-blade instrumentation at both on- and off-resonant conditions to establish the unsteady pressures due to known distortion patterns.The resulting steady and unsteady flow in the fan blade passages has been predicted by three methods, all three-dimensional. The first is a linearised unsteady Euler method; the second is a non-linear unsteady Navier-Stokes method; the third method uses a similar level of aerodynamic modelling as the second but also includes a coupled model of the structural dynamics. The predictions for the 3 methods are presented against the test data, and further insight into the problem is obtained through post-processing of the data. Predictions of the blade vibration response are also obtained. Overall the level of agreement between calculations and measurements is considered encouraging although further research is needed.Copyright

Towards long length scale unsteady modelling in turbomachines

Abstract The modelling issue of long length scale unsteadiness in turbomachines is discussed in this paper and a new three-dimensional CFD method with a hierarchy of body force models is proposed. A program has been developed to deal with the long length scale problem using efficient coarse meshes and large time steps, together with body force models. The distributed viscous body force can be calculated either using a very simple viscous wall shear stress model or extracted from a ‘stand-alone’ three-dimensional single passage steady calculation using a fine mesh. By eliminating the need to compute fine viscous scales, the proposed method is several orders of magnitude more efficient than ‘standard’ fine mesh N-S unsteady calculations while maintaining respectable resolution down to the scale of blade-to-blade variation, including wake/potential interactions between blade rows. Two sample cases with high-frequency low-amplitude and low-frequency high-amplitude, respectively, are used to illustrate the accuracy of the model.