The hub boundary layer of an axial flow compressor

In this thesis an investigation of flow in the hub region of a single stage axial flow compressor has been made. This study represents the initial portion of a program being undertaken at the University of Tasmania, aimed at improving the understanding of the flow mechanism and reducing the losses resulting from this region. The viscous effects resulting from blade passage end wall boundary layer growth are taken into account in axial flow compressor design by the use of empirical factors applied to inviscid flow theory. Servoy (Ref. 1) in a review of recent progress in the field states \that most designers in the United States extrapolate main passage velocity profiles to the inner and outer walls as if no boundary layers were present changes due to the presence of the boundary layers are accounted for by a blockage factor the value of which is poorly defined\". British designers use a similar system introducing a work done factor (Howell (Ref. 2) and Horlock (Ref. 3)) to estimate the decrease in temperature rise per stage due to wall effects. In addition to causing deformation of the mainstream and hence making the factors discussed above necessary the hub and tip regions account for the major portion of the losses occurring in a machine. An example of the importance of these regions is given by Howell (Ref. 2) shown in Fig. 1. At the design point the losses occurring in the end wall region i.e. the annulus and the major portion of the secondary flow losses account for 60% of the total loss. If a significant reduction in the losses in turbomachinery is to be made a reduction in this major component will be necessary. A better understanding of the mechanism of the flow in the wall boundary layers is necessary to permit the development of a model of the flow which will allow the influence of these regions to be accounted for in design and to determine the main sources of loss and the factors controlling these sources. The flow in the end regions of a blade passage is complex. The main features contributing to this complexity are the blade passage secondary flow tip clearance flows effects due to relative motion between moving blade rows and the stationary walls flows resulting from radial pressure gradients and the influence of flow separation which occurs at the junction of blade suction surface and the end wall. These various influences are illustrated in Fig. 2; a detailed discussion of each will be found in Chapter 2. Qualitative and limited quantitative information is available on passage secondary flows and tip leakage effects but the flow separation originating in the corner bounded by the end wall and the suction surface of a blade appears to be the major cause of loss. Data on this phenomenom are limited. In this thesis a detailed study of the boundary layer on the hub wall downstream of the rotor and through the stator row of a single stage axial flow compressor is reported. The flow in the stator hub region is dominated by a separation region in the suction surface/hub corner which sheds low energy air in the form of a streamwise vortex. The boundary layer downstream of the rotor has been found to consist of three distinct regions. Next to the wall there is a region controlled by the wall in which the flow angle remains constant and the velocity profile can be described by a logarithmic distribution. Further from the wall the flow is dominated by vorticity generated by the turning of the end wall boundary layer and undergoes considerable over turning. On the outer edge of the boundary layer a third region dominated by a second vortex rotating in the opposite direction to the passage vortex exists. This vortex appears to originate from a separation region similar to that found in the stator row and it contains a major portion of the losses occurring in the hub region. Measurement of the distribution of turbulence components downstream of the rotor indicate distinct directional properties which appear to be due to the rotor wakes. As a result a model of the hub boundary layer as a quasi turbulent layer has been developed."