Annular Sector Heat Transfer Facility

Investigators: Professor Tom Povey

Students: Ben Kirollos, Salvador Luque

Sponsors: EPSRC, Rolls-Royce Turbines

The Annular Sector FacilityThe Annular Sector is an advanced aerodynamic technique for establishing pressure boundary conditions transonic cascade experiments. The technique represents an improvement over previous methods, and provides the first means by which annular sector boundary conditions that are representative of those which develop in an annular cascade can be established with a high degree of satisfaction. The technique enables cascade designers to exploit the obvious advantages of annular sector cascade testing: the reduced cost of both facility manufacture and facility operation, and the use of engine parts in place of two-dimensional counterparts.

By employing an annular sector of deswirl vanes downstream of the annular sector of test vanes, the radial pressure gradient established in the swirling flow downstream of the test vanes is not disturbed. The deswirl vane exit-flow – which has zero swirl velocity – can be exhausted without unsteadiness, and without the risk of separation, into a plenum at constant pressure. The pressure ratio across the annular sector of test vanes can be tuned by adjusting the throat area at the deswirl vane exit plane.

Schematic of the Annular Sector FacilityFlow conditioning systems which utilise the Oxford deswirl vane technology have previously been used to set pressure boundary conditions downstream of fully annular cascades in both model and engine-scale (the Isentropic Light Piston Facility at Farnborough) experimental research facilities.

The deswirl vane is particularly suited to the control of highly whirling transonic flows. It has been demonstrated by direct comparison of aerodynamic measurements from fully annular and annular sector experiments that the use of a deswirl vane sector for flow conditioning at the exit of an annular sector cascade represents an attractive novel solution to the boundary condition problem.

In the annular cascade experiment, the nature of secondary flow development depends upon both inlet and exit-flow conditions. Where a nozzle guide vane row is tested in isolation – in a stationary cascade – engine-representative pressure boundary conditions are often difficult to achieve. Radial pressure gradients, established in regions of swirling flow, affect the growth of secondary flows and, indeed, the span-wise velocity distribution at the vane exit-plane. Several methods have been developed to establish the correct radial pressure gradient at the exit of annular cascade facilities, although many of these are somewhat unsatisfactory – a common problem is that of unsteady flow separation downstream of the test vane.

The deswirl system and sidewallsAn alternative to annular cascade testing, is the use of a fixed linear cascade of vanes, a technique that is attractive because testing is possible at lower mass flow rates – a smaller number of test vanes can be used at the same chord size. In the linear cascade, radial pressure gradients are not established, and secondary flows do not develop as they would in a working engine. This means that radial distributions of total pressure loss and whirl angle are un-representative of engine conditions. In addition, surface heat-transfer rate distributions, which are strongly influenced by the secondary flow structure, are non-representative of engine conditions. An additional shortcoming is that only geometrically simplified (usually two-dimensional) designs can be tested, whereas in annular cascade experiments the exact three-dimensional vane profiles, even engine parts, can be used. Another difficulty is that of establishing truly periodic inlet and exit-flow conditions, which, being established automatically in the annular facility, can be achieved in a linear cascade only when a very large number of vanes is used.

The sidewalls of the inflow and exit ducts of a linear cascade exert a strong influence on the streamline pattern within the cascade, and techniques such as variable wall-suction and flexible sidewalls have been employed to tune the inlet and exit-flow conditions. Achieving even approximately periodic conditions is generally problematic however. A very comprehensive review of advanced techniques applicable to both linear and annular cascade testing has been published by AGARD (Hirsch, H., (Editor), 1993, “Advance methods for cascade testing,” AGARDograph 328, AGARD-AG-328.).

The deswirl system and test vanesAnnular sector cascade testing is a technique that combines the advantages of both the annular and linear cascade testing methods. Actual engine components can be tested, and the radial pressure gradients established in regions of swirling flow ensure secondary flows develop as they would in an operating engine. Model and testing costs for annular sector cascades are considerably lower than for their fully annular equivalents. Annular sector cascade testing is, however, relatively uncommon, as the designer is faced with the dual challenges of establishing vane-to-vane periodicity, and of maintaining the radial pressure gradient in the sector cascade exit-flow. To date, these problems have not been satisfactorily resolved, and, consequently, the potential advantages of annular sector testing have so far not been fully enjoyed.