Investigators: Professor Tom Povey

Sponsor: Rolls-Royce Turbines.

The supersonic cascade facility is a new two-dimensional mini-cascade blow-down tunnel, with 8 vane passages of only 15 mm pitch and 40 mm span. It has been developed for testing gas turbine nozzle guide vane profiles at transonic exit Mach numbers and realistic Reynolds numbers with a quick turnaround and high accuracy.

The small size of the cascade has many advantages. Vane profiles can be tested at adjustable engine representative Reynolds numbers at room temperature with exit static pressures above atmospheric and Re and Mach number can be independently varied. The mass flow is small and the long (> 3 minutes) run times allow downstream traversing with a 3-hole miniature probe. The inaccuracies inherent in the traditional method of mounting the blades on pegs in the schlieren windows has been eliminated by accurately CNC machining the whole cascade of blades integral with the tunnel side walls, with re-usable, plug-in Schlieren windows. Cascades of new test blade profiles are quickly machined and tested, with a fast turn-around for comparative studies. A new Schlieren photography system captures multiple flow visualisation images per run directly into a digital SLR camera.

A number of transonic cascade profiles have been investigated, and demonsrate that the tunnel performs well in the subsonic and low supersonic region, but, as the exit Mach number increases, shock wave reflections from the free jet boundary, seen on the schlieren, cause non-periodicity. Interestingly, if the downstream pressure is reduced further to give even higher (but unrealistic) Mach numbers, the cascade becomes axially supersonic and excellent periodicity is restored.

Using this tunnel the flow in supersonic cascades is investigated using downstream traverse measurements (for loss), schlieren photographs for detailed understanding of the shock structures, and CFD simulations of the tunnel flow.