Transonic turbine vane tests in a new miniature cascade facility

Abstract

A new two-dimensional mini-cascade blow-down tunnel, with eight vane passages of only 20 mm true chord, 15 mm pitch, and 40 mm span, has been developed for testing gas turbine nozzle guide vane profiles at transonic exit Mach numbers ( M) and realistic Reynolds numbers ( Re) 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 Re at room temperature with exit static pressures above atmospheric and Re and M can be independently varied. The mass flow is small and the long run times (>3 min) allow downstream traversing with a three-hole miniature probe. The inaccuracies inherent in the traditional method of mounting the vanes/blades on pegs in the Schlieren windows have been eliminated by accurately computer numerical control (CNC) machining the whole cascade of vanes integral with the tunnel side walls, with re-usable, plug-in Schlieren windows. Cascades of new test vane profiles are quickly machined and tested, with a fast turn-around for comparative studies. A new Schlieren photography lens arrangement is demonstrated, which captures numerous flow visualization images per run directly into a digital SLR camera. Lower-than-atmospheric exhaust pressures were achieved by using a long exhaust designed to maximize exit flow dynamic pressure recovery using the self-ejector pumping effect. Exhaust pressures as low as 0.7 bar were achieved using the self-pumping exhaust, allowing M up to 1.6 with 3 bar inlet pressure and an exhaust at atmospheric pressure. An analytical model is developed for the mildly supersonic exit flow condition and validated against experimental data. Excellent agreement is achieved. Results presented for a typical transonic cascade show that the tunnel performs well in the subsonic and low supersonic region, but, as the exit M increases, shock-wave reflections from the free jet boundary, seen on the Schlieren, cause non-periodicity. Deep in the cascade, the flow is still periodic and can be used for investigations of loss and shock boundary-layer interaction. Interestingly, if the downstream pressure is reduced further to give even higher (but unrealistic) M, the cascade becomes axially supersonic and excellent periodicity is restored. Schlieren results are compared with computational fluid dynamics (CFD) simulations of the tunnel flow, and the loss measurements are validated against measurements conducted using a large-scale cascade at Göttingen.