Design and testing of a transonic linear cascade tunnel with optimised slotted walls

In linear cascade wind tunnel tests, a high level of pitchwise periodicity is desireable to reproduce the azimuthal periodicity in the stage of an axial compressor or turbine. Transonic tests in a cascade wind tunnel with open jet boundaries have shown to suffer from spurious waves, reflected at the jet boundary, that compromise the flow periodicity in pitch. This problem can be tackled by placing at this boundary a slotted tailboard with a specific wall void ratio s and pitch angle alpha. The optimal value of the s-alpha pair depends on the test section geometry and on the tunnel running conditions. An inviscid two-dimensional numerical method has been developed to predict transonic linear cascade flows, with and without a tailboard, and quantify the non-periodicity in the discharge. This method includes a new computational boundary condition to model the effects of the tailboard slots on the cascade interior flow. This method has been applied to a six-blade turbine nozzle cascade, transonically tested at the University of Leicester. The numerical results identified a specific slotted tailboard geometry, able to minimise the spurious reflected waves and regain some pitchwise flow periodicity. The wind tunnel open jet test section was re-designed accordingly. Pressure measurements at the cascade outlet and synchronous spark schlieren visualisation of the test section, with and without the optimised slotted tailboard, have confirmed the gain in pitchwise periodicity predicted by the numerical model.