An investigation of flush off-takes for use in a cooled cooling air system

The design and evaluation of off-takes has traditionally focused on increasing ram pressure recovery with little consideration given to flow uniformity. Preliminary studies on a proposed cooled cooling air system for a large aero gas turbine indicated that the offtake represented a weak point in the design with the non-uniformities it generated negatively affecting system performance. High levels of diffusion and a uniform flow are required to minimise loss and to maximise the effectiveness of the downstream heat exchanger. This paper presents a numerical and experimental parametric study of parallel wall flush off-takes with focus placed on the quality of the downstream flow and its uniformity. A realisable k-omega turbulence closure was employed with a standard wall function to examine the pressure recovery and uniformity of flush off-takes. The performance of the off-take was investigated with different inflow boundary layer thicknesses in conjunction with changes in various design parameters. The current investigation highlights that there exists a direct trade-off between the diffusion and uniformity that can be achieved by a flush offtake. Nevertheless, the work provides an improved understanding of how each performance parameters can be maximised with respect to uniformity and this knowledge is currently being applied to the development of an optimal off-take design.