Two methods of evaluating the loss of mechanical energy in CFD predictions are described. It is shown that the difference between the two methods is a good measure of grid dependence for the prediction of the flows in 2D plane and annular model dump diffusers. The additional loss of mechanical energy due to sector end walls is investigated for 3D plane and annular model dump diffusers over a range of sector widths. The results show a strong dependence on the size of the boundary layer on the sector end walls at the inlet to the pre–diffuser. With no boundary layer on the sector end walls, the measured loss on the central symmetry plane is within 15% of the of the underlying 2D loss for pre–diffuser inlet width to height ratios of greater than 10:1. The deviation from the underlying 2D loss increases with the size of the boundary layer on the sector end walls.

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