Two annular diffusers of different conical angles of expansion but constant outer diameters have been investigated with rotating flow behind a fan. The performance characteristics have been determined and the rotational-kinetic-energy effects on the over-all energy transfor¬mation were observed over a range of inlet Mach numbers from 0.1 to 0.55 and at angles of flow up to 28°. A wide range of flow distri¬butions was encountered as a result of changes in operating conditions. The over-all performance of the 8° diffuser is shown to be substantially-better than that of a 16° diffuser under comparable conditions for the range of Mach numbers and angles of rotation tested. Regions of maximum efficiency were found at angles of inflow approximately equivalent to the conical angle of expansion of the diffusers and again under conditions approaching axial flow. Sharp reductions in efficiency were recorded in both diffusers at the maximum values of stream rotation. The radial pressure gradient caused by the rotation of air assisted divergence of the flow; however, at the large angles of rotation, an adverse condition resulted from the inflow of low-energy air which in turn caused separation of flow on the inner wall. The rotational-kinetic -energy loss was negligible at the low angles but became appreciable at the maximum angle for low Mach numbers. This loss was greater for the 8° diffuser, but the over-all diffuser efficiencies remained higher than those of the 16° diffuser for corresponding test conditions.
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