THE FLOW PATTERN AND SEPARATION IN ANNULAR DIFFUSERS

Abstract

Annular diffusers are often used in turbomachines as exducers in turbines, diffusing elements in compressors and interstage ducts in multi- stage turbines. The design and performance of these diffusers are dependent on large number of geometrical and fluid dynamical parameters, if they are not carefully designed the flow pattern within them frequently shows large energy losses and stall. The flow pattern is further complicated if the flow entering such diffusers is swirled, which is commonly the case for flow leaving turbomachines rotors. In view of adverse pressure gradient and the complexity of the flow pattern in annular diffusers with swirled flow, a complete theoretical flow analysis through annular diffusers is rarely possible. Therefore, experimental methods have been of great importance in achieving some understanding of the flow behaviour.

In this thesis, an experimental study of the influence of non-swirling

:i / and swirling flows on the performance of annular diffusers is presented. ;• I Five diffusers were tested over a range of casing wall angles from 8 to 30 degrees, and a cylinderical hub. In addition, 180 degrees annular sudden expansion, as a short annular diffuser is tested. The experimental study conducted for a range of entry Reynolds number from 9. lxl 04 to 2.15xl 05

Two different methods of swirl generation are separately used

for adding the swirl velocity component to the incoming axial flow in the tested annular diffusers. The first swirl generator (free-swirl) imparts a swirl component via passages through swirl vanes installed upstream of the diffuser. Variuos swirl vanes with different angles of vane settings were fabricated. Swirl angles are varied from Oto 45 degrees were separately tested for each diffuser configuration. In the second method of swirl generation (forced-swirl), the swirl component is presented in the axial-air