Flow in annular diffusers. II - Flow analysis and effect of inlet boundary layer thickness

Inviscid flow in an annular diffuser composed of a cylindrical casing and a conical hub was analyzed theoretically, and effects of the boundary layer thickness on the flow pattern and the performance of the diffuser were examined experimentally. When the inlet flow has a whirl component, theoretical results show that the axial velocity comes to increase near the hub side with a spread of the free vortex flow region independently of the inlet whirl condition. The agreements between the theory and the experiment are good except for effects of the boundary layer. On the other hand, the flow has a tendency to run toward the hub side with an increasing displacement thickness of the boundary layer on the casing wall even if the flow does not have a whirl component. The performance of the diffuser is improved by increasing the thickness on the casing wall and generating a mild whirl flow in the inlet cross section.