Studies of Flow Patterns in a Diffuser Designed to Generate Longitudinal Vortices

Abstract

This paper describes flow visualization studies on a three-dimensional diffuser that generates two longitudinal vortices. The premise is that the three-dimensional flow patterns of this diffuser may have attributes that are superior to the two-dimensional flows of conventional diffusers. The diffuser geometry consists of a wide-angle, plane-wall diffuser with a pyramid-shaped insert attached to its expansion wall. The upsweep on the insert upper surface increases the pressure of the incoming flow, which rolls over into the expansion region in the presence of a transverse pressure gradient to form two symmetric, longitudinal vortices. At low Reynolds numbers (below 6000 based on the diffuser inlet height), the flow exhibited three flow patterns with subtle distinctions. The flowfield contained longitudinal vortices that modified and reduced in extent the separated regions that were present along the diffuser expansion wall. At high Reynolds numbers (above 6000), the separated regions along the diffuser expansion wall were replaced by a turbulent vortex. The lack of closed separated regions in the flow implies the absence of low-frequency effects that are characteristic of such regions. In addition, the flowfield exhibited little or no hysteresis with respect to changes in Reynolds number. These flowfield features are desirable in certain applications, such as exhaust diffusers for turbomachinery.