On part load recirculation of pumps and fans-a generic study

At part load in turbo machinery, there is a boundary layer separation resulting in a large vortex structure called part load recirculation. Up to now the influence of Reynolds number, relative roughness and degree of turbulence on this important stability limit of machines is not sufficiently understood. To shed some light onto these phenomena, in this work the simplest ''machine'' is considered by numerical and experimental studies. The apparatus we examine is a circular pipe at rest followed by a rotating co-axial pipe segment. By doing so, we have a generic test case which serves to study the critical flow number, defined by the onset of the separation and formation of a ring vortex