Developing Swirl Boundary Layer and Flow Separation at the Inlet of a Coaxial Rotating Diffuser or Nozzle

When an axial flow enters a rotating diffuser or nozzle, a swirl boundary layer appears at the wall and interacts with the axial boundary layer. Below a critical flow number φ_c, there is a flow separation, known in the turbomachinery context as part load recirculation. This paper extends the previous work for a cylindrical coaxial rotating pipe still considering the influence of the centrifugal force by varying the pipe's radius, yielding a coaxial rotating circular diffuser or nozzle. The integral method of boundary layer theory is used to describe the flow at the inlet of a rotating circular diffuser or nozzle, obtaining a generalized von Kármán momentum equation. This work conducts experiments to validate the analytical results and shows the influence of Reynolds number, flow number, apex angle, and surface roughness on the boundary layers evolution. By doing so, a critical flow number for incipient flow separation is analytically derived, resulting in a stability map for part load recirculation depending on Reynolds number and apex angle. Hereby, positive apex angles (diffuser) and negative apex angles (nozzle) are considered.