Suppression of Rotating Stall by Wall Roughness Control in Vaneless Diffusers of Centrifugal Blowers

Abstract

By positioning the completely rough wall locally on the hub side diffuser wall alone in the vaneless diffuser, the flow rate of rotating stall inception was decreased by 42% at a small pressure drop less than 1%. This is based on the fact that the local reverse flow occurs firstly in the hub side in most centrifugal blowers with the backswept blade impeller. The 3-D boundary layer calculation shows that the increase in wall shear component normal to the main-flow direction decreases markedly the skewed angle of the 3-D boundary layer, and results in suppression of the 3-D separation. It is also clarified theoretically that the diffuser pressure recovery is hardly deteriorated by the rough wall positioned downstream of R = 1.2 because the increase in the radial momentum change, resulting from reduction in the skewed angle of the 3-D boundary layer, supports the adverse pressure gradient.

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“…It is obvious that the pressure rise from R = 1.00 to 1.05 is not affected by the introduction of the wall roughness, since the differences are insignificant. This is in agreement with the results reported by Ishida et al (2001). There are differences between C p (smooth) and C p (rough) downstream of R = 1.05.…”

“…The methods employed in this study are similar to those used by Ishida et al (2001) to stabilize the flow in the diffuser. These methods involve using rough surfaces (i.e., sandpaper) attached to the diffuser walls.…”

“…The flow is attached to the other diffuser hub wall. For better control, the rough surface should be attached to the shroud wall to eliminate the reverse flow, as recommended by Ishida et al (2001) for the back-swept impeller.…”

“…This is achieved by optimally implementing both a ring groove arrangement and a narrowed diffuser width. Ishida et al (2001) also examined the use of rough surfaces simulated by sandpaper stuck on the hub surface in controlling the inception of stall. They found that the flow rate of rotating stall inception was decreased by 42% at a small pressure loss of less than 1%.…”

Linear stability analysis of rotating stall in a wide vaneless diffuser

“…It can be seen that the wall boundary layers in narrow vaneless diffusers can merge together and occupy the whole flow channel. This kind of diffusers has been widely studied by Jansen (1964), , , Caignaert et al (1982), Dou and Mizuki (1998), Frigne and Van Den Braembussche (1985), Ishida et al (2001).…”