Active Control of Rotating Stall by use of Swirl Breaker

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Rotating stall in a vaneless or a vaned diffuser not only makes the stably operating range of pumps, fans and compressors very narrow, but also gives severe damages to bearings, and thus many studies have been performed so far and the method has long been pursued to suppress this unstable phenomena. However, most of the methods so far proposed need complicated mechanisms, and thus reduce overall efficiency of a turbomachine. It is still strongly required to find a simple method of suppressing a rotating stall.In order to passively control and suppress a rotating stall in a vaneless diffuser and in a vaned diffuser, a new device by use of radial shallow grooves is proposed and its effect is studied experimentally and theoretically. The results show that the radial grooves of 3 mm depth suppress a rotating stall completely in a vaneless and a vaned diffusers over almost all flow range. The radial grooves of only 1 mm depth on both sidewalls can also suppress a rotating stall.Theoretical considerations have revealed that these tremendous effects of radial grooves are caused by two mechanisms ; one is a remarkable decrease of tangential velocity due to the mixing between the main flow and the groove flow, and the other is a remarkable increase of radial velocity due to the reverse flow in the grooves.