Passive control of rotating stall in a parallel-wail vaneless diffuser by radial grooves

In order to control and suppress rotating stall in diffuser of a centrifuga l turbomachine a passive method of utilizing radial shallow grooves is prop osed anti its effect is studied theoretically and experimentally. The resul ts show that radial grooves of 3 mm depth on one wall or of only 1 mm depth on both walls can suppress rotating stall in a vaneless diffuser for the e ntire flow range. Theoretical considerations have revealed that this remark able effect of radial grooves is caused by two mechanisms; one is a signifi cant decrease in tangential velocity at the diffuser inlet due to mixing be tween the main flow and the groove flow, and the other is a remarkable incr ease in radial velocity due to the groove reverse flow. Both effects have t he same contribution to increase the flow angle. [S0098-2202(00)02901-1].