THE TIP CLEARANCE EFFECTS ON THE CENTRIFUGAL COMPRESSOR VANELESS DIFFUSER FLOW FIELDS AT OFF-DESIGN CONDITIONS

Three centrifugal compressor vaneless diffuser configurations, varying in diffuser width, were studied experimentally. Each diffuser configuration was tested with four different tip clearances. The stage overall performance along with the diffuser flow fields were studied. The diffuser flow fields were studied with probe traverses for total temperature, total pressure, and flow angle, along with static pressure measurements. The diffuser flow fields were studied at the diffuser inlet and outlet at the compressor design rotational speed near the stall (1.04 q(m,stall)) and choke (0.96 . q(m,choke)) conditions. The increasing tip clearance is seen as a larger secondary flow region present at the diffuser inlet near the shroud. This causes additional losses which are seen as lower compressor stage efficiency. The effect is more profound at higher mass flows. The flow velocity distributions at different circumferential locations showed significant differences at off-design conditions. One particular circumferential location showed higher or lower velocities depending on whether the mass flow was higher or lower than the design flow, and the phenomenon is stronger at the low flow.