A series of unsteady numerical simulations was conducted for an after-burning oil pump and the effect of annular chamber radial width on the pump performance and inner flow characteristics was investigated.Decrease of the radial width can increase the pump head under different flow-rates and the head increase under small flow-rate condition is larger than that under large flow-rate condition

which improves operation stability under the small flow-rate.Under the large flow-rate condition

annular chamber head loss is reduced by the decrease of circulation flow-rate in annular chamber resulted from the decrease of the annular chamber and following circumferential section area.Accordingly pump head is raised.While

under the small flow-rate condition

decreasing annular chamber radial width makes the incoming flow of the diffuser approach to annular chamber

which reduces the impeller passage head loss.As a result

pump head is enhanced significantly.