Positively Sloped Head-Flow Characteristics and Inlet Recirculation of a Mixed-Flow Pump.

A flow mechanism that leads to the onset of positively sloped head-flow characteristics (referred to as stall onset) was investigated for a series of unshrouded mixed-flow pump impellers having the nominal specific speed of 560 (rpm, m3/min, m). A criterion that indicates the stall onset was proposed using the critical value of a theoretical pump head. The stall onset was caused by the discontinuous reduction of the theoretical pump head due to the blockage effects of impeller inlet recirculation, which was induced by the extensive flow separation on the casing wall at the latter half of the impeller. The stall onset was prevented when the impeller inlet recirculation occurred before the theoretical pump head reached its critical value as the flow rate was reduced. The blockage effects of the impeller inlet recirculation, in this case, developed continuously and decreased the diffusion of the flow within the impeller flow passages, thus suppressing the onset of extensive flow separation on the casing wall.