Research On Hump Suppression Method Of Axial Flow Pump Based On End Wall Treatment

When the axial flow pump runs to a small mass flow rate condition,the external characteristic curve has an unstable hump area,which restricts the actual operating performance of the axial flow pump.The sudden drop in the head of the small flow condition pump has become a difficult problem in the design and application of the axial flow pump.The study was to find a method that can effectively depress the hump in an axial-flow pump.Try to reveal the formation mechanism of the hump area of the axial flow pump through numerical simulation,theory analysis and experimental investigation,and explore ways to suppress the hump area of the axial flow pump based on the flow control method,broaden the efficient operation range of the pump,and improve the stability of the pump operation.The main target of the thesis is:1.Research on the flow characteristics of the axial flow pump in the hump operating condition.When the mass flow rate decreases to Q/Qdes=0.82,the head begins to decline,and this point was the near stall condition.When the flow was Q/Qdes =0.61,the head drops to the lowest point and enters the stall condition,and there was an obvious hump area in the flow-head curve.In the near stall condition,tip leakage vortex was appeared in the tip passage of the axial flow pump,accompanied by leading edge overflow and secondary leakage;At lower mass flow rates,the angle between the tip leakage vortex and the blade bone line keeps increasing,and the initial position of the leakage vortex developed from the chord length in the middle of the blade to the impeller inlet,which leaded to a significant increase in the proportion of inlet pipe entropy production under small mass flow rate conditions.After entered a deep stall condition,the tip leakage vortex was broken,causing the blockage of the tip passage and the deterioration of the flow field inside the impeller,which intensified the swirling flow at the inlet of the impeller,resulting in an increase in flow loss and a decrease in the pump head.2.Different end wall treatment technologies improve the mechanism of axial flow pump hump.Five different end wall treatment technologies have different effects on the hydraulic performance of the pump.The end wall slots had the strongest hump suppression ability.The head improvement under stall conditions is 83.4%,and the design condition efficiency is reduced by 4.3%;re-circulation structure at the design condition,the efficiency is only reduced by 2.9%,but the stability expansion effect is only 65.8%.The internal fluid of the end wall slot,re-circulation and staggered re-circulation structure all flow from the downstream to the upstream of the end wall,forming a flow circulation,to achieve the purpose of suppressing the tip leakage vortex.The tip of the water jet generates a high-energy jet through an external water source,thereby reducing the load on the tip of the blade.Except for the treatment technology of the circumferential groove end wall,other structures can effectively reduce the blade tip blockage.3.The end wall slots model with better improvement effect was studied parametrically.The results show that the hump area of the Axial-Flow Pump can be effectively suppressed by the axial slots.The pump head and efficiency were increased by 73.9% and 10.1% in the hump area respectively.The ability of the axial slots in depressing hump enhances when increasing the slot numbers and the slot length.However,when the number of slots was more than 25 and the length of the slots was more than 50%Ca,the efficiency of the pump under design conditon will be reduced by 4.3% and 13.6%,respectively,that had a greater negative impact on pump efficiency.When the radial skewed angle of the slot was not exceed 45°,it was beneficial to suppress the hump of the pump,but an excessively large radial skewed angle will reduce the head of the pump.Placing the slot at the impeller inlet and appropriately increasing the radial skewed angle was more conducive to suppressing the hump of the pump.But an excessively large radial skewed angle will weaken the restraining effect on the hump.4.Research on the mechanism of end wall slot to improve hump of axial flow pump.The flow recirculation of injection and suction is found to be established in the slots.Under the effect of the flow recirculation,the relative flow angle above 0.9 span is significantly reduced under the stall condition,maximum drop 67°.The tip leakage vortex is controlled successfully by the slots,and the average leakage intensity is reduced by 41.4%.The thickness of the surface layer in the middle of the blade tip was reduced by 15 mm.Effectively suppressed the blockage caused by the interaction between the tip leakage vortex and the main flow.and the pressure fluctuations induced by the tip leakage vortex near the blade tip is remarkably weakened by the slots.Consequently,the axial slots have great potential in improving the hump area and enhancing the efficiency under the stall condition for the axial-flow pump.