Numerical simulation of pressure fluctuation in postpositional bulb tubular pump
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Abstract:
Bulb tubular pump is widely used in South-to-North Water Transfer Project because of high efficiency,large flow rate,and excellent hydraulic performance.Abnormal hydraulic resonance could cause when large pumping stations operate under off-design conditions,and the operation of pumping stations could disturbed by abnormal hydraulic resonance.Therefore,it is urgent to study and prevent abnormal hydraulic resonance due pressure fluctuation of impeller and guide vane of postpositional bulb tubular pump in South-to-North Water Transfer pumping station under different working conditions. The pressure fluctuation in the vane area of the impeller under the condition of partial flow and water level was calculated and analyzed by CFD method.To analyze the pressure fluctuation law of impeller and guide vane area in bulb tubular pump,18 pressure fluctuation monitoring points were arranged.The frequency-domain diagram of pressure fluctuation was obtained by fast Fourier transform of time-domain data of pressure fluctuation.The variation characteristics of amplitude and frequency of pressure fluctuation were analyzed by frequency domain diagram of pressure fluctuation. The results showed that the time-domain diagram of pressure fluctuation in the impeller and guide vane was periodical.The primary and secondary frequency of pressure fluctuation was influenced by the number of impeller blades.The primary and secondary frequencies of the impeller and guide vane area were all integral multiples,and the pressure fluctuation amplitude of the impeller and guide vane area was decreased from the rim to the hub.The pressure fluctuation amplitude in the impeller area was significantly greater than that in the guide vane area.Under the partial water level condition,the amplitude of pressure fluctuation in the impeller and guide vane area was slightly larger than that under the design condition,and the primary and secondary frequencies were not changed.The pressure fluctuation amplitude of small and large flow conditions under partial flow conditions was greater than the design condition.The pressure fluctuation amplitude of the small flow condition at each monitoring point was about 23 times of the design condition.The low-frequency fluctuation was obvious under the small flow condition.The pressure fluctuation amplitude of the pumping units was greatly influenced by the operation under the partial working condition.Pumping unit operation efficiency was seriously affected by the long-term off-design operation. Under the design condition,the time-domain diagram of pressure fluctuation of each section was obvious periodicity.The law of fluctuating pressure in a cycle was affected by the number of impeller guide vanes,and the pressure fluctuation amplitude in the guide vane area was less than that in the impeller area as a whole.The primary and secondary frequency of pressure fluctuation at the inlet and outlet of the impeller and the outlet of the guide vane were affected by the impeller,guide vane and front and rear support plates,which was an integral multiple of the conversion frequency.The distribution law of pressure fluctuation was affected by the dynamic and static interference between the impeller and the guide vane or support plate.The pressure fluctuation amplitude at the impeller inlet changed little from the rim to the hub,the pressure fluctuation amplitude at the hub was slightly larger than the rim,and the pressure fluctuation amplitude at the impeller outlet and guide vane outlet was decreased gradually from the rim to the hub.The pressure fluctuation amplitude under partial water level conditions was large than that under design conditions,and the pressure fluctuation was affected by the amplitude of the water level difference between upstream and downstream.Under the off-design conditions,the pressure fluctuation amplitude of each section monitoring point was large than that of the design condition both under small flow rate and large flow rate,and frequent low-frequency fluctuation occurred under small flow rate.Abnormal hydraulic resonance was easy to occur under off-design conditions,which aggravated the cavitation of the pump impeller and generated adverse flow patterns.During the operation of the pumping station,off-design conditions,especially small flow conditions,should be avoided as far as possible.
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