Pressure pulsation and shaft system modal optimization of a large shaft tubular pump
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Abstract:
Pumping station is an important water conservancy infrastructure,which plays a key role in flood control and drainage,cross-basin water diversion for farmland,and regional irrigation.The analysis of the natural vibration characteristics of the shaft system in water and air makes the natural frequency of the structure avoid the possible excitation frequency of the outside world and prevent resonance,which is of special importance to improve the safety and stability of the pump unit in the process of operation and prolong the life of the unit. A large shaft tubular pump is taken as the research object,based on the equation of SST(shear stress transport)model considers the transmission of turbulent shear stress,and through the modal analysis,it can provide a reference basis for the optimization of the vibration characteristics of the structure and in the process of structural design so that the inherent frequency of the structure can avoid the possible external excitation frequency,which can prevent the structure from resonance. It is found that:the water flow in the outlet channel will produce some vortex bands,which have some influence on the performance of the pump,among which the angle of placement +2° vortex band is the most obvious.The runner inlet pressure pulsation has good periodicity,and its main frequency is 8.6 Hz,which is 2 times the runner blade frequency,and the second frequency is 4.3 Hz,which is the runner blade frequency.Under 3.2 and 3.8 m head conditions,the main frequency of pressure pulsation at the outlet of the runner is 4.3 Hz,i.e.the blade passing frequency,while the opposite is true under the low head condition of 0.5 m,which indicates that the main pressure pulsation at the outlet of the runner is caused by the rotation of the runner,whereas the flow is more unstable under the low head condition.The inherent frequencies of the impeller rotor in the air at different angles are calculated for the first eight orders.It can be seen that more blade is deflected to the negative angle (-2°),its inherent frequency has a decreasing trend,but the difference is small.The inherent frequencies of the first eight steps of each scheme are calculated,with the increase of shaft diameter,the inherent frequencies of each step gradually increased,and the first step frequency increased by 18%.With the increase of shaft length,the inherent frequencies of each step gradually decreased,while with the gradual growth of bearing length,the inherent frequencies of each step also increased,and the first step frequency increased by 83%.It can be seen that the influence of the change of support position and shaft diameter on the inherent frequency is more obvious. Conclusions(1) Comparing the flow field distribution under different heads and different placement angles,the pump streamlines flow pattern is better under the design head and placement angle of -2° working condition.(2) The pressure pulsation at the inlet of the runner has good periodicity,its main frequency is 2 times the runner blade frequency.The second frequency is the runner blade frequency,the rotation of the runner is the main cause of the pressure pulsation.The main frequency at the outlet of the runner is the blade passing frequency,indicating that the main pressure pulsation is caused by the rotation of the runner,and the flow is more unstable under low head conditions.The amplitude of pressure pulsation at the exit point of the runner is smaller than that at the inlet of the runner,and the runner has the role of stabilizing the water flow.(3) Comparing the inherent vibration pattern of the runner in air and water,it can be seen that part of the kinetic energy is transferred to the water body,which leads to the decrease of the inherent frequency of the runner in the water body,but the decrease is small,and the higherorder frequencies of the dry and wet modes are the same.Comparing the nonconstant calculation results and modal analysis results,the main frequency of pressure pulsation is mostly the rotational frequency,which has a large gap with the inherent frequency of the runner and is not easy to cause resonance and meet the structural strength requirements.(4) The effect of increasing the support position and increasing the shaft diameter on the inherent frequency is more obvious,and the firstorder frequency increases by 83% and 18%,respectively.
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