Research On Characteristics Of Centrifugal Pumps At Shut-off Condition

With the rapid development of social economy, the characteristics of centrifugal pumps at shut-off condition (SOC), such as nuclear power pumps, marine pumps, renewable energy pumps and so on, have become indispensable to the hydraulic design. The characteristics of centrifugal pumps at SOC are not considered in traditional design, which results in that the present study on characteristics at SOC is very few. Therefore, it is urgent to research characteristics at SOC to develop the modern design methods of centrifugal pumps. Under the financial support from National Natural Science Foundation of China (51079062,51179075), the characteristics of centrifugal pumps at SOC are researched deeply in this paper by using theory analysis, numerical simulation and test. The main research contents and important conclusions of this paper include:1. The present situations and development trends on related aspects of centrifugal pumps were summarized, such as characteristics at SOC, inner flow simulation, PIV test, fluid structure interaction, flow induced vibration and performance prediction and so on.2. The effects of impeller main parameters on characteristics at SOC, including blade outlet angle, blade outlet width, blade numberm, blade wrapping angle and blade inlet attack angle, are studied by experiment test for the first time. The effects of those parameters are compared with each other in detail.3. The main calculation methods of head at SOC for centrifugal pums are summarized and the concrete formula of each mthod is presented. Based on experimental data of23centrifugal pumps, the7formulas of head at SOC which are used often are modified, and the coefficients are calculated by regression analysis method. The practical applications show that the modification can improve the accuracy of each formaula obviously.4. Based on BP artifical neural network, the prediction model of power at SOC for centrifugal pumps is established firstly, and the input model and topology of the network are presented. The different data of46centrifugal pumps are used to train the network model, and the different data of another3centrifugal pumps are used to check the network model. The resulats indicate thet the average prediction deviation of the networl is4%, and the predition model can be used in engineering application.5. The numercial simulation method for inner flow in centrifugal pumps at SOC is established, and the method is used to simulate the inner flow in3different centrifugal pumps. According to the simulation results, the charactersistics and inner flow structure of the3pumps are analyzed. The main conslusions are following.(1) The prediction deviations of head at SOC of the3pumps are all less than5%.(2) The unsteady flow in impeller has significant imapcts on flow in inlet pipe, and the influence distance is ten times pipe diameter. Only the rotation frequency has a few effects on pressure fluctuation in let pipe.(3) With the increase of specific speed, the vortex at impeller outlet gets smaller, while the vortex at impeller inlet becomes larger, and the "jet-wake" at impeller outlet gets more obvious. The rotation frequency has obvious impacts on pressure fluctuation at impeller inlet, while the pressure fluctuation at impeller outlet and volute is mainly affected by blade passing frequency.(4) The pressure fluctuation curves at all points in outlet pipe are same. The number of wave crest on each curve is equal to blade number, and the maximum fluctuation amplitude occurs at blade passing frequency.6. The inner flow in a centrifugal pump at SOC, whose specific speed is65, is tested by PIV technology, and the experiment results are compared with simulation results in detail. The comparison shows that except the flow field at the impeller inlet, the inner flow distribution obtained by PIV is basically similar to that obtained by numerical simulation. The difference of flow field at impeller inlet probably results from sedimentation of tracking particles.7. At SOC, the structure field and flow field of a centrifugal pump, whose specific speed is46.2, are calculated by fluid structure interaction (FSI), and the effects of FSI on head and inner flow field are discussed. Also, the impeller distortion is analyzed in detail. The following3results are obtained.(1) The calculation of FSI can improve the prediction accuracy of head at SOC.(2) The pressure fluctuation in impeller obtained by calculation of FSI presents more obvious periodicity, and the fluctuation amplitude gets bigger.(3) The displacement and stress at the location which is near to the volute tongue are bigger. The displacement increases gradually from blade inlet to outlet, but the biggest stress is not at the blade outlet.8. The flow induced vibration in a centrifugal pump at SOC, whose specific speed is65, is tested by using acceleration sensors, and the test results are analyzed carefully. The main conclusions are as follow.(1) The axial vibration presents weak periodicity, while the radial vibration has no periodicity.(2) The fluctuation amplitude at volute tongue is the biggest.(3) The biggest acceleration of each test point is at1400Hz, which is about10times blade passing frequency.(4) The axial vibration at section5of volute is the most violent, while the radial vibration at section7is weakest.