Study On Cavitation Characteristics Induced By Inlet Backflow In Centrifugal Pumps

When a centrifugal pump runs at low flow rates, backflow occurs near the impeller eye due to the pressure difference of the blades near the leading edge, which will cause flow oscillation and pressure pulsation, as well as induce vibration and noise. The shear layer between the swirling backflow and the main flow rolls up and forms a backflow vortex at the impeller’s inlet. The backflow vortex cavitation occurs if the pressure drops lower than the vaporization pressure. In order to predict the cavitation characteristics associated with the inlet backflow of the centrifugal pump, the hydraulic performance tests were firstly carried out to verify the accuracy of the corresponding numerical results obtained from three pieces of commercial CFD software, one of which with suitbable numericl models was therefore applied to analyze the characteristics of backflow at the impeller inlet under non cavitation conditions. Moreover, the flow patterns of backflow vortex cavitation and its influence on performances of low specific speed centrifugal pump were futher studied. The main work and achievements can be as follows:1.The reseach background on backflow vortex cavitation and its generation mechanism were described. A brief introduction of the inlet backflow, backflow vortex cavitation and numerical methods on cavitating flows both at home and abroad were given, respectively.2.The current cavitation models included in three pieces of commercial software like CFX, Fluent and Pump Linx were summarized. Compared with the test data of hydraulic performances of the model pump, it can be found that the numerical models in Pump Linx were more accurate to predict the first critical cavitation numbers, while the second critical cavitation numbers predicted by Fluent were more closed to its corresponding experimental data. The results predicted by CFX were more accurate than the other two especially at low flow rates, and therefore CFX was chosen to study the backflow vortex cavitation which is the main concerning in this paper.3.The generation of backflow and its effect on the inlet flow under non-cavitation conditions were analyzed in detail. Moreover, the numerical simulation of cavitating flow was carried out by adding a cavitation model to analyze the effect of the vapor volume on the head dropping of the pump, and the characteristics of cavitation induced by backflow. The results showed that:(1)Due to the occurring of backflow, the cavities in the main flow may grow intensely, and the maximum total pressure shows a counterclockwise distribution in the cross secton of the inlet pipe. Meanwhile, it can also be found that the lowest static pressure occurs in the place of the blade leading edge where the ratio of the parameter r/R1 is about 0.5, with the areas of cavitation inception changing at the same time.(2)The vapor volume distribution inside the blades shows uniform at the design flow rate(Qd), which becomes asymmetrical at a lower flow rate of 0.4Qd. When the cavitation number σ is 0.249, backflow vortex cavitation occurrs near the blade leading edge, while cavities between blade No.1 and blade No.6 disappear when the value of σ is 0.049.(3)Backflow vortex cavitation occurs firstly at free state, and then develop and attach to the blades as the cavitation number decreases. It shows a slight increase before head dropped sharply due to the developing and collapsing of the backflow vortex cavitation bubbles.4.Unsteady numerical simulation of cavitating flow were carried out based on the relevant steady numerical results. The fluctuation of head, cavity volumn, axial speed and pressure induced by backflow vortex cavitation were analyzed. Meanwhile, the developing of backflow vortex cavitation as time changes was studied. The results showed that:(1)Under the condition when the flow rate is 0.4Qd and the cavitation number σ is 0.056, the backflow vortex cavitation rotates with the rotating impeller. There are three rotating components around the blade leading edge. The occurring of backflow vortex cavitation has a strong impact on the fluctuation of head, cavity volume and axial speed.(2)The main frequency of pressure fluctuation in the center of the inlet pipe changes from high to the low frequency, with its amplitude increasing as the cavitation number decreases when the flow rate is 0.4Qd. The main frequency is the blade passing frequency fb when the cavitation numbers σ are 1.923 and 0.764, while it becomes 0.2 of the shaft frequecy fn when the value of σ is 0.056.(3)The phase coherence analsis of the pressure fluctuation both at the impeller inlet and blade leading edge were carried out. Under the condition when the flow rate is 0.4Qd and the cavitation number σ is 0.056, there were no rotating components near the impeller eye, while three rotating components around the blade leading edge were found whose corresponding propagating radios are 0.35, 0.66 and 1.95, respectively.