| The centrifugal pump is one of the most widely used fluid machinery. The low-specific-speed centrifugal pump exist three problems for lower efficiency, bad cavitation performance and unsteady flow. With the development of modern test technique and the CFD technology, the PIV measurement and the 3D unsteady turbulent flow simulation of a centrifugal pump considering of the impeller-volute interaction have attracted many researchers in the pump field recently. In this paper the unsteady flow phenomena and characteristics were analyzed, the PIV measurement and the numerical simulation of the 3D unsteady turbulent flow of the various centrifugal pumps were carried out. Through comparative studies, the flow field structures and the unsteady flow characteristics were obtained, which not only laid the theoretical basis for the optimization design of the low-specific-speed pump; but also help realize the accurate performance prediction. The main work and the achievements are as follows:(1) By critical review the trend of the inner flow measurement and numerical simulation for centrifugal pumps, the main flow phenomena in centrifugal pumps was thoroughly analyzed. The numerical simulation models and computational methods for 3D viscous turbulent flow in the centrifugal pump were discussed.(2) Using the advanced PIV measurement technology, the flow instantaneous velocity in the centrifugal pump was measured. According to the special requirements for the centrifugal pump PIV measurement, a PIV testing rig was established to measure the instantaneous velocity in the testing pump with four impellers by different design methods at different running conditions. The measurement results showed that: the fluid in the impeller designed by the low-specific-speed design methods flows along the blade and there is no wake zone at the suction side of the blade; for the impeller by the normal design method, the fluid is heavily squeezed at the impeller blades inlet and then the bubbles would be easier to appear; for the impeller with splitter blades, if the splitter blade is a bit shorter, the fluid would flow from the suction side of the blade to the pressure side of the blade and there exists obvious vortex zone which would be smaller with the increase of the flow rate. While if the splitter blade is longer, the fluid in the impeller passage flow irregularly and the vortex appeared at the suction side of both the long blade and the splitter blade. Thus, from the PIV results, the different design schemes were evaluated.(3) In order to comparatively study the flow characteristics in the centrifugal pump with different impellers by various design methods, the steady flow in the single impeller, the steady and unsteady flow in the impeller and volute considering the rotor-stator interaction were simulated, and the evaluation of the five impellers based on different design method was obtained.The Reynolds-averaged Navier-Stokes equations closed by the two-equation RNG k-εturbulence model were solved. The SIMPLEC pressure-correction technique on a non-staggered gird arrangement was employed. Then the numerical simulations of the 3D viscous turbulent flow in the five impellers based on different design schemes were carried out. This numerical simulation model was proved accurate for engineering through the comparisons between the simulation results and the measurement results. The distributions of the velocity, pressure and turbulent intensity of the flow in the five impellers at design and off-design conditions were showed and compared, which reveals the flow field structures of the centrifugal pump impeller. The fluid in the impeller designed by the low-specific-speed design method flows mainly along the blade, and the relative velocity near the suction side of the blades is larger. There is no low-speed wake zone at the outlet of the blade suction side, which agrees with the PIV results. While for the normal-designed impeller, there exists a wake zone at the outlet of the blade suction side, and there also appears a large-area low-speed vortex zone at the pressure side of the blades, which increase the relative velocity of the main flow. For the impeller with splitter blades, there exists an obvious vortex zone near the pressure side at the blade inlet section and fluid flows from the suction side to the pressure side of the blade, which also agrees with the PIV results. Because the existence of the splliter blade prevented the forming and developing of the wake, there is no wake zone near the suction side of the long blade outlet. When the splliter blade leaned to the suction side of the long blade, the fluid in impeller passages on both sides of the splliter blades are balanced and the flow is more even. These evaluations to the five design schemes are helpful for optimization design of the centrifugal pump impeller.(4) A CAD hydraulic design software for the pump volute was developed under the AutoCAD platform. The 3D steady turbulent flow simulation considering the impeller-volute interaction was finished. The 3D unstructured grid was generated by the multi-block and the local-refinement grid techniques. A frozen rotor method and the multi-reference coordinate model were used. The numerical simulation results showed that the inlet velocity and static pressure of the volute along the circumference obviously presents the periodic fluctuation characteristics, the appearance and its location of the jet-wake zone are related with the flowrate and impeller structure. All these results further verified the strong non-symmetrical pattern of the flow field from the impeller to volute. The numerical comparisons of the flow for the pump with five impellers operated at different conditions revealed the effects of the rotor-stator interaction on the flow field. The characteristics were obtained through these numerical simulations and compared with the corresponding testing results. Accordingly, the accuracy of our numerical models and the reliability of the performance prediction were further verified, and all these could be references for the performance optimization of the centrifugal pump.(5) 3D unsteady turbulent flow field was simulated for the centrifugal pump with the arbitrary sliding mesh technology. The periodic fluctuation phenomenon and the unsteady flow characteristics in the centrifugal pump caused by the interaction between impeller and volute tongue were investigated. The static pressure fluctuation in the volute and at the volute outlet and the instantaneous head rise changes were showed for the pumps with five different impellers operated at design and off-design conditions. The pressure fluctuation is strongest near the tongue and is slighter at the volute outlet section. The transient head fluctuation increases with the flow rate. The transient head fluctuation of the low-specific-speed designed impeller is the heaviest, while that of the normal-designed impeller is the smallest among five impellers. Among three splitter-blade impellers, the transient head fluctuation of the impeller with splitter blades leaned to the corresponding suction side of the long blades is the smallest. Eventually, the evaluation to the different design schemes was realized from the unsteady flow analysis. The study results can be further used to study the hydraulic noise and vibration induced by the unsteady flow in the centrifugal pump.(6) The performance test, the PIV measurement of the inner flow and various numerical simulations were systematically analyzed for the centrifugal pump. All these research results showed that the low-specific-speed design method and the splitter-blades design method will be better and more appropriate than the normal design method for the low-specific-speed centrifugal pumps. For the splitter-blades design method, if the splitter blades are appropriately leaned to the suction side of the long blades in peripheral direction, the flow performance of the centrifugal pump will be better.
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