| This work was supported by the State Key Program of National Natural Science Funds of China （Grant No.51239005）, National Science&Technology Pillar Program （Grant No.2011BAF14B04） and Jiangsu Provincial Project for Innovative Postgraduate of China （Grant No. CXZZ110564）.The running cost of low specific-speed centrifugal pumps are higher compared with those of high specific-speed ones, due to their comparatively lower efficiency. Therefore any small improvement in the low specific-speed centrifugal pump’s efficiency will make significant savings to the running cost. One effective way to improve pump efficiency is to increase the speed and thereafter to increase the specific speed, while that will cause the negative effect on pump reliability and cavitation performance. Cavitation is a common fault in centrifugal pumps and, if a pump operates under cavitating conditions, the following situations will always be observed:reduction in pump capacity, head degradation, high noise level and high levels of vibration. Based on numerical and experimental methods, the leading edge cavitation attached on the radial impeller, which is known as the most erosive type and the origin of the head drop phenomenon, was studied in the aspects of cavitation detection, numerical simulation of cavitation characteristics, head drop caused by cavitation and unsteadiness behaviors with rotor-stator interaction （RSI） effects. The cavitation instabilities and flow instabilities under part flow rates were also discussed in the paper. The main work and creative achievements of the dissertation include:（1） To investigate effects of grid quality on the calculation accuracy for flow field simulation in centrifugal pumps, taking structured grid as research objects, the near-wall mesh of pumps were constructed and evaluated in three aspects from the number of grid, subdivision of near-wall mesh and blocking strategies. The relationship between the near-wall mesh and the turbulence models were also analyzed to find a reliable and feasibility numerical method for the centrifugal pumps. Numerical simulation of whole flow field model and simple model in CFX for the pump were also carried out and the reason of the difference between two models was compared. A significant improvement for the numerical results of cavitating flows has been obtained with the whole flow field model. The whole flow field model shows a high accuracy than the simple model, and the flow pattern of two models were also different. （2） A closed loop was established to investigate the pump cavitation phenomenon. Different kinds of methods were used to detect the cavitation performance, including vibration signal obtained from four accelerometers on the pump casing, the pressure fluctuation signal from two pressure transducers close to the inlet and outlet of the pump, the waterborne acoustic signal from four hydrophones in the suction and discharge pipe close to the pump. The stator current of the motor driving the pump was also described in the thesis. The statistical parameters for PDF （Probability Density Function）, Variance, Standard Deviation and RMS （Root Mean Square） were used to analyze the time domain signal to determine whether they could be used to detect cavitation. The results show that different statistical parameters used in this thesis are able to detect cavitation breakdown condition. Both time and frequency domain analysis on stator current were conducted with respect to the cavitation phenomenon, and it was found that it is sensitive to the cavitation incipience and therefore can be used to identify the cavitation incipience condition.（3） The origin of the cavitation head drop was discussed and an analysis of its mechanism in the pump was proposed. The hydraulic losses caused by cavitating flow can be divided into two types:One type is cavitating flow that modifies the flow pattern strongly inside the blade-to-blade channel, which increases the kinetic energy at the trailing edge of the suction blade and directly induces the head drop. The other is the pressure distributions around the blade modified by the sheet cavity attached on the blade suction side. This clearly reduces the blade load, which influences the energy transfer and causes an increase of the losses. For a centrifugal pump, a small decrease of the head drop can be associated with the flow pattern near the blade leading edge, while head breakdown is clearly associated with the vortex structure near the blade trailing edge.（4） The cavity volume increases rapidly when the NPSHA reducing to a certain value, the cavitation zone appears at the inlet of impeller near the wear ring under serious cavitation condition. As for the cavitation region located at the pressure surface of the blade, the cavity is unstable and prone to induce head drop, the largest area is near the shroud. For the suction surface, the recirculation and the re-entrant jet in the cavity wake are responsible for the cavitation instabilities. The relationship between the transient cavitating flows and pump head instability was discussed in the thesis. With the pressure decreasing at the pump inlet, vapor will gradually fill the spaces inside the inter-blade passage along the impeller blades and causes the relative velocity to increase. The collapse of the cavity due to the local higher pressure leads to an increasing in turbulent velocity fluctuations, the amplitude of the pump head fluctuation increases for unsteady cavitating flow, which is absent in cavitation-free case.（5） The cavitation performance is deterioration and the cavitation instabilities are increasing due to the existence of volute. In order to verify the relationship between the cavitation performance and the intensity of rotor-stator interaction （RSI）, four impellers with different diameters were measured and the experimental results show that the changes of impeller diameter affect the impeller’s ability to control the fluid. The results confirm that the intensity of RSI is one of the main reasons that account for cavitation instabilities.（6） The large-scale vortical structures observed in the cavitating region and stall caused by the flow decreasing have been systematically summarized and analyzed, and the results show that it is necessary to find the relationship between the cavitation instabilities and flow instabilities when centrifugal pumps operate under part flow rates. Pumps with a positive slope in the head-flow performance curve are investigated, the relationship between the head-flow characteristic instability and design parameter is discussed in detail, and the change in flow topology in the pumps are also discussed by theoretical analysis, experiments and numerical simulation. Understanding these areas will clarify the origin of the positive slope of the head-flow performance curve for a centrifugal pump. The cavitation performance under part-load flow rates which has a relationship with the attack angle of the blade and the large-scale vortex in the impeller was also analyzed in the paper. The proposed conclusions can provide basis for pumps operating under cavitation instabilities and flow instabilities. |
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