Investigation Of Stall Characteristics In Centrifugal Pumps

Stall often occurs at part load condition in the pump. Under the stall condition, the periodic generation and shedding of stall cells always induces the low frequency pressure fluctuation, and makes the noise stronger and intensifies the vibration, which has severe influence on the safety and stability of pumps. It is necessary to increase knowledge of stall phenomenon to improve the safety and stability operation. This dissertation focuses on stall phenomenon in the centrifugal pump impeller based on large eddy simulation method.Firstly, the computation model for stall flow in a centrifugal pump is builted. The current study assesses the influence of some typical SGS models suitable for predicting stall phenomenon in pumps. The suitable computation strategy is given based on the dynamic mixed nonlinear model (DMNM), which establishes the foundation for predicting the stall flow in the pump accurately.Secondly, the characteristic parameters and occurrence condition for two types of stall phenomenon are put forwarded.The results showed that the blade number had significant effects on the stall types. As the blade number is even, the alternative stall phenomenon occurs. Otherwise, when it is odd, the rotating stall phenomenon appears. Furthermore, the blade number also has influence on the characteristic parameters of stall cells. Under the alternative stall condition, as the increase of blade number, the number of stall cells increases, but the stall frequency decreases. Under the rotating stall condition, with the increase of blade number, the number of stall cells also increases.Thirdly, the flow structures and pressure fluctuation characteristics are further analysed based on the previous model. For alternative stall, the stall cells are fixed relative to the impeller, but a large vortex in the stalled passage is always swaying. The outlet vortex generates from it, and then develops and sheds periodically. The pressure fluctuation caused by outlet vortex motion, which acting on blades, appears as a "broadband" with high frequency. Further, the large vortex shows obvious life cycle including decay, split, mergence and growth, which results in a low frequency compared with the impeller passing frequency. For rotating stall, the stall cells first occur in the suction side of the blade. With the growth of the stall cells, the block area gradually increases until almost blocking the inlet region, then moves to the pressure side with the continuous decay. When the rotating stall occurs, the amplitude of pressure fluctuation is much higher than that under alternative stall condition. The propagation of stall cells have significant effect on the pressure fluctuations in the impeller. The dominant frequency of pressure fluctuation at blade is rotating stall frequencyLastly, The impeller-volute interaction around the tongue region in centrifugal pumps is investigated, which reveals mechanism of pressure fluctuations propagation under stall condition. After the alternate stall occurs, a vortex always appears in the fixed position (near the tongue) when the blade passing the tongue, which is called "fixed stall" phenomenon. Three stall cells near the entrance of passages can be observed in the pressure distribution. At initial stall condition, the stall cells play the leading role. The dominant frequency is3times of the rotation frequency, which is50%of the blade passing frequency. However, when the flow rate is further decreased, the "fixed stall" cells play the leading role. The dominant frequency of the pressure fluctuations near the tongue is6times of the rotation frequency, which is the blade passing frequency.The investigation achievements provide a valuable guidance to improve the hydraulic design and operating stability for centrifugal pumps.