Abstract
The RNG k-ε turbulence model combined with cavitation model was used to simulate unsteady cavitating flows inside a double-suction centrifugal pump under different flow rate conditions based on hexahedral structured grid. The numerical external characteristic performances agree well with the experimental performances. The predicted results show that the turbulence kinetic energy and the turbulence dissipation rate inside the impeller at design flow rate are lower than those at other off-design flow rates, which are caused by various vortexes. Based on frequency-domain analyses in the volute casing, the blade passing frequency is the dominant one of the pressure fluctuations except the vicinity of volute tongue for all operating cases, and the dominant frequency near the volute tongue ranges from 0 to 0.5 times the blade passing frequency for other off-design points, while the blade passing one near the volute tongue is the dominant one of the pressure fluctuations at design point. The increase of flow rate reduces the pressure fluctuations amplitude. For cavitation cases, the blade loading of the middle streamline increases a bit during the initial stage, but, for serious cavitation, the blade loading near the blade inlet reduces to 0 and even negative values, and the serious cavitation bubbles block the blade channels, which results in a sharp drop in pump head. Under noncavitation condition, the predicted power related to the pressure in the impeller channels increases from the inlet to the exit, while, under different cavitation conditions at the design flow rate, these power-transformation distributions in the impeller channels show that these power conversions are affected by the available NPSHa and the corresponding work in leading regions of the blades increases increases gradually a bit, and then it increases sharply in the middle regions, but it decreases in the blade trailing regions and is greatly influenced by secondary flows.
Highlights
Centrifugal pumps are the most commonly used type among various pumps, where double-suction centrifugal pumps have advantages of large flow rate, high efficiency, high head, and so forth, which occupies a large proportion of the pump products
Based on the RNG k-ε turbulence model and the hexahedral structured grid, the unsteady flow inside a double-suction centrifugal pump was numerically investigated under different flow rate conditions
The blade loading and power distributions related to the cavitation at design flow rate were analyzed systematically
Summary
Centrifugal pumps are the most commonly used type among various pumps, where double-suction centrifugal pumps have advantages of large flow rate, high efficiency, high head, and so forth, which occupies a large proportion of the pump products. The internal flow inside a double-suction centrifugal pump is unsteady and extremely complex. Chakraborty et al [7] analyzed the static pressure distribution and characteristics of the two-dimensional (2D) incompressible flow inside a centrifugal pump’s impellers with different blade numbers, and the predicted results displayed that the head and static pressures of the pump increase with the increase of the blade number, while the efficiency with seven-blade. Gonzalez et al [8] investigated numerically the internal flow inside a double-suction centrifugal machine in the pump operation mode and found that the flow in the double-suction chamber is uniform at design flow rate but has a strong unsteady characteristic at off-design flow rates. When the pressure inside the pump is below the saturation water vapor pressure, cavitation may occur It will cause noise, performance breakdown, and costly damage to hydraulic machineries [9]. For different NPSHa, the blade loading and the vapor volume fraction for cavitation flows in the impeller were simulated numerically and analyzed systematically at design point, and the studies on the influence of the cavitation and secondary flows on power distributions were carried out
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