Effects of the Circulating Jet Flow on the Suction Flow Field and Cavitation in a Canned Motor Pump

Abstract

Cavitation is a general phenomenon in centrifugal pumps. When the inlet pressure near the leading edge of the blade is lower than the saturated pressure, cavitation would develop in the impeller. As cavitation occurs, the pump head will drop rapidly and the pump efficiency will decrease. In addition, severe vibration and noise will be induced. Cavitation performance is considered as an important factor in many industrial applications, and affected by various conditions. The canned motor pump is a special type of non-seal centrifugal pump. The pump and motor are integrated. In order to cool the motor and lubricate the bearing during the operation, a portion of fluid, called the circulating flow, is withdrawn from the impeller outlet, and then flows along the cooling circle within the motor. Finally, the circulating fluid moves through the hollow shaft and merges with the main suction flow near the impeller inlet, which can be defined as the circulating jet flow. The jet flow will alter the uniform velocity distribution at the impeller inlet as its direction is opposed to the main suction flow. Consequently, it is expected that the cavitation performance of the pump will drop drastically. It is necessary to analyze the effect of the jet at the pump inlet on the cavitation performance. In this paper, in order to illustrate the jet flow on the pump performance, a numerical simulation method is applied to depict the fluid flow field and cavitation performance of a canned motor pump. For the turbulent model, the standard k-ε turbulent model is adopted. To capture the cavitation performance of the pump, the Zwart-Gerber-Belamri cavitation model was used to investigate the steady cavitation flow through the entire flow channel. It can be seen from the numerical results that the internal jet flow formed by the coolant circulation has a significant effect on cavitation performance. At the pump inlet, the velocity field is divided into three regions: the internal jet flow region, the main-stream region, and the backflow region. The internal jet presents a typical submerged jet structure and its existence results in the non-uniform inlet flow distribution. For the jet flow, it extends to the pump inlet and exhibits an asymmetric characteristic. The static pressure near the impeller inlet with the internal jet is drastically reduced compared to the case without the internal jet structure, and a local low-pressure region occurs around the outlet of the jet nozzle. The cavitation performance of the pump with the internal jet drops obviously. At the off-design condition, the cavitation performance of the pump is seriously degraded. From quantitative data, it indicates that the NPSH3 increases by more than 1.51 times compared with that of the original impeller under design condition. The cavitation inception occurs on the suction side of the leading edge of the impeller near the hub, and then cavitation also occurs near the outlet of the jet nozzle. Finally, the cavitation occurs in the transition region between the internal jet and the main-stream flow regions. So, it is believed that the deterioration of cavitation performance is caused by the combined effect of the non-uniform flow distribution and the cavitation at the internal jet region.