Transition process characteristics of centrifugal pump with power-off based on entropy production theory

In this paper, a transient numerical calculation method is established based on ANSYS CFX software and user-defined program, which can simulate the power-off runaway transient characteristics of a centrifugal pump. The variation rule of external characteristic parameters with time can be obtained through numerical calculations and compared with experimental data. The comparison of results show that the simulation results agree well with experimental results, the proposed numerical method has been therefore validated. During the power-off runaway transient process, the centrifugal pump system will successively undergo the pump condition, braking condition, turbine condition and runaway condition, with external characteristic varying significantly during the process. Internal flow structure is closely related to external characteristic and affects each other. The hydraulic loss due to friction and unstable flow patterns in the pump causes a drop in hydraulic efficiency. The traditional method for analyzing the hydraulic loss is by evaluating the total pressure drop, which has certain limitations and cannot determine the exact locations at which the high hydraulic loss occurs. In this study, entropy production theory has been adopted to obtain a detailed distribution of the hydraulic loss in the centrifugal pump. Through the use of the entropy production theory, the variation of the entropy production was obtained. The distribution of energy loss of flow components during this process is obtained by using the entropy generation theory, and the energy loss is analyzed. The results show that there is a clear correlation between the entropy production and the external characteristics of the centrifugal pump during the entire runaway process. The entropy production also undergoes dramatic changes at the moment of sudden changes in flow rate, rotational speed and torque. The main losses of the impeller, guide vanes and suction pipe are caused by turbulent dissipation, and the wall entropy production accounts for about 10%-15% of this total entropy production, which means the wall effect could not be ignored. However, the loss of the volute is mainly due to the strong wall effect in the near wall region, which is different from other three flow components, denoting the main loss generation mechanism is different. In addition, it is found that both the energy loss and variation range of the impeller and guide vanes are relatively larger than those of other flow components in the whole process. By analyzing the distribution of the local entropy production of the impeller and the vanes at different stages, it is intuitively obtained from the energy point of view that the high loss region of the internal flow field is the largest in the braking condition, and the high loss area fills the entire impeller flow path when the rotational speed drops to 0. The results obtained in this paper can improve the understanding of transient characteristics of a centrifugal pump during the power-off transient condition. © 2020, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.