Transient Internal Flow And Performance Of Centrifugal Pumps During Startup Period

Centrifugal pumps usually run at steady operating conditions in which the rotational speed or the operating point is basically stead or changed slowly in a short time. Hereby the existing investigations on centrifugal pumps mainly are limited to the steady operating conditions. However, with the expansion of applications of centrifugal pumps and the increasing complexity of flow systems, centrifugal pumps have to be operated in transient operating conditions, such as startup, stopping and rapid change of rotational speed etc. As such, the hydraulic performance of centrifugal pumps at transient conditions has been drawn more and more attention in recent years. Therefore, it has become more necessary and urgent to study the hydraulic performance of centrifugal pumps during transient operating periods.In this thesis, the study object is the centrifugal pumps operating at transient working conditions. Their hydraulic performances during the startup period are studied systematically and deeply. And the transient characteristics are obtained in the case of delivering solid-liquid two-phase flow by using CFD. Firstly, a new generalized Euler equation is proposed according to the theorem of moment of momentum, which can be used to describe the performance of incompressible fluid machinery during transient operating periods. By means of the proposed equation, the additional theoretical heads of a centrifugal pump can be calculated quantitatively in all kinds of transient operating periods. Secondly, the unsteady flow inside a centrifugal pump during rapid startup period is numerically simulated using the dynamic mesh method. As such, an analytical method for transient performance of centrifugal pump is ultimately established based on the numerical simulation. Thirdly, the experiments on the performances of centrifugal pumps during startup periods are conducted. The transient behavior of centrifugal pump during startup periods is revealed further by the dimensionless analysis method. Finally, the transient flow in pump models in startup period are numerically calculated for the pumped media with solid particle based on numerical simulation, and the transient characteristics are obtained at last.The main contents of this thesis are briefly stated as followings:1. A generalized Euler equation is theoretically deduced based on the theorem of moment of momentum and theory of flow in hydraulic machinery. The proposed equation fully reflects the influences of blade thickness and blade profile on additional theoretical heads, and is able to predict the additional theoretical heads (or pressure rise) of incompressible fluid machinery, including pumps, fans, and turbines and so on, during all kinds of transient operating periods. 2. By using the dynamic mesh method, the numerical simulations are carried out in startup process in the case that a centrifugal pump transports water. As such, the study method for obtaining transient performance of a centrifugal pump during all kinds of transient operating periods is established eventually. In order to simulate the unsteady flow inside the centrifugal pump during startup period, a closed-loop pipe system with a centrifugal pump is established, a constant pressure point is set up in the system to make the flow resemble the real status. Through numerical simulation, many important characteristics, such as variations of the performances and the internal flow fields, are obtained. The static pressure at the inlet of the pump firstly declines and then rises. At the very beginning of the startup, the rising rate of flow rate is relatively slow compared with the rising rate of rotational speed. The dimensionless transient flow rate shows a rapid rise trend at the beginning of the startup, and then arrives at a stable value. The dimensionless transient head is of extreme at the very beginning of startup, and quickly falls below the quasi-steady value, then gradually rises to the stable quasi-steady value.3. A test rig is established in this thesis to measure the transient performance of centrifugal pumps during startup period. For water, three pumps with three typical centrifugal impellers, namely ordinary closed, splitter-compound, and open impellers, are tested to obtain their performance during startup period. The variations of rotational speed, flow rate, head, and shaft-power with time are recorded in the experiments, and then the variation characteristics of each parameter are analyzed. The transient characteristics of rotational speed are basically stable in startup and are independent of the steady working point after startup. Compared with the rotational speed, the rising rate of flow rate is relatively low at the beginning of the startup. Moreover, the flow rate lags behind the rotational speed to arrive at their final steady value, and the delay becomes more severe with increasing discharge valve opening. Generally, there exist the pressure and haft-power impact phenomena in startup.4. Firstly, the calculations of solid-liquid two-phase flow inside a centrifugal pump are accomplished by using the solid-liquid two-phase flow model based on the Euler-Euler approach. The influence of the solid phase properties on the hydraulic performance is emphatically analyzed. Moreover, the characteristics of the flow fields, including the distribution of solid particles in the pump, are also analyzed in detail. Subsequently, the experimental results of rotational speed and flow rate of the pump during startup period are fitted by using a function of time. Then the fitted two functions are encoded by means of user defined function to serve as the boundary conditions to the impeller and the inlet of the pump, respectively. Finally, the unsteady solid-liquid two-phase flows inside the pump are numerically simulated during startup period using the dynamic mesh method. The results show that the difference in performance of centrifugal pump during startup period is very obvious for pure water and the solid-liquid two-phase flow. At the early stage of startup, the transient performances are basically the same, while the difference in performance is very obvious at the later stage of startup. The transient head of delivering pure water always shows a steadily rising effect during startup period, however, that of delivering solid-liquid two-phase flow doesn’t, namely a maximum peak occurs in somewhere during the time course. A longer time is needed for a solid-liquid two-phase flow to get a steady performance compared with the water single phase flow.