Numerical Simulation And PIV Measurement Of Salt-out Two-phase Flow In Centrifugal Pump

Research work in this paper is financially supported by "Study on the liquid-solid two-phase flow with salt-out in turbo-machine", the project of National Natural Science Foundation of China. (No.50476068)Salt-out two-phase flows are widely found in petroleum exploitation, irrigation and chemical industry, which have aroused inconvenience and enormous energy waste to the industrial production. To acknowledge this phenomenon extensively in centrifugal pump, numerical simulation and experiment were adopted. The goals are to explore the relationship between salt-out and inner flow of turbo-machine and supply theoretical foundation of optimum design. The innovative achievements and important conclusions obtained are as follows:1、Basic types crystal nucleation during salt-out process were comprehensive analyzed. According to practical operation, it can be concluded that the heterogeneous nucleation and secondary nucleation mechanism are dominant during salt-out process. The fundamental theories of crystal growth were also described, crystal growth rate during salt-out is controlled by both diffusion and surface reaction.2、Salt-out delay time concept is proposed through studying the salt-out process of sodium sulfate in the pipe. When the flow velocity is below 1 m/s, the delay time increases with the increase of flow velocity. However, when the velocity exceeds 1 m/s ,the delay period will not change obviously. We find that the delay time increases as the temperature increases. The influences of temperature and concentration on salt-out layer thermal resistance are also researched. The salt-out process is divided into five stages based on lots of mechanism experiments. The calculation model of crystal deposition rate is built, which is applied to predict the salt-out thermal resistance variation.3、The influences of salt-out process on centrifugal pump performance are studied. During salt-out delay time period, the performance almost maintains stable. However, the pump head decreases, the power increases, and the efficiency decreases during the crystal growth because of the appearance of large numbers of crystal. From the crystal particle motion equation, velocity triangles of liquid and crystal particle at the impeller outlet are obtained, from which we obtain the conclusion that absolute velocity circumference component of crystal particle is less than that of liquid phase. And the head decrease is verified by theory.4、The fundamental governing equations of salt-out turbulent flow are established based on two-fluid model. The exchange rate of mass, momentum, and energy between the two phases are obtained by averaging technique. The chemical reaction is also considered, the population balance equation is introduced to describe nucleation, crystal growth, crystal coalescence and break during salt-out process.5、The general governing equations with body-fitted coordinate are derived through coordinate transformation. Combing the commercial software FLUENT with user-defined functions, salt-out turbulent flow in centrifugal pump is simulated. The influence of temperature on salt-out flow in pump passage is analyzed, and the mechanism of salt-out flow in centrifugal pump is illustrated preliminarily.6、According to special requirements of PIV measurement in centrifugal pump, a testing platform was established to measure the instantaneous velocity distribution in the impeller under different operating conditions. The image processing program is compiled to treat the salt-out flow field. The particle size distribution is successfully obtained. The main conclusions include:(1) No crystal particle appears in the solution during delay period, which is concluded through image processing. And the flow field in the pump does not change obviously during this period.(2) After delay period, lots of particles appear in the system. The particle size obtained by image processing is around 30μm, by comparing the flow field of different phases, it can be concluded that the crystal particles have better tracing property and have little influence on liquid phase.(3) Because of the damping action of shroud , the velocity distributions are different in different sections ,and the velocity of crystal has been affected obviously.(4) Crystal particles growth larger as the temperature decreases from 40℃to 34℃. The particles move to pressure side obviously, and the main deposition region is middle part of pressure side. There are two crucial reasons: a low velocity region exists in this area, which is in favor of crystal growth ,on the other hand , composite force of crystal particles is leaned to the pressure side because of the high density of crystal.