Salt-Out Two-Phase Flow Theory And Investigation On Internal Flow In A Vortex Pump

The salt-out two-phase flow phenomenon exists widely in the salt solution transporting process in relative industrial domain departments.It is a difficult issue both in academic and project field that costs vast waste of energy and resource.At present,researches on the salt-out two-phase flow theory are still on exploration road, especially in the internal field of impeller machinery.Under the support of National Natural Science Fund(No.50476068) and Postgraduate Cultivation Innovation Project of Jiangsu Province(No.CX07B₀93z),vortex model pump suitable for salt solution transporting is adopted as the study object,and sodium sulfate supersaturated solution with representative characteristics of salt-out as the conveying medium in this paper. Systematic study on salt-out two-phase flow theory and salt-out flow mechanism in the vortex pump has been done.The major works and achievements are as following.1.Based on the basic theories of salt-out process,detailed descriptions about crystal nucleation and growth,crystal particle transportation and the relation with flow conditions are given.It is concluded that supercooling(or supersaturated) is the direct driving force of phase change in the nucleation process.Heterogeneous and collision nucleation become dominant mechanisms during pump salt-out.Nucleation velocity,crystal particle size and concentration distribution are affected mostly by local flow conditions.2.The idea of interface phase theory is helpful for proposing and defining the near interface zone.It is divided into three parts to describe the salt-out layer and zones nearby,such as salt-out interface,adsorption and transition layer.Each part with various structures and features makes different functions on salt-out process.Analysis on internal characteristics of salt-out layer is also taken.Mathematical and experimental methods on the layer increasing are illuminated.An assumption on looking for "pump head-time" relation is proposed for pump salt-out research.3.According to the practical process,two-fluid model is used to set up the physical model of salt-out flow.Then the whole salt-out two-phase turbulent flow equations are established by fluid mechanics methods.Constitutive relations are also given,such as mass,momentum,energy and so on.Equation group is closed by k-ε-k_p model.In order to establish the mathematics model in impeller machinery, centrifugal and coriolis forces due to the impeller rotation are involved in the equations.4.Population Balance Model(PBM) is introduced to describe the particle microscopic behaviors of coalescence and breakage,relate to the macroscopic features as well.It shows that PBM-CFD coupling may improve the precision of two-fluid model.Ways and existing problems of application on salt-out flow flied calculation are discussed.It provides a new way for studying the salt-out two-phase flow.5.Based on the Eulerian multiphase model and mixture RNG k-εturbulent model,three dimensional unsteady turbulent salt-out flow field in the pump is simulated by moving mesh technique.It is a preliminary study on the movements both liquid and salt-out particle phases at different time and temperature conditions. According to the particle concentration distribution,the salt-out process in the pump can be proposed that salt-out layer may formed on the internal wall of pump chamber and pressure side of impeller at first,and thickening until blocked the whole passage finally.Performance prediction of the pump when transporting water and salt solution is also studied.Results are compared and analyzed with experimental data.6.The salt-out flow field is measured on three dimensions by Particle Image Velocimemter(PIV) via radial and axial two test models.After image processing, velocity distribution of crystal particles in the pump is obtained.Particle flow field is visualized to reveal the particle motion law intuitionally.Except for the numerical difference,computational results are basically consistent with PIV data.7.Phase Doppler Particle Anemometer(PDPA) is used to measure the salt-out flow field in the pump in detail and completely.An effective method is groped for impeller machinery internal flow measurement.Turbulent flow features,crystal particle size and number density distribution in the impeller and volute are obtained. Evolution rule of salt-out flow with different external conditions is found and analyzed via changing the major factors that affecting flow system,such as pump running condition,conveying medium temperature and concentration.The salt-out flow mechanism in the pump is basically grasped at last.Research achievements and methods from this paper can be extended and applied on the salt-out two-phase flow in other types of impeller machinery.