Research On The Cavitation-Induced Gas Blockage Characteristics In Helical Axial-Flow Multiphase Pump

Helical axial-flow multiphase pump is a novel concentrated mixing and transportation technology that offers compact structure and large displacement advantages.It not only saves complex separation equipment and special gas-liquid transport pipelines but also lowers production costs and capital investment,providing a significant increase in economic benefits for oil fields.However,when the pressure during medium transportation is lower than the local saturation vapor pressure,cavitation can occur in helical axial-flow multiphase pump.The generated cavitation bubbles in two-phase medium can easily induce gas blockage in the impeller flow passage.The gas blockage phenomenon can cause serious harm to the stable operation and service life of hydraulic machinery.Although many improvement schemes have been developed to reduce the negative impact of gas blockage,it is difficult to completely eliminate cavitation in real-life situations due to unavoidable non-design factors.Therefore,it is necessary to study the cavitation-induced gas blockage characteristics of spiral axial-flow multiphase pumps under two-phase gas-liquid conditions,especially analyzing the process of gas blockage induced by different cavitation phenomena.The analysis of the distribution pattern of bubbles on the impeller blades and the response change of global variables in the pressurization unit under gas blockage conditions is of great significance for the safe and stable operation of helical axial-flow multiphase pump.In this paper,a numerical simulation method was used to investigate the cavitation-induced gas blocking characteristics of a helical axial flow multiphase pump under gas-liquid two-phase conditions.The main research contents are as follows:Firstly,the phenomenon of cavitation generates bubbles mainly in the impeller of the spiral axial mixed-flow multiphase pump.To study in detail the morphological changes of bubbles from formation to detachment,the impeller consisting of a ringshaped blade surrounding the hub is unfolded into five different airfoil shapes for unsteady analysis.It is found that during the bubble detachment process in the airfoil cavitation,bubbles first appear in the low-pressure area produced by flow separation at the leading edge of the airfoil suction surface,demonstrating an attached cavitation and adhering to the airfoil surface as they extend backward.Moreover,changes in the pressure field on the airfoil surface during the bubble detachment process also exhibit periodic variations,resulting in complicated flow in the bubble-liquid phase interface area and significant turbulence energy dissipation.Furthermore,the evolution of the performance of the helical axial-flow multiphase pump with cavitation was investigated.It was found that the trends of the cavitation characteristic curves were similar under the three flow conditions,where the head remained unchanged at first,then gradually decreased after the critical cavitation coefficient was reached,and exhibited slight fluctuations after reaching the critical point.Finally,blockage occurred and the head dropped rapidly.In the cavitation inception stage under the design condition,the air bubbles first appeared at the inlet of the blade suction surface.The distribution range in the low-pressure region of the impeller coincided with that of the air bubbles.The shape and distribution range of the air bubbles were not uniform due to the influence of the high-pressure region of the flow field,resulting in the occurrence of multiple flow reversal phenomena on the suction surface of the guide vane,thereby significantly reducing the operational efficiency of the helical axial-flow multiphase pump.Finally,the phenomenon of gas blocking induced in the impeller channel during cavitation of single-phase fluid and gas-liquid two-phase fluid was analyzed.The air distribution and pressure distribution of the impeller boosting unit in the initial cavitation nucleation stage,the cavitation-induced gas blocking stage,and the gas blocking-induced head break stage were analyzed.It was found that compared with the single-phase fluid condition,the appropriate introduction of gas into the incoming medium can help to inhibit the occurrence of gas blocking phenomena and improve the distribution of bubbles generated by cavitation in the impeller domain.The nonuniformity of the blade gas blocking phenomenon is superimposed with the cavitation phenomenon,causing a downward trend in the head and performance of the helical axial flow pump when micro-bubbles form.As the gas blocking phenomenon develops,the flow inside the duct becomes more unstable,especially at the large blade wrap angle of the helical axial flow multiphase pump,which is extremely prone to the phenomenon of the duct being blocked by a large number of bubbles,resulting in a discontinuous drop in the head of the helical axial flow multiphase pump.