Research On Transient Flow Characteristics Of Gas-liquid Two-phase In Helicon-axial Multiphase Pump

The advantages of helicon-axial multiphase pump in engineering applications are not only shortening the platform construction cycle,reducing the cost of crude oil extraction and achieving long distance transport,but also transporting gas-liquid mixed medium with a small amount of sand.Due to these advantages,multiphase pump has been widely used in domestic and foreign countries.Multiphase pump often transport mediums containing oil,gas,water,sand,and so on.Moreover,the gas volume fraction is varied from 0-100% randomly,which makes it difficult for the pump to operate in a wide efficient range.Furthermore,the complexity of the mediums leads to low efficiency,low head,poor stability,and even vibration and noise.The main reasons are as follows: First,the design method of screw-axial multiphase pump is not mature enough;Second,the understanding of the two-phase flow mechanism in rotating machinery is not clear enough,especially at high gas volume fractions;third,both the complex internal flow of multiphase pump and the unique structure of the compression unit make the helicon-axial multiphase pump not able to operate in high efficiency.However,the study of the interaction mechanism between phases is rarely reported in literatures.Therefore,this paper studies the transient flow characteristics of gas-liquid two-phase in helicon-axial multiphase pump to deepen the understanding of the flow mechanism in the pump and to provide a theoretical reference for the optimization design of the structure,the improvement of performance and the control of flow induced vibration.The main works and achievements are as follows:1)Design of helicon-axial multiphase pump and flow field calculationThe model of the pump with multi-stage compressing unit and tandem structure was designed.The numerical simulation method was used to calculate the flow characteristics of the whole passage in the three-stage helicon-axial multiphase pump.The mechanism of phase separation and gas phase local aggregation in the multiphase pump was revealed.The gas-liquid two phase distribution pattern and the flow characteristics when the multiphase medium flows through the pump,as well as the hydraulic loss in the pump were obtained.2)Calculation of gas-liquid two phase transient flow in the helicon-axial multiphase pump.In order to ensure the reliable operation of the multiphase pump,the transient flow characteristics of the pump at different gas volume fractions were calculated.The transienthydrodynamic characteristics and the dynamic characteristics of the impeller were studied.The variation of the transient axial force and radial force with the gas volume fraction,as well as the pressure fluctuation characteristics on multiphase pump were revealed.3)Study on transient characteristics of cavitation in multiphase pump with gas-liquid flowCavitation will cause serious harm to the impeller of hydraulic machinery,so the two-phase transient cavitation flow pattern in the multiphase pump at different gas volume fractions was analyzed to reveal the position of cavitation and the evolution of vapor bubbles at different degree of cavitation,and the relationship between gas volume fraction and cavitation.In addition,the influence of cavitation flow on the hydrodynamic characteristics of the multiphase pump was analyzed by comparing that with no cavitation flow.The result shows that cavitation can also induce the increase of unsteady force fluctuation amplitude and substantial increase of force.So cavitation affects operation stability and reliability of pumps.