Gas-liquid Two-phase Fluid Separation And Gas Blockage Mechanism In The Impeller Of The Helical Axial-flow Pump

The exploitation and transportation of deep-sea and remote oil and gas fields have become an important part of national energy strategy.The great superiority of multiphase mixed transportation system in oil and gas production and transportation provides a better choice for the development of oil fields.Multiphase pump is the core equipment of the mixed transportation system,in which the helical axial-flow gas-liquid mixed transportation pump is widely used because of its high flow rate and insensitive to solid particles.Due to the large density difference between gas and liquid,gas-liquid separation and gas blockage are the most important factors affecting the safe and reliable operation of the production and transportation system.In this paper,the compression unit of a single-stage helical axial-flow pump designed by ourselves is taken as the research object,the Euler two-fluid multiphase flow model and SST k-ω turbulence model are used to calculate the internal flow of the pump.Firstly,monitoring points were set up in the impeller channel.Based on the radial velocity of gas-liquid two-phase in the flow surface and the distribution of gas-phase volume fraction in the channel,the trajectory of bubbles in the channel and their location on the channel wall were investigated.Secondly,based on the stress analysis of bubbles in the flow channel,the extrusion coefficient and separation coefficient of gas-liquid two-phase were introduced to characterize the effect of liquid on bubbles and the separation degree of gas-liquid two-phase in the flow channel.The internal radial pressure gradient and the dynamic mechanism of gas-liquid separation in different regions were discussed.Finally,the variation of gas volume fraction and pressure on the impeller hub surface in a period of impeller rotation was analyzed.The mechanism of gas blockage at the outlet of the channel was revealed by introducing the difference between the impeller’s work on the fluid and the static pressure of the channel.The main conclusions are as follows:(1)On the meridional surface of the impeller,the trajectory of the bubble is a kind of horizontal throwing curve,and finally the synthetic air mass gathers at the hub of the pressure surface in the non-coincident area of the impeller blade.(2)When the gas-liquid two-phase medium is transported by the multiphase pump,bubble motion is mainly affected by a combination of inertia force,flow resistance force,virtual mass force and pressure gradient force.The reason for the separation of the gas-liquid two-phase state is that the liquid in the rotating impeller caused by the imbalance of centrifugal force and centripetal force pushes the bubbleout;the centripetal force of the bubble is mainly provided by the radial pressure gradient,and its size affects phase separation degree of gas-liquid two-phase.(3)The separation of gas and liquid leads to the accumulation and combination of small air masses in the hub of the flow channel.Affected by the shear flow of phase separation,the axial vortex is formed at the air mass accumulation place at the outlet of the flow channel.The work done by the impeller to the air mass is not enough to overcome the overpressure gradient formed by the liquid phase accumulation of the adjacent flow channel at the outlet of the flow channel,resulting in the stagnation of the vortex air masses near the wall of the hub of the flow channel,forming the gas blockage phenomenon.In this paper,the mechanism of gas-liquid separation based on crowding effect and the mechanism of gas blockage based on the effect of over reverse pressure gradient were discussed.It is found that eliminating the radial pressure gradient in the non-coincident areas of adjacent blades can effectively slow down the separation of gas-liquid two phases;controlling the over reverse pressure gradient at the trailing edge of impeller can reduce the degree of gas blockage at the outlet of the flow channel,and the idea and direction of solving the gas blockage in the flow channel are given,which provides theoretical support for the safe and reliable design of high efficiency helical axial flow gas-liquid multiphase pump.