Research On The Internal Flow Pattern And Stage-by-stage Design Of A Multi-stage Rotodynamic Gas-liquid Pump

Helico-axial multiphase pump is one of the most important equipments in the multiphase transportation technology.It has broad prospects in engineering application with the advantages such as large capacity,adapting to the sand-containing environment,and strong anti-drying ability.Because of the compressibility of gas and the complexity of two-phase rotating flow,the study on the internal flow pattern of multiphase pump is not comprehensive,thus affects the improvement of pump performance.In this paper,a comprehensive and in-depth research on the internal gas-liquid flow pattern in each stage of a multi-stage multiphase pump is carried out.And a stage-by-stage optimization design method for multi-stage pump and the modification of impeller structure at high inlet gas volume fraction(GVF)are proposed furthermore in order to improve internal flow in multiphase pump and increase the pressure increment.The main contents and achievements are as following:Firstly,in order to test the pump performance at different working conditions and provide experimental data for the validation of numerical simulation method,the experiments on a three-stage helico-axial multiphase pump with both water and gas-liquid phase are conducted and the pressure increment and efficiency at different conditions are investigated.The results show that pump differential pressure decreases slowly as inlet GVF increases when the inlet GVFs are less than 10%.The differential pressure decreases significantly when inlet GVF becomes higher.Increasing the rotational speed can effectively slow down the tendency of the pressure reduction when inlet GVF increases.Subsequently,numerical simulations are conducted based on the model of the three-stage pump used in experiment.The influence of different turbulence models,surface roughness,drag force models between gas and liquid and gas bubble diameters on the predicted differential pressure and efficiency as well as the pressure and velocity distribution in flow passage are both investigated.An improved numerical simulation method for multiphase pump is proposed.By using this modified method,the velocity reflow phenomenon near the shroud of impeller at the meridional plane can be captured,and the error between the predicted pressure increment and efficiency and the experiment data can be reduced to less than±5%,which shows the improved numerical simulation method is reliable and accurate.Then,based on the modified numerical simulation method,the variations of the condition parameters such as flowrate,inlet GVF,axial velocity and inlet flow angle at impeller shroud in each stage of a five-stage multiphase pump are analyzed.For the problem that the impeller structure of the multistage pump does not match well with the corresponding inlet flow parameters,which causes the increase of inlet attack angle and leads to hydraulic loss,a stage-by-stage optimization design method is developed according to the variation characteristics of flow parameters at different inlet GVFs.When the inlet flowrate is 100 m~3/h,the rotational speed is 4500 rpm and the inlet GVF are 30%and 50%.The pressure increments of the modified five-stage multiphase pump based on the stage-by-stage design method increases by 53.72 k Pa and 58.57 k Pa respectively,and the efficiency increases by 1.23%and 1.53%as well.The feasibility of the stage-by-stage optimization design method is verified.Finally,a new modified impeller with split-vane blades is developed to solve the problem that there are serious gas accumulation in the flow channels of multi-stage pump under high GVFs which leads to the deterioration of pump performance.The gas distribution,velocity distribution and pressrue increment at different GVFs are compared between the modified three-stage multiphase pump and the original pump.The results show that the gas accumulation disappears in the first stage of the modified pump and then decreases rapidly in the last two stages.The performance of modified pump is general better than the original pump at different inlet GVFs.The maximum pressure difference can be 51.34 k Pa when the inlet GVF is 60%.Therefore,the split-vane impeller can help to improve the pump boost performance especially at high inlet GVFs conditions.