The Transient Gas Liquid Flow In Helical Axial Pump Diffuser And Its DMD Model Analysis

With the development of offshore oil fields,gas-liquid multiphase mixed transport technology has attracted more attention by various countries for its remarkable economic benefits.There are two kinds of pumps usually used in the oil-gas mixed transportation system at present,which are the screw pump and helical axial gas-liquid multiphase pump.The helical axial gas-liquid multiphase pump is widely used in oilfield development for its advantages of large flow rate,compact structure and insensitivity to solid particles.The gas content can vary from 0 to 100%,high gas content in the pump is easy to produce gas blocking phenomenon and cause the flow channel blocked.It is required that helical axial gas-liquid multiphase pump must have the function of both pump and compression stage.The complex gas-liquid two-phase flow in the helical axial gas-liquid multiphase pump results in the immaturity of its design theory.It is of great significance to study the complex gas-liquid two phase flow in the helical axial gas-liquid multiphase pump.In this paper,the flow in helical axial gas-liquid multiphase pump was numerically simulated and analyzed.The DMD method was applied to analyze the velocity field and phase field of helical axial gas-liquid multiphase pump,and the mechanism of gas-liquid two-phase flow in helical axial gas-liquid multiphase pump was further investigated,which provide some reference for the structure design and performance optimization of the pump.The main research of this paper including the following parts:Structured grids were used to discretization the calculation domain of YQH-90helical axial gas-liquid multiphase pump.The Eulerian-Eulerian two-fluid multiphase model and SST k-?turbulence model were used for the numerical simulation of transient gas-liquid two-phase flow.Based on three sets of grids,the error of numerical calculation was analyzed by using Verification and Validation methods.Based on the analysis of the phase structure in the impeller and guide blade,it is found that under the action of centrifugal force in the impeller,the liquid volume fraction in the impeller increases gradually from hub to shroud,and the distribution of gas and liquid phase in the whole guide blade from the hub to the shroud tends to be uniform.With the increase of flow rate,the relative velocity in the impeller gradually increases,and the gas-liquid separation in the impeller is strengthened.The analysis of velocity field shows that with the increase of blade span,the circumferential velocity in the impeller increases gradually,which leads to the increase of gas-liquid two-phase velocity.The analysis of the pressure field shows that the pressure in the impeller gradually increases from the hub to the shroud.FFT analysis of the GVF,pressure and velocity field pulsation signals of guide blade inlet and outlet can analyze the unsteady characteristics of flow filed,but can not effectively express the dynamic evolution information of the whole unsteady field structure.Dynamic modal decomposition method was used to analyze oil-gas pumps guide blade unsteady velocity field and the phase field.It is found that the reduced order model based on DMD method can reconstruct the flow field with sufficient accuracy.The gas-liquid two phase flow field can be decomposed into the base mode field with zero frequency,the model of rotor-stator interaction with the blade passing frequency and its multiple frequency,and the dissipative model field.The frequency of the velocity basic modal is zero,which reflects the steady flow characteristics caused by the geometric characteristics of the flow channel and the guide vane profile.The 3 and 4 order modes respectively reflect the unsteady characteristics of blade passing frequency and its multiple frequency caused by rotor-stator interaction.Modes 3 and 4 form positive and negative alternating vortices with the same frequency at the guide blade inlet and influence the flow field in the guide blade,and the vortex gradually dissipates as it develops downstream of the guide vane.The basic modes of the phase field indicate that on the section with higher span,the higher the flow velocity is,the lower the gas volume fraction is.The higher order dynamic modal characteristics of phase field is the same as that of the velocity field.The characteristics of the velocity field mode and phase field mode of 2f_b are basically in harmony with its blade passing frequency mode(f_b),but its energy is significantly lower than that of the blade frequency mode.