| Electrical submersible pumps(ESP)are kind of multi-stage centrifugal pumps that are used to convert kinetic energy into hydraulic pressure head for oil production.Compared to traditional centrifugal type ESP,the mixed flow type ESP have advantages of significant flowrate increase and high efficiency,which are highly desirable in deep well oil extraction.However,the internal degassing of the oil layer still causes the generation of free gas in the oil well,which produces an incoming gas-liquid two-phase flow condition.A higher gas content could lead to the degradation of pump performance ranging from mild to severe deterioration(surging/gas locking).Surging will lead the pump to vibrate and shorten its lifespan,while the gas-locking can entirely cease the oil production.However,the change law and mechanisms of pump performance deterioration with mixed flow type under different inlet gas void fractions(IGVFs)is still unclear.Therefore,a deeper insight is needed to fully understand the adverse effects of gas-liquid two-phase on the pump performance,and unsteady internal flow characteristics of ESP,which has theoretical significance and engineering value.For this purpose,this paper takes a five-stage mixed-flow ESP as the research object,and conducts an in-depth study on the internal flow characteristics of the ESP under single-phase and gas-liquid two-phase flow using theoretical analysis,experimental research and numerical calculation.The main research contents and innovative results are as follows.1.A multi-stage ESP gas-liquid two-phase flow experimental rig was built to study the external characteristics for both pure water and air-water two-phase flow conditions.The experiments for both single and two-phase flow were conducted under three flow conditions(0.8Q_d,1.0Q_d,and 1.2Q_d).The single-phase flow results were conducted under different rotational speeds.Then,the two-phase flow experiments were carried out at 1475r/min speed with different inlet gas void fractions(IGVFs).The results show that the flow-head characteristic curves of the ESP are highly similar at different rotational speeds for pure water conditions.The flow range of the ESP decreases gradually with the decrease of rotational speed,and the optimal operating point at each rotational speed shifts to a smaller flow condition with the decrease of rotational speed.The pump performance drops rapidly after the IGVFs is greater than 5%in the experiment.However,at this point,the ESP can still operate normally.But when the IGVFs exceeds 9%,the whole system is particularly unstable at this time.2.In order to enhance the imbalance rate issue in experimental data,an intelligent computing methodology is developed based on artificial neural network(ANN)and SMOTE(Synthetic Minority Oversampling Technique)algorithm for analyzing the two-phase flow performance of ESP.The test data was oversampled to a reasonable extent using the SMOTE algorithm,which has shown precision until 10 decimal points,showing that the SMOTE algorithm is a useful tool for increasing the artificial neural network model’s precision.Additionally,the statistical analysis performed on the oversampled data showed that the BP-ANN approach developed in this study is better to be applied for predicting the multiphase performances of complex turbomachines such as ESP.3.Then,the experimental data of conventional single-stage centrifugal pump and multistage ESP is taken to perform the application and comparison of existing empirical models for analyzing their feasibility,versatility and application range in predicting two-phase performance of pumps.ⅰ.The comparison has shown different trends regarding pump performance prediction and surging mechanism.Romero’s model has shown a pretty good prediction for both head and surging related to multistage ESP.ⅱ.Some empirical models offer a good approximation,while some models either underestimate or overestimate the predicted values because most of their models were established for high inlet pressure(for example,in the case of Turpin,344.73 kpa,689.47 kpa,and 1378.95 kpa),no water but oil and CO2,multi-stage arrangement,different rotational speed,and different pump geometries.Therefore,these empirical models are more specifically adapted for the pump types and working conditions they beyond to.4.The numerical simulation method of pure water and gas-liquid two-phase flow is established to analyze the performance and unsteady internal flow field of ESP.The twophase simulations are based on Euler-Euler two-fluid model,and the computational fluid dynamics(CFD)calculation of the ESP under different IGVFs is carried out.ⅰ.The numerical findings on pure water indicate a decent agreement between the simulated and experimental values.Under the design flow rates,the relative errors of the head,efficiency,and power between the numerical calculation and the test ones were 1.08%,3.2%,and 3%,respectively.Which are less than 5%,proving the credibility of simulation method.ⅱ.The simulation results on two-phase flow show that the Euler-Euler two-fluid model can more accurately predict the performance of the ESP under different IGVFs.With the gradual increase of the IGVFs,the bubble accumulation phenomenon first appears near the suction surface of impeller and in the area near the middle and rear part of the impeller hub.ⅲ.When the IGVF is 5%,there is obvious gas-liquid separation phenomenon in the impeller flow channel.At this stage,the two-phase flow pattern in the impeller flow passage is turbulent,and the bubble occupy some part of the impeller flow passage,impacting the performance of the ESP significantly.However,at higher IGVFs(9%),the bubble accumulation area gradually moves from leading edge of the impeller to the trailing edge,which covers the major part of the impeller flow channel,resulting in the serious degradation of ESP performance.5.The unsteady simulations are conducted to analyze the impact of gas on the radial force distribution in ESP.The results indicate that the magnitude and direction of the radial force on the impeller blades were dramatically influenced by changes in IGVFs for all flow(0.8Q_d,1.0Q_d,and 1.2Q_d)conditions.The highest radial force value was found in the pure water condition.The radial force decreased as the IGVFs increased.The changing pattern was similar when the gas content was low,and the number of wave peaks was equal to the number of impeller blades.After the gas content reached a certain level,the radial force fluctuated inconsistently.The radial force’s strength and direction frequently changed,which had an adverse impact on the functional capability of ESP.In addition,the transient simulations are conducted to analyze pressure pulsation characteristics of ESP at rotating speed of 1475r/min,different flow rates(0.8Q_d,1.0Q_d,and 1.2Q_d)and IGVFs(0%,5%,and 9%).The main findings have been obtained as follows:ⅰ.The value of Cp steadily rises from the impeller’s leading edge to the trailing edge on both the suction(S1-S5)and the pressure sides(P1-P5)of the blade for all flow conditions.As the IGVF increases,the Cp curve became increasingly disorganized.This meant that the increase in IGVFs disrupted the pump,s stable operation,and more the gas void fraction is,the more unstable pulsating signal will be inside the ESP impeller.The strength of pressure pulsations in both the impeller and diffuser is mostly caused by flow interactions between the rotating impeller and the static diffuser,resulting in the performance reduction of ESP.ⅱ.Concerning frequency-domain charts,the primary frequencies at all operating conditions were at blade passing frequency(BPF),and the pulse amplitude was almost negligible at each harmonic frequency.When the IGVF increased,the strength of the pulsation at one-time frequency steadily increased.Other than the main-frequency and low-frequency signal amplification,the increase in IGVF values did not result in the creation of a new pulsing signal.6.Finally,the outcomes from unsteady simulations on entropy production study indicates that the impeller flow channel generates more energy losses than the diffuser flow passage.The main contributor of energy losses inside the rotor flow channel is the velocity caused by vortices and turbulent flows.ⅰ.Overall,both the total pressure loss(TPL)and total energy loss(TEL)approaches exhibit similar trends,revealing that the entropy generation technique is a valid and reliable approach for investigating the flow losses produced under multiphase flow conditions.ⅱ.Furthermore,it is obvious that the amount of entropy generated from direct dissipation is far less than that generated from turbulent dissipation.This reveals that the large portion of the overall entropy generated by the ESP is due to turbulent dissipation,leading to higher degradation in the ESP performance under multiphase flow conditions. |
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