| The semi-open impeller centrifugal pump has attracted attention because of its simple structure and wide application range.However,due to the tip clearance in the semi-open impeller,the flow situation is more complicated than that in the closed impeller,and there are unstable flow phenomena such as vortex and backflow.In practical engineering applications,the phenomenon of semi-open impeller centrifugal pumps conveying gas and liquid mixtures often occurs.Since the addition of the gas phase will change the distribution of the flow field in the pump,the change of the gas-liquid twophase flow pattern is complicated.With the increase of the gas content,the performance of the semi-open impeller centrifugal pump decreases,and the vibration and noise increase.In severe cases,the gas will block the impeller flow-path,or even cut off the flow,which endangers the safety and reliability of the system.This thesis takes IS65-65-174 semi-open impeller centrifugal pump as the research object,and uses the method of combining numerical simulation and experiment to study its internal flow conditions.The main research contents and innovations of this thesis are as follows:1.Through the analysis of the gas-liquid two-phase flow numerical model and experimental research methods,it is found that the two-fluid model widely used nowadays has higher accuracy under small inlet gas volume fractions,but its accuracy is lower under high inlet gas volume fractions.Reflecting the aggregation and breaking of bubbles is the key to improve the accuracy of simulation.Visualization experiment is an important means to observe flow pattern changes and verify numerical simulation.The tip clearance and dynamic and static interference of the semi-open impeller centrifugal pump under air-liquid two-phase are the key factors of the changes of internal flow pattern.2.This thesis established a full flow field numerical simulation method for the semiopen impeller centrifugal pump under pure water conditions and carried out steady and unsteady calculations.Obtained the hydraulic characteristic curve,the flow conditions of the middle section of the impeller and tip clearance layer,and pressure fluctuation characteristics.Finally compared with the experimental results,the research results show that the hydraulic characteristics comply with the rotational speed similarity law,there is a local low pressure area on the suction side at the inlet of the impeller,the turbulent kinetic energy near the outlet of the impeller-path and the tongue of the volute is relatively large.The low pressure area of the tip clearance layer is located at the corresponding position of the volute tongue,and obvious vortices can be observed in the streamline diagram of the middle section of the tip clearance layer.With the increase of the flow rate,the flow condition in the tip clearance is improved,and the tip clearance vortex is gradually weakened.3.Use the Musig model to numerically simulate the internal flow characteristics of the semi-open impeller centrifugal pump under gas-liquid two-phase,and compare the results with Euler-Euler model.Obtained the hydraulic characteristic curves calculated by two models and gas distribution cloud diagram of the impeller.Through the verification of hydraulic characteristics and visual experimental results under the condition of gas-liquid two-phase flow,it is found that the calculation results of the Musig model are closer to the real flow conditions in the semi-open impeller and the simulation accuracy is higher under high flow rates.The change of the inlet gas volume fraction directly affects the flow pattern in the impeller,and the change of the flow pattern causes the pump performance to change.The flow pattern of bubbles in the impeller will appear uniform bubbly flow,polymerized bubbly flow,cavitation flow and separation flow as the inlet gas volume fraction increases.The performance deteriorates with the change of flow pattern.The model pump can deliver a maximum inlet gas volume fraction of 4.6%at 1000 r/min.4.Based on the Musig model,the flow field characteristics under different inlet gas volume fractions are analyzed.The research results show that: as the inlet gas volume fraction increase,the pressure drops.There are large-scale vortices and backflow phenomena in the impeller flow-path,this phenomenon becomes more obvious with the increase of the inlet gas volume fraction.At the same time,areas with high gas content have greater turbulent kinetic energy,indicating that the entry of gas aggravates the unstable flow inside the centrifugal pump,resulting in an increase in turbulent pulsation.5.Based on the Musig model,unsteady calculations are performed on the semi-open impeller centrifugal pump under different inlet gas volume fractions conditions,and the gas distribution of the middle section and the axial surface during 1/6 cycle of the impeller rotation obtained,and the results are compared with the visual experiment results.The results of the study show that: under small inlet gas volume fraction,after the gas enters the pump,they are concentrated in discrete spherical bubbles at the outlet of the impeller near the pressure surface.When the inlet gas volume fraction increases,the bubbles collide and fuse with each other,and the free bubbles in the pump gather into a larger air mass,which is mainly attached to the inlet of the suction side of the blade.As the impeller rotates,the attached bubbles gradually move downstream.In the drainage area near the tongue of the volute,the bubbles move in a whirlpool.With the further increase of the inlet gas volume fraction,the attached gas on the suction surface of the blade increases and extends from the outlet to the blade inlet.The gas occupies most of the flow path of the impeller,forming a cavitation flow or even a separated flow.And the characteristics of pressure pulsation are analyzed.The research results show that as the inlet gas volume fraction increases from 1% to 4.5%,the surface of the blade is almost attached by gas,and a large number of air bubbles formed at the impeller flow-path,outlet and volute.The effect of the blade on water is weakened and the interference between the water flow and the volute tongue is reduced,so the maximum value of the pressure pulsation amplitude at the blade frequency decreases with the increase of the inlet gas volume fraction. |
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