| This study is financial supported by National Natural Science Foundation of China “The Internal Flow Mechanism of the Centrifugal Pump and Its Flow Induced Characteristics under Gas-Liquid Two-Phase Flow Conditions”(Grant No.51779170)?National Key Research Project “Research and Application Demonstration of Key Technologies for Complex Environmental Engineering Rescue”(Grant No.2018YFC0810505)and the Senior Foreign Expert Project “Study on Internal Flow Characteristics of Pumps in Gas-Liquid Mixed Transportation Conditions”(Grant No.GDW20153200139).The phenomenon that gas-liquid two-phase flow of centrifugal pump is often encountered in engineering practice.Its performance will deteriorate sharply with the increase of inflow void fraction,and even cause a cutoff of flow rate,resulting in a great threat to the safety and stability of the system,and its internal flow characteristics and performance deterioration mechanism have not yet been revealed.The combination of experimental research and numerical simulation is used to test the hydraulic characteristics,pressure pulsation and high-speed photography of model pumps at different rotational speeds and different inflow void fractions in this thesis.The Euler-Euler inhomogeneous model is used to calculate the internal flow field of model pump under gas-liquid mixture conditions.The followings are the main research contents and conclusions:1.By summarizing and analyzing the research status of domestic and international,the thesis points out the development trend and existing problems of gas-liquid two-phase flow for centrifugal pumps at present.The fundamental reason for the deterioration of performance under gas-liquid two-phase conditions is the change of internal flow patterns.2.An open test rig for gas-liquid two-phase flow was built.The hydraulic characteristics of single-stage and single-suction stainless steel centrifugal pump with specific speed of 88.6 under different rotational speeds and different inflow gas void fraction were tested.At the same time,the PMMA centrifugal pump of semi-open impeller with the same geometrical parameters was designed for visualization test of internal flow field with different inflow gas void fraction.The conclusions are as follows: The stainless steel centrifugal pump can still work normally when the inflow gas void fraction is 10% at rated speed,and the maximum inflow gas void fraction will decrease with the decrease of the rotational speed under other rotational speed conditions.The performance of PMMA centrifugal pump has the similar rule.It is also found that the maximum inflow gas void fraction of PMMA centrifugal pump is 7.5% at 1450 r/min and 4.6% at 1000 r/min.3.The pressure pulsation of model pump were tested under different working conditions and different inflow gas conditions.The conclusions are as follows:(1)The main frequency of pressure fluctuation of stainless steel centrifugal pump is blade frequency under different inflow gas void fraction conditions,and the larger the inflow gas void fraction,the larger the amplitude of pressure fluctuation main frequency.When the inflow gas void fraction exceeds 5%,the amplitude of pressure fluctuation main frequency increases obviously,and the amplitude of low frequency signal amplitude is also obvious.With the increase of inflow gas void fraction,the Root Mean Square(RMS)value of pressure fluctuation increases first and then decreases.With the decrease of flow rate,the RMS value of pressure fluctuation increases gradually,and the RMS value of pressure fluctuation is the smallest at the design operating point,which can be used as an important basis for flow regime monitoring.(2)The main frequency of pressure fluctuation of PMMA pump is blade frequency under different inflow gas void fraction conditions.With the increase of inflow gas void fraction,the RMS of pressure fluctuation decreases gradually,which shows a difference when compared with stainless steel centrifugal pump.This is due to the existence of tip clearance,which makes gas more likely to stay in the tip clearance.The frequency domain analysis of pressure fluctuation at inflow gas void fraction of 0% and 2% shows that the head decreases and the amplitude of pressure fluctuation increases obviously at axial frequency due to the increase of unstable flow after ventilation,which indicates the intrinsic correlation between the amplitude of pressure fluctuation and the head.The uncertainty analysis shows that the test results have certain credibility.4.The Euler-Euler inhomogeneous model was used to calculate the model pump.The conclusions are as follows: The accumulation of gas in the volute leads to the generation of vortices,which intensifies the accumulation of gas,and then affects the energy exchange and transmission of the impeller,resulting in the decline of pump performance.The gas always gathers at the suction side of the blade.With the increase of the inflow gas void fraction,it gradually gathers in the whole flow passage.Some of the gas flow from the impeller outlet to the volute,and finally to the outlet pipe.The phenomenon of gas-liquid separation has begun to occur when the inflow gas void fraction is 5%.5.The high-speed photographic experiments of PMMA centrifugal pump in the dynamic and static interference region between the tongue and impeller were carried out under different inflow gas void fraction conditions,and the force of bubbles in the impeller was analyzed.The conclusions are as follows:(1)The flow pattern of bubbles in impeller will appear uniform bubbly flow,aggregated bubbly flow,gas pocket and separated flow with different inflow gas void fraction.The performance changes are different because of the different flow patterns,especially when the gas pocket occurs in the impeller,it is found that the performance decreases rapidly and the pressure pulsation fluctuates violently.(2)Bubbles always gather near the suction side of the blade,and then move towards the outlet of the impeller along the pressure of the blade.This is because the uneven force exerted by the bubbles in the impeller causes the bubbles to flow out along the high-pressure side which deviates from the liquid streamline.Therefore,the bubbles gathered at the low pressure side of the suction surface of the blade will gradually move towards the high pressure side and gradually flow to the volute outlet. |
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