| In view of the problem that harmful vortices are easy to occur in the pump sump,the velocity field of the evolution process of the floor-attached vortex(FAV)under the bell mouth of axial-flow pump from the initial stage to the disappearance stage was measured by the volumetric velocity measurements system(V3V).and uses VOF method to simulate the vortex flow in the pump sump,and analyzes the dynamic characteristics of FAV during the evolution process of FAV.The experimental measurement of pressure fluctuation was carried out at different cross-section positions of axial-flow pump with or without FAV in the pump sump.The characteristics of pressure fluctuation induced by FAV are studied,and the mathematical model of pressure fluctuation induced by FAV was established.The method of using the volume fraction of fluid to show the shape of vortex tube is proposed.According to the dynamic characteristics of the floor-attached vortex,a new method of vortex elimination is proposed.The three-dimensional velocity field of FAV under the bell mouth of axial-flow pump in the pump sump during the whole period from occurrence to disappearance was measured byV3 V.The VOF method is used to calculate the steady and unsteady state of the axial-flow pump device in the pump sump.The dynamic characteristics of the whole development space of FAV were analyzed by the experimental results and CFD results.A high-speed camera was used to capture the real shape of the vortex tube in the whole evolution process of FAV.From analysis of velocity gradient distribution in the vortex region,it was recognized that the large velocity gradient is key factor for the formation of a vortex.FAV originates from the singularity area where the flow direction of main stream changes sharply with great velocity gradient,causing the FAV generation and vortex shedding.The continuous shedding leads to the gradual increases of intensity of FAV to form a continuous vortex rope.It can be assumed that the process of FAV formation and development is the process of vortex shedding.The evolution process of FAV can be divided into five stages:inception,development,continuance,collapse,and disappearance.The regularity of intensity of FAV of the measurement results meet basically the vorticity transfer equation from the STOKES Theorem.The regularity of intensity of FAV increases first with time,remains constant in continuance stage after reaching at maximum value,the vortex intensity meets the vortex tube intensity conservation theorem at this stage,and then decreases rapidly.However,the increasing rate of FAV is less than the decreasing rate about 80%,which is different from the calculation results of the vorticity transfer equation.The process of the eddy kinetic energy increasing,developing,and decreasing denotes the vortex energy accumulation,retention,and dissipation.The occurrence frequency of FAV was between 1.2Hz and 2.6Hz.In view of the problem of pressure pulsation induced by FAV,pressure pulsation sensors are installed in the area where vortex occurs at the bottom of pump sump,impeller inlet,impeller outlet and guide vane outlet to measure the pressure pulsation with and without FAV in the pump sump.The characteristics of pressure fluctuation in different positions of axial flow pump with and without vortex were analyzed.The main frequency of the pressure pulsation at the impeller inlet is the blade frequency,which is independent of whether there is a floor-attached vortex in the pump sump.When FAV occurs,the time-domain curve of pressure fluctuation at the bottom of the pump sump is irregular compared with that without FAV,and the amplitude of pressure fluctuation is 1.98 times of that without FAV.The number of pressure pulsation peaks and troughs at the impeller inlet without FAV is equal to that of the impeller blades in one rotating period of the impeller,but the number of peaks and troughs increased with FAV,which is larger than that without FAV.The pressure pulsation at the impeller inlet changes the most at different periods with FAV.The pressure pulsation amplitude at the impeller inlet with FAV is larger than that without FAV.According to the Biot-Savart Law,the pressure fluctuation induced by FAV was analyzed.It was obtained the theoretical formula of pressure fluctuation induced by FAV.The pressure pulsation induced by FAV changed periodically with time and the shape of the pressure pulsation curve was a trigonometric function curve,which was verified by the experimental results.The pressure pulsation induced by FAV is a low frequency pulsation,and the frequency of the vortex is 2.12Hz,which is very harmful to the safe operation of the pump unit.The application conditions and limitations of various vortex criterion were analyzed by the experimental results and CFD results.According to the characteristics of gas-liquid distribution in the vortex tube,the method of displaying the vortex tube shape by the volume fraction of fluid is put forward,which is more consistent with the experimental situation.The gas-liquid boundary is the vortex tube wall,which is clearer and more accurate than other vortex criteria.The influence of the suspended height of the horn tube on the vortex characteristics is analyzed.When the height of suspension decreases,the frequency of vortex increases,the intensity of vortex increases,the duration of vortex increases,and the pressure in vortex core decreases.However,the characteristics of the floor-attached vortex are the same under different height of suspension.An anti-vortex guide wheel with vortex elimination blades is designed to suppress the occurrence of the vortices.The hub of the anti-vortex guide wheel and the vortex elimination blade are parabolic.The number of blades has the greater influence on the pump.When the number of vortex elimination blades is 6,there is no vortex near the guide wheel,and the operation efficiency of the pump device increases by 1.0-1.3%.The pressure fluctuation intensity at the impeller inlet is reduced by nearly 10%. |
