Exploration of the mechanism of cavitation vortex rope and vortex development in the draft tube of tubular turbine units

Draft tube vortex rope is considered a special cavitation flow phenomenon in tubular turbine units.Cavitation vortex rope is one of the most detrimental factors affecting the safety of hydraulic turbines.In this study,ANSYS CFX software was utilized to numerically simulate the internal cavitation flow of a hydraulic turbine draft tube.The evolution of the cavitation vortex core was characterized by vortex line distribution and vorticity transport equation.The shape and number of blades influenced the revolving direction and distribution characteristics of the vortex close to the runner cone,which formed a counterclockwise-clockwise-counterclockwise distribution pattern.Simultaneously,there were many secondary flows in the draft tube.Mutual cancellation and dissipation between the flows was one of the reasons for reduction in vorticity.When the cross-sectional shape of the draft tube was changed,the vorticity was distributed from the center of the vortex rope to all parts of the cross-sectional draft tube,with extreme values at the center and at the walls.The vortex stretching and dilatation terms played a major role in the change in vorticity,with the baroclinic torque having an effect at the center of the vortex rope,this study is helpful to understand the flow of water in the draft tube and guide the design and optimization of the draft tube in engineering application.

Study on S-shaped region of pump turbine based on Omega vortex analysis method and entropy production theory

In order to comprehensively analyze the operation instability of the pump turbine S-shaped region,this paper uses DDES turbulence model to calculate the model pump turbine from the perspective of the evolution law of runner vortex and draft tube vortex rope and entropy production rate,combined with experiments.The results show that the numerical simulation is in good agreement with the experiment.Omega vortex analysis method is more accurate than other vortex recognition methods because it is not affected by the threshold value.The vortices at the runner region under the runaway condition and the turbine brake condition develop towards the vaneless space and the blade pressure surface respectively,which will cause the flow obstruction and blade separation.The overall vorticity of the reverse pump condition is the largest.The vortex rope of the draft tube under runaway and turbine brake conditions is columnar in shape and has very high rotational strength.The vortex rope under reverse pump conditions is prone to fracture and form scattered vortices,impeding the normal movement of the fluid.The entropy production rate of the spanwise surface near the upper ring and the lower crown is greater than the middle spanwise surface due to the boundary layer effect.And the energy dissipation in the runner under reverse pump conditions is characterized by high at both ends of the runner and low in the middle.The energy dissipation near the wall of the straight cone section of the draft tube is large due to the squeezing effect of the vortex rope on the flow.

Effects of medium fluid cavitation on fluctuation characteristics of magnetic fluid seal interface in agricultural centrifugal pump

The fluctuation law of magnetic fluid seal interface of an agricultural centrifugal pump is theoretically unknown;the pressure and velocity fluctuations are crucial factors that cause interface fluctuation.In this study,the pressure and velocity fluctuations of the sealing interface on an agricultural centrifugal pump during cavitation were investigated based on the methods of Ansys CFX numerical calculation and experimental verification.The results demonstrated that at the same flow rate,the pressure fluctuation amplitude of the sealing interface decreased gradually from the shaft surface to the bottom of the polar tooth.At different flow rates,the amplitude of the pressure fluctuation decreased with an increase in the flow rate.The cavitation of the medium aggravated the impact and water hammer of the liquid,leading to the occurrence of the jitter phenomenon in the sealing interface to accelerate the fluctuation frequency of the axial velocity of the sealing liquid,which accelerated the emulsification of the magnetic fluid.This law can provide a reference for the design of magnetic fluid sealing devices for agricultural centrifugal pumps.