Investigation On Geometric And Dynamic Characteristics Of The Bubble In Stationary Water And Cross-flow

An investigation on geometric and dynamic characteristics of the bubble in liquid environment was carried out.A preliminary work was performed for single bubble trapped in stationary water;then the study was advanced,which was substantiated by the exploration of the initial bubble morphology,bubble trajectory,bubble size distribution and carrier flow features under the conditions of liquid cross-flow.Two sets of visualization test rigs were established;one provided stationary water and the other produced liquid cross-flows.Bubbles were observed and analyzed quantitatively.In stationary water,with six ventilation tubes of different inner diameters,the motion of bubble was traced using the high-speed photography.The bubble information was extracted statistically using an in-house code developed based on the commercial code MATLAB.The bubble movement in the entire development process was described.The dynamic characteristics of the bubble under various operation conditions were comparatively analyzed.Based on the combination of statistical results obtained from a large amount of bubble data and the existing empirical formulae,the bubble aspect ratio was related to the dimensionless number Eo,We and Ta.An emphasis was placed on the effects of buoyancy,surface tension and the inertia force.Liquid crossflows were generated in the test section of a water tunnel;the water flow velocity was varied in the range of 0.21-4.04 m/s and the air flow rate between 200 and 2000 m L/h.Bubble images were captured using the high-speed camera.The carrier flow was measured using particle image velocimetry technique.Moreover,the effects of the liquid cross-flow on the bubble were analyzed by the corresponding algorithm and the developed software.Major conclusions obtained as follows:(1)In stationary water,the trajectory of the rising bubble evolves from the initial straight line to the eventual curved line after it detaches from the ventilation tube.The length of the straight trajectory segment for different bubbles varies.At the stage of steady rise,bubble trajectory can be approximated by the sinusoidal function,and small bubbles are featured by considerable deviation from the vertical symmetrical line.Based on the bubble velocity and the bubble aspect ratio,the bubble rising process can be divided into four stages,namely velocity increasing stage,zero acceleration stage,velocity descending stage and stable movement stage.The bubble velocity during the bubble rising process is inversely proportional to the bubble aspect ratio.Moreover,with the increase in the bubble equivalent diameter,the bubble terminal velocity increases linearly at first and then decreases gradually towards some certain range.The bubble aspect ratio decreases on the whole.Bubble size determines the most predominant force exerted on bubble;for different ranges of bubble equivalent diameter,both the distribution and the decline rate of the bubble aspect ratio are different.(2)In liquid cross-flow,according to the magnitude of the upstream flow velocity and the air flow rate,initial bubble development regimes can be divided into six typical patterns,namely smooth fine bubble flow,throat-neck fine bubble flow,stratified bubble-ligament flow,Taylor bubble cavity flow,diffused and fragmented bubble flow and intermittent atomized bubble flow.Such a demarcation is novel in consideration of the findings published in relevant studies.Various bubble development regimes are explained.Moreover,the evolution of bubble morphology is depicted.The variation of initial bubble geometry depends greatly on the upstream velocity and it is related to the air flow rate to some extent.In this context,the regimes of throat-neck fine bubble flow and stratified bubble-ligament flow are sensitive to the cross-flow parameters.(3)For low upstream flow velocities,the bubble trajectory is composed of linear and oscillating segments,which is similar to that in stationary water.Furthermore,the wake vortices following the bubble in the stationary water are also witnessed under the cross-flow condition,and the vortex intensity attenuates with the increase in the upstream flow velocity.By correlating the bubble aspect ratio with E?tv?s number and Weber number,the dynamic characteristics of bubbles are explained.The evolution of bubble in the liquid cross-flow proves to be relevant to multiple factors.With high upstream flow velocities and certain air flow rate,the volume fraction assumed by small bubbles increases with the increase in the cross-flow velocity.Exposed to high turbulence intensity and velocity gradients downstream of the ventilation tube,small bubbles are generated owing to the collapse of large bubbles.With the increase in the upstream flow velocity,the total bubble volume decreases continuously.(4)Based on the carrier flow data obtained using particle image velocimetry,it proves that the initial bubble geometry,bubble separation from the ventilation tube,bubble trajectory,bubble size distribution and bubble collapse mechanisms are closely related to carrier flow characteristics.The initial bubble geometry and the detachment process are highly dependent on velocity gradients and vorticity distributions immediately downstream of the ventilation tube.It indicates that the position where streamlines gather near the ventilation tube is the deepest position that the bubble attached to the ventilation tube can reach.For the regime of stratified bubble-ligament flow,large bubbles are confined in the wake area near the outlet of the ventilation tube due to the twist of streamlines.Whether the bubbles could be positioned near the ventilation tube depends on the velocity difference between the near-wall flow and the undisturbed flow.The vorticity distribution in the carrier flow and the bubble profile are found to vary synchronously.