Experimental Study And Numerical Simulation Of Single Bubble Rising Behaviors

Experiment and numerical simulation was used to study a bubble rising through still water,the rising behaviors of bubble trajectory,velocity,tilt angle,forced and wake evolution was analyzed qualitatively and quantitatively,and can provide basic parameters and theoretical guidance for numerical simulation and practical process which involving gas-liquid two-phase flowing and mixing.Under the experimental conditions,the shape of bubble is spherical or nearly spherical when the diameter of the bubble leaving the nozzle is less than 3.38 mm,and the shape of bubble is spherical-cap or nearly spherical-cap when the diameter is larger than 3.38mm.When the initial diameter of the bubble is less than 1.60 mm,its trajectory is linear.When the diameter is 1.70～2.00 mm,the trajectory of steady rise is spiral.When the diameter is 2.00～4.30 mm,the trajectory of steady rise is zigzag or spiral.However,when the diameter is larger than 4.30 mm,the bubble trajectory is difficult to maintain stability.When the bubble oscillation rises and the trajectory is stable,bubble trajectory,velocity and tilt angle with time can be quantitatively characterized by sine or cosine function,and they have the same oscillation frequency in the horizontal direction,but oscillation frequency of the total and vertical velocity is twice fast as oscillation frequency of horizontal velocity when the bubble rises zigzag.If the oscillation frequency of rising trajectory is smaller,the horizontal amplitude or projection area of the bubble rising trajectory along zigzag or spiral is larger.The horizontal amplitude or projection area increases with the increase of the value of bubble tilt angle.In this experiment,the horizontal amplitude of 2.45 mm bubble rising along zigzag trajectory is largest(5.64 mm),and the oscillation frequency is smallest(30.15 rad·s-1);when the bubble of 1.83 mm rises steadily along spiral frequency,the horizontal projection area is largest(95.87 mm2)and the oscillation frequency is smallest(25.12 rad·s-1).When bubble is rising with a constant velocity and the rising process is stable,the forced can be calculated by combining the traj ectory,tilt angle and initial diameter of the bubble with Newton’s second law.There are three motion states of bubble wake:symmetrical shedding,unstable shedding and periodic shedding during the bubble freely rising.The symmetrical shedding is the initial state of the wake,and the symmetry of the vortex is good and both the left and right vortices shedding on the same horizontal line.The unstable shedding is transitional state between symmetrical shedding and periodic shedding,the symmetry of the wake decreases with the rise of the bubble and the left and right vortices do not shed on the same horizontal line.The periodic shedding is the final state,and the left and right vortices shedding alternately with the rise of the bubble.The transition of the motion state of bubble wake is closely related to the change of its rising trajectory and tilt angle.When bubble rises in a straight line,the wake evolution is same and shedding symmetrical mainly from different angles of view,and shape of the bubble is a spherical and does not tilt.When the bubble rises in a zigzag path,the wake evolution is different and may be shedding symmetrically,or may be shedding symmetrically,unstable shedding and periodic shedding from different angle of view,and shape of the bubble changes to an ellipsoid and there is tilt angle during the rising process.When the bubble rises along a spiral trajectory,the wake evolution all is shedding symmetrically,unstable shedding and periodic shedding from different angle of view,and shape of the bubble changes to an ellipsoid and occur to tilt during the rising process,but the transition time of the wake motion and the tilt angle of the bubble all may be different.