Study On The Dynamics And Wake Structure Of Single Bubbles Contaminated By Surfactant Near A Vertical Wall

Adding a small amount of surfactant to gas-liquid two-phase flow can greatly change the behavior of bubbles,which has important application value in nuclear industry,petrochemical industry,and environmental engineering.In this paper,particle image velocimetry and shadow method are used to study the rising movement of a single deforming bubble(Re ≈ 470～800)contaminated by surfactant near a vertical wall in stationary liquid.Different concentrations of sodium dodecyl sulfate solution(100 ppm,400 ppm,and 800 ppm)and between the initial dimensionless distance(S*)a needle and the wall on kinematics and dynamics characteristics and change law of bubble wake bubbles are comparatively analyzed.The trajectory parameters,velocity,shape oscillations and energy changes of contamination bubbles near a vertical wall are studied,and the relationship between bubble wake and dynamics is explored.In the study of bubble kinematics,it is found that surfactants inhibit the transverse movement of bubbles and reduce the trajectory amplitude and wavelength.It accelerates the transition from a zigzag trajectory to a spiral trajectory.Different from clean bubble law,the difference of Reynolds number does not lead to significant change of contamination bubble trajectory parameters.Surfactants reduce the velocity of bubbles,and the wall-normal velocity of contaminated bubbles has a relatively stable period.With the increase of concentration,the difference of mean velocity at different wall distance decreases,and the variation range of instantaneous velocity peak in bubble movement direction caused by collision decreases gradually.Through the normalized power spectrum analysis,it is found that there is no secondary peak in the contamination bubble power spectrum.The main peak collision frequency of the contamination bubble is less than that of the free-rising bubble.In the study of bubble dynamics,it is found that surfactants inhibit bubble deformation.The shape oscillation of the bubble is analyzed by shape mode decomposition.It is found that the rotational oscillation of the bubble is promoted by Marangoni stress,which leads to the increase of the frequency of the contaminated bubble mode(2,2).With the increase of surfactant concentration,the wall effect is inhibited and the Marangoni effect is dominant.At 800 ppm,the bubble mean velocity and drag coefficient remain stable,and the lift coefficient does not change with the increase of concentration.The surface energy and kinetic energy fluctuations of contaminated bubbles decrease,and the energy dissipation caused by the collision decreases,which weakens the impact of the bubble-wall collision.The vorticity of bubble wake is negatively correlated with surfactant concentration.The liquid velocity of bubbles is inhibited by adding surfactant.As a result,the minimum liquid film thickness increases,the liquid film area decreases,and the wall induced lift decreases.The surfactant thickens the velocity boundary layer near the wall,resulting in the bubble unable to quickly discharge the surrounding liquid in the collision process,thus inhibiting the bubble movement trajectory and increasing the drag.