科研之友
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摘要
Turbulence plays a critical role in detachment process of bubble from a solid surface. To investigate this effect, detachment process of a stationary air bubble from a nozzle in both quiescent and turbulent liquid field was studied. A stationary vertical (flat-ended) needle of ID 1.24mm was used as a nozzle to generate a bubble which was anchored to the needle tip. Different sizes of bubbles were generated in quiescent liquid. Volume and contact angle for these bubbles were measured precisely using microscopic imaging technique and correlated. In the quiescent case experiments, a constant contact angle of 90 degrees and bubble diameter of 3.05 +/- 0.004mm were obtained consistently. A simple force balance approach was proposed assuming bubble in equilibrium to determine this maximum bubble diameter during detachment. The detached bubble size calculated using this approach agreed fairly well with the experimental results. An oscillating grid device capable of operating at different frequencies was then applied to generate a homogeneous, near-isotropic turbulent velocity field around the anchored bubble. It was observed that for detachment of smaller bubbles, higher turbulence intensity was indeed necessary. The turbulent flow field was quantified using particle image velocimetry (PIV) technique and resolved into flow structures (eddies) of different length scales using a Gaussian filter. It was concluded that smaller eddies perturbed the bubble interface whilst the larger eddies contributed to weakening of the capillary force causing the bubble detachment. Energy dissipation profile obtained from the PIV images indicated significant energy dissipation near the bubble compared to the bulk fluid which supported the fact that strong interactions between bubble and eddies were indeed responsible for bubble detachment.
- 出版日期2014-12
