The effects of ripples, bubbles, droplets and contamination on mass transfer across tne breaking air-water interfaces

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The accurate estimation of the mass transfer rate across the sheared air-water interface has attracted a special interest in the global warming problem related to the exchange of carbon dioxide between atmosphere and ocean. The purpose of this study is to experimentally and numerically investigate the effects of ripples, bubbles entrained into water and dispersed droplets by wave breaking on the mass transfer across the air-water interface. The effects of heavy-oil surface contamination on the CO_2 transfer across the air-water interface were also investigated through laboratory experiments in an oscillating-grid tank. The bubble entrainment rate and relative velocity between a bubble and liquid were measured using both a phase Doppler anemometer (PDA) and a laser Doppler velocimeter (LDV). Further, the mass transfer velocity between a bubble and liquid was estimated by means of direct numerical simulation (DNS) over the wide range of the particle Reynolds number Re_p. The size distrib … More utions and mean velocities of dispersed droplets by wave breaking were measured using a PDA.The results show that the net contribution of the bubbles to the total mass transfer across the breaking air-water interface is at most 7% even at the free stream wind velocity of U=18m/s where wind waves are intensively broken. This result supports our previous study that the rapid increase of the transfer velocity in the high wind speed region with breaking waves is due to both the increase of the surface renewal frequency of ripple-like eddies and the disappearance of surface contamination of very tiny surface-active impurities by wave breaking. The CO_2 transfer velocity across the non-breaking interface for 3.5wt% saltwater(or natural seawater)contaminated with heavy-oil is damped to 46% of that for 3.5wt% saltwater without heavy-oil contamination. When the interface is intensively broken, the CO_2 transfer velocity across the air-water interface contaminated with heavy-oil approaches to that for tap water or saltwater without heavy-oil contamination. Less