Gas transfer to and across an air-water interface

Abstract

A treatment based on Reichardt's formulation of the velocity profile in turbulent flow over a smooth plane surface is shown to give good agreement with published data on the boundary layer transfer of heat and mass over a wide range of Prandtl (or Schmidt) number, s̀. Applied to transfer to a water surface, agreement with published laboratory results is also good for low air speeds (smooth water). Comparison with observations for the sea shows there is little difference between the calculated evaporation coefficient and those reported for the sea with winds of ∼7 m s−1. This is consistent with sea trials having so far detected little increase of evaporation and heat transfer coefficients with wind speed over the range 4–10 m s−1. The air/water transfer of non‐reactive gas is governed by the resistance of the viscous sublayer of water, and the smooth surface treatment gives the transfer velocity (VL) on a liquid phase basis as VL = 0.082 (pa/pw)1/2s̀−2/3u* where papw is the density ratio, air/water, and u* = friction velocity of the surface air flow. The agreement between this formula and published wind tunnel results is good for the smooth water condition. At higher wind speeds transfer exceeds the calculated value and appears then to increase roughly in proportion to the square of the wind speed.

Gas exchange across the air—sea interface

“…Calculation of transfer velocities is then straightforward (see Deacon, 1977 andHasse, 1971). This is similar to the film model, but avoids the unrealistic notion of a purely viscous layer near a fully turbulent layer and the necessity of having different thicknesses for the diffusive sublayer depending on the molecular diffusivity of the property.…”

“…a layer of molecular transport at the interface, bordering on the bulk fluid side with a region of fully turbulent transport (Whitman, 1923), or more refined, with a smooth transition from molecular to turbulent transport, reviewed by Deacon (1977), or (ii) surface renewal model, where the fluid, which is at rest at the interface, is removed into the bulk at random intervals (Higbie, 1935;Danckwerts, 195 1).…”

Uncertainty of the global oceanic CO<sub>2</sub> uptake induced by wind forcing: quantification and spatial analysis

“…The theoretical background of the gas transfer is well established and extensively described in Deacon (1977), Liss and Merlivat (1986) and in Sarmiento and Gruber (2006). In a nutshell, gas transfer that occurs at the air-water interface, FCO 2 10 [mol C m -2 yr -1 ], in a micrometric water and air boundary layer, can be estimated by Fick's first law of molecular diffusion:…”

Optimizing the Air Dissolution Parameters in an Unpacked Dissolved Air Flotation System

“…The increased volume of air released from the exiting flow at warmer temperatures can be explained by the Stokes-Einstein equation for diffusivity [16]. T = the absolute temperature, °K α = radius of the solute molecule, m µ = viscosity of the solvent, kg/m*s The rate of transfer of gas between water and air was controlled by the thickness of a thin layer of water through which gas was transferred by molecular diffusion [17].…”