Air‐water gas transfer in open channels

Abstract

Laboratory flume experiments on waterside‐controlled gas transfer, such as reaeration, are described. The experiments are well described by Danckwerts' surface renewal (1951) model, as originally proposed by O'Connor and Dobbins (1958), for an open channel flow. The primary cause of surface renewal in flumes is proposed to be the upwelling of large streamwise vortices and is documented by relating prior velocity measurements of these vortices and observations of surface upwellings to gas transfer measurements. A dimensionless liquid film coefficient (Stanton number) is related to a shear Peclet number and a shear Reynolds number. The shear Reynolds number represents the effectiveness of the large streamwise vortices in penetrating the water surface. The proposed hypothesis is essentially a large eddy model, except that coherent structures in the flow have been identified as the predominant mechanism of surface renewal, rather than random turbulence. The coherent structures of importance to surface renewal are the large streamwise vortices with a size similar to the depth that occurs in an open channel flow. Thus a streamwise vortex model of gas transfer in open channels is developed. The implications of this model on the prediction of gas transfer in open channels is discussed.