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Experiments on gas transfer at the air–water interface induced by oscillating grid turbulence

Published online by Cambridge University Press:  14 December 2007

HERLINA  and
G. H. JIRKA
HERLINA
Affiliation:
Institute for Hydromechanics, University of Karlsruhe, Germany
G. H. JIRKA
Affiliation:
Institute for Hydromechanics, University of Karlsruhe, Germany
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Abstract

The gas transfer process across the air–water interface in a turbulent flow environment, with the turbulence generated in the water phase far away from the surface, was experimentally investigated for varying turbulent Reynolds numbers ReT ranging between 260 and 780. The experiments were performed in a grid-stirred tank using a combined particle image velocimetry – laser induced fluorescence (PIV-LIF) technique, which enables synoptic measurements of two-dimensional velocity and dissolved gas concentration fields. The visualization of the velocity and concentration fields provided direct insight into the gas transfer mechanisms. The high data resolution allowed detailed quantification of the gas concentration distribution (i.e. mean and turbulent fluctuation characteristics) within the thin aqueous boundary layer as well as revealing the near-surface hydrodynamics. The normalized concentration profiles show that as ReT increases, the rate of concentration decay into the bulk becomes slower. Independent benchmark data for the transfer velocity KL were obtained and their normalized values (KLSc0.5/uHT) depend on ReT with exponent −0.25. The spectra of the covariance term c′ w′ indicate that the contribution of c′ w′ is larger in the lower-frequency region for cases with small ReT, whereas for the other cases with higher ReT, the contribution of c′ w′ appears to be larger in the higher-frequency region (small eddies). These interrelated facts suggest that the gas transfer process is controlled by a spectrum of different eddy sizes and the gas transfer at different turbulence levels can be associated with certain dominant eddy sizes. The normalized mean turbulent flux profiles increase from around 0 at the interface to about 1 within a depth of approximately 2δe, where δe is the thickness of the gas boundary layer. The measured turbulent flux (c′ w′) is of the same order as the total flux (j), which shows that the contribution of c′ w′ to the total flux is significant.

Type
Papers
Information
Journal of Fluid Mechanics , Volume 594 , 10 January 2008 , pp. 183 - 208
Copyright
Copyright © Cambridge University Press 2008

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