On the use of the active infrared technique to infer heat and gas transfer velocities at the air-water free surface

A comparison study of the experimental and theoretical transfer velocities of heat ands gas transfer at a wavy air-water interface is undertaken using an active infrared technique and two gas tracers. Applying the surface renewal model formalism [Danckwerts, 1951], we find that the experimentally evaluated heat transfer velocity is roughly a factor of 2 higher than the transfer velocity of a gas with a low solubility in water when both are referenced to Sc=600. Potential origins of such a discrepancy are investigated and we propose the use of the random eddy model [Harriott, 1962] to explain our results. The model is an extension of surface renewal to include the eddy approach distance as a new parameter. Numerical simulations of the random eddy model have been performed using a timescale evaluated from the Active Controlled Flux Technique (ACFT) and the characteristics of heat as well as the two gases used in the experiments (He and SF6). The simulation results show that the transfer velocities of two species, referenced to the same Schmidt number, are different and that their ratio depends on the average value of the approach distance and its distribution. The model as implemented in the present work also predicts changes in the Schmidt number exponent when the hydrodynamics conditions are varied.