Laboratory experiments on interfacial heat and gas exchange promoted by surface cooling: novel simultaneous thermal imaging and optical oxygen-concentration measurement

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To be able to properly model the global cycling of carbon (and thus the global heat budget), detailed knowledge is needed on the amount of greenhouse gases absorbed or released by large bodies of water. At present, most models used to estimate gas transfer across the air-water interface only consider wind-shear and do not take buoyancy into account, which is a major contributor at low to moderate wind speeds. To improve the accuracy of the predictions, a detailed study of buoyancy-driven gas transfer in deep waters is necessary. As the interfacial mass transfer of low to moderate soluble gases (e.g. carbon dioxide, oxygen, methane) is characterized by an extremely thin gas concentration boundary layer, elucidating the physical mechanisms of the process is immensely difficult. Despite advanced development in optical measurement techniques, detailed simultaneous mappings of the highly dynamic temperature and gas concentration distributions promoted by buoyant-convection under well-controlled laboratory conditions have not been reported yet. We therefore propose to investigate the heat and gas transfer process driven by a buoyant-convective instability triggered by surface cooling through non-intrusive (optical) and simultaneous measurements of gas (oxygen) concentration and temperature fields (i) at the water surface and (ii) in a vertical plane on the liquid side. A complete lifetime-based laser induced fluorescence system suitable for resolving the oxygen dynamics including in the diffusive sublayer will be developed. To capture the distribution of the thermal structures at the surface, a high-precision infrared camera will be used, while an intensity-based LIF-thermometry system will be employed to obtain 2D thermal fields within the water column. The results will provide unprecedented experimental data in the form of synoptic two-dimensional thermal and gas concentration mappings under buoyant-convectively driven flow conditions in a relatively deep water body. The correlation between the thermal and gas-saturated plumes will be studied and their geometrical characteristics both at the water surface and in the water column will be explored and related to the heat and gas fluxes. A series of gas transfer velocity (k) measurements over a wide range of temperature differences between the bulk of the air and of the water will be carried out allowing k to be related to the bulk Rayleigh number and compared with k obtained from our new detailed simultaneous measurements. In addition, to provide information on the flow-field, particle image velocimetry (PIV) measurements will also be carried out for selected cases, which mainly focus on providing an overall(bulk)-view.