Introduction

Mass transfer at air-water interfaces has a great influence on the climate, since it acts on the atmospheric proportion of gazes (oxygen, carbon dioxide...). It is also involved in many industrial applications such as chemical industry, energy production and cleaning processes (water purification, CO2 absorption).

Those systems' performances therefore partly depend on mass transfer between the two phases. An ideal way to optimize the process would be to estimate this transfer using CFD. Nevertheless, a better understanding of the mechanisms ruling the transfer, like the influence of interface's shape or flow regime, is still needed to create reliable models. The aim of the present work is to study CO2 mass transfer at a flat gas-liquid interface, enhanced by turbulence.

Experimental setup

Gas dissolution from air to water is controlled by water turbulence characteristics [1,2] and by surface deformations under the action of wind. Three possible turbulence sources may be considered [3]: gaseous phase turbulence, similar to that of wind above oceans or lakes; liquid turbulence caused by bottom shear found in rivers and stirred cells, and convective turbulence created by temperature gradients between the free surface and the bottom of the fluid. The goal of this study is to better understand the mechanisms controlling CO2 absorption process in an environment dominated by shear generated liquid phase turbulence. The experimental bench is a prismatic tank with transparent Plexiglas walls and lid allowing flow visualization. Liquid phase turbulence is generated by an oscillating grid. Coupled Stereoscopic Particle Image Velocimetry (SPIV) and Quenched Planar LASER Induced Fluorescence (QPLIF) techniques make possible to measure simultaneously velocity and concentration fields at a good spatial resolution, and provide a better insight to the turbulent mass fluxes.

Results

From PIV measurements, it has been shown that, for a given grid oscillation frequency, velocity magnitude and fluctuations decrease toward the free surface. Vertical distribution of turbulence can be related to the grid oscillation frequency f, the stroke S, the mesh parameter M (distance between two consecutive grid bars), and the distance to the interface [4]. The Q-PLIF allows to obtain instantaneous CO2 fields and this shows the existence of a thin subsurface layer in which the concentration rapidly decreases from the surface saturation concentration. Under that layer, in the region affected by turbulence, CO2 concentration is globally lower, but one can notice high CO2 concentration patches being carried to the bottom by whirling structures: this is called peeling [5]. These observations show that turbulent dissipation plays an essential role in gas transfer. The rate at which CO2 is transported through the interface is set by the thickness of the viscous sublayer, and by the speed at which the turbulent structures replace the high CO2 concentration liquid close to the interface by low CO2 concentration fluid coming from the bulk. Finally, coupled SPIV and Q-PLIF measurements show the impact of 3D velocity structures on local mass transfer events.

References:

[1] Theofanous, T.G. (1984). Conceptual models of gas exchange, Gas transfer at water surfaces, 160, edited by W. Brutsaert and G. H. Jirka, D. Reidel Publishing Company

[2] Kitaigorodskii, S.A. (1984). Wind-wave effects on gas transfer, Gas transfer at water surfaces, 160, edited by W. Brutsaert and G. H. Jirka, D. Reidel Publishing Company

[3] Herlina, H. (2005). Gas Transfer at the Air Water Interface in a Turbulent Flow Environment. PhD Thesis

[4] Hopfinger, E.J., Toly, J.A. (1976). Spatially decaying turbulence and its relation to mixing across density interfaces. J. Fluid. Mech, vol 78, part1, 155-175

[5] Variano, E.A., Cowen E.A. (2013). Turbulent transport of a high-Schmidt-number scalar near an air-water interface. J. Fluid. Mech, vol 731, 259-287