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Systematic Measurements of Bubble Induced Gas Exchange for Trace Gases with Low Solubilities

Mischler, Wolfgang

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This thesis reports on a systematic experimental study on bubble-mediated gas transfer. Trace gas concentrations were measured using a quadrupole mass spectrometer with two silicone membrane inlets in the gas and water phase of the gas tight bubble tank. Fourteen trace gases with a wide range of solubilities and diffusivities – SF6, Neon, N2, HD, D2, O2, Krypton, Pentafluoroethane, Xenon, N2O, C2H2, CH3Cl, Benzene and DMS – were used to investigate the dependency on these two physico-chemical parameters. Bubbles were generated by a water jet with adjustable kinetic energy, which entrained a controllable gas volume flux into the water tank. Bubble size distributions in a radius range from 10 μm to 5000 μm and velocity distributions in a range from 70 μm to 3000 μm were measured at 60 positions by a telecentric optical setup. Invasion and evasion experiments with a variety of conditions were conducted including salt water (1.75% NaCl), the addition of the soluble surfactant Triton X-100, n-butanol and glycerol. All used additives tend to increase the fraction of small bubbles generated and therefore increase the transfer velocity at constant gas volume flux and energy input. The addition of salt had the strongest effect, leading to an enhancement of the transfer velocity up to a factor of approx. 4. The increase of the transfer velocity adding Triton X-100 was found to be lower than 5 %. The transition between diffusivity controlled and solubility controlled transfer occurs at solubilities ranging from 0.05 to 0.44. The transition is shifted to lower solubilities by a factor of approx. 3 by the addition of salt. Existing models for bubble mediated gas transfer are tested. Simple power law dependencies turn out to be incapable to describe the transfer for the whole range of solubilities and diffusivities. An extension of the parametrization proposed by Woolf requiring 4 parameters fits the data best. A simple model using only 2 parameters is proposed. Its performance is almost as good as the extended Woolf model.

Document type: Dissertation
Supervisor: Jähne, Prof. Dr. Bernd
Date of thesis defense: 12 November 2014
Date Deposited: 25 Nov 2014 08:39
Date: 2014
Faculties / Institutes: The Faculty of Physics and Astronomy > Institute of Environmental Physics
DDC-classification: 500 Natural sciences and mathematics
530 Physics
Controlled Keywords: Gasaustausch, Gasblase, Optische Messtechnik
Uncontrolled Keywords: gas exchange, telecentricity, bubble size distribution, salt water vs fresh water, gas transfer velocity, bubble mediated gas exchange
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