Thesis Open Access

Entwicklung eines Experiments zur Messung von Blasendichten und blaseninduziertem Gasaustausch

Mischler, Wolfgang

Thesis supervisor(s)

Jähne, Bernd

In this thesis, an experimental setup for the measurement of the contribution of air bubbles to gas exchange is presented. In the ocean, bubble clouds are generated especially by breaking waves and potentially contribute to the exchange of trace gases between ocean and atmosphere in a significant way. The lab experiments aim at modeling this contribution on a physical basis. In the experiment, ocean-like bubble clouds can be created with a water jet pump in a glass tank (130 cm x 15 cm x 70 cm (H x D x W)), which was constructed for that purpose. The flow rate of air can be controlled, leading to a bubble distribution that is stationary in a statistical sense. For the measurement of the probability density of bubbles as a function of bubble radius and position in the tank, an imaging method was developed. It is based on the depth from focus principle and allows for the optical measurement of bubble radii between 100 and 1000 mum with simultaneous determination of the size-dependent measuring volume. The apparatus can be shifted automatically horizontally and vertically, allowing for capturing of the whole bubble cloud. For image evaluation, several image processing algorithms have been implemented and compared. A second optical setup for the measurement of bubbles penetrating the water surface was prepared and tested. Gas exchange rates were measured by integration of a UV absorption spectrometer in the water cycle. This allows the simultaneous measurement of the change of the concentration of multiple tracers (in the experiment: hexafluorobenzene, 1,4-difluorobenzene and phenylacetylene) in time. A pilot experiment demonstrates that transfer velocities can be computed from a mass balance and that e.g. the dependence on tracer solubility can be studied with respect to the bubble size distribution./p>

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