Visualisation of Oxygen Concentration Fields in the Mass Boundary Layer by Fluorescence Quenching

Abstract

Laser-Induced Fluorescence (LIF) is applied to observe directly the mechanism of gas exchange in the aqueous viscous boundary layer at a free water surface. In order to make dissolved oxygen (DO) visible, a new class of dyes with a long phosphorescent lifetime in the order of microseconds is used. This property makes the quenching constant for DO sufficiently high for sensitive measurements. Depth profiles of the O2 concentration near the water surface are obtained by a vertical laser light sheet at a rate of 185 frames per second. This technique is capable of visualising a measurement window of some centimetres down from the water surface with a resolution in the order of 50–100 µm. For a small circular wind-wave facility a correlation between wind speed and gas-transfer velocities calculated from the extracted mean boundary-layer thickness are presented and compared to the results of parallel measurements with a mass balance method for other gases with given Schmidt numbers.KeywordsWind SpeedTransfer VelocityPhosphorescent LifetimeMass Balance MethodCircular WindThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.