Methode der laser-induzierten Lumineszenz zur experimentellen Analyse des Stofftransportes in laminar-welligen Flüssigkeitsfilmen

  • Laser-induced luminescence method for the experimental analysis of mass transfer in laminar-wavy liquid films

Schagen, André; Modigell, Michael (Thesis advisor); Bardow, André (Thesis advisor)

Aachen : Shaker (2015)
Book, Dissertation / PhD Thesis

In: Berichte aus der Verfahrenstechnik
Page(s)/Article-Nr.: VIII, 118 S. : graph. Darst.

Aachen, Techn. Hochsch., Diss., 2015

Abstract

Enhanced heat and mass transfer in wavy liquid films is not clarified in detail. A laser-induced luminescence method is developed to investigate these phenomena experimentally. This method enables the non-invasive, inertia-free and simultaneous measurement of local film thickness and the distribution of a scalar field quantity in high spatio-temporal resolution. Biacetyl is used as luminescence indicator. Its fluorescence emission correlates with the film thickness. The decay of phosphorescence emission depends on the distribution of a transported scalar in the liquid film. Experiments are performed in excited laminar-wavy water films flowing down an inclined plane. Mass transfer is observed for the liquid side controlled absorption of molecular oxygen into liquid films. Bottom-side heated liquid films are used to look at the heat transfer. Even laminar-wavy liquid films flowing down a slightly inclined plane, show enhanced mass transfer compared to liquid films with smooth surfaces. Evaluations of measured data exhibit maxima of heat and mass transfer coefficients between wave hump front and capillary waves region in solitary waves. Minimal transfer coefficients are located beneath the wave humps. Higher transfer coefficients are found in the capillary waves regions. Fields of effective diffusion coefficients are calculated from measured concentration distributions. At the end of the hydrodynamic transitional region maxima occur below the liquid surface. In the hydrodynamic developed region the effective diffusion coeffient at the interface is higher than the molecular diffusion coefficient. Its value increases with higher Reynolds number. Within the first iteration of the structured working process model based experimental analysis a model is proposed which describes the fields of effective diffusion coefficients. By implementing such a model into appropriate design tools, falling liquid film apparatus can be designed efficient and more accurate than using integral design correlations.

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