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Third International Topical Team Workshop on TWO-PHASE SYSTEMS FOR GROUND AND SPACE APPLICATIONS, Brussels, Belgium, September 10-12, 2008 Evaporation of water-ethanol mixtures in a Hele-Shaw cell : an experimental interferometry-based study Sam Dehaeck, Christophe Wylock and Pierre Colinet Université Libre de Bruxelles, Faculty of Applied Sciences, Chemical Engineering Department, Av. F.D. Roosevelt 50, CP 165/67, B-1050 Brussels, Belgium, [email protected] , [email protected] Heat transfer applications such as heat pipes, boilers and refrigerators use evaporation and condensation to evacuate the energy dissipated or stored in the system. To this end, pure liquids are primarily used as the use of mixtures leads to concentration gradients which generally lower the heat transfer coefficient. On the other hand, recent work (Y. Abe, 2007) has shown that well-chosen mixtures can have special properties which can delay the onset of burn-out of heat pipes, leading to an important enhancement of the maximum achievable heat transfer rate. Studying the evaporation of mixtures in micro-channels and thin films therefore appears crucial both on the fundamental and applied points of view. In this work, the evaporation of a water-ethanol mixture in a 2D Hele-Shaw cell is examined with a Mach-Zehnder interferometer. Concentration changes inside the Hele-Shaw cell change the optical path of one of the laser lines, thus creating a visible phase shift ∆φ of the fringes, which can be transformed into a refractive index difference ∆n. Through calibration, this ∆n can then be translated in an ethanol concentration variation. Experiments have shown that, as ethanol is preferentially evaporated, a boundary layer rich in water is formed near the interface. Now, as this layer is heavier than the underlying mixture, dense water plumes tumble down, like in the classical Rayleigh-Taylor instability. This creates extra mixing and replenishes the ethanol in the upper layer, which in turn enhances the evaporation rate. Experimental results illustrating this instability are shown in figure 1 for two different bulk ethanol concentrations. Note that a larger initial ethanol concentration leads to larger concentration gradients in the flow as this also increases the evaporation rate. In addition to a Rayleigh-Taylor instability, Marangoni convection was also observed in this system. These motions can occur because the surface tensions of water and ethanol strongly differ. Therefore, ethanol concentration variations along the surface result in the appearance of convective vortices, which also aid the replenishment of ethanol. In the present contribution, the onset of these instabilities and their evolution in function of the bulk ethanol concentration and the gas flow rate will be examined. This will also be done for different configurations of the gas channel of the Hele-Shaw cell. Acknowledgements The authors gratefully acknowledge financial support of this study by the CIMEX-PRODEX Program, managed by ESA in collaboration with the Belgian Science Policy Office. References Y. Abe, Terrestrial and Microgravity Applications of Self- Rewetting Fluids, Microgravity Science and Technology, Vol. 19, pp. 11-12 (2007). Figure 1: Relative concentration during evaporation for two different bulk ethanol concentrations
