Self-ReWetting capillary flow under evaporation and condensation processes in parabolic flight conditions

This work deals with experimental results obtained during the 67th Parabolic Flight Campaign of the European Space Agency. The capillary flow of self-rewetting fluids i.e. dilute aqueous solutions of long chain alcohols with an unusual surface tension behaviour, has been investigated onboard a zero-g plane under different gravity levels. Such mixtures have been extensively investigated on ground as working fluids for two-phase heat transfer devices. The presence of small proportions of alcohols in water changes both the wetting and surface tension properties of the mixture. Contrary to ordinary liquids, the surface tension becomes an increasing function with temperature that, in addition to the variation induced by the preferential evaporation of the more volatile component, provides a reverse Marangoni flow along the liquid-vapor interfaces driven towards the hotter regions. As working fluids for heat pipe systems, self-rewetting fluids show better properties, i.e. lower thermal resistance, enhanced dry-out limit and more stable behaviour. The parabolic flight experimental configuration includes a V-shaped groove channel partially filled with a water/butanol mixture and equipped with a top transparent window and a lighting system, enabling visualization of the liquid in the groove with a CCD camera. Results show that the liquid film distribution is affected from the gravity levels. During the parabolic manoeuvres, the liquid remains confined inside the groove channel and increasing the power level the thickness gradually decreases. The results are explained with respect to the thermo-physical properties of the self-rewetting mixture and discussed in relation to the experiments carried out in normal gravity condition.