Bubble rise in a non-isothermal self-rewetting fluid and the role of thermocapillarity

Abstract

We report on the motion of a buoyancy-driven bubble in a vertical micro-channel and the significant role of thermocapillarity. A series of experiments have been carried out using a circular micro-channel filled with pure liquids (pure water and pure 1-butanol) and a self-rewetting fluid (water – 1-butanol 5% vol.) under isothermal and non-isothermal controlled conditions. In both cases, different mass fluxes and heat fluxes were applied on the micro-channel within the same temperature gradient field (18 °C–75 °C) which was increasing linearly in the same direction with the liquid flow. We have shown that the behaviour of the bubbles in a self-rewetting fluid departed considerably from that of pure liquids. The anomalous property of the alcohol mixture, i.e. the quasi-parabolic dependence of the surface tension with the temperature, drastically modified the movement (promoted or inhibited) and the shape (spherical or deformed) of the migrating bubbles. These phenomena were explained in terms of the location of the bubble associated with the well-defined surface tension minimum, and as a function of dimensionless numbers. Heat transfer coefficient calculations in the single and two-phase flows were acquired for all the liquids used. We demonstrated that the presence of Marangoni stresses resulted in the enhancement of the heat transfer distribution in the self-rewetting fluid flows compared to the pure liquids.