Experimental study of the onset of the 3D oscillatory thermocapillary convection around a single air or vapor bubble. Influence on heat transfer C. Reynard * , M. Barthe `s, R. Santini, L. Tadrist Laboratoire IUSTI UMR 6595, Ecole Polytechnique Universitaire de Marseille, CNRS—Universite de Provence, 5 rue Enrico Fermi, Technopo ˆle de Cha ˆ teau Gombert, 13453 Marseille Cedex 13, France Abstract In a first section, Marangoni convection induced by a temperature gradient is studied around a single air bubble. The bubble is introduced under a downward facing heating element into a liquid layer heated from upper and cooled from below (silicone oils Pr = 16.7 and Pr = 228 or FC-72, Pr = 12.3). In this configuration stationary thermocapillary convection can be followed by the oscillatory convection. Firstly the 3D oscillatory thermocapillary convection is presented for a silicone oil (Pr = 16.7). Then, the critical threshold between the stationary state and the first oscillatory mode is determined. The influence of the liquid kind on this threshold is presented for one layer height. Moreover, other parameters (horizontal wall temperature, layer height) are tested for one liquid. At least, we consider heat transfer induced by this convection, and we discuss the results versus the convection state. The second section of this paper concerns preliminary results on Marangoni convection with boiling around a single vapor bubble grow- ing under a downward facing heating element into a subcooled liquid (FC-72). The oscillatory thermocapillary convection is observed. It occurs during the growth from a critical bubble radius. The influence of the subcooling level on its occurrence threshold is shown. Ó 2005 Elsevier Inc. All rights reserved. 1. Introduction With the increase in the power and the decrease in the exchange surface size, particularly in the field of space, a lot of scientific works are being performed to study the mechanism of boiling. With the latest experimental re- sults on pool boiling obtained under microgravity, this mechanism seems to be efficient to transfer heat [1–3] under microgravity as under normal gravity. These re- sults are in contradiction with classical theories which explain the importance of the heat transfer under nor- mal gravity with bubble detachment induced by the buoyancy force. This leads to new questions on the mechanism of boiling. Thus, in order to determine which phenomenon is responsible of this equivalence, a new kind of experiment was performed in the last years. They were carried out around a single bubble [4,5], to avoid interactions with other bubbles, and sometimes without a liquid–vapor phase change, to avoid interactions between the different phenomena. For instance, a particular flow induced by a surface ten- sion gradient along an interface, the Marangoni effect, is studied in these cases because of its independence with the gravity level and its role under microgravity, which can be important [6–8]. However several questions remain still open on this phenomenon. For these reasons, we studied in our laboratory this phenomenon around single bubbles. Two complemen- tary experiments (without or with liquid–vapor phase 0894-1777/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2005.03.001 * Corresponding author. Tel.: +33 4 91 10 68 90; fax: +33 4 91 10 69 69. E-mail address: christelle.reynard@polytech.univ-mrs.fr (C. Rey- nard). www.elsevier.com/locate/etfs Experimental Thermal and Fluid Science 29 (2005) 783–793