Combined Surface Tension and Buoyancy Driven Convection around a Bubble on a Heated Surface
- Paper number
IAC-05-A2.4.08
- Author
PhD Stefano Fico, University of Naples "Federico II", Italy
- Coauthor
Prof. Raffaele Savino, University of Naples "Federico II", Italy
- Coauthor
Ph.D. Fabrizio Nota, University of Naples "Federico II", Italy
- Year
2005
- Abstract
The behaviour of bubbles in terrestrial and space applications has been subject to extensive research, especially related to pool boiling processes. The dynamic of formation, growth and detachment of a bubble in typical boilers has proved to be influenced by both gravity and surface tension. In this work a combined experimental and numerical analysis is carried out to study the effect of gravity and surface tension gradients on a bubble formed under a heated wall. In this configuration, the interaction between buoyancy and surface tension driven convection, produces complicated fluid dynamic structures. An instability occurs in the form of an oscillatory 3D fluctuation of the thermal and flow field when a critical Marangoni number is exceeded. In previous works the dynamic of this instability has been accurately investigated from an experimental point of view, and partially explained by means of 2D axi-symmetric numerical simulations. In this work a 3D numerical model has been developed to study the structure of this complex flow regime with transient simulations. The numerical model takes into account the presence of both thermocapillary and natural convection in the surrounding of the bubble. The Navier Stokes equations have been numerically solved using a time implicit control volume method. A structured grid with 300000 quadrilateral cells has been considered. Experiments have been carried out, forming a bubble of relatively small dimensions (diameter ranging from 1mm to 3mm) under the upper wall of a cell filled with n-octane (Pr=8,85). A temperature difference is imposed, between the upper and lower wall (the height of the cell ranges fro 10 to 30 mm), producing a temperature gradient along the bubble interface, which causes thermocapillary convection. For low Marangoni numbers steady conditions are attained. With increasing Marangoni numbers a transition to an unsteady regime takes place and an oscillation of the temperature and velocity field appears around the bubble (with frequencies of about 0,3 Hz). The velocity field around the bubble is evaluated monitoring the motion of tracers, using a “laser sheet” technique for flow visualization. A Wollaston prism interferometer has been used to capture the oscillating temperature field. The Wollaston interferometer (which is sensible to inhomogeneities of the index of refraction gradient) has been chosen because of its accuracy in pinpointing the onset of the instability. An excellent agreement has been found between numerical and experimental results, which has allowed to obtain new original conclusions on this complex phenomenon. In particular the limitations of an axi-symmetric model have been shown and proved by comparing the 3D numerical solutions with the axisymmetric ones. Gravity has proved to be fundamental for the onset of the oscillations.The influence on the heat transfer through the heated wall (where the bubble is placed), has been shown both in normal gravity and no gravity conditions.
- Abstract document
- Manuscript document
IAC-05-A2.4.08.pdf (🔒 authorized access only).
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