Bubble dynamics and substrate thermalization during boiling in water saturated porous matrix

• Optical recordings of bubbles during boiling in porous matrix.
• Interaction between bubble dynamics and temperature inside porous.
• Heat transfer coefficient dependence on gravity.

Boiling over and just below the surface of a porous matrix is experimentally studied during the immersion of ceramic porous matrix saturated with water inside a bath of a hot immiscible liquid (i.e. oil with temperature well above the water boiling point). To simplify the geometry of the problem, the porous matrix has only one surface exposed to hot oil, the others being thermally insulated. Therefore, the hot oil triggers boiling solely over the exposed porous surface.Continuous temperature measurements inside the oil bath, on the oil-porous interface and inside the porous matrix (i.e. 0.5, 1.0 and 1.5 mm below the surface) are acquired along with optical images of bubbles activity (based on fast-video recordings) over the exposed porous surface. Temperature profiles along with images are cross-examined aiming at identifying a possible relation between bubbles behavior and boiling heat transfer inside the porous matrix. The influence of the oil bath’s temperature, Toil, on the above phenomena is studied by testing various Toil values (i.e. 150, 160, 170 and 180 °C). Increased levels of gravity in the range from 1 g to 9 g are used as a tool (experiments conducted in the Large Diameter Centrifuge at ESA/ESTEC) to modify bubble dynamics over the porous surface.The results reveal the influence of Toil on the evaporation front propagation beneath the porous surface. In addition, the analysis of the experimental results elucidates the relationship between the heat transfer coefficient and the gravitational acceleration parameter. Moreover, the data analysis indicates a strongly non-linear effect of increasing gravity on heat transfer coefficient over porous media.