Experiments on natural convection in complex enclosed spaces containing either two fluids or a single fluid

Heat transfer coefficients were determined experimentally for natural convection in the enclosed space between two concentrically positioned vertical cylinders having different finite heights. The space was filled with a pair of fluid layers, one atop the other. The investigated two-fluid systems included air and water, air and hexadecane, and hexadecane and water. Experiments were also performed for the case in which the intercylinder space was filled with a single fluid-air, water, or hexadecane. Parametric variations were carried out for the height of the fluid-fluid interface and for the Rayleigh number. Comparisons were made between the experimental results and predictions obtained from numerical solutions of the problem. Very good agreement prevailed for the single-fluid cases and also for the two-fluid cases where meniscus and interfacial tension effects were either negligible or small. When these effects were more significant, as for the hexadecane-water system (the only liquid-liquid system investigated), it was found possible to achieve satisfactory agreement by making use of a relatively simple interface model. For the single-fluid results, the effect of the Prandtl number variation (between 0.71 and 40) was highlighted.