Buoyancy-driven convection in two superposed fluid layers in an octagonal cavity

Abstract

Buoyancy-driven convection in a differentially heated cavity containing superposed layers of air–water, air–silicone oil and silicone oil–water has been experimentally studied. The apparatus is octagonal in plan. It has a nominal diameter of 130.6 mm and a height of 50 mm. The individual fluid layer heights have been kept equal to 25 mm in all the experiments. The cavity temperature difference has been varied over a range of 0.4 to 18 K, depending on the combination of fluid layers studied. Measurement of the thermal field in the octagonal cavity has been carried out using laser interferometry. Four different view angles namely 0, 45, 90, and 135° have been considered. The interface shapes have been recorded in the form of shadowgraphs. In the air–water experiments, the thermal fields in both fluid layers show a loss of axisymmetry, while progressing from a lower to a higher Rayleigh number. In air–oil experiments, a steady axisymmetric thermal field is observed in the oil layer, while a sufficient number of fringes did not form in air. In oil–water experiments, the thermal field in oil is close to axisymmetric. In water, the transition sequence to three dimensionality and unsteadiness is similar that of a single fluid layer. In most experiments, the cavity temperature difference is found to split across the fluid layers inversely in proportion to the effective thermal conductivity of the fluid layers. The experimental interface temperature matches the estimated value from single fluid correlations fairly well. Discrepancy with correlations is observed when the fluid layers are mechanically, rather than thermally coupled.