Buoyancy-induced convection in differentially heated superposed two fluid layers in a rectangular cavity

Buoyancy-driven convection in a differentially heated rectangular cavity containing layers of air—water, air—silicone oil, and silicone oil—water has been experimentally studied using laser-interferometry. The cavity has dimensions 32 × 32 mm² in the vertical plane and length 447 mm. The layer thicknesses were taken to be equal. Experiments are conducted with three cavity temperature differences of 10, 15, and 18 K. The flow field is mapped in the direction parallel to the cavity length using a Mach-Zehnder interferometer. The flow regimes (such as steady two-dimensional, time-dependent, etc.) established in the individual fluid layers were compared with the published regime diagrams of single fluid layer at different Rayleigh numbers. Quantities such as the interface temperature, average Nusselt number, and the temperature profiles were determined from the interferograms. The interface shapes are recorded in the form of shadowgraphs. These results were also looked upon in terms of the coupling mechanism established at the interface.

The major conclusions arrived in this study are as follows. In the experiments involving air, the layers were found to be thermally coupled. The unsteadiness in water could however be transmitted to air in the mechanical coupling mode. The presence of silicone oil over water led to mechanical coupling in the sense that the convective field in water was visibly retarded. The interface temperature determined from the experiments matched those from correlations for a single fluid whenever the coupling was thermal in origin. The differences were higher during mechanical coupling. The interface deformation correlated well with the roll movement visible in the fringes.