TEMPERATURE-MEASUREMENTS DURING HEAT-UP OF A CONTAINED HOMOGENEOUS FLUID

Abstract

Laboratory measurements of the evolution of temperature field of a fluid contained in a square cavity during heat-up are reported. At the initial state, the fluid is motionless and at uniform temperature. Flows are induced by changing abruptly the boundary wall temperatures to a vertically linear profile. The transient adjustment process of the thermal structure is described. Experiments were performed using water as the working fluid at large system Rayleigh numbers. The Mach-Zehnder interferometry technique was used to portray the evolving thermal field. The results clearly disclosed the formation of boundary layers at very early times. In the intermediate stages, the presence of the vertically propagating temperature front is discernible. Ahead of the temperature front, the fluid remains unstratified, retaining the original uniform temperature. The adjustments of the global thermal structures are substantially accomplished over the convective time scale O(Pr1/2Ra1/4N(f)-1). The transient process is shown to proceed faster at a lower Ra because of the enhanced role of the diffusive effects. These observational findings are consistent with the previous analyses and numerical predictions.