Melting of a porous medium saturated with water near its density maximum

Abstract

We report on an experimental and theoretical study of a liquid-saturated porous medium in a test cell with large dimensions, instrumented with a grid of 100 thermocouples. Well characterized, uniform-size glass beads with a nominal diameter of 0.6 cm have been chosen as the porous medium to facilitate the modelling of the transport processes in both the liquid and frozen regions. Experiments have been conducted to determine the variation of temperature as a function of position and time when a melting front initiated at the top of the test cell propagates downward. In these experiments, the water-saturated frozen porous medium is heated from above through the sudden imposition of a constant high-temperature boundary condition (i.e. T = 4°C at the top isothermal plate). Throughout the experiments, the mixture of ice and the porous matrix of glass beads is initially at 0°C, sharing the same temperature with the low-temperature bottom isothermal plate. The remaining boundaries have been thermally insulated. The experiments have provided evidence that conduction had been the dominant mode of heat transfer, initially. Comparison between our experiments and an analytically derived solution for front propagation based purely on heat conduction has indicated that at times larger than 25 h, natural convection prevails. The experiments also have suggested the establishment of two, counter-rotating flow circulation loops due to natural convection which move the fluid down along the outside bounding walls and up in the middle portion of the heat-transfer domain. The shape and size of the observed loops change with time as the melting front sweeps through the frozen fully-saturated porous medium.