Study on Natural Convection of Melted Fluid in Porous Media with High Porosity

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Installation of high conductivity metal wires between heat transfer surfaces is effective for heat transfer augmentation during solidification of latent heat storage. This method, however, suppresses natural convection during melting, which decreases heat transfer. In order to investigate the effect of the wires between the surfaces on heat transfer, the critical Rayleigh number Rac of natural convection for horizontal layers with wires and heat transfer characteristics were studied by an analysis and an experiment.1.Linear stability analysis : The critical Rayleigh number was obtained for horizontal layers heated from below with wires installed vertically between the layers. Darcy flow was assumed and flow resistances of the wires obtained by Stokes flow assumption were used in the analysis. Results show that Rac was decreased compared with the sphere filled porous media for the same permeability.2.Rayleigh number at the onset of natural convection and heat transfer characteristics were studied by an experiment. Silicon oil and vinyl chloride rods were used as a fluid and a wire, respectively. Porosity was changed from 0.82 to 1.0. The results are as follows : i)The Rac was almost constant at 1707 for the porosity above 0.97. It decreases with decrease in porosity below 0.97. ii)After the onset of natural convection, heat transfer rate increases with Rayleigh number Ra. Increase rate of heat transfer decreased with decrease in the porosity. The increase rate, however, did not depend on the porosity when Nusselt number is plotted against Ra/Rac. iii)The critical Rayleigh number Rac for the porosity of 0.82 was about 6000-9000 for the experiment and 4550 for the analysis. The agreement is not bad. The cause of the slight disagreement will be non-uniformity of the flow, effect of wall thermal conductivity and so on.