Natural Convection Heat Transfer in a Horizontal Porous Enclosure with High Porosity Yasuaki SHIINA*4, Kota ISHIKAWA and Makoto HISHIDA Nuclear Applied Heat Technology Division, Japan Atomic Energy Agency, 4002 Oarai-machi, Higashi-Ibaraki-gun, Ibaraki, 311-1394 Japan Heat transfer characteristics near the critical Rayleigh number for the onset of natural convection of horizontal anisotropic porous fluid layer heated from below with thin cylindrical rods inserted with high porosity above 0.822 to unity were studied by an analysis and an experiment. The critical Rayleight number Rac and dependence of Nusselt number on the Rayleight number were analysed by linear stability theory and series expansion method on the basis of Darcy flow assumption. Critical Rayleight number, increased with decrease in the porosity, was higher for the experiment than for the analysis but the difference was decreased with decrease in Darcy number. Experimental Nusselt number was considerably decreased with decrease in the porosity near unity although Rac is rather same as the case of Benard convection without cylindrical rods. The experimental Nusselt number can be expressed by one correlation curve when Nusselt number is plotted against Ra/Rac Darcy number of the present experiment was rather large to satisfy Darcy flow assumption. Experiment shows that Darcy flow assumption will be satisfied for Darcy number Key Words: Benard Convection, Critical Rayleight Number, Anisotropic Porous Layer, Natural Convection, Nusselt Number, Porosity, Heat Transfer, Heat Storage, Latent Heat E-mail : shiina.yasuaki@jaea.go.jp
Fig. 1 Physical coordinate of the analysis
Table 1 Rod diameter, number of rods and porosity Table 2 Thermal properties of fluid and cylindrical rods Fig. 2 Schematic of experimental apparatus
(n=25, in line) (n=41, staggered) Fig. 3 Example of the configuration of the cylindrical rods: black circles are inserted by the cylindrical rods Fig. 4 Plot of Nusselt number against Rayleight number Fig. 5 Plot of Nusselt number against Ra(Ki/h2)
Fig. 6 Critical Rayleight number vs. K1/h2 Fig. 8 Cell patterns for the case of Ra=8400 on the heated surface Fig. 7 Plot of Nu against Ra/Rac
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