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

将高导热金属丝安装在换热表面之间，可有效地强化潜热蓄热固化过程中的换热。然而，这种方法抑制了熔化过程中的自然对流，从而减少了热传递。为了研究层间导线对换热的影响，通过理论分析和实验研究了水平层间导线自然对流换热的临界瑞利数RAC和换热特性。1.线性稳定性分析：得出了水平层间垂直铺设导线层间加热时的临界瑞利数。分析中采用了达西流假设，并采用了由斯托克斯流动假设得到的导线的流动阻力。结果表明，在相同的渗透率条件下，RAC比球形多孔介质要小。2.实验研究了自然对流开始时的瑞利数和换热特性。硅油和氯乙烯棒分别用作流体和导线。孔隙率从0.82变为1.0。结果表明：1)当孔隙率大于0.97时，RAC几乎不变，为1707。当孔隙率低于0.97时，孔隙率减小。Ii)自然对流开始后，换热速率随瑞利数Ra的增大而增大。换热增加率随气孔率的减小而减小。然而，当Nusselt数与Ra/Rac作图时，增长率并不取决于孔隙率。Iii)孔隙率为0.82的临界瑞利数RAC，实验约为6000-9000，分析约为4550。这份协议还不错。引起微小不一致的原因是流动的不均匀、壁面导热系数的影响等。

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.