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

在传热表面之间安装高导电性金属丝对潜热储存凝固过程中的传热增强是有效的。然而，这种方法抑制了熔化过程中的自然对流，从而减少了传热。为了研究面间导线对传热的影响，通过分析和实验研究了带导线水平层自然对流的临界瑞利数Rac及传热特性。线性稳定性分析：从下加热水平层，层与层之间垂直安装金属丝，得到临界瑞利数。假设达西流动，采用斯托克斯流动假设得到的导线流动阻力进行分析。结果表明，在相同渗透率条件下，与球体填充多孔介质相比，Rac值有所降低。通过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。孔隙度从0.82变为1.0。结果表明：1)孔隙度大于0.97时，Rac在1707时基本不变。在0.97以下随孔隙度的减小而减小。ii)自然对流开始后，传热速率随瑞利数Ra增大。随着孔隙率的减小，传热的增加速率减小。然而，当努塞尔数与Ra/Rac作图时，增加速率不依赖于孔隙度。iii)孔隙度为0.82时，实验临界瑞利数Rac约为6000-9000，分析临界瑞利数Rac约为4550。协议不错。造成微差的原因有流动不均匀、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.