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

在相变蓄热体凝固过程中，在传热面之间设置高导热金属丝是强化传热的有效方法。然而，这种方法抑制了熔化过程中的自然对流，从而降低了热传递。为了研究表面间金属丝对传热的影响，通过理论分析和实验研究了金属丝水平层自然对流的临界Rayleigh数Rac及其传热特性。1.线性稳定性分析：得到了金属丝垂直放置水平层的临界Rayleigh数Rac。在分析中，假设了达西流动，并使用斯托克斯流动假设获得的线的流动阻力。结果表明，在相同渗透率下，多孔介质的Rac比球形填充多孔介质的Rac有所降低。2.通过实验研究了多孔介质自然对流开始时的Rayleigh数和传热特性。硅油和氯乙烯棒分别用作流体和线。孔隙率从0.82变更为1.0。结果如下：i）对于大于0.97的孔隙度，Rac几乎恒定在1707。当孔隙度低于0.97时，它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而增加。随着孔隙率的减小，传热的增加速率减小。然而，当努塞尔数对Ra/Rac作图时，增加率不依赖于孔隙度。iii）对于0.82的孔隙度，临界瑞利数Rac对于实验为约6000-9000，对于分析为4550。协议还不错。造成微小差异的原因可能是流动的不均匀性、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.