EFFECTS OF FLOW CHANNELING ON HEAT TRANSFER CHARACTERISTICS OF POROUS INSULATION LAYERS

由于多孔材料的不均匀填充或多孔层与边界壁之间的热膨胀差异，沿着多孔绝缘层可能出现外围间隙。本研究的目的是使实验和理论检查的影响的热和流阻的不均匀性引起的间隙对自然对流的多孔绝缘层加热从侧面和加热从下面。在最大气压为8 MPa的500 mm高的压力容器中，用由玻璃珠和氮气组成的多孔介质进行实验。实验结果表明，在较高的气体压力或较大的间隙宽度下，通过差距导流可降低绝热层的总热阻。当气体压力较低时，由于差距区热阻较大，提高了涂层的隔热能力，从而消除了这一不利影响。通过假设在间隙和多孔层之间的可渗透界面处的海狸-约瑟夫滑移边界条件或通过假设基于窄间隙和多孔介质中的流动之间的Hele-Shaw类比的具有表观渗透率的等效多孔子层来进行分析。分析结果证实，在近壁不均匀性的增加产生局部增强对流主要局限于周边间隙。因此，在多孔绝缘系统中的周边间隙的外观可以增加传热速率，由于局部强化的流动在近壁区域或降低速率，由于伴随的高热阻。这些变化的传热特性的垂直和水平系统的一般相关性进行了讨论的瑞利数和无量纲多孔和几何参数。

Peripheral gaps may arise along porous insulation layers due to the inhomogeneous packing of porous material or the thermal expansion difference between the porous layer and the boundary walls. The purpose of this study is to make an experimental and theoretical examination on the effects of the thermal and flow resistance inhomogeneity induced by the gaps on natural convection in both porous insulation layers heated from the side and heated from below. An experiment is conducted with a porous medium composed of glass beads and a nitrogen gas in a pressure vessel of 500 mm in height with a maximum gas pressure of 8 MPa. The experimental results show that at a relatively high gas pressure or at a large gap width flow channelinng through the gap reduces the total thermal resistannce of the insulation layer. The adverwse effect is obserbed at a low gas pressure where the thermal insulation capacity of the layer is increased due to the large thermal resistance of the gap region. An analysis is made by assuming the Beavers-Joseph slip boundary condition at the permeable interfaces between the gaps and the porous layer or by assuming assuming equivalent porous sublayers with apparent permeability based on the Hele-Shaw analogy between flows in narrow gaps and porous media. The analytical results confirm that the increase in the near-wall inhomogeneity produces locally intensified convection mostly confined to the peripheral gaps. Thus, the appearance of peripheral gaps in porous insulation systems can either increase the heat transfer rate due to the locally intensified flow in the near wall region or decrease the rate due to the accompanying high thermal resistance. General correlations for these change in heat transfer characteristics for both the vertical and horizontal systems are discussed in terms of the Rayleigh number and nondimensional porous and geometrical parameters.