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

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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.