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Analytical and numerical studies of heat and moisture transfer through porous insulation Zheng, Bi-Feng


This work contains both analytical and numerical studies of heat and moisture transport through a porous insulation in the presence of condensation, with impermeable, adiabatic vertical boundaries, and with one horizontal boundary facing a warm humid ambient and the other facing a cold impermeable surface. The analytical model is developed for heat and water vapor transfer in flat-slab and round-pipe thermal insulations. The model is validated by comparing its predictions with available experimental data. The effective thermal conductivity of the insulation in the presence of condensation depends on seven design and operating variables. The effect of these variables is determined by a parametric study. For practical operating conditions, the effective thermal conductivity varies from about 1.5 to 15 times the dry-state value. The computed data are presented in the form of design curves which may be used to estimate the effective thermal conductivity for flat-slab and round-pipe insulation systems. The analysis in this work quantifies the process of energy and mass transport in a porous insulation. A rigorous and fundamental formulation of heat and mass transfer in the insulation system is presented. The problem is modeled as one-dimensional, transient, multiphase flow with variable properties. Four stages in the energy and moisture transport process are identified, and they are formulated by a system of transient intercoupled equations and several thermodynamic relations using a local volume averaging technique. The numerical results are compared with experimental data for five different operating conditions and for times up to 600 hours. The model predicts the temperature distribution, heat transfer rate, the total moisture gain successfully. The predicted liquid distributions agree well with measured data for a period of up to 70 hours. The interesting effects of pertinent parameters on the energy and moisture transfer in the porous insulation are investigated. The present study, which for the first time presents a full simulation of the problem considering the mobile condensate, can be applied to other classes of problems on heat and mass transfer with phase change through a porous medium.

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