VAPOR BOUNDARY LAYER GROWTH DURING TRANSIENT HEAT AND MOISTURE TRANSFER IN CELLULOSE INSULATION
Olutimayin, Stephen Oluwaytoyin
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In this thesis, a new experimental test facility that permits the continuous measurement of transient heat and moisture transfer in porous media is applied to study the vapor boundary layer in cellulose insulation. The experimental facility can easily be applied to study other materials. In the experiment, the relative humidity, temperature and moisture accumulation are measured continuously within a bed of cellulose insulation with a fully developed flow of air at a controlled temperature and humidity provided above the bed. These experimental results are used to verify a mathematical model that calculates the transient heat and moisture transfer in the cellulose insulation bed. There is good agreement between the measured and simulated results. Sensitivity studies are performed to investigate the effect of material and other properties used in the numerical simulation on the simulated results. The properties that are considered are the adsorption isotherm, effective thermal conductivity, external convective heat and moisture transfer coefficients, heat of adsorption and effective diffusion coefficient. The results from the sensitivity studies show that the sorption isotherm is the most sensitive property in the numerical simulation of the vapour boundary layer, while the heat of phase change is the most sensitive property in the numerical simulation of the thermal boundary layer. The results from the sensitivity studies also help verify the mathematical/numerical model. A major contribution of this thesis is the development of an expression for moisture diffusivity that is analogous to thermal diffusivity and takes into consideration moisture storage. The moisture diffusivity is used to calculate the vapor density in the boundary layer and the size of vapor boundary layer in cellulose insulation using the analytical expressions for heat transfer from the literature. It is found that the moisture storage effect has a very significant effect on the vapor boundary layer and cannot be ignored. For cellulose insulation, the size of the vapor boundary layer may be over predicted by a factor often when moisture storage is neglected.