Experimental Evaluation of the Effect of Cell Shape on Heat and Mass Transfer through Elastomeric Foams

Wide usage of cellular foam as low temperature thermal insulating material warrants good understanding of its thermal and water vapor transmission behavior. An attempt has been made to experimentally observe the effect of cell shape on heat and mass transfer characteristics of cellular foams. The foam samples used in this study were derived from elastomeric foam The thermal conductivity and water vapor transmission data were taken on foams with three different cell shapes. one having spherical cells and two having elliptical cells, with varied degree of elon gation.

The thermal conductivity measurements on the samples described above were ob tained using Thin-Heater thermal conductivity apparatus, otherwise known as the ASTM C-1114 test. The thermal measurements were performed in the temperature range of 35 to 153°F. The experimental data indicated no significant effect of the cell elongation on thermal conductivity of foam. This could be attributed to the follow ing : a) the densities of the foam were roughly the same, and therefore conduction heat transfer was not significantly changed; b) the possible decrease in the radiation component of thermal conductivity because of elongated cells was nullified by the increase in transmission of heat attributable to relatively thinner walls resulting from the stretching of the foams during sample preparation. Additionally, the change in the shape of cells could have changed the radiation heat transfer characteristic of the samples.

Water vapor permeabilities of the foams with elongated cells were measured with Permatran® and compared to that of the foam with uniform cells. It was found that the foams with elongated cells had almost twice the permeability as the foams with uniform cells Also, the two foams with elongated cells had about the same perme abilities, although their degrees of elongation differed. The difference between the permeabilities of the foams with elongated cells and foams with uniform cells could be attributed to the different strain and thermal histories undergone during the prep aration of the foam samples

The results of water vapor transmission measurements of the foams underscore the importance of transmission properties of the cell walls in offering the primary resis tance to mass transport From thermal conductivity measurements, it appears that the thermal properties of the foam arc not affected as significantly by the changes in properties of the cell wall.