Annals of the Assembly for International Heat Transfer Conference 13

ISBN 1-56700-225-0 / CD 1-56700-226-9

Year 2006

C. Bonacina
Università degli Studi di Padova, Padova, Italy

M. Campanale
Università degli Studi di Padova, Padova, Italy

L. Moro
Università degli Studi di Padova, Padova, Italy

The theory of heat transfer through semi-transparent insulating materials explains the importance of thermal radiation in the overall heat transfer. As a matter of fact, in many semitransparent insulating materials, the influence of radiation is such that the boundaries surface emissivity ε has a considerable influence on measured thermal conductivity. The thermal resistance of a flat specimen of low density material may be expressed through a mathematical model as: R = R_o + d/λ_t where R_o is a factor that depends on many other parameters included the emissivity e of the surfaces, d is the thickness of the specimen while λ_t is the transmissivity, that may considered the sum of two terms: λ_cd and λ_r. The first one λ_cd is the thermal conductivity due to the heat transfer in the solid matrix and in the gas enclosed in the cells or among the fibres and λ_r is the heat transfer due to radiation.
In this work, the heat transfer through some low density insulating materials has been investigated, both theoretically and experimentally. A preliminary experimental work to determine the emissivity of several reflective foils (aluminium, silver and gold) by an heat flow meter apparatus has been done. Then several measurements of the thermal conductivity on low density (10 kg/m³, 14 kg/m³, and 21 kg/m³) expanded and extruded polystyrene boards has been performed at a mean test temperature of 283 K and 293 K and with a temperature difference, between the main surfaces, of 20 °C. The first measurements had been done on an extruded polystyrene with a density of 51 kg/m³, but in that case it was immediately clear that the different emissivity of the surfaces did not influence the thermal behaviour of the material. The measurements were done with the surfaces of the heat flow meter apparatus uncoated (the emissivity of the surfaces of the apparatus was already well known) and then with the specimens enclosed in the three different reflective foils.
It was interesting to find out that a simple thin reflective aluminium cover, placed on a low density insulating material, may decrease its thermal conductivity from 5.5% to 9%.

CND-12 pages DOI: 10.1615/IHTC13.p27.110 Download article

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