Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.
The theory of heat transfer through low density insulating
materials explains the importance of thermal radiation in the
overall heat transfer. As a matter of fact, in many
semitransparent insulating materials, radiation has a
considerable influence on measured thermal conductivity. In
this work we continue the investigation (both experimentally
and theoretically) on the heat transfer through low density
insulating materials that we started some years ago and whose
results have been presented in some International Conferences.
Test have been done on low density insulating materials:
expanded polystyrene with a density of 10 kg/m3 and polyester
fibres with a density of 9 kg/m3 (these last tests are not yet
completed and will not be discussed in this paper). The
transfer factor was measured in the heat flow meter
apparatus of our laboratory [1]. The first measurements have
been done with the two surfaces of the apparatus uncoated
(emissivity = 0.91) at a mean test temperature of 283 K (10
°C) and then with the specimens enclosed in the aluminium
foils (emissivity = 0.045) at the same mean test temperature.
The results obtained from the measurements of the transfer
factor pointed out that a change of the emissivity of the
surfaces from 0.91 to 0.045 caused a considerable decreasing
of the transfer factor. Then the same panels have been cut into
two slices and the aluminium foil has been interposed also
between the slices, as shield, and the transfer factor was
measured again in both cases: at first with the surfaces of the
apparatus uncoated and then with the surfaces of the apparatus
coated with the aluminium foils. In both cases the decreasing
of the transfer factor t was not negligible. The radiation
extinction parameters have been then measured with a
spectrometer and a model has been found to predict the
transfer factor in function of the testing conditions. We can
conclude that in presence of a thin reflective metal cover
placed on a low density insulating material, the thermal
conductivity decrease immediately (about 7%) with a
significant improvement of the thermal characteristics of the
insulating material. If we put then some low emissivity foils
between the slices of the material, another considerable
decreasing of the thermal conductivity can be obtained (about
10%) and in presence of both aluminum foils (inside and
outside). This fact confirms the importance of the contribution
of radiation in thermal transmissivity of low density insulating
materials and gives the possibility to reduce and to predict the
thermal performances
