Thermal radiation and conduction properties of materials ranging from sand to rock-fill

This paper presents an experimental study on thermal radiation and the thermal conductivity of rock-fill materials using a 1 m x 1 m x 1 m heat transfer cell. Testing temperatures are applied by temperature-controlled fluid circulation at the top and bottom of the sample. Heat flux and temperature profiles are measured to establish the effective thermal conductivity lambda(e), which includes contributions from both conduction and radiation heat transfer mechanisms. The materials studied had an equivalent particle size (d(10)) ranging from 90 to 100 mm and porosity (n) ranging from 0.37 to 0.41. The experimental results showed that thermal radiation greatly affects the effective thermal conductivity of materials with lambda(e) values ranging from 0.71 to 1.02 W.m(-1).K-1, compared with a typical value of 0.36 W.m(-1).K-1 for conduction alone. As expected, the effective thermal conductivity increased with particle size. An effective thermal conductivity model has been proposed, and predictions have been successfully compared with the experimental results. Radiation heat transfer becomes significant for d(10) higher than 10 mm and predominant at values higher than 90 mm. The results of the study also suggest that the cooling potential of convection embankments used to preserve permafrost conditions may not be as efficient as expected because of ignored radiation effects.