Decrease in thermal conductivity with increasing temperature in nearly dry sandy soil

Abstract

SummaryTo clarify the role of the water bridges between soil particles on the transfer of heat we studied the dependence of thermal conductivity (λ) and electrical conductivity (E) on temperature between 278 and 338 K of sand and sand mixed with kaolin in the nearly dry state. The thermal conductivity decreased as temperature increased in the sand at volumetric water contents less than 0.07 m3 m−3, but it increased in the sand–kaolin mixture over the measured range of water content. In the sand, the ratio of E in the soil solution to the electrical conductivity of pure water increased gradually with increasing water content at the water contents less than 0.05 m3 m−3 and was almost constant at larger water contents. The ratio of E of the sand–kaolin mixture increased with increasing water content, particularly at the lower temperature. For both samples the ratio of E decreased as temperature increased, which suggested that the conduction of heat decreased through the decrease in the water bridges as temperature increased. Because the decrease in λwith increasing temperature could not be explained by the transfer of latent heat transfer, we considered that the temperature dependence of λwas due not only to the transfer of latent heat but also to the thermal bridge of water. We conclude that the condensation, conduction and evaporation in series involved in the latent heat transfer take place mainly through the water films. Our experimental results will help to understand the mechanism of the latent heat transfer in soil with the water films surrounding the soil particles.