To clarify the role of the water bridges between soil particles on the transfer of heat we studied the dependence of thermal conductivity (l) 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 m 3 m ÿ3 , 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 m 3 m ÿ3 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 l with increasing temperature could not be explained by the transfer of latent heat transfer, we considered that the temperature dependence of l was 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.
We measured the heat flux, temperature distribution and water content of an unsaturated Ando soil under a constant temperature gradient and reduced air pressure to investigate the mechanism of latent heat transfer in the soil and its relationship to the distribution and circulation of soil water. As the air pressure decreased, the heat flux increased for the soil samples with an initial volumetric water content (y ini ) greater than 0.30 m 3 m À3 , but did not change for y ini less than 0.20. While the temperature gradient of the sample did not change for y ini greater than 0.30 m 3 m À3 , it did increase on the hotter side of the sample and decreased on the colder side for y ini less than 0.20. The water content did not change, and a homogeneous distribution of water content was observed for y ini greater than 0.30 m 3 m À3 . For y ini less than 0.20, the water content decreased on the hotter side and increased on the colder side, forming a large water content gradient. The large transfer of latent heat was caused by the circulation of water vapour and liquid water, which resulted in the homogeneous water distribution. We concluded that the soil functions as a heat pipe through a series of micro-heat pipes centred on the soil pores. Our experimental results will help to explain the transfer mechanism of latent heat in soil as a heat pipe phenomenon.
One of the best ways to evaluate the coupled heat and mass transfer in soil is to measure the heat flux and water distribution simultaneously. For this purpose, we developed an apparatus for measuring the onedimensional steady-state heat flux and water distribution in unsaturated soil under reduced air pressure. The system was tested using four samples with known thermal conductivity (0.6-8.0 W m À1 K À1 ). We confirmed that the system could measure the one-dimensional steady-state heat flux under a fixed temperature difference between ends of the samples over a wide range of thermal conductivity values. Time domain reflectometry was used to measure the water distribution with a repeatability of less than AE 1.0%. We used the apparatus to measure the soil heat flux and distribution of water content and temperature under steady-state conditions with reduced air pressure. The initial volumetric water content, y ini , of the soil samples was set at 0.20 and 0.40 m 3 m À3 . For a y ini of 0.20, the heat flux was not significantly affected by air pressure, and the water content on the hot side decreased whilst that on the cold side increased, i.e. a pronounced water content gradient was formed. For a y ini of 0.40, the heat flux increased sharply with reduced air pressure, and the water content did not change, i.e. a homogeneous water distribution was observed. The increase in the heat flux with air pressure reduction is caused by the vapour transfer in soil pores. We found that a large vapour transfer took place in the soil with the homogeneous water distribution, and that the vapour transfer was less in the soil with the pronounced water content gradient. These experimental facts were entirely different from the traditional knowledge of vapour transfer in soil under temperature gradients. A lack of data on heat flux must have resulted in the previously incorrect conclusions. The new apparatus will serve to clarify the intricate phenomena of thermally induced vapour transfer in unsaturated soil in further experiments.
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