Heat pipe phenomenon in soil under reduced air pressure

Abstract

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.

Issues of conventional model of transfer of latent heat in soil

“…1 includes the air-filled porosity of 0.5-0.25 m 3 m -3 (Sakaguchi et al, 2009) and the tortuosity factor of 0.66 (Penman, 1940). The product of them, shown in Fig.…”

“…(2). The estimated J air and the J soil in Ando soil (313 K, 101-10 kPa) of Sakaguchi et al (2009) provided C which was the slope between them and did not include the dT dx -1 .…”

“…From the measured heat flux in unsaturated Ando soil (Haplic Andosols, sampled at surface layer in Shinjyo-city, Yamagata-prefecture, Japan) under temperature of 313 K, pressure of 101-10 kPa, and the steady-state temperature gradient of 100 K m -1 conditions (Sakaguchi et al, 2009) and the thermal conductivity of air of 0.0272 W m -1 K -1 at 313 K, the dT dx -1 of soil pore was estimated because the heat flux under the steady-state temperature gradient was uniform across the soil sample. The calculated dT dx -1 of soil pore as a function of water content and air pressure ranged within 1,600-6,100 K m -1 .…”

“…Even after the experimental elucidation of the mechanism of transfer of latent heat in soil under steady-state temperature gradient (Sakaguchi et al, 2009), no formula or analytical method considering the elucidated mechanism of the latent heat transfer (LHT) has been developed yet. Even though the LHT has been recognised as an error in measuring the thermal conductivity of unsaturated soil with the transient-state heat probe method (Tarnawski et al, 2013) and it is minor compared to heat conduction under atmospheric pressure and low temperature, the LHT is one of the typical phenomena of heat and mass transfer in soil.…”

“…Also, it was mentioned that a major component of C was the ratio of temperature gradient across soil pore to that of overall soil. Sakaguchi et al (2009) measured the LHT, the distributions of temperature and water content in soil simultaneously under the steady-state temperature gradient for several initial water contents, and the C was not estimated. From those measured data, the temperature gradient as the uncertainty factor can be eliminated from the C, and the C as a function of water content can be estimated.…”