Development of an apparatus for measuring one‐dimensional steady‐state heat flux of soil under reduced air pressure

Abstract

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.

Heat pipe phenomenon in soil under reduced air pressure

“…We used a thermo-module (6301/071/030; Ferrotec Corp., Tokyo, Japan) as a Peltier device, and a Peltier controller (DPC-100; Ferrotec Corp.) for controlling the electrical heat supplied to the sample and generating a fixed temperature difference between the sample ends. Momose et al (2008) confirmed a linear relationship between the square of the voltage supplied to the thermo-module and the heat flux in the sample under steady-state conditions. Several hours after setting the air pressure, the voltage supplied to the thermo-module (i.e.…”

“…As shown in Figures 2‐3 and 4, the vapour flux did not change and the large gradient of water content was formed for the samples with θ ini less than 0.20 m 3 m −3 . As noted by Momose et al (2008), this moisture gradient was caused by the vapour flow under the transient state that took place immediately after reducing the air pressure. Although this moisture gradient would seem to cause liquid water to flow from the cold side towards the hot side of the sample, the liquid water hardly moved.…”

“…We measured heat flux using the apparatus for 1-D steady-state heat flux measurement under reduced air pressure that had been developed by Momose et al (2008). Figure 1(a) is a schematic diagram of the apparatus for measuring the heat flux, distribution of water content and temperature.…”

“…We measured the volumetric water content of the samples using a time domain reflectometry (TDR) system (TDR100; Campbell Scientific Inc.). As explained by Momose et al (2008), the TDR probe consisted of two stainless steel tubes, one of which contained the thermocouple for measuring the temperature of the samples. The TDR probes were set at the distances of 1.0, 3.0, 5.0, 7.0 and 9.0 cm from the hot end of the sample.…”

“…The water flow caused by the temperature gradient would be much clearer through the simultaneous measurement of the latent heat transfer and the distribution of water content in solute‐free soil under steady‐state conditions. Momose et al (2008) developed an apparatus for measuring the one‐dimensional (1‐D) steady‐state heat flux and distribution of water content in unsaturated soil under reduced air pressure, and examined the vapour flow that caused the transfer of latent heat. They found that a large vapour flow took place in soil with a homogeneous distribution of water content, and that the vapour flow was less in soil with a pronounced moisture gradient.…”