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ARS Home » Midwest Area » Ames, Iowa » National Laboratory for Agriculture and The Environment » Soil, Water & Air Resources Research » Research » Publications at this Location » Publication #204942

Title: Reducing contact resistance errors in measuring thermal conductivity of porous media

item Sauer, Thomas - Tom

Submitted to: Journal of Applied Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/24/2006
Publication Date: 7/21/2008
Citation: Akinyemi, O.D., Sauer, T.J., Olowofela, J.A. 2008. Reducing contact resistance errors in measuring thermal conductivity of porous media. Journal of Applied Science and Technology. 12:58-64.

Interpretive Summary: The property of a soil that expresses how well it conducts heat is called thermal conductivity. Since soil temperature affects many biological and chemical processes, knowing a soil's thermal conductivity is important. If you know how much energy is going into a soil and the thermal conductivity of that soil you can predict the temperature at different depths in the soil. Soil thermal conductivity is measured by inserting a heated needle-shaped probe that releases heat. How fast the needle loses heat is used to calculate the soil's thermal conductivity. A problem with this method is that not all of the heat released by the needle is transferred to the soil. This experiment evaluated the use of grease-like materials called heat sink compounds on the needle to improve heat transfer and obtain more accurate thermal conductivity values. Experiments were completed with glass beads, quartz sand, stone dust, and clay soil at varying water content, density, and particle sizes. Two heat sink compounds with differing thermal conductivities were used. The results showed that the heat sink compounds improved thermal conductivity measurements when the materials had low water contents but had little effect when the materials were wet. The heat sink compound with the higher thermal conducitivity generally increased thermal conductivity values more than the heat sink compound with lower thermal conductivity. The results are important to scientists who are interested in improving measurements of soil thermal properties as the use of a heat sink compound is shown to produce more accurate values of soil thermal conductivity.

Technical Abstract: Values of thermal conductivity (k) of glass beads, quartz sand, stone dust and clay were determined using a thermal probe with and without heat sink compounds (arctic silver grease (ASG) and white grease (WG)) at different water contents, bulk densities and particle sizes. The heat sink compounds (HSC) increased k at air-dry condition, thereby reducing the thermal contact resistance (TCR), but reduced values of k when water was added. The increase in values of k was higher with application of high-conductivity ASG than low-conductivity WG. The maximum percentage increase in k with application of ASG was 25, 118, 22 and 25 % in glass bead, quartz sand, stone dust and clay, as compared with increase of 10, 22, 23 and 22 % respectively for WG. The increase in k for 1 mm samples exceeded the increase in 2 mm samples. At water saturation, application of HCS on the probe did not change the values of k suggesting almost a non-existent TCR at such conditions. The values of k increased with water content (Vw) and bulk density (p). The highest increase was attained when 200 mL of water was added to air-dry samples, where k in quartz sand was increased by 132 % on application of WG as compared to increase of 307 % on application of ASG.