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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 #219109

Title: Effects of heat sink compounds on contact resistance of porous media

item Sauer, Thomas

Submitted to: Thermal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/2007
Publication Date: 12/9/2007
Citation: Akinyemi, O.D., Sauer, T.J. 2007. Effects of heat sink compounds on contact resistance of porous media. Thermal Science. 11:113-124.

Interpretive Summary: The ability of porous materials like soils to conduct heat is affected by its density and water content. One technique for measurement of these changes in thermal properties with changing density or water content involves inserting a metal needle-shaped probe into soil, heating it, and measuring how fast the needle cools down. This technique assumes that there is good contact between the needle and soil so that the soil can efficiently conduct heat away from the needle. It is well-known that for coarse particles like sand, this assumption isn't always valid. A series of experiments were conducted with a needle probe used the typical way and with one of two different greases applied to the needle to improve the contact between the needle and soil particles. The results indicate that when the soil is dry, applying grease to the probe reduces variation between readings and generally increases the values of the thermal properties. These effects are reduced as the soil becomes more wet. These results are important for scientists making thermal property measurements as the errors associated with the needle probe techinque can be significant, especially when the soil is coarse or dry.

Technical Abstract: High and low-conductivity heat sink compounds were applied in succession on a thermal probe, which was then used to determine the thermal conductivity and thermal diffusivity of some porous media at room temperature. The experiment was conducted separately under different packing densities and water contents to see the effects of the heat sink compounds on the thermal properties at such conditions. High conductivity grease increased the values of thermal conductivity considerably and thus reduces the contact resistance, with simultaneous increase in the bulk density at air-dry conditions, but had virtually no effects on its thermal diffusivity. It however decreased both the thermal conductivity and thermal diffusivity with water content increment. The thermal properties obtained without thermal grease vary considerably from those with the heat sink compounds as water was being applied. The variation however reduced also considerably towards saturation.