ARS | Publication request: Revisiting the block method for evaluating thermal conductivities of clay and granite

Submitted to: International Communications in Heat and Mass Transfer
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 1, 2011
Publication Date: July 19, 2011
Citation: Akinyemi, O.D., Sauer, T.J., Onifade, Y.S. 2011. Revisiting the block method for evaluating thermal conductivities of clay and granite. International Communications in Heat and Mass Transfer. Available: http://sciencedirect.com/science/article/pii/S0735193311001333.

Interpretive Summary: Energy from the sun or heat from buried wires or pipes can warm the soil or other geologic materials. It's important to understand this process for agricultural production and for the design of efficient engineering applications. The key property is the thermal conductivity of the material, literally how effective the material is in transferring heat. There are several methods to measure thermal conductivity of uniform materials but it is more difficult to make measurements on a surface where the properties, like water content, often change with depth in the material. The block method was developed several decades ago for these types of measurements. It involves heat a block of plastic, placing it on the surface, and measuring the decrease in temperature of the block as the heat flow into the material. Work with this method ended because some heat was lost due to poor contact between the block and the surface. In this study, the block method is revisited to evaluate the effect of adding a paste to the surface of the block to improve the heat transfer. The results show significant improvement in the performance of the block method when using this paste for measurements on a clay soil and granite rock. The results of this study are of interest to scientists involved with heat flow in natural environments as it demonstrates an improved method for making thermal property measurements for non-uniform surfaces.

Technical Abstract: Determination of thermal conductivities of porous media using the contact method is revisited and revalidated with consideration of thermal contact resistance. Problems that limit the accuracy of determination of thermal conductivities of porous media are discussed. Thermal conductivities of granite (rock sample) and clay (soil sample) are determined in the laboratory with and without application of thermal interface material TIM (Arctic Silver®) to reduce thermal contact resistance. The thermal properties analyzer KD2 (line- source heat dissipation probe) is also used with and without TIM to measure thermal conductivities of the samples. The results from the contact method and KD2 analyzer, with and without TIM, are compared with the standard values. Results indicate significant differences with consideration of thermal contact resistance. Thermal conductivity of the clay sample increased from 0.68W/mK to 0.85W/mK while that of the granite increases from 2.95W/mK to 3.95W/mK with the standard values for clay ranging between 0.25 and 1.8W/mK and for granite ranging from 2.0 and 7.0W/mK. The difference in thermal conductivities with or without TIM is significant at (P < 0.05) which implies the effectiveness of the thermal interface material used in this study to reduce thermal contact resistance.