Thermal property standards using granular media with air-pluviation and heat pulse probe measurements

Abstract

The fundamental thermal properties of soils and granular media include volumetric heat capacity (Cv), thermal conductivity (λ) and thermal diffusivity (κ), which play key roles in the transfer and maintenance of heat in the environment. Thermal properties are highly dependent upon the granular medium volumetric water content (θw), and thus thermal property functions are critical for modeling soil and land surface thermal processes. However, few available standard granular materials provide a source of ‘known’ thermal property values, perhaps foremost because of the difficulty in achieving a repeatable bulk density (ρb) value in addition to other packing and wetting complications. Our objectives were to identify and evaluate a range of readily available granular media that could be used to calibrate and validate thermal property measurements (i.e., Heat Pulse Probes, HPP) and to establish packing and wetting/draining methods that result in repeatable granular media conditions for thermal property determinations. We identified and tested a variety of granular media including spherical- (quartz sand, glass beads), angular- (crushed media) and aggregated- materials. Thermal property values were determined under uniform θw of oven-dried, saturated conditions and the drained-water content at −5.9 m suction using combined methods of HPP measurements for thermal property estimations and the multiple sieving pluviation (MSP) method, yielding highly repeatable ρb in coarse granular media. Overall, thermal property values ranged from 0.51 to 3.68 MJ m−3 K−1 for Cv, 0.13 to 3.73 W m−1 K−1 for λ and 0.17 to 1.5 mm2 s−1 for κ. Optimized parameters were presented for selected Cv(θw) and λ(θw) thermal property models. We demonstrated that with the MSP method, it was possible to achieve highly repeatable ρb values and thermal property values for several commercially available granular materials. We recommend this approach as a standard calibration method based on 1) repeatable granular media packing and 2) standardized thermal property values for coarse granular media at targeted uniform θw distributions.