Thermal Conductivity of Binary Sand Mixtures Evaluated through Full Water Content Range

Abstract

A soil's grain-size distribution affects its physical and hydraulic properties; however, little is known about its effect on soil thermal properties. To better understand how grain-size distribution affects soil thermal properties, specifically the effective thermal conductivity, a set of laboratory experiments was performed using binary mixtures of two uniform sands tightly packed with seven different mixing fractions over the full range of saturation. For each binary mixture, the effective thermal conductivity, l, capillary pressure, h c , and volumetric water content, q, were measured. Results demonstrated that the l-q relationship exhibited distinct characteristics based on the percentage of fine-and coarse-grained sands. We further compared measured l-q properties with independent estimates from two semi-empirical models (Campbell Model and Lu and Dong Model) to evaluate the models' applicability in relation to physically based parameters associated with changes in soil mixing (e.g., porosity and grain size). Both models were able to fit experimental data but to varying degrees based on the number of physically based parameters used. In general, model improvements are needed to capture the l-q relationship solely on physically based parameters.Abbreviations: SWRC, soil water retention curve. U nderstanding soil thermal properties is important to properly determine the impact of heat transfer on the distribution, circulation, and evaporation of water in soil. Heat transfer within soil influences micrometeorological phenomena (Hanks et al., 1967), engineering efforts such as cooling electronic devices with heat pipes and insulating buildings (Bussing and Bart, 1997;Sakaguchi et al., 2009;Moradi et al., 2015), agricultural production (Al Nakshabandi and Kohnke, 1965;Usowicz et al., 1996;Lipiec et al., 2007), and landmine detection efforts that rely on thermal contrast (e.g., Garcia-Padron et al., 2002;Martínez et al., 2004). Previous studies provide insight into the effect of soil moisture, temperature, and soil physical characteristics on soil thermal properties, specifically the effective thermal conductivity, l (W m -1 K -1 ; e.g., De Vries, 1963;Johansen, 1975;Hopmans and Dane, 1986;Ochsner et al., 2001;Tarnawski and Gori, 2002). However, to our knowledge, none of the studies to date has investigated the effect of mixing differently sized particles on controlling the l behavior under varying soil water contents (q, cm 3 cm -3 ). Knowledge of the l-q relationship for differently sized soil fractions is particularly important in many engineering applications such as soil borehole thermal energy storage (SBTES), landfill covers, and engineered material stabilization.Previous research provides strong evidence that l is affected by a wide range of parameters including q (e.g., Philip and de Vries, 1957;De Vries, 1963;Yadav and Saxena, 1977; Core Ideas• Thermal conductivity and water content relationship based upon mixtures are distinct.• Systematic change in particle fine fraction directly affects thermal cond...

Thermal Conductivity of Sand–Silt Mixtures

“…Nonetheless, the presence of water significantly improves heat conduction in the interstitial space at grain contacts because water readily fills interstices and forms a meniscus around contacts in hydrophilic grains (Cho and Santamarina 2001;Yun and Santamarina 2008;Tang et al 2008). Therefore, even low water saturation in the pendular regime has a marked effect on grain-to-grain heat transfer (Wallen et al 2016).…”

“…On the other hand, pore fluids have considerably lower thermal conductivities, e.g., k water ¼ 0.6 W • m −1 • K −1 for water and k air ¼ 0.024 W • m −1 • K −1 for air at room temperature and atmospheric pressure (Lide 2010). Overall, the contact resistance is a function of the thermal properties of solids in contact, the solid-solid contact area, the average distance between neighboring surfaces, and interstitial fluid properties, i.e., simply wetting the interstitial space significantly improves the bulk thermal conductivity (Gori et al 2001;Wallen et al 2016).…”

“…Conduction governs heat flow in soils where small pores (D 50 < 6 mm) and moderate temperatures (T < 400°C-500°C) diminish the relative effects of free convection and radiation (Yagi and Kunii 1957;Johansen 1977;Farouki 1981). In fact, the number and so-called quality of contacts determine heat transport in soils; in turn, they reflect the state of stress, packing density, mineralogy, and water content (Salmone and Kovacs 1984;Beziat et al 1988;Ould-Lahoucine et al 2002;Weidenfeld et al 2004;Yun and Santamarina 2008;Wallen et al 2016).…”

Evaporation from undulating soil surfaces under turbulent airflow through numerical and experimental approaches

“…The drying characteristic of a sand can be described by the van Genutchen parameters (van Genuchten, 1980): α (m −1 ), which is related to the inverse of the air entry suction; and n (dimensionless), which is connected with the pore size distribution. The van Genutchen parameters of the #50/70 sand used in this experiment are α = 5 m −1 and n = 7 (Gao et al, 2018), and for a coarser sand #12/20, they are approximately 8 m −1 and 11 (Wallen, Smits, Sakaki, Howington, & Chamindu Deepagoda, 2016), respectively. The influence of the combined effect of the air suction and the pore size is numerically studied by comparing the evaporation from #50/70 and #12/20 sand.…”

Effects of “soil‐like” particle size on gas transport and water retention properties in aged municipal solid waste from a Sri Lankan open dumpsite

“…At each corresponding suction level, the water level in the reservoir was maintained constant for 2 days to ensure the gravity-capillary equilibrium within the sample before moving to the next suction level. Further details of the measurement method can be found in Wallen et al (2013) who also shows a schematic of the experimental apparatus. To measure soil moisture at each matric suction, a dielectric soil moisture sensor (ECH2O EC-5, 5.5-cm prong length, 70-MHz measurement frequency, Decagon Devices) was installed within the cell in such a way that the long axis of the sensor was horizontal and the flat plan was vertical.…”