Closed-Form Equation for Thermal Conductivity of Unsaturated Soils at Room Temperature

Abstract

Thermal conductivity is a fundamental physical property governing heat transfer in soil. It depends on soil types, pore structure, temperature, and moisture content, and can vary over an order of magnitude. Some theories have been established to address the effect of temperature on thermal conductivity. Other theoretical works focus on the effect of moisture content on thermal conductivity of different sandy and loamy soils. This work addresses the effect of all soil types and moisture content on thermal conductivity for ambient temperatures from 20 to 25°C. Thermal conductivity varies little within the hydration and capillary regimes, varies moderately within the funicular regime, and varies greatly within the pendular regime. A closed-form equation is proposed and validated by explicitly considering the soil-water retention regimes. Results taken from the literature for 25 soils and from the current study for two clay soils show that the closed-form equation can accurately predict thermal conductivity of all types of unsaturated soil.

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

“…Although much effort has been invested in the development of l-q models (e.g., De Vries, 1963;Johansen, 1975;McCumber and Pielke, 1981;Campbell, 1985;Campbell et al, 1994;Lu et al, 2007;Chen, 2008;Konrad, 2005, 2009;Tarnawski and Leong, 2012;Haigh, 2012;Lu and Dong, 2015), none of the models specifically address the applicability to soils that are mixed, including binary mixtures. Additionally, few l-q models are physically based and rely on the use of fitting parameters.…”

“…The focus for this research was to obtain experimental data on l-q for binary mixtures whose soil-water retention relationships are well characterized and examine how the l-q relationships are affected by mixing fractions. Therefore, two physically based models, the Campbell et al (1994) and Lu and Dong (2015) Models were selected to obtain insight on how existing models can or cannot reproduce the l-q relationships for binary mixtures.…”

Storage of Solar Thermal Energy in Borehole Thermal Energy Storage Systems

“…The subsurface at the site consists of 1 m of silty sand underlain by conglomerate bedrock, with a groundwater table more than 30 m deep. The thermal conductivity of the unsaturated colluvium measured using the TRIM method (Lu and Dong 2015) and was found to be 0.5 W/mK for the average degree of saturation with depth. The conglomerate has a relatively high hydraulic conductivity of approximately 10-5 m/s, indicating that infiltration and thermally induced water flow may affect heat transfer processes.…”

“…This project focuses on the development of new coupled constitutive equations for the thermo-hydraulic properties of soils (Lu and Dong 2015) as well as numerical efforts to understand the impacts of borehole array geometry, ground properties, heat injection magnitudes, and heat injection duration on the heat storage (Başer et al 2015), characterize the behavior of borehole system behavior (Başer et al 2016a), understand the impacts of different modes of heat transfer (Başer et al 2016b(Başer et al , 2016c, and understand the role of incorporating a thermal insulation layer (Başer et al 2016d). However, data from a field-scale installation is still necessary to understand the variable boundary conditions for heat injection, to provide a baseline data set for calibration of numerical simulations, and to assess different injection schemes to improve the efficiency of heat storage.…”

Experimental Investigation of Thermo-Physical Properties of Soil Using Solarisation Technology

“…They strongly depend on soil bulk density (ρ b ), porosity (Φ) and gravimetric water content (θ) for soil (Alrtimi et al, 2016;Levy and Schmidtm, 2016;Lu and Dong, 2015;Mondal et al, 2016;Ravazzani, 2017;Usowicz et al, 2013;Zhang et al, 2017). There are several studies in the literature that estimate TPP under different conditions.…”