An Adiabatic Boundary Condition Solution for Improved Accuracy of Heat-Pulse Measurement Analysis Near the Soil-Atmosphere Interface

The infinite line source model (ILS model) is widely used in the dual-probe heat pulse (DPHP) method for measuring the thermal conductivity (k), specific heat capacity (c), and thermal diffusivity (alpha) of soil. However, there exists errors when sensors are located very close to the soil-atmosphere interface. The purpose of this study is to develop a new method to accurately determine k and c near the soil-atmosphere interface. The method of images was used to obtain the ILS solution with an adiabatic boundary condition (ILS-ABC), when DPHP sensors are near the soil-atmosphere interface. Both finite element simulations and DPHP experiments verified that the soil-atmosphere could be approximated by the adiabatic boundary condition (ABC). The new solution performed well in predicting the thermal properties of the samples when compared with the ILS model, which ignored the existence of the soil-atmosphere interface. If the ILS model was used to compute k and c rather than the new ILS-ABC model, the magnitude of the error in k and c increased rapidly with decreases in burial depth (h) and could be larger than 30% at h <= 3 mm. The solution may be helpful for better extracting soil thermal properties near the soil-atmosphere interface.