Soil drying characteristics around a buried heated pipe

Research on ground coupled heat pumps began in the late 1940s, but they became commercially available only in the 1980s. The system components are off-the-shelf items used in other applications, but the ground heat exchanger is unique for each installation. Ground heat transfer is complicated by soil porosity and the many variables needed to characterize soil properties. Another complication is the coupling of the heat and mass transfers. A finite difference numerical model was developed for a horizontal buried pipe where the heat and mass transfer relationships in the unsaturated porous soil were fully accounted for. The comparison of predicted temperature profiles and controlled laboratory measurements produced significant deviations after several days from the start of the test. These differences were likely due to the underprediction by the model of the moisture movement away from the pipe. Nearly complete drying out of the soil around the pipe was predicted by the model and was observed experimentally. The primary difficulty in obtaining good agreement with experimental results over all time periods was due to soil physical property variation with moisture content.