Experimental and numerical investigations on heat transfer in stratified subsurface materials

Based on a laboratory apparatus, this paper investigates the transient heat transfer performance of ground heat exchangers (GHEs) in a layered ground experimentally and numerically. The 6.25 m × 1.5 m × 1 m experimental box was filled with sand and clay, consisting of two U-tubes and precisely positioned thermocouples. Predictions from numerical models with layered and equivalent thermal properties were compared and validated with experimental data from a 24 h heat injection experiment. Numerical models with layered and equivalent thermal properties gave similar water temperatures while different thermal exchange and ground temperature distributions along the depths of tubes. Compared to the homogeneous numerical model, both the measurements and layered model can well elaborate the effect of ground stratification, where an obvious temperature decline can be observed in the interface from sand to clay, the slope of which doubled during the last 12 h heat injection period. Application of the material with a large thermal diffusivity such as sand results in lower water and ground temperatures, as well as a more significant interference effect of multi-tubes. Therefore, the well-controlled laboratory apparatus is validated to provide reliable and detailed data, and can be used for further investigations on thermal performances of the layered subsurface.