Thermal properties of porous media: with applications in multi-layer ground-coupled heat pumps

The primary objective of this study was to develop a more thorough understanding of the thermal conductivity characteristics and predict the overall thermal conductivity for mixtures of two porous materials. An experiment was conducted to measure the thermal conductivities for mixtures of fine sand and white lime, fine sand and coal powder, and salt and sawdust at various volume ratios using a transient hot-wire device. Results showed that the low thermal conductivity material controls the heat conduction process in the two-material mixture. This mixed thermal conductivity does not obey any simple mixing law. The concepts of basic and bypassing heat transfer elements were introduced to explain the heat transfer mechanism and a probability, group contribution theory was developed to predict the overall thermal conductivity for mixtures of two porous materials. The developed model predicted measured data with an R value greater than 0.96 in all cases with highly significant level (alpha = 0.001).

Another objective was to investigate the heat conduction process in multi-layer soils for vertical ground-coupled heat pumps (GCHP). An experiment was conducted to measure the heat rejection operation from hot water in a copper tube to coarse sand, clay, and fine sand three-layer soils. A two-dimensional transient heat conduction equation was solved numerically. The experimental data and computer simulation results agreed well (R > 0.87, alpha = 0.001) and showed that the heat rejection rates were discontinuous from soil layers. The coarse sand dissipated about 70% more heat than the clay and 25% more heat than the fine sand. It was found that vertical temperature gradients existed between coarse sand and clay and the fine sand and clay layers in the early stages of heat rejection. These temperature gradients decreased as the operation time increased due to the vertical heat flow.