Development of an Experimental Apparatus for Studying High-Temperature Heat and Mass Transfer in Soils

The objective of this study was to design and build an experimental apparatus for studying heat and moisture transport phenomena in soils at temperatures greater than 40°C up to 90°C. An experimental soil cell was designed and constructed for experimental studies of one-dimensional heat and moisture transfer within a vertical soil column. The interference effect between two proximate TDR probes was examined for three types of soils and it was found that parallel TDR probes can interfere with each other if the distance between them is around 1 cm. Also, for the samples with higher water contents, the effect of interference on the electromagnetic waveform signals is more prominent, which can result in 15% uncertainty in the measurement of water content. Through the numerical study, four stages of design analysis were carried out to eventually reach a satisfactory design which was deemed to meet the research objective, i.e. less than 5% variation of heat fluxes in the radial direction along the soil cell. The experimental assessment of the final soil cell was first performed using dry Matilda soil. The temperature profile along the soil cell deviated from the linear temperature profile by 18.6% when the temperature level and gradient was high at 82.6°C and 90°C/m, respectively. At this condition, the difference of heat fluxes between the top and bottom heat flux meters was recorded to be 34%. This case is the worst case due to the low thermal conductivity of the dry soil. The experimental assessment of the final soil cell was also done for a wet Matilda soil at a degree of saturation of about 65%. The temperature profile along the soil cell had a maximum deviation of 7.7% from the linear temperature profile even when the temperature level of the soil cell was high at 82.1°C. At this condition, the difference of heat fluxes between the top and bottom heat flux meters was recorded to be 4.2%. After the reliability of the apparatus was assessed, nine cases of the wet soil were studied. The results show that the temperature gradient is the main driving force to cause moisture migration.