Comparison of Two Laboratory Methods for Measuring Soil Critical Temperature

Abstract

Abstract Coupled heat and moisture transfer above a critical temperature is important in various geothermal applications (e.g., geothermal heat exchangers, buried power cables, and energy piles) because of its impact on nonsteady soil thermal properties. In this study, temperature and moisture content of seven sandy soils were measured using two laboratory approaches—vertical and radial—to evaluate the critical temperatures. The vertical approach measured temperature and moisture data from top to bottom of a soil column, and the radial method obtained the measurements from center to outside of the specimen. To validate the test results, computational modeling was also carried out in MATLAB. Although the results obtained from radial testing included rapid changes in temperature gradients near the boundaries between dry and wet zones, ambiguous temperature profiles (i.e., gradual gradients over the entire range) were observed in vertical testing possibly because of horizontal boundary heat losses. In contrast to the gradual temperature gradients, moisture content profiles showed significant variations near the transitional regime, thus the critical temperatures were determined based on both temperature and moisture profiles. Average critical temperature for seven soils was 45.8°C, and the critical temperature was dependent on the void ratio. As void ratio decreased, critical temperature increased with a corresponding decrease in the size of the dry zone. The critical temperatures obtained from radial testing were higher (up to 5.2°C higher) and took more time to reach steady-state condition (up to 9 h longer) than those obtained from vertical testing. Because there are limited data and methodology in published studies that discuss a standard methodology to measure soil critical temperature, a consistent, universally accepted, procedural-based method is needed for the measurement or determination of the critical temperature.