Impacts of Local Thermal Non-Equilibrium on Heat Transport Interpretation in Shallow Aquifer

Local thermal equilibrium (LTE) assumption, which assumes immediate thermal equilibrium between solid and neighboring fluid, has generally been applied to analyze thermal behavior in the subsurface environment. In terms of time to equilibrate, the high flow velocity can disturb the instant thermal equilibrium. So far, however, velocity criteria for LTE assumption have rarely been suggested and especially have yet to be verified experimentally. In this study, heat and solute tracer experiments were performed on a lab scale using sand and were analyzed through mathematical models. Thermal front velocities and dispersion coefficients estimated by heat tracer tests were compared to those predicted by a retardation factor with solute transport parameters. The results provided the experimental evidence of local thermal non-equilibrium (LTNE) in the experimental velocity range (Reynolds number, Re < 0.37). The impact of LTNE was larger on the estimates of thermal dispersion coefficients than on those of velocities. This result suggests that the LTE assumption needs to be verified when the thermal dispersion coefficients could have a crucial role in heat transport interpretation. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017R1A2B3002119). This work was also supported by Korea Environment Industry & Technology Institute(KEITI) through "Activation of remediation technologies by application of multiple tracing techniques for remediation of groundwater in fractured rocks" funded by Korea Ministry of Environment (MOE)(Grant number:20210024800002/1485017890). This work was also supported by the Nuclear Research and Development Program of the National Research Foundation of Korea (NRF-2021M2E1A1085200). This work was also supported by the Institute for Korea Spent Nuclear Fuel (iKSNF) and National Research Foundation of Korea (NRF) grant funded by the Korean government (Ministry of Science and ICT, MSIT) (Grant number: 2021M2E1A1085200).