Natural convection effect on heat transfer in saturated soils under the influence of confined and unconfined subsurface flow

Recently, it has been proven that natural convection significantly impacts heat transfer close to geothermal systems in saturated soils even in the presence of groundwater flow. However, this phenomenon has only been investigated in the presence of confined groundwater flow. Therefore, in this study, a fully coupled hydrothermal model was developed to quantitatively determine the natural convection influence on heat transfer in the presence of an unconfined groundwater flow and to compare the hydro-thermal response of soil under these two different scenarios. Results showed that natural convection influence depends on the coupled effects of soil permeability, groundwater velocity, and heater temperature. According to the results, in the case of a groundwater flow velocity less than 10-7 m/s, natural convection role is non-negligible when Rayleigh number is greater than 50, while for other groundwater velocities, that matters only when Buoyancy ratio is greater than 20. It was also shown that choosing a confined groundwater flow over an unconfined one led to an error of 47 % to 173 % in the explored scenarios. Furthermore, it was revealed that when the relative distance of the heater from the surface alters from 0 to 15 and 30 m, the error in the soil temperature due to neglecting natural convection can be as high as 43 %, 48 %, and 60 %, respectively.