Investigation of heat transfer characteristics and effective thermal conductivity in a 3-D water–rock fracture structure

Abstract

The rapid expansion of mid-deep geothermal energy and underground thermal storage faces significant challenges due to the variability in geological conditions. The impact of fracture morphology on thermal storage and heat transfer has become a critical area of research, driven by the lack of comprehensive characterizations of the heat exchange processes within these systems. This research examines the heat transfer characteristics and effective thermal conductivity (ETC) of a 3-D water–rock fracture structure. Firstly, the convective heat transfer coefficient (h) at fracture surfaces within the 70–100 ℃ range is compared and selected for accuracy. Secondly, the coupled flow and heat transfer processes of water and rock in a 3-D rough fracture are quantified under varying amplitude factors (A_mn). Finally, the ETC at different A_mn ranges is quantitatively characterized. The study reveals that with A_mn escalating from 0.01 to 0.035, the effective heat exchange area within the fracture increases by 22.73 %. Concurrently, the normalized velocity (UN) exhibits a non-linear rise of 67.64 %, escalating from 2.2 to 6.8. Afterwards the selection of proper calculation formulas for h in water–rock systems, the correlation between ETC and h is considered. It is found that ETC positively correlates with A_mn, increasing by 83.33 % with rising A_mn. Finally, An empirical model to describe ETC in fracture structures is proposed, contributing to the understanding of fluid flow and heat transfer in fractured rocks at high temperatures and enriching the theory for mid-deep geothermal energy exploitation.