The study provides a comprehensive numerical investigation into the coupled fluid flow and heat transfer within a three-dimensional groundwater seepage system featuring cold circular tunnels. To address the escalating demand for energy-efficient thermal management, the research explores the performance of both water-based fluids and advanced ternary hybrid nanofluids (THNFs) composed of water, ferric oxide, copper oxide, and molybdenum disulfide. Using COMSOL Multiphysics, a robust finite element software, the analysis meticulously combined conduction and convection heat transfer phenomena. A primary contribution of this research is a systematic evaluation of key parameters, Reynolds number ( \({\text{Re}}\) ), fracture aperture ( \(f_{a}\) ), aspect ratio ( \(P_{r}\) ), nanofluid total volume fraction ( \(\phi\) ), and tunnel displacement ( \(D_{s}\) )—on thermal and fluid dynamic performance. The results provide valuable insights into optimal cooling configurations. For instance, the findings indicate that increasing the Reynolds number significantly enhances convective heat transfer, boosting heat dissipation and the local Nusselt numbers. Optimal thermal performance, quantified by a notable 9.6215% temperature change, was observed at a low flow rate ( \({\text{Re}}\) = 10) and specific geometry ( \(f_{a}\) = 0.01 m, \(P_{r}\) = 0.04, \(D_{s}\) = 0, \(\phi\) = 0.1). Conversely, geometric factors proved more dominant for flow acceleration, with a maximum 51.861% velocity change observed at \(f_{a}\) = 0.05 m and \(P_{r}\) = 0.1. For overall heat transfer maximization, the highest average Nusselt number of 83.673 was achieved at a higher flow rate ( \({\text{Re}}\) = 40) and with base fluid ( \(\phi\) = 0). This research offers critical insights for optimizing the thermal management of complex geothermal and seepage systems.
