An analytical model of construction duct ventilation for high-geothermal tunnel based on non-uniform convective heat transfer coefficient

Abstract With the advancement of tunnel and underground engineering, the geological environment has grown increasingly intricate, posing significant challenges to existing technologies. In particular, when tunnels traverse high geothermal areas, elevated temperatures within the tunnel not only compromise construction quality but also pose health risks to workers. Duct ventilation stands out as a widely employed mitigation measure. However, currently the temperature field variation induced by duct ventilation in construction tunnels can only be analysed through numerical simulations, requiring case-by-case modelling and computation, and the analytical assessment of the cooling efficacy of duct ventilation predominantly relies on steady-state analyses of operational tunnels, neglecting the consideration of the longitudinally differentiated distribution of heat transfer efficiency and the dynamic changes in the temperature field during tunnel construction. Motivated by these shortcomings, this study introduces a novel semi-analytical model for construction duct ventilation in high-thermal tunnels, utilizing the Green's function method (GFM) and Dirac's function. The model incorporates a trough equivalent ring heat source at the air-rock interface and a non-uniform convective heat transfer coefficient (CHTC) distributed along the longitudinal direction via a Gaussian function. This approach enables the determination of the unsteady temperature field within a dead-end tunnel, facilitating a comprehensive evaluation of the cooling effect. The findings reveal substantial variations in the temperature field of the surrounding rock along the longitudinal direction in a short timeframe, underscoring the significance of dynamic cooling assessments. The derived temperature distribution aligns well with numerical simulations and field-test results in high-geothermal tunnels. Further parametric analysis emphasizes the pronounced boundary effect of ventilation cooling, leading to suggested optimal ventilation rates tailored to different rock masses. This research provides valuable insights for optimizing tunnel construction in high-thermal environments.