Experimental study on the coupled effects of ventilation conditions and GHEs on the thermal performance of cold region tunnels

• A physical model reveals how tunnel ventilation affects ground heat exchangers. • Water temperature impacts heat transfer much more than fluid flow speed. • During ventilation, hotter water and slower flow reduce heat loss to the air. • The surrounding rock warms up slowly, especially deeper away from the tunnel. • These findings help optimize tunnel heating systems to prevent frost damage. To address the structural and operational risks induced by frost damage at tunnel entrances in cold regions, active heating has emerged as a crucial engineering solution, in which the use of tunnel lining ground heat exchangers (GHEs) plays a pivotal role. While the heat transfer characteristics of GHEs have been extensively studied, a systematic understanding of their multifactor thermal response under self-heating tunnel conditions is still insufficient. This study developed a laboratory physical model that couples a GHE system with airflow to simulate the dynamic evolution of temperature fields in typical cold-region tunnels, systematically investigating how inlet temperature, circulation flow rate, and ventilation influence the heat transfer performance. The results indicate that without ventilation, inlet temperature exerts a significantly stronger influence on both heat exchange efficiency and surrounding rock temperature than flow rate. A relatively high thermal utilization was achieved when the inlet temperature reached 35 °C with a flow rate of 0.6 m/s. When ventilation is present, wind speed amplifies the heat transfer rate to the surrounding rock and increases thermal losses, particularly under high-temperature and low-flow scenarios. Under these conditions, optimal heat utilization was achieved with an inlet temperature of 40 °C and a flow rate of 0.4 m/s. Consequently, to optimize anti-freezing operations, this study recommends a dynamic parameter-tuning strategy: employing high inlet temperatures coupled with low fluid velocities during ventilation to actively suppress convective heat loss to the airflow, thereby maximizing effective thermal penetration into the surrounding rock.