Experimental design of a “coupled heat transfer” model for surrounding rock–heat exchanger–airflow in high geothermal tunnels

With the increasing construction of deep and long tunnels, high geothermal environments have becomea criticalissueaffecting both construction safety and thermal comfort. Although such environments are often associated with thermal hazards, the stable geothermal potential of thesurrounding rock can be exploited as a renewable heat source. However, the thermal performance of ground heat exchangers (GHEs) in high-temperature tunnelsremainsinsufficientlyunderstood from anexperimental perspective. This study develops a scaled laboratory platform to investigate the coupled heat transfer behavior amongsurrounding rock, heat exchangers, and tunnel airflow.

A scaled experimental platform with a geometric scale ratio of 1 : 30 wasdesigned and constructed, consisting of a model test box, model tunnel, heat exchange system, heating system, ventilation system, seepage simulation system, and multipoint data acquisition system. This platform allowsforcontrolled simulation of coupled thermal processes under variable environmental conditions. Experiments wereconducted to evaluate the effects of ventilation speed (0 – 3 m/s), seepage water temperature (60 – 80 ℃), and working fluid velocity (0.4 – 0.8 m/s) on thermal performance. The outlet water temperature wasused as the primary indicatorofcoupledheat transfer behavior.

The results showthat ventilation speed hasa noticeable influence on heat exchange: the outlet water temperature decreasessignificantly when the airflow speed exceeds 2 m/s. High-temperature seepage markedly increases the outlet temperature during the initial stage of heat exchange (e.g., seepage at 80 ℃ increased the outlet temperature by 87% comparedwith the no-seepage case). However, this effect gradually weakensdue tothe limited water retention of the surrounding rock. Increasing the working fluid velocity enhancesboth heat-exchange efficiency and system stability, with 0.6 m/s identified as the optimal operating parameter balancingthermal performance and energy efficiency.

This study experimentallyrevealshowventilation, seepage temperature, and fluid velocity influence the thermal performance of GHE systems in high-geothermal tunnels. The findings provide valuable guidancefor the design and operationof geothermalenergy utilization systems in underground engineering.