Experimental study on smoke temperature distribution and thermal stratification of fire sources at the excavation face of high-geothermal tunnels

Excavation faces exhibit elevated fire risks owing to the accumulation of combustible materials, especially for high-geothermal tunnels. In this study, small-scale tunnel fire experiments were performed to investigate the smoke movement and temperature distribution under geothermal conditions. The results revealed that the geothermal activity induces a distinct asymmetrical temperature field, characterized by thermal stratification with heated airflow beneath the tunnel ceiling and cold external airflow at the bottom. In the event of a construction tunnel fire, the geothermal-induced thermal stratification confined the extent of the high-temperature smoke layer beneath the tunnel ceiling, while promoting the smoke diffusion across the cross section near the excavation face. Furthermore, the effects of geothermal temperature and pool fire size on longitudinal temperature distribution and vertical smoke stratification were revealed. The excess ceiling temperature decayed non-monotonically in high-geothermal tunnels, with a secondary peak at 4.1 m from the excavation face. The predictions of the excess ceiling temperature were provided for high-geothermal tunnel fires. The results revealed that geothermal conditions enhanced the intensity of vertical smoke stratification by acting as sustained external heat sources. By contrast, the positive effect of pool fire size on vertical smoke stratification was attenuated within the geothermal-affected zone.