Road tunnel fires can produce rapid heat accumulation and severe thermal loading, particularly when fixed firefighting systems are not activated during the early stages of fire development. Although previous tunnel fire studies have examined ventilation effects and individual fire scenarios, only a limited number have quantitatively evaluated the performance of water-mist fixed firefighting systems under substantially different fire intensities using identical tunnel geometry and operating conditions. This gap restricts the ability to assess suppression efficiency across both moderate and severe tunnel fire scenarios. Computational fluid dynamics modelling, particularly the FDS–LES framework, enables controlled comparison of such scenarios that would be difficult, costly, or unsafe to reproduce in full-scale tunnel experiments, while providing detailed information on temperature field development and heat propagation. This study evaluates the influence of a water-mist fixed firefighting system on temperature development and the spatial extent of high-temperature zones in a road tunnel. Numerical simulations were performed in PyroSim using the Fire Dynamics Simulator (FDS) and the Large Eddy Simulation (LES) approach. Four scenarios were analyzed under identical tunnel geometry, ventilation conditions, and operational settings, combining two heat release rates (30 MW and 200 MW) with suppressed and unsuppressed fire conditions. The 30 MW case represented a passenger vehicle or light commercial vehicle fire, whereas the 200 MW case represented a severe heavy goods vehicle fire. The results showed that, in the 200 MW scenario, activation of the fixed firefighting system reduced the maximum temperature from 950 °C to 700 °C (−26%), while in the 30 MW scenario the maximum temperature decreased from 310 °C to 160 °C (−48%). Minimum temperatures were reduced from 550 °C to 200 °C in the 200 MW scenario and from 290 °C to 110 °C in the 30 MW scenario. The water-mist system also limited the propagation of the high-temperature layer beneath the tunnel ceiling, with a more pronounced relative effect under the lower heat release rate. Although complete suppression of the 200 MW fire was not achieved, the system reduced peak temperatures and limited the extent of critical high-temperature zones. The main contribution of this study is the quantitative comparison of water-mist suppression performance under moderate and severe tunnel fire conditions using the same tunnel configuration, which provides practical evidence for assessing peak-temperature reduction, high-temperature zone limitation, and thermal loading mitigation in road tunnel fire safety design.
Rusnák et al. (Tue,) studied this question.