Parametric simulations and full-scale experimental validation of fire conditions in naturally ventilated road tunnels

The Ofenegg Tunnel Fire Test was a full-scale experimental study conducted to gain a better understanding of fire behavior in road tunnels. Documented and validated experiments of this nature are limited in the literature, and comprehensive numerical validation studies remain relatively scarce, highlighting the unique contribution of this research. For validation, three fire scenarios were selected, involving 100, 500, and 1000 litres of gasoline, corresponding to maximum heat release rates of 17, 43, and 37 MW, respectively, under natural ventilation conditions. The Fire Dynamics Simulator (FDS) was used to compare the predicted temperature distribution, oxygen levels, carbon monoxide concentrations, and gas flow rates with experimental data. The findings showed good agreement between FDS predictions and experimental values. Following validation, a total of 54 parametric simulations were conducted to investigate the effects of fire sizes (1.5, 75, and 150 MW), tunnel slope (0.5%, 3%, and 6%), and cross-sectional configuration (unidirectional and bidirectional) on the time-temperature profiles reaching the tenability threshold of 60°C, measured at 2 m to represent human breathing height and determine the Available Safe Egress Time (ASET). The analysis tracked temperature development both 10 m upstream and downstream from the origin of the fire and along tunnel lengths of 100, 450, and 1000 m, demonstrating the combined influence of fire size, slope, tunnel length, and geometry on temperature magnitudes. Results showed that small fires (1.5 MW) remained within tenable limits regardless of slope, tunnel length, or cross-section configuration, whereas larger fires (75 and 150 MW), combined with steeper slopes (3% and 6%) in the downstream direction, represented the most critical scenario, leading to rapid temperature rises and reduced ASET. For large fires with a slope of 0.5%, the time to reach 60°C was similar in both directions, yielding longer ASETs compared to steeper slopes. Furthermore, the effect of cross-section was consistent: bidirectional tunnels exhibited slightly lower peak temperatures than unidirectional ones. • FDS validated against Ofenegg tunnel fire tests under natural ventilation. • 54 parametric cases: fire size, slope, length, and cross-section were varied. • Small fires remained within safe tenability limits across all tunnel configurations. • Large fires in long and steep tunnels caused rapid downstream tenability failure.