Estimation of maximum ceiling temperature rise in tunnel fires with natural ventilation at various longitudinal fire locations

The confined geometry of tunnels and the uncertainty of fire location can produce markedly different ceiling thermal loads under natural ventilation. This study numerically investigates the maximum ceiling temperature rise (MCTR) in a 100 m long tunnel with natural ventilation by varying longitudinal fire location and tunnel slope. A total of 147 simulation cases are conducted by combining tunnel slopes of 0°-12°, downstream tunnel lengths of 20-80 m, and HRRs of 2-4 MW. The results show that in horizontal tunnels, as the fire source deviates from the midpoint, MCTR decreases monotonically. This is driven by increased convective heat loss resulting from enhanced asymmetric air entrainment. In inclined tunnels, the longitudinal airflow induced by stack effect dominates the smoke exhaust through the downstream tunnel portal. Under the coupled stack effect and Coanda effect, increasing tunnel slope or downstream tunnel length will intensify the downstream tilt of the flame, resulting in a monotonic decline in MCTR. Accordingly, two empirical models are developed: an asymmetric-flow-based model for horizontal tunnels and a coordinate-transformation-based model for inclined tunnels incorporating an equivalent velocity to account for coupled stack and Coanda effects. Validation against literature data indicates that these models may fail when the fire source deviates significantly from the tunnel midpoint. The research findings enable timely risk assessment and ventilation control in naturally ventilated tunnel fires. • Analyzed MCTR in naturally ventilated tunnels considering steep slopes. • Clarified mechanisms: asymmetric flow and coupled stack-Coanda effects. • Proposed coordinate-transformation-based models for inclined tunnels.