In-Depth Assessment of Cross-Passage Critical Velocity for Smoke Control in Large-Scale Railway Tunnel Fires

Demand for underground railways has rapidly increased due to accelerated urbanisation and population growth. This has elevated the importance of tunnel designs with adequate fire safety and protection measures. However, due to intricate modern rail tunnel designs, prescriptive codes are often difficult to implement and lead to over-conservative design. In this study, the current state of tunnel fire analysis was reviewed with a focus on Australia. A large-eddy simulation (LES)-based fire model was applied to investigate the temperature and smoke dispersion from a 2 MW metro tunnel fire case scenario to the cross-passage. A total of 28 cases with various cross-passage ventilation settings were examined, including longitudinal tunnel velocity, cross-passage velocity, train location relative to the cross-passage and fire location. The modelling showed that a 0.84 m/s critical velocity was sufficient for smoke control in the cross-passage. Furthermore, two empirical methods for cross-passage critical velocity were performed, which showed utilisation of the Froude number produced a less conservative critical velocity (0.610 m/s) compared to the dimensionless method (0.734 m/s). Nevertheless, both numerical and empirical results were significantly lower than the standard 1.0 m/s minimum flow rate for smoke control (AS1668.1). The results provide preliminary evidence towards the need for revision of current tunnel fire standards and response protocols.