Convective-Buoyancy Scaling and Thermal Attenuation of Two-Stage Smoke Propagation in an Open-Door Metro Carriage–Tunnel Fire

Distributed door openings couple ceiling-smoke transport in a metro carriage with subsequent tunnel propagation, so a source-based buoyancy scale may not represent the advancing carriage front over its full path. FDS simulations were conducted for steady 1–5 MW fires at three locations under adiabatic-wall and heat-transfer conditions and evaluated against reduced-scale ceiling-temperature data. For the 5 MW middle-source adiabatic case, the source-scaled carriage velocity coefficient decreased from 0.790 to 0.614 with distance. For the middle-source adiabatic cases, the local convective-buoyancy velocity scale decreased more rapidly than the carriage smoke-front velocity, resulting in a streamwise increase in the locally scaled velocity coefficient. Compared with the corresponding adiabatic-wall cases, the inclusion of solid-boundary heat transfer was associated with additional distance-dependent deceleration of both carriage and tunnel fronts. Smoke entering the tunnel formed a secondary ceiling front, whose normalized ceiling-temperature rise followed an exponential decay. The results characterize the scaling and attenuation of two-stage smoke propagation and reveal the limitations of using a constant local convective-buoyancy velocity coefficient in the examined open-door configuration.