Time-dependent multiscale simulations of fire emergencies in longitudinally ventilated tunnels

This paper applies a novel and fast multiscale approach to model ventilation flows and fires in tunnels. The complexity and high cost of full CFD models and the inaccuracies of simplistic zone or analytical models are avoided by efficiently combining mono-dimensional (1-D) and CFD (3-D) modeling techniques. The multiscale model couples a 3-D CFD solver with a simple 1-D model allowing for a more rational use of the computational resources. The 1-D network models tunnel regions where the flow is fully developed (far field), and detailed CFD is used where flow conditions require 3-D resolution (near field). The multiscale method has been applied to model steady-state fires and tunnel ventilation systems, including jet fans, vertical shafts and portals, and it is applied here to study the transient flow interactions in a modern tunnel of 7 m diameter section and 1.2 km in length. Different ventilation scenarios are investigated to provide the timing to reach the critical velocity conditions at the seat of the fire and to remove the upstream back layering. The much lower computational cost is of great value, especially for parametric and sensitivity studies required in the design or assessment of real ventilation and fire safety systems. This is the first time that a comprehensive analysis of the transient fire and ventilation flow scenarios in a long tunnel is conducted.