Design of Tunnel Ventilations Systems for Fire Emergencies using Multiscale Modelling

This paper presents a novel and fast modelling approach to simulate tunnel ventilation flows during fire emergencies. 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 (1D) and CFD (3D) modelling techniques. A simple 1D network approach is used to model tunnel regions where the flow is fully developed (far field), and a detailed CFD representation is used where flow conditions require 3D resolution (near field). This multi-scale method has previously been applied to simulate tunnel ventilation systems including jet fans, vertical shafts and portals (Colella et al 2009, Build. Environ. 44(12): 23572367) and it is applied here to include the effect of fire both in steady state and transient situations. The methodology has been applied to a modern tunnel of 7 m diameter section and 1.2 km in length. Different fire scenarios ranging from 10 MW to 100 MW are investigated with a variable number of operating jet fans. Emphasis has been given to the discussion of the different coupling procedures for steady state and transient calculations. An accurate discussion on the computational cost reduction as well as on the control of the numerical error is also presented. Compared to the full CFD solution, the maximum flow field error can be reduced below 2%, but providing a reduction of two orders of magnitude in computational time. The much lower computational cost is of great engineering value, especially for parametric and sensitivity studies required in the design or assessment of ventilation and fire safety systems.