Design fires for vehicles in road tunnels
Fire in road tunnels is a unique design problem which can lead to serious consequences if not addressed appropriately. Observations from full scale tunnel fire experiments have indicated the heat release rate depends on the ventilation conditions, tunnel geometry and fuel load. Although these experiments have provided valuable information, they are generally very expensive to conduct and the data are limited. The experiments are often based on a specific test condition such as air velocity, geometry or tunnel slope which may be different from the design conditions present for an actual tunnel project. The design of smoke extraction systems for tunnels often uses prescriptive values for the heat release rate (HRR) from a vehicle fire which does not account for the tunnel conditions. This paper presents an overview of the methodology to estimate the heat release rate of a credible vehicle fire in a road tunnel using a performance-based approach. The analysis consists of two stages; stage one involves the use of a probabilistic approach (risk analysis) to identify the potential cause and type of vehicle which could result in a tunnel fire. Findings from the risk analysis are used in stage two in which Computational Fluid Dynamics (CFD) modelling is used to establish the heat release rate in a tunnel considering factors such as fuel load, ventilation condition, tunnel geometry and ignition location. An urban tunnel in Singapore is used to illustrate this methodology.