This paper relates the probable fire heat release rate (FHRR) for a given combustible load in a road tunnel to its geometry and identifies some issues that need to be considered when selecting an appropriate FHRR to be used in the design of life-safety systems for road tunnels. Recent fire tests have achieved peak FHRR's in excess of 200 MW using mockups of heavy goods vehicles (HGV's) with high combustible loads in a tunnel with a relatively small cross-section and under an imposed longitudinal airflow. These test results may not be directly applicable to a road tunnel with conventional cross-sectional dimensions. In order to evaluate this situation, an approach developed by Carvel, Beard and Jowitt at Heriot-Watt University has been used to study the effect of extrapolating from test conditions to real world dimensions. To investigate the trend in FHRR suggested by this methodology, a series of computational fluid dynamic (CFD) simulations were performed using the Fire Dynamic Simulator (FDS). Burning of identical combustible loads was simulated in tunnels with only the tunnel width being varied. The results of these simulations showed good correlation with the Heriot-Watt results.
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