CFD Investigation of Large Scale Pallet Stack Fires in Tunnels Protected by Water Mist Systems

Abstract

A series of full-scale fire suppression tests was conducted at the San Pedro de Anes test tunnel facility near Gijon, Asturias, Spain in February 2006. The fuel was wooden pallets or a mixed load of wood and high density polyethylene pallets. Fire protection was provided by water mist systems in different configurations. Because of facility restrictions, some scenarios of great interest, such as a free burn fire, could not be investigated. However, in order to complement the experimental results, a number of computational fluid dynamics simulations were conducted on a 140 m section of the tunnel facility. The Fire Dynamics Simulator, version 4, was used for the numerical investigation. An algorithm was developed to allow the fire to spread along the top of a series of pallet loads in such a way that the measured heat release rate was reproduced. Verification and validation studies confirmed that the model predicted the measured ventilation speeds and peak temperatures. The agreement between the simulations and the field measurements was very good prior to activation of the water mist. Back-layering was modeled well. After activation of the mist, the simulations predicted a large drop in gas temperatures, and retreat of the back-layer, but under-predicted the thermal cooling by the water mist downstream of the fire. With the suppression system, high temperatures and heat fluxes were limited to the immediate vicinity of the burning pallets. The model was then used to simulate a free burn fire in the tunnel. The simulation demonstrated the catastrophic conditions created by an unsuppressed fire in a tunnel when compared against the thermally managed conditions under suppressed conditions.