A COMPUTATIONAL FLUID-DYNAMICS STUDY OF WOOD FIRE EXTINGUISHMENT BY WATER SPRINKLER

A Computational Fluid Dynamics (CFD) model is developed to predict ext inguishment times of an array of wood slats by water sprinkler. The mo del predicts flow field, combustion of wood volatiles and radiation tr ansfer. The gas-phase model is coupled with the wood pyrolysis model t o predict a volatile release rate. A sprinkler water spray is modelled using a Lagrangian particle tracking procedure, coupled with the gas flow model by a Particle-Source-In-Cell algorithm. A simple model of i nstant droplet evaporation at the burning surface is employed. The exp erimental program includes full-scale experiments in a fire gallery wi th a commercial sprinkler system installed in the roof In some tests a water restrictor is used to vary the waterflow rates. Water droplet s ize and velocity distributions are measured to serve as inputs to the spl ay model. A vertical nr ray of wood slats is ignited uniformly in a slight draft of about 0.7 m/s. A few minutes after self-sustained bu rning is developed, the sprinkler is activated. Thermocouple and heat flux measurements in the vicinity of the flame, as well as a video rec ord, are used to determine flame shape and to provide data for validat ion of the CFD model. Burning sates are measured by load cell and by C O2 measurements. Extinguishment happens primarily due to fuel cooling, which is indicated bit long extinguishment times (two orders of magni tude longer than for plastic materials). The predictions of burning ra te and flame shape are reasonably accurate. Extinguishment times are m odelled for different water flow rates. The dependence on water pow ra te is found to be weak because the extinguishment process is controlle d by the thermal time constant of the whole wood sample. (C) 1997 Publ ished by Elsevier Science Ltd.