A Computational Fluid Dynamics study of wood fire extinguishment by water sprinkler

Abstract A Computational Fluid Dynamics (CFD) model is developed to predict extinguishment times of an array of wood slats by water sprinkler. The model predicts flow field, combustion of wood volatiles and radiation transfer. The gas-phase model is coupled with the wood pyrolysis model to 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 instant droplet evaporation at the burning surface is employed. The experimental program includes full-scale experiments in a fire gallery with a commercial sprinkler system installed in the roof. In some tests a water restrictor is used to vary the water flow rates. Water droplet size and velocity distributions are measured to serve as inputs to the spray model. A vertical array of wood slats is ignited uniformly in a slight draft of about 0·7 m/s. A few minutes after self-sustained burning is developed, the sprinkler is activated. Thermocouple and heat flux measurements in the vicinity of the flame, as well as a video record, are used to determine flame shape and to provide data for validation of the CFD model. Burning rates are measured by load cell and by CO 2 measurements. Extinguishment happens primarily due to fuel cooling, which is indicated by long extinguishment times (two orders of magnitude longer than for plastic materials). The predictions of burning rate and flame shape are reasonably accurate. Extinguishment times are modelled for different water flow rates. The dependence on water flow rate is found to be weak because the extinguishment process is controlled by the thermal time constant of the whole wood sample.

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