Numerical Modeling Of The Effect Of Fine Water Mist On The Small Scale Flame Spreading Over Solid Combustibles

Abstract

A CFD model has been developed to predict the behavior of the flame spread over solid fuels in water mist environment. A coupled analysis involving gas phase and solid fuel is considered using unsteady two-dimensional conservation equations describing the self sustained flame propagation. Due to the analysis is focused on the mechanism of flame suppression at the flame leading edge, which is explicitly exposed to the mist, finite-rate chemical reaction is taken into account. The equations for water mist and vapor mass fractions are added to the basic flame spread statement, which includes corresponding term describing energy consumption due to water evaporation. Horizontal flame spread over thick fuel beds of polymeric material (PMMA) under water mist suppression has been investigated. The results have shown that self-sustained energy balance in the heat release zone in the flame is highly sensitive to the external energy loss, which in this case is due to water droplet evaporation. Thus, flame struggles against the presence of water mist on the flame leading edge and either continues to propagate with almost the same velocity (as of without water mist), or complete extinction occurs. The extinguishing characteristics of fine water mist with the droplet diameter of the order of 30 microns are investigated. A critical concentrations of initial water mist mass fraction required for extinguishment have been achieved for the various conditions of flame spread phenomenon.