Large eddy simulation of flame extinction in a turbulent line fire exposed to air-nitrogen co-flow

The general objective of this project is to support the development and validation of large eddy simulation (LES) models used to simulate the response of fires to the activation of suppression systems. The focus here is on suppression by gaseous agents. The present experimental configuration is a two-dimensional, plane, buoyancy driven, methane-fueled, turbulent diffusion flame with a controlled co-flow. The co-flow is an air-nitrogen mixture with variable oxygen dilution conditions, including conditions that lead to full flame extinction. Experimental measurements include the global combustion efficiency and global radiative loss fraction. The numerical simulations are performed with a LES-based fire model developed by FM Global and called FireFOAM. In this study, FireFOAM is modified to include a flame extinction model based on the concept of a critical flame Damkohler number and a flame reignition model based on the concept of a critical gas temperature. The numerical simulations are found to successfully reproduce the rapid change that is observed experimentally when exposing the flame to a co-flow with decreasing oxygen strength: the change corresponds to an abrupt transition from a strong flame with a global combustion efficiency close to one to a residual flame with a global combustion efficiency close to zero.