Local Extinction of Diffusion Flames in Fires

Abstract

The objective of the present study is to use large activation energy asymptotic (AEA) theory to bring basic information on the extinction limits of non-premixed flames. The AEA analysis leads to an explicit expression that predicts the occurrence of flame extinction in the form of a critical Damköhler number criterion; the criterion provides a unified framework to explain the different extinction limits that are observed in non-premixed combustion (i.e., aerodynamic quenching, thermal quenching, and dilution quenching). The critical Damköhler number criterion is then formulated in terms of six input variables; these variables characterize the magnitude of flame stretch, the magnitude of the flame heat losses, and the composition and heat content of the fuel and oxidizer supply streams; these input variables thereby contain information on (laminar or turbulent) flow-induced perturbations, deviations from adiabatic combustion, and air and fuel vitiation. Different two-dimensional flammability maps are then presented using different assumptions aimed at reducing the dimension of the parameter space from six to two. While providing a limited view point, these flammability maps provide valuable insights; it is found for instance that diffusion flames are more sensitive to air vitiation than fuel vitiation.