CFD Simulations on Extinction of Co-Flow Diffusion Flames

Abstract

An improved flame extinguishing criterion is implemented to the Fire Dynamics Simulator (FDS) for gaseous suppressants under the lumped species model with transport-limited combustion (infinitely fast chemistry). The criterion considers the total enthalpy of gas mixture as a function of temperature, and evaluates whether a limiting adiabatic flame temperature can be obtained due to combustion in a cell.The model is validated against a number of experiments in the cup-burner apparatus, a standard means of obtaining the minimum flame-extinguishing concentrations (MEC) for a variety of fuel-suppressant combinations. The results indicate that the model can reproduce the MEC values for all common inert gas agents, as well as some of the fluorinated halocarbon agents.It is further demonstrated in the paper that the flame extinguishment criterion, although validated using grid cells on the order of 1 mm, is equally applicable to larger cell sizes more appropriate for full-scale simulations. This improves the capability of FDS to predict the performance of full-scale fire suppression systems.

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

“…Available models used to describe flame extinction in fire problems are based on the concepts of a critical flame temperature [1,2] or a critical flame Damköhler number [3][4][5][6][7]. Models based on the concept of a critical flame temperature choose to ignore the importance of chemical time scales and are not consistent with known laminar flame phenomenology [8].…”

Modeling flame extinction and reignition in large eddy simulations with fast chemistry

“…The baseline flame extinction algorithm available in FDS uses the concept of a critical flame temperature [10], which accounts for the thermal quenching mechanism, but neglects the aerodynamic and kinetic quenching mechanisms. For extinction via nitrogen dilution of the oxidizer, thermal quenching acts as the dominant mode of extinction; therefore a critical flame temperature based extinction model is sufficient for the present study.…”

“…The criterion for flame extinction in FDS incorporates an enthalpy balance where in order for combustion to occur, the potential heat release due to combustion must be sufficient to increase the mean temperature of the reactant mixture above a critical value, T ext , defined as the critical flame temperature [10, 23]. Combustion is allowed to proceed if the inequality given by $${\hat{Y}}_{\mathit{\text{fuel}}}false(h_{\mathit{\text{fuel}}}false(\stackrel{\sim }{T}false)+\mathrm{normal\Delta }{h}_{\mathit{\text{comb}}}false)+{\hat{Y}}_{\mathit{\text{ox}}}{h}_{\mathit{\text{ox}}}false(\stackrel{\sim }{T}false)+{\hat{Y}}_{\mathit{\text{dil}}}{h}_{\mathit{\text{dil}}}false(\stackrel{\sim }{T}false)>{\hat{Y}}_{\mathit{\text{fuel}}}{h}_{\mathit{\text{fuel}}}false(T_{\mathit{\text{ext}}}false)+{\hat{Y}}_{\mathit{\text{ox}}}{h}_{\mathit{\text{ox}}}false(T_{\mathit{\text{ext}}}false)+{\hat{Y}}_{\mathit{\text{dil}}}{h}_{\mathit{\text{dil}}}false(T_{\mathit{\text{ext}}}false),$$is satisfied, where [ Ŷ fuel , Ŷ ox , Ŷ dil ] and [ h fuel , h ox , h dil ] are the mass fractions and mass-specific sensible enthalpies of the fuel, oxidizer, and diluent (defined as any non-reactant species, including any inert agents or products of combustion) present in the reactant mixture, T̃ is the initial cell temperature, and Δ h comb is the mass-specific enthalpy of combustion of the fuel.…”

“…These works have successfully identified the primary physical mechanisms for extinction (thermal, aerodynamic, and kinetic quenching) [1–9], while others have made progress toward developing simple formulations to model flame extinction in cases applicable to realistic fire scenarios [10–13]. Additional studies have highlighted the primary features of flame reignition events, which may follow localized extinction in large-scale turbulent flames [14–19].…”

Local Extinction of Diffusion Flames in Fires

“…[17,18], which assumes constant heat capacities; see Ref. [19] for a recently modified version of the FDS flame extinction model that accounts for variable heat capacities and is formulated as an enthalpy-based criterion. In order to allow direct comparisons between FDS and AEA, and since the FDS model does not account for effects of flame stretch, the AEA model uses the oxygen- .…”