A numerical investigation of the gas-phase dynamics of small pool fires

Abstract

This work is an investigation, through direct numerical simulation, of the gas-phase dynamics of small circular pool fires. The objectives are: to determine the mechanisms that contribute to vortex formation and their relative magnitude as dependent upon the Froude and Reynolds number, the frequency response of the flame as dependent upon the Froude and Reynolds number; and to determine the diameter limit of the axisymmetric assumption by formulating a local Reynolds number. The parametric range investigated is 10-4 ≤ Fr ≤ 10-2 and 10 ≤ Re ≤ 10 2. The effect of heat release rate, flame temperature, fuel molecular weight and air-fuel ratio on the vortex formation mechanisms is also investigated. The vortical formation mechanisms include baroclinic torque, density gradient/gravity interaction, stratified shear layers, and density inversion. Stratified shear layers and density inversion are investigated by formulating a kinetic energy of fluctuation equation and evaluating the pertinent production terms. Time-averaged, as well as Favre-averaged, spatially integrated values are obtained as a function of elevation. The flow is modeled as axisymmetric, and an infinitely fast chemical reaction, thin-flame sheet model is used along with a low Mach number approximation. The time-dependent governing equations are solved in axisymmetric coordinates with an explicit, projection-based algorithm. The results indicate that radial density gradients interacting with gravity and unstable density stratification are the dominant mechanisms effecting vortex formation. Baroclinic torque is also significant. The net effect of shear is to provide stability, though local areas exist which are destabilizing. Local regions of stabilizing density stratification also occur. The Froude number, Reynolds number, heat release rate, flame temperature, fuel molecular weight and air-fuel ratio effect the investigated flame dynamics. The Froude number is the dominant parameter effecting the investigated vortex generation mechanisms. An approximate critical local Reynolds number can be identified for which an axisymmetric model is valid. Using available experimental evidence, this value as been found to be around 3000. It is concluded, for this range that an axisymmetric, single-step, irreversible, reaction model with finite rate chemistry is capable of capturing essential features of the gas-phase of small pool fires.