General correlations of chemical species in turbulent fires

Abstract

The scaling with fire size and intensity of stable molecular species in turbulent pool and buoyant jet propane fires has been investigated. The experimental apparatus consisted of porous sintered bronze burners of various diameters supplied with a gaseous fuel, a specially designed constant mass flow gas sampling probe, and H2O, O2, CO2, CO, and total hydrocarbon analyzers. Maps of time-averaged species concentrations in 190, 381, and 762 mm diameter gaseous fuel pool fires over a range of combustion heat release rates were obtained through axial and radial sampling probe traverses. Concentrations for each species throughout the flame were found to be a function only of the mixture fraction, ξ, which is the fraction of atomic species present which originated in the fuel supply. Two correlations of the assembled centerline mixture fraction data are presented. The correlating parameters are respectively 1) a virtual source term parametric in combustion heat release rate, pool diameter, and lip size; and 2) a fire “Froude” number which controls the dimensionless flame height. In the latter correlation, the data are presented in plots of ln (ξ/Q*) vs ln (z/D), where Q* is a fire “Froude” number which controls the dimensionless flame height, and z/D is the dimensionless height. These results indicate four distinct entrainment regimes: 1) counterflowing flames near the fuel surface; 2) columnar, above where the flames neck in to form a mid-axis column; 3) buoyant jet, below the flame tip; and 4) asymptotic plume, above the flame tip.