ASYMPTOTIC ANALYSIS OF THE STRUCTURE AND EXTINCTION OF SPHERICALLY SYMMETRICAL NORMAL-HEPTANE DIFFUSION FLAMES

A minimal chemical-kinetic mechanism for the oxidation of n-heptane is reduced to a global two-step mechanism by the systematic application of partial-equilibrium and steady-state assumptions, with the global reaction rates related to the elementary rates. From this mechanism, the structure of spherically symmetrical diffusion flames around n-heptane droplets is analyzed using rate-ratio asymptotics. The outer transport zones are described by the classical flame-sheet analysis with Lewis numbers of unity. The inner structure consists of a thin fuel-consumption zone on the rich side of the flame and a broader but still thin layer on the lean side where H-2 and CO are oxidized. The theory identifies a scalar dissipation rate, related to the droplet diameter, appropriate for droplet burning. From the analysis, the variations in flame temperature and in species concentrations with the stoichiometric scalar dissipation rate chi(st) were obtained. Since extinction occurs where chi(st) reaches a maximum, the extinction diameters for n-heptane droplets can be estimated from the results and are given for different pressures and ambient oxygen concentrations.