Combustion characteristics of primary reference fuel (PRF) droplets: Single stage high temperature combustion to multistage "Cool Flame" behavior

We report experiments and detailed numerical modeling of mixtures of primary reference fuel (PRF) droplets consisting of n-heptane and iso-octane with initial droplet diameters of 0.5 and 3.51 mm. The results show a quasi-steady, low temperature (or "Cool Flame" (CF)) droplet burning mode that stems from a varying two-stage chemical kinetic behavior of the combustion chemistry. The simulations further illustrate, that the CF droplet burning mode in 1 atm air is dependent upon the iso-octane fraction and droplet size. CF droplet burning is predicted to be absent for large diameter droplets containing more than 50% (by volume) iso-octane (> PRF50), and for all droplet diameters that exhibit hot flame burning without radiative extinction. The model predictions are in agreement with new, large diameter PRF50 experiments reported here, as well as previous ground-based PRF50 sub-millimeter diameter experiments. The effects of PRF mixture fraction are further analyzed numerically. Additional simulations show that replacing small amounts of in-ert (nitrogen) with ozone can sufficiently modify the low temperature kinetic activity of PRF50 droplets to promote CF droplet burning, even for sub-millimeter droplet diameters (with no hot flame transition). The implications are that with proper experimental configurations, CF droplet burning might be studied in ground-based, sub-millimeter diameter, isolated droplet burning experiments. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.