The effects of multicomponent fuel droplet evaporation on the kinetics of strained opposed-flow diffusion flames

With the increasing use of alternative fuels, it becomes important to understand the impacts of their different chemical and physical properties on combustion processes. The objective of this paper is to explore the impact of the vaporization of a multicomponent liquid fuel on the combustion kinetics using an opposed-flow diffusion flame model. The model fuel consisted of a n-heptane, n-dodecane, and n-hexadecane mixture, selected to represent a Fischer-Tropsch fuel. A computational model is developed to describe the multicomponent vaporization process. Gas-phase chemical kinetics is modeled using a reduced mechanism containing 196 species. Results compare pre-vaporized fuel streams with those containing monodispersed initial droplet sizes of 20, 25 and 30 μm. The separation distance between the fuel and air inlets is either 5 and 10 mm. In all cases the fuel is carried in nitrogen, the pressure is 10 atm, and the fuel and air inlet velocities are 1 m s−1. The fuel loading is set to achieve an overall equivalence ratio of unity. Results show that the finite evaporation rate significantly impacts the chemical kinetics. In particular, if the combination of separation length, stream velocity, and fuel volatility is such that fuel droplets penetrate into the higher temperature region near the flame-front, the rapid increase in evaporation rate significantly enhances the local vapor phase fuel mole fraction. The high temperature increases reaction rates, leading to higher peak temperatures as well as increased pyrolysis in the pre-flame region. For example, the peak temperature predicted for 30 μm droplets is 330 K higher than that for the pre-vaporized case. This increase occurs in spite of an initial decrease in temperature as a consequence of fuel vaporization. A similar effect is observed for the pre-flame pyrolysis products; ethylene, acetylene, and butadiene all increase by about a factor of two for the 30 μm droplet case. The implications of these findings regarding the use of alternative fuels is discussed.