Droplet Combustion in a Reactive Environment

Abstract

The transient processes of evaporation, ignition, and combustion of a fuel droplet in a hot, reactive fuel-oxidizer environment simulating the spray interior are studied numerically, with emphasis on the relative importance of diffusional burning in the droplet vicinity and premixed burning in the ambience. It is shown that for a fuel-lean environment diffusional burning is generally possible and is initiated rapidly, whereas for a fuel-rich environment only premixed burning is possible. For a sufficiently reactive environment one of the reactants is rapidly depleted such that during most of the droplet lifetime the environment is really nonreactive. The present numerical study yields much physical insight useful for further analytical modeling efforts. A new transient process, involving the need to build up the fuel vapor concentration in the droplet vicinity from an initial state of low concentration, has also been identified to be important during droplet combustion and warrants further study.

Numerical Study on Combustion of a Liquid Fuel Droplet

“…T, (11) where T, is the normal boiling point evaluated at the normal atmospheric pressure, P" and PI is the prevailing total pressure. The fuel mass fraction at the interface can be determined by (12) The oxygen mass fraction Yo, and normal velocity~,n at the interface are obtained by the conservation of species: (13) …”

“…Botros et al [11] showed that for a fuel-lean environment, diffusional burning is generally possible and is initiated rapidly, whereas for a fuel-rich environment, only premixed burning is possible. Dwyer and Sanders [12] Downloaded by [ Rah et al [13] observed experimentally that ignition of a droplet is closely related to the formation of soot and NO z ' As the ignition delay time is large, the formation of soot is restricted.…”

Droplet Autoignition in a Reactive Oxidant/Fuel-Vapor/Inert Environment

“…However, experimental observations and theoretical studies 2,5,14 show that ignition occurs only a few droplet diameters away from the droplet surface. Further, the fuel vapor is almost always present in the environment prior to droplet ignition in practical sprays.…”

“…Therefore, prediction of ignition event is necessary for the simulation of spray combustion in practical energy conversion systems. Although many experimental measurements and numerical simulations of the single droplet ignition process have been carried out, [1][2][3][4][5] the ignition processes of droplets in various reactive environments remain far from well understood. Even with recent advances in computational facilities, 6 the detailed numerical simulation of individual droplet vaporization and ignition in practical spray combustion modeling is beyond the current computational capability because of the large number of droplets present in such sprays.…”

“…Such a simple criterion is very useful as a submodel for realistic spray combustion modeling, 7 and is the objective of the present study. Therefore, an analytical approach will be employed instead of detailed numerical simulation, such as the work of Botros et al 2 and Samuel and Karim. 5 The quasisteady basic droplet model, sometimes known as the d 2 -law, has been studied both experimentally and theoretically.…”