Dynamics of ignition transience and gasification partition of a droplet

Abstract

Thermochemical evolution of a droplet suddenly exposed to a hot environment is studied to assess the transient characteristics of the ignition, flame bifurcation and scavenging combustion, transition of premixed flame to nonpremixed combustion, and the ultimate burnout of an isolated droplet. Canonical theory of droplet gasification gives a general criteria of ignition and serves to identify all the gasification submechanisms of an arbitrary geometry in a stationary or convective environment. The theory is used in conjunction with numerical analysis for prediction of the transient flow-field structures and the gasification rates of all the submechanisms of gasification. The results reveal that the ignition transience exhibits flame bifurcation in a broad range of the environmental temperature, which lies between 930 K and 1700 K, for an n-heptane droplet with the simulated reaction rate model. At temperature higher than 1700 K, flame splitting does not occur. There are, in general, seven gasification submechanisms for a droplet: however, the net gasification rate during the ignition, flame bifurcation, and scavenging combustion is primarily controlled by the exothermic reaction and thermal energy accumulation, each of which has the effective gasification rate of nearly 10 to 40 times of that of the conventional Godsave-Spalding gasification rate: whereas the other submechanisms contribute at nearly the same order of magnitude of the Godsave-Spalding gasification rate. The droplet combustion is also classified into fully evolved and partially evolved combustion depending on the state of the droplet at the burnout. The predicted ignition delay time for n-heptane droplets in the size range of 600–2200μm are in good qualitative agreement with available experimental data. Areas of future research are also discussed.