Simulations of kerosene droplet combustion in vitiated air

Abstract

Numerical simulations of isolated kerosene droplet combustion in an environment of air diluted with hot combustion products, for a wide range of such dilution, have been performed with the aim of exploring fundamental aspects of the possible behaviour of liquid fuel sprays in gas turbine combustors. Kerosene was modelled as a mixture of three pure components for which detailed chemistry is available. Autoignition transient has been analysed in both physical and mixture fraction space for different values of the droplet initial diameter and various hot product-air mixtures. Then, the flame structure long time after autoignition has been compared with the flame structure of a gaseous counterflow flame in the same conditions of pressure, temperature and composition of the oxidiser. Results show that droplet autoignition is strongly influenced by the level of dilution of the oxidiser and initial droplet diameter. The increase of the amount of hot products in the vitiated air reduces the ignition time delay. Comparisons with the standard gaseous flame show a different behaviour of the two flames in the pyrolysis region possibly due to the different shape of the scalar dissipation rate in the mixture fraction space, which in the droplet case differs from the bell-like shape of standard counterflow flames, and to the conditions at the fuel side boundary which in the case of droplet combustion are related to the evaporation process. The results offer insight for turbulent combustion models and for the structure of moderate to intense low-oxygen dilution (MILD) combustion.