CHAPTER 4 - THE COMBUSTION OF DROPLETS OF LIQUID FUELS

Abstract

Publisher Summary This chapter discusses the combustion of droplets of liquid fuels. In the combustion of a liquid fuel in an oxidizing atmosphere such as air, the liquid droplet evaporates and acts as a source of vapor. Because the fuel vapor and air are initially separated, they burn in the form of a diffusion flame. Three principal techniques have been used to investigate experimentally the rate of combustion of single droplets, these are: (i) the captive or suspended drop method, (ii) the supporting sphere technique, and (iii) the free drop technique. The captive drop technique is used to obtain the rate of change of droplet diameter or the size as a function of time. Typically, a single droplet or array of droplets is suspended from a silica fiber, ignited and the rate of combustion observed by a video camera or photographically. Theoretical models of droplet combustion give insight into the dependence of the rate of combustion on the various chemical and physical processes that occur. All commercial fuels consist of mixtures of hydrocarbons which may have widely differing boiling points. Their behavior when burning may thus be markedly different from single component volatile liquid droplets in a number of respects. Selective distillation of the components may occur and if the lighter fractions are very volatile then these may flash evaporate forming bubbles within the droplet which disintegrate the droplet. This is termed as disruptive evaporation. The multicomponent combustion of mixtures containing high boiling point components may result in droplet temperatures that are so high that thermal decomposition occurs resulting in the formation of coke-like residues which are termed cenospheres.