Combustion of droplets in sprays

Abstract

The factors affecting the combustion of single droplets of various fuels have been fairly well elucidated in the past. However, there is a considerable lack of knowledge of burning rates of droplets in sprays. Knowledge is required to fill the gap between single droplet combustion and practical spray combustion in complex aerodynamic environments. This paper sets out those variables considered to be significant to single droplet combustion and compares their effect in spray systems. These variables include the effects of oxygen concentration, ambient temperature, droplet spacing, and forced convection. In order to appreciate the role of single droplet theories in spray systems it is necessary to have knowledge of the frequency distributions of sizes, angles and velocities, and their interrelations. A description of these parameters is given for a number of sprays which have been investigated experimentally and variables which produce significant changes in these parameters are highlighted. The influence of the aerodynamic environment of the spray on the trajectories and velocities of drops and the difference between burning and cold sprays are illustrated. The effects of air fuel ratio and temperature distributions within the system are then superimposed. Models of spray combustion are discussed and compared with gaseous diffusion flames.