An Overview of Spray Modeling

Abstract

T role of sprays in combustion systems is to break up a liquid fuel so that there is more surface area for evaporation to provide gaseous fuel for combustion. Some spray properties that are important for the design of combustion systems are the evaporation rate of the fuel, mixing of the fuel by entrainment and diffusion, and local cooling processes. To be useful tools for understanding and predicting the behavior of sprays, numerical models must be capable of providing this type of information, given the basic properties of the incoming liquid jet and the background gas. The physical development of a spray occurs in three stages corresponding to three spatial regimes. First, given a liquid jet, the process of atomization breaks up the jet and forms a dense cluster of droplets. In this second dense droplet region, droplets interact with other droplets, leading to coalescence and further droplet breakup. The spray then diffuses and evaporates, forming a third dilute droplet region in which droplet-droplet contact is minimal and the motion of droplets and changes in their properties are governed by the local gas flowfield. The physical mechanisms leading to atomization depend on the type of atomizer used. Pressure atomizers convert the energy associated with pressure to kinetic energy of thin liquid sheets. These sheets become unstable due to frictional surface tension and fluid dynamic effects, and disintegrate into droplets. The most common type of pressure atomizer is the centrifugal pressure nozzle in which a swirling component of velocity is imparted to the liquid forming a conical spray. Two-fluid (or pneumatic) atomizers subject the liquid to a high-velocity gas, which breaks up the liquid into droplets. In general, the breakup occurs in two stages. First, the high-velocity gas stream fragments the liquid into filaments and large droplets, and then the filaments and large droplets are broken into smaller droplets. The process is very complex. The resulting droplet size is influenced by liquid properties and the velocity and density of the gas. The droplets emerge from the atomizer as a dilute spray.