Modelling superheated fuel sprays and vaproization

Abstract

One of the major challenges to the gasoline direct injection (GDI) engine design is to understand the behaviour of fuel atomization and vaporization at a variety of chamber conditions and injection strategies. As gasoline is a volatile fuel, it is often unintentionally superheated before injection in GDI engines. This can cause significant changes in the fuel spray distribution and fuel-air mixing. To account for these situations, this paper presents a comprehensive superheat fuel spray and vaporization model. In the model, it is assumed that, under superheat conditions, a hollow cone spray sheet still forms from a pressure-swirl atomizer. The sheet flash boiling is considered to be constrained by the transient heat conduction process inside the sheet with an effective thermal conductivity. Hydrodynamic instability, cavitation and bubble growth inside the sheet eventually break up the sheet to form droplets. Models of the subsequent droplet vaporization account for heat transfer under both flash boiling and sub-boiling conditions. Simulation results have been compared with Mie scattering images of the spray in an optical engine and good agreement is found at all the operating conditions considered. Finally, changes in the spray characteristics, evolution and vaporization behaviour under superheat conditions are explored with GDI applications in mind.