On The Evaporation And Spray Characteristics Of Liquid Hydrocarbon Fuels In Supercritical Environments

Spray combustion of liquid fuel is one of the most widely used combustion modes in modern internal combustion engines.In gasoline direct injection(GDI)engines and diesel engines,fuel-air mixing is accomplished by jet break-up and evaporation.With the increase of in-cylinder pressure,the injected fuel may find itself above its critical point,and as a result,the transition of the jet from a conventional two-phase spray to a supercritical jet become an extensive phenomenon.In this dissertation,key issues of the mechanism of supercritical transition and the evolution of the liquid-vapor interface,evaporation characteristics of fuel droplet under supercritical environment,and characteristics of trans-/super-critical jets are systematically studied through a combination of Molecular Dynamics(MD)simulations and optical diagnostics.Using the open source MD code LAMMPS(Large-scale Atomic/Molecular Massively Parallel Simulator),the simulation system of a one-dimensional liquid hydrocarbon fuel film evaporating into nitrogen ambient is built.Evaporation rate and lifetime of the liquid film are obtained.Thermodynamic state of the film is analyzed and transition of the fluid to supercritical state is determined.The temporal evolution of the characteristics of the liquid-vapor interface during the supercritical transition is analyzed.It is found that lifetime of the film first decreases and then increases with increasing ambient pressure under low ambient temperatures,and decreases monotonically with ambient pressure under high ambient temperatures.The expanding of the liquid-vapor interface makes itself enter the continuum-fluid regime,and surface tension force vanishes during the supercritical transition of the liquid film.Then,hydrocarbon fuel droplet evaporation systems are built,and the evaporation characteristics of fuel droplet under supercritical environment are analyzed using MD simulations.The results indicate that the evaporation process of a hydrocarbon fuel droplet can be divided into an initial heat-up period and a quasi-steady evaporation period.With the increase of ambient pressure and/or ambient temperature,droplet initial heat-up time and lifetime decrease.Droplet evaporation rate constant,however,increases first and then decreases with increasing ambient pressure.Droplet supercritical transition time decreases with the increase of ambient pressure and/or ambient temperature.With the increase of droplet size,supercritical evaporation regime expands to lower ambient pressure.The comparison of a single-component diesel surrogate fuel droplet and a three-component diesel surrogate fuel droplet indicates that the three-component fuel droplet has a shorter lifetime and heat-up time.Finally,a high-pressure constant volume chamber test bench is designed and built.Optical diagnostics like high-speed microscopy and ballistic imaging are carried out to visualize the near-field of hydrocarbon fuel jets under various ambient conditions.It is found that,with the increase of ambient pressure,detachment of ligaments from the jet liquid core is inhibited.Droplet formed at the end of injection is found to transition from a subcritical evaporation phenomenon to a diffusion dominated supercritical mixing phenomenon,and finally disappear completely with the increase of ambient pressure.The observations of experiments are combined with the predictions of MD simulations,to help build a regime diagram which depicts the influence of ambient condition on jet characteristics,and the jet characteristics in the sub-,trans-and supercritical regimes are summarized.