Numerical modelling of flash-boiling fuel sprays

Abstract

Flash-boiling of fuel sprays can occur when a liquid is discharged into an ambient environment with pressure lower than the fuels saturation pressure. The fuel enters a metastable superheated state which promotes atomisation and evaporation due to rapid phase change. The flash-boiling mechanism typically refers to high temperature thermally driven phase-change in the form of superheat, as opposed to mechanically driven low pressure phase-change associated with cavitation. Flash-boiling can exist in direct-injection spark-ignition engines operating at low in-cylinder pressures and/or high fuel temperatures. The addition of even small amounts of novel high volatile additives/fuels may also promote the onset of flash-boiling. Within multi-hole injectors flash-boiling can significantly vary spray formation, causing individual plumes to interact and collapse which can be both advantageous and detrimental to engine operation. The modelling of flash-boiling fuel sprays using a Lagrangian Particle Tracking framework within the CFD code of STAR-CD in the context of direct-injection spark-ignition engines was investigated in the current work. This included parametric studies into important aspects of flash-boiling as well as the development of flash-boiling sub-models which introduced thermal mechanisms such as in-nozzle phase-change, near-nozzle plume expansion through droplet shattering and increased evaporation rates. The parametric study highlighted the importance of increased atomisation and evaporation rates within flash-boiling sprays. This led to the development of two flash-boiling models which were applied to a wide range of operating conditions, single-component fuels and injector configurations. The models included a flash-boiling spray effective nozzle model and a flash-boiling breakup model which attempted to quantify both in-nozzle phase change effects and near-nozzle thermal atomisation, respectively. Both modelling techniques showed promise individually but also as a combination of in-nozzle and near-nozzle mechanisms. Important flash-boiling spray characteristics including complex plume interactions, spray collapse, droplet recirculation and effects on penetration lengths were captured and where possible quantitative and qualitative comparisons were made to experimental data. The developed flash-boiling capability was used to investigate spray and mixture formation within a direct-injection spark-ignition engine and the potential improvements in mixture homogeneity were confirmed.