Numerical Modelling of Ethanol Direct Injection (EDI) Sprays of a Multi-Hole Injector under Non-Evaporating, Transition and Flash-Boiling Conditions

摘要

Ethanol direct injection (EDI) has great potential in facilitating the downsizing technologies in spark ignition engines due to its strong anti-knock ability. The fuel temperature may vary widely from non-evaporating to flash-boiling sprays in real engine conditions. In this study, a CFD spray model was developed in the ANSYS Fluent environment, which was capable to simulate the EDI spray and evaporation characteristics under non-evaporating, transition and flash-boiling conditions. The turbulence was modelled by the realizable k-ε model. The Rinzic heterogeneous nucleation model was applied to simulate the primary breakup droplet size at the nozzle exit. The secondary breakup process was modelled by the Taylor Analogy Breakup model. The evaporation process was modelled by the Convection/Diffusion Controlled Model. The droplet distortion and drag, collision and droplet-wall interaction were also included. The spray model was verified against the spray experimental results in a constant volume chamber. The developed spray model well simulated the EDI spray evolution and evaporation processes under non-evaporating, transition and flash-boiling conditions. The simulation results showed that the non-evaporating spray's characteristics were similar to those of the normal-evaporating spray in terms of spray structure and spray tip penetration. The spray plumes converged towards the middle one with the increase of fuel temperature and finally collapsed completely when the spray superheat degree was higher than 9 K. This was caused by the increased ambient air speed and stronger vortices entrained by the spray jet, and additionally by the significantly reduced droplet size. The EDI spray could be considered as non-evaporating when the fuel temperature was lower than 325 K at 1 bar. The evaporation rate increased slightly with the fuel temperature increased from 275 to 360 K, but significantly from 360 to 400 K. Although it reduced the cooling potential, the cooling effect of EDI was in fact enhanced by fuel heating.