Abstract
Flash boiling occurs when a superheated liquid fuel is injected into a sub-atmospheric pressure environment. This phenomenon is often seen in a gasoline direct injection (GDI) engine under part-load conditions. The injector tip is exposed to the combustion environment, increasing the fuel temperature inside the injector up to 80–90° C. Under these flash boiling conditions, the injected liquid fuel will undergo rapid vaporization, affecting the spray structure. The flash boiling spray helps achieve better atomization of the fuel spray. However, in some conditions, due to flashing, a dense vapor cloud will form in the in-cylinder and disturb the air-fuel mixture, and lead to unwanted pollutants. Several numerical models have been developed for the flash boiling sprays. However, most of these models are hard to implement in a multi-cycle GDI engine combustion simulation. The existing flashing-based evaporation models in the CONVERGE code are not dimensionally consistent. In that regard, a phenomenological model of flash boiling spray is developed using the relevant non-dimensional numbers to help GDI engine combustion modeling under such conditions and address the issue of dimensional inconsistency in the source term formulations.
A multi-hole GDI injector from the engine combustion network (ECN) called Spray G is considered for this work. The model is developed using the CONVERGE v2.4 user-defined functions (UDFs). The blob injection method is considered for this work, and the simulations are carried out using the Eulerian-Lagrangian framework. An Unsteady Reynolds Averaged Navier-Stokes (URANS) RNG k-ε turbulence model is considered in this study. The KH-RT breakup length model is adopted for modeling spray breakup. The developed model has achieved a reasonable agreement with the experimental spray images. Comparisons of the predictions are made with the data available through ECN, such as spray penetrations and spray patternations.
