Numerical Study on Wall Impingement and Film Formation in Direct-Injection Spark-Ignition Condition 2020-01-1160
Since the amount of emitted CO2 is directly related to car fuel economy, attention is being drawn to DISI (Direct-Injection Spark-Ignition) engines, which have better fuel economy than conventional gasoline engines. However, it has been a problem that the rich air-fuel mixtures associated with fuel films during cold starts due to spray impingement produce particulate matter (PM). In predicting soot formation, it is important to predict the mixture field precisely. Thus, accurate spray and film models are a prerequisite of the soot model. The previous models were well matched with low-speed collision conditions, such as those of diesel engines, which have a relatively high ambient pressure and long traveling distances. Droplets colliding at low velocities have an order of magnitude of kinetic energy similar to that of the sum of the surface tension energy and the critical energy at which the splash occurs. Therefore, the kinetic energy of parent parcels can be successfully reduced by applying an empirical formula consisting of the Weber number and surface tension as applied in an existing model. However, the kinetic energy in DISI engines is much larger than the dissipation energy, which is calculated by the Weber number and surface tension. Thus, in this paper, the amount of dissipation energy is determined under a realistic range. To consider the 2-D spray-wall impingement phenomenon more accurately, 2-D child droplet generation was considered. Finally, both film and spray behavior were measured to validate the Seoul National University (SNU) model. The Mie-scattering images of the gasoline spray near the wall were acquired to measure the rebound spray radius. Then, laser-induced fluorescence (LIF) methods with total internal reflection (TIR) were used to capture the film thickness and shape. Compared to the existing models, the SNU model shows the better agreement with the experimental results without case-dependent changes to the model constant.