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, the attention is being drawn to DISI engine which has better fuel economy than conventional gasoline engine. However, it has been problem that rich air-fuel mixture which is oriented from fuel film during cold start due to spray impingement make particulate matter (PM). Because simulating air-fuel mixture field precisely is base information to predict soot formation, it can be seen that accurate spray model and film model are prerequisite. The existing models show diesel spray behavior with good accuracy. On the other hand, they tend to over-estimate DISI spray rebound radius and under-estimate the area of the fuel film. The disagreement between the simulation results and the experimental results might be based on assumptions used in the previous models. The previous models were well matched with the low speed collision condition such as diesel engine which has relatively short penetration length due to its injection pressure. The energy can be successfully reduced by Weber number and surface tension in previous model. However, droplet kinetic energy in DISI engine is much larger than dissipation energy which is calculated by Weber number and surface tension. Thus, in modified model, the amount of dissipation energy is determined under realistic range. To consider 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 modified model. The Mie-scattering images of iso-octane spray near wall were acquired at various temperature and injection pressure to measure rebound spray radius and height. Then, Laser Induced Fluorescence(LIF) methods with TIR was used to capture film thickness and shape. Compared to the existing models, the modified model shows the best agreement with the experimental results without case-dependent changes to the model constant.
Junghyun Kim, Jongwon Chung, Woojae Kim, Kyoungdoug Min