Transient Internal Nozzle Flow in Transparent Multi-Hole Diesel Injector 2020-01-0830

An accurate prediction of internal nozzle flow in fuel injector offers the potential to improve predictions of spray computational fluid dynamics (CFD) in an engine, providing a coupled internal-external calculation or by defining better rate of injection (ROI) profile and spray angle information for Lagrangian parcel computations. Previous research has addressed experiments and computations in transparent nozzles, but less is known about realistic multi-hole diesel injectors compared to single axial-hole fuel injectors. In this study, the transient injector opening and closing is characterized using a transparent multi-hole diesel injector, and compared to that of a single axial hole nozzle (ECN Spray D shape). A real-size five-hole acrylic transparent nozzle was mounted in a high-pressure, constant-flow chamber. Internal nozzle phenomena such as cavitation and gas exchange were visualized by high-speed long-distance microscopy. Through optical observation, we find that the initial sac condition is mostly occupied by gas, and the gas remains relatively long after the start of injection, even longer than the case from a single axial hole, thereby affecting the ramp-up in ROI. Also, pronounced cavitation occurs at the hole inlet for the multi-hole nozzle, unlike the single axial hole. Using different initial levels and positions of gas in the sac, based upon experiments, CFD simulations were performed to predict ROI and internal cavitation. While the experiment showed continuous cavitation during injection, the RANS simulations showed cavitation only at lower needle lift. In addition, less bulk cavitation and gas ingestion into the nozzle are predicted at the end of injection compared to experiment, which affects the sac state for the next injection.