Heat and mass transfer effects in the nozzle of a fuel injector from the start of needle lift to after the end of injection in the presence of fuel dribble and air entrainment

Abstract

The design of fuel injectors is key to achieving high-efficiency engine combustion with low tailpipe emissions. The small dimensions of injector nozzle holes make the manufacturing of real-size optical injectors aimed at fundamental understanding of in-nozzle processes at design stage very challenging, especially for operation under realistic in-cylinder thermodynamic conditions. Therefore, faithful numerical predictions based on complete multiphase flow simulations upstream and downstream of the nozzle exit of a real injector geometry are highly sought after. In this paper, numerical studies of a Diesel injector nozzle with moving needle were performed using transient Reynolds Averaged Navier-Stokes (RANS) modelling with compressibility of all phases accounted for. A Volume of Fluid (VOF) method was employed, coupled to cavitation and evaporation submodels, along with a complete set of pressure and temperature dependent thermophysical fuel properties. The aim was to understand the flow inside the nozzle both during injection and after the end of injection, including fuel dribble and air backfilling effects. A range of fuel injection and air chamber pressures and temperatures were simulated, namely 400 and 900 bar upstream and 1, 35 and 60 bar downstream. Fuel, air and wall temperatures were varied in the range 300 K to 550 K. The results showed that the flow during injection carried hysteresis effects. After the end of injection, the state of the nozzle varied from being filled with a large amount of liquid to being filled mostly with air. Some form of immediate fuel dribble existed in all test cases, whilst late liquid fuel mass expulsion was also predicted under certain conditions. The latter prediction highlighted sensitivity to the models enabled. The use of a transient pressure outlet based on an engine's expansion stroke pressure trace affected the process of late fuel expulsion by pulling fuel out of the nozzle in multiphase form faster. These processes are of particular importance as they can contribute directly to unburned hydrocarbon emissions and/or the formation of deposits inside the holes. Starting a second injection from the resulting state of the nozzle at the end of the original injection resulted in a deformed liquid jet tip without the classic mushroom shape and a temporarily lower liquid jet penetration.