Numerical and experimental investigation of early stage diesel sprays

Abstract

Experimental and numerical investigations of primary atomization in a high-pressure diesel jet are presented. Information on flow processes and structures inside and near nozzle exit are described at early and quasi-steady stages of injection. The numerical method is based on the Volume Of Fluid (VOF) phase-fraction interface capturing technique, in an Eulerian framework. The influence of grid resolution, convection interpolation scheme and temporal integration scheme on the modelling of jet physics are investigated. The present flow setup includes in-nozzle disturbances with the no-slip condition at the walls. All experimental operating conditions are replicated in the numerical models. The early stage liquid jet leading edge demonstrates an umbrella-shaped structure in the numerical results which is in qualitative agreement with experimental imaging. Data obtained provide insight into the flow behavior in the dense region including commencement of fragmentation and early spray angle formation. Experimental images show a cloud of air-fuel mixture at the early stage of injection. The existence of ingested air inside the injector after needle closure could be the source of the observed deviation between experimental and numerical results. The results show that the jet break-up rate and liquid core length increase in cases with higher grid resolutions. The early spray angle from the numerical results at the quasi-steady stage, shows good agreement with experimental data.

Zooming into primary breakup mechanisms of high-pressure automotive sprays

“…Once inertia is too small to propel further fuel into the vessel, air inside the nozzle is trapped until start of a consecutive injection. Similar observations where are observed by Blessing [20] and Ghiji et al [27]. Observed bubbles inside liquid structures must consist either of i) fuel vapor or ii) ambient gas.…”

“…As soon as air bubbles leave the nozzle hole, pressure drops significantly and bubbles abruptly decompress and disturb the jet. This mechanism was analysed in detail by Ghiji et al [27]. However, this latter mechanism has not been shown in this work.…”

Development of a CFD Solver for Primary Diesel Jet Atomization in FOAM-Extend

“…ii) The simulated conditions; iii) The insufficient grid resolution of 2 µm near the interface (as similarly observed by Ghiji et al [5] or; iv) Too short simulation that did not achieve quasi steady-state conditions. All of these factors are currently under investigation and we hope to find the cause of the discrepancy and publish it in future publications.…”

“…A similar shape has been observed both numerically and experi- Figure 10: Side view of the whole computational grid. mentally by Ghiji et al [5]. The perspective view of the primary atomization processes is presented in Figure 14.…”

“…The computational grid had approximately 17 million elements, providing quite rich flow details. Another set of high-fidelity numerical simulations has been performed by Ghiji et al [5] in their combined numerical and experimental analysis of early stage diesel sprays. They have shown that the first break-up pattern obtained with their VOF code qualitatively matches the one observed in the experiments.…”