Velocity measurements in the near field of a diesel fuel injector by ultrafast imagery

TL;DR

This study uses ultrafast imaging to measure velocity profiles in diesel fuel sprays, revealing asymmetries that impact atomization and spray morphology, with implications for injector design and spray modeling.

Contribution

It introduces a novel ultrafast shadow imaging technique for detailed velocity mapping in diesel sprays, highlighting asymmetric flow behaviors in real injectors.

Findings

Significant velocity asymmetry (~100 m/s) across the spray.

Asymmetries influence droplet dispersion and macrostructure.

Flow structures resemble cavitation effects observed in scaled tests.

Abstract

This paper examines the velocity profile of fuel issuing from a high-pressure single-orifice diesel injector. Velocities of liquid structures were determined from time-resolved ultrafast shadow images, formed by an amplified two-pulse laser source coupled to a double-frame camera. A statistical analysis of the data over many injection events was undertaken to map velocities related to spray formation near the nozzle outlet as a function of time after start of injection. These results reveal a strong asymmetry in the liquid profile of the test injector, with distinct fast and slow regions on opposite sides of the orifice. Differences of ~100 m/s can be observed between the 'fast' and 'slow' sides of the jet, resulting in different atomization conditions across the spray. On average, droplets are dispersed at a greater distance from the nozzle on the 'fast' side of the flow, and distinct macrostructure can be observed under the asymmetric velocity conditions. The changes in structural velocity and atomization behavior resemble flow structures which are often observed in the presence of string cavitation produced under controlled conditions in scaled, transparent test nozzles. These observations suggest that widely used common-rail supply configurations and modern injectors can potentially generate asymmetric interior flows which strongly influence diesel spray morphology. The velocimetry measurements presented in this work represent an effective and relatively straightforward approach to identify deviant flow behavior in real diesel sprays, providing new spatially resolved information on fluid structure and flow characteristics within the shear layers on the jet periphery.