Stereoscopic high-speed microscopy to understand transient internal flow processes in high-pressure nozzles

• Published in: Experimental thermal and fluid science 2020-06, Vol.114, p.110027, Article 110027
• Main Authors: Manin, J., Pickett, L.M., Yasutomi, K.
• Format: Article
• Language: English
• Subjects: Cavitation; Combustion; Deformation analysis; Diameters; Diesel; Diesel engines; Diesel injection; Discharge coefficient; Elastic analysis; Elastic deformation; ENGINEERING; Fuel injection; Gas exchange; High speed; Ingestion; Injection; Injectors; Internal combustion engines; Internal flow; Microscopy; Nozzles; Orifices; Outlet flow; Stereoscopy; Transparent nozzles
• ISSN: 0894-1777; 1879-2286
• DOI: 10.1016/j.expthermflusci.2019.110027

•Stereoscopic high-speed microscopy was applied to investigate the internal flow details of high-pressure injections.•Detailed analysis of the geometry showed good agreement with target metal ECN Spray D injector.•The results supported needle lift profiles measured via x-ray for Spray A and showed that sac pressure transients were essentially shorter than 100 ms.•Time-resolved visualizations revealed a fluid hammer-induced bulk cavitation (gasification) at the end of injection.•The gas present in the sac affects the following injection, with chamber gas being mixed with the liquid fuel for a substantial amount of time. The flow and cavitation behavior inside fuel injectors is known to affect spray development, mixing and combustion characteristics. While diesel fuel injectors with converging and hydro-eroded holes are generally known to limit cavitation and feature higher discharge coefficients during the steady period of injection, less is known about the flow during transient periods corresponding to needle opening and closing. Multiple injection strategies involve short injections, multiplying these aspects and giving them a growing importance as part of the fuel delivery process. In this study, single-hole transparent nozzles were manufactured with the same hole inlet radius and diameter as the Engine Combustion Network Spray D nozzle, mounted to a modified version of a common-rail Spray A injector body and needle. Needle opening and closing periods were visualized with stereoscopic high-speed microscopy at injection pressures relevant to modern diesel engines. Time-resolved sac pressure was extracted via elastic deformation analysis of the transparent nozzles. Sources of cavitation were observed and tracked, enabling the identification of a gas exchange process after the end of injection with ingestion of chamber gas into the sac and orifice. We observed that the gas exchange contributed widely to disrupting the start of injection and outlet flow during the subsequent injection event.