Ballistic and Optical Imaging of Transient Fuel Sprays for Dual-Fuel Combustion

Pilot sprays play a vital role in dual-fuel (DF) combustion applications, where a small quantity of liquid fuel is used to ignite the primary gaseous fuel. The quantity and distribution of the pilot spray have a significant influence on combustion efficiency and the formation of harmful exhaust emissions. This study examines the fuel spray characteristics of heavy-duty marine diesel injectors using optical diagnostics. In dual-fuel operation, these injectors operate in a transient mode due to shortened dwell times and limited needle lifts, which directly impact the spray breakup and atomization processes. The objective is to investigate the behavior of transient sprays under high-pressure and high-temperature ambient conditions. Experiments were conducted in optically accessible spray test rigs that replicate in-cylinder environments. Initially, multi-hole (MH) injectors were investigated using the Mie-scattering technique to analyze plume-to-plume variations under non-evaporative and non-reactive conditions. Afterward, the focus shifted to studies of single-spray plumes; to achieve this, the thimble method was implemented. A thimble is a small metal cap placed on the nozzle tip that isolates a single spray plume from a multi-plume spray, while collecting the remaining plumes and directing them into a drain passage. This method enables the study of individual spray characteristics without interference from adjacent plumes and without altering the nozzle geometry. Single spray experiments were carried out under various conditions, including non-evaporative and non-reactive sprays, evaporative and non-reactive sprays, and reactive sprays. A diffuse back-illumination (DBI) technique was used to visualize the liquid phase, while Schlieren imaging captured evaporative and reactive sprays. Natural luminosity imaging was used to capture both the low-temperature combustion (LTC, or cool-flame) and high-temperature combustion (HTC) regimes in the reacting spray. The low-temperature combustion (LTC) or cool-flame phase, observed at approximately 850 K (±50 K), appeared as a blue flame, indicating gas-phase oxidation with negligible soot formation due to lower flame temperatures and relatively uniform fuel–air mixing. In contrast, the high-temperature combustion (HTC) phase, observed at around 1150 K (±50 K), exhibited a yellow flame resulting from higher local temperatures, locally fuel-rich regions, and notable soot formation, whose thermal radiation dominated the luminosity signal under these conditions. Lastly, near-field atomization behavior was investigated using time-resolved ballistic imaging (BI), which captures spray evolution and breakup phenomena within 10 mm of the nozzle exit. These experiments (BI) were conducted under ambient back-pressure conditions only, with fuel injection pressures systematically varied from 1700 to 2100 bar to study their influence on spray development. Throughout all experiments conducted in this research, the ambient density was varied up to approximately 28 kg/m3, and the injection pressure was varied up to 2100 bar. The results emphasize the combined influence of ambient density, fuel injection pressure, and temperature on the transient development of the spray.