High-speed quantitative droplet sizing in transient marine sprays using SLIPI-polarization ratio imaging

This study presents the first time-resolved, two-dimensional droplet sizing measurements in a single-hole marine fuel spray, thanks to the previously developed SLIPI-Polarization Ratio technique. Experiments were conducted in a large optical spray and combustion chamber representative of marine-relevant spray conditions, using n-heptane as a diesel surrogate. Five operating points were investigated to isolate the effects of injection duration, injection pressure, ambient pressure, and liquid temperature on droplet size evolution. A numerical calibration based on Lorenz-Mie theory was implemented to convert polarization ratios mapping into droplet size distributions. The results reveal average droplet diameters of approximately 3.5 μm during the injection phase for most operating conditions, while increasing the ambient pressure from 15 to 35 bar (OP3 to OP4) produces significantly larger droplets, with average D 21 values exceeding 6 μm. Increasing injection pressure enhances spray penetration, reaching approximately 150 mm, and promotes the formation of a dense liquid core containing droplets with diameters up to 15 μm. After the end of injection, average droplet diameters decrease slightly under cold conditions, while elevated fuel temperature accelerates evaporation and leads to a faster reduction in droplet size. The overall uncertainty of the droplet-sizing measurements was estimated to be below 10.9%.