Reacting fuel droplet number density measurements in a lean premixed prevaporized combustor

Lean prevaporized premixed (LPP) combustion is a promising technology for reducing aeroengine emissions. While injectors for these systems are designed to vaporize fuels before combustion, droplets can still be present at different operating conditions. In order to design better mixers and injection systems, droplet size distributions and number densities in the reacting flow must be fully characterized. Limited optical access, however, can prevent the application of traditional phase Doppler anemometry techniques for droplet sizing. Many imaging methods – like inline holography, off-axis holography, LED backlight imaging, and fluorescent dye cell methods – are also unable to obtain clear droplet images due to distortions from density gradients in the reacting high pressure flame. To overcome these limitations, we present the first imaging measurements of fuel droplet size distributions and number densities in a reacting LPP combustion system. To eliminate distortions, a spoiled cavity laser backlight imaging method is employed to directly capture droplets as small as 9.4 μm from a standoff distance of 400 mm. A unique edge gradient image processing technique is then developed and calibrated to automatically identify particles within the depth-of-focus to estimate droplet number densities. Results show similar droplet size distributions for different fuel air ratios (FAR) and injection locations. However, higher droplet number densities are observed for higher FAR and for fuel injection locations with less vaporization. By using the techniques developed in this work, reacting fuel sprays from various injector, mixer, and combustor designs can be studied in higher detail and new concepts can be developed for future aircraft propulsion applications. Novelty and significance statement: This work demonstrates novel distortion-free imaging measurements of fuel droplets in a reacting high-pressure combustion environment. In these systems, existing methods like holography and LED backlighting are often plagued with flame distortions that obscure droplets. To overcome these challenges, this work performs backlight imaging using a low coherence nanosecond pulsed spoiled cavity laser, which reduces motion blur and interference pattern distortions. This work also pioneers new methods to estimate and compare fuel droplet number densities in a reacting environment. This novel technique enables the generation of more accurate fuel spray models and improved injector and combustor designs for high efficiency and low emission operation.