POLARIZATION RATIO WITH STRUCTURED LASER ILLUMINATION PLANAR IMAGING FOR DROPLET SIZING IN FLASH-BOILING AMMONIA SPRAY

Liquid-phase ammonia, a carbon-free alternative fuel, is increasingly being considered for improving overall efficiency, reducing system complexity, and lowering costs for various thermal applications. However, the physical properties of ammonia differ significantly from carbon-based fuels, as ammonia is prone to flash boiling upon injection. Therefore, experimental characterization of flash-boiling liquid ammonia sprays, specifically the spatial distribution of droplet sizes, is essential to improve the understanding of the evaporation process and its mixing with ambient air to optimize it for applications. Under flash-boiling conditions, the rapid breakup of the liquid ammonia spray produces a highly optically dense droplet cloud. In addition, the strong interference from the large amount of generated vapor makes droplet sizing even more challenging. Hence, flash-boiling sprays severely suffer from multiple scattering, limiting the use of investigatory optical techniques. Structured laser illumination planar imaging (SLIPI), however, offers an effective and widely accepted technique developed to suppress such parasitic optical phenomena. Upon combination with a novel polarization ratio technique, droplet sizing can be performed for such highly scattering flash-boiling events. This manuscript presents a first attempt to measure droplet sizes, D21, across a flash-boiling ammonia spray injected into an optical vessel using a Spray G multi-hole injector. While the conventional planar polarization Mie scattering imaging is highly affected by the detection of multiple light scattering, studies show that the use of 2-phase SLIPI allows for an efficient suppression of these unwanted photons to obtain. To the best of the authors' knowledge, it is the first reliable measurement of droplet sizes for flash-boiling ammonia spray under such challenging conditions.