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Recent trends in decarbonizing efforts have brought ammonia to the forefront of research as a fuel for energy and transportation. But several previous studies have strongly suggested that ammonia has difficulties in ignition and heat release when used in a gaseous form due to its physical properties. On the other hand, working with liquid ammonia presents countless problems like difficulty in pressurizing, damage to elastomers and other metals, etc. In this study, pure liquid ammonia is injected into a CVCC (constant volume combustion chamber) using a hollow cone injector to understand the behavior of liquid ammonia when pressurized. Specifically, varying ambient temperature and pressure conditions encompassing the fuel’s subcritical to the supercritical regimes are studied as liquid ammonia tends to rapidly vaporize and flash-boil at pressures and temperatures above 50 bar and 315 K. High-speed shadowgraph and Schlieren imaging techniques are used to characterize the spray and understand the effects of varying conditions. Based on the formation of the central plume due to collapsing spray, many measurements like the plume ratio and penetration are studied to indicate the fuel's transition into the transcritical regime. A measurement of the flash-boiling spray plume ratios along with the spray penetration data give us a correlation of the environmental conditions to the spray transitioning into the supercritical regime. Interestingly, increasing the injection pressure from 75 bar to 150 bar shows 3 distinct regimes forming in the central spray plume penetration and the sprat plume ratio. This study has novel contribution to the development of direct injection of liquid ammonia spray for applications in high-power density engine systems. 

Understandings on soot formation and evolution in pressurized flames are of significant interest due to the increasing operating pressures in different combustors and the accompanying increased soot emissions. In this study, a series of pressurized turbulent sooting flames at 1 bar, 3 bar and 5 bar, are simulated to study the pressure effect on the soot formation and evolution. The inflow conditions are chosen such that the Reynolds number at different pressures keeps constant. Via a Radiation Flamelet Progress Variable (RFPV) approach with a conditional soot sub-filter Probability Density Function (PDF) to consider the turbulence-chemistry-soot interaction, quantitatively good agreements (e.g., maximum discrepancy within one order of magnitude) are achieved for soot volume fraction predictions compared with the experimental data at different pressures. Soot volume fraction source terms are then discussed to show the pressure effect on nucleaion, condensation, surface growth and oxidation at different axial positions in these flames.