Search for: All records

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. 

As fuel injection systems advance towards higher injection pressures and the combustor environment increases in both temperature and pressure in the pursuit of improved emissions and efficiency, advanced combustion strategies are required. Injecting fuel as a supercritical fluid has the potential to improve fuel/air mixing and eliminate steps in the spray vaporization process. Experiments are carried out on a heated fuel injector in an open-air test cell using Mie scattering, Schlieren imaging, and long-distance microscopy to investigate changes in spray characteristics with varying temperature and pressure. Spray angle, spray penetration length, and vapor-liquid ratio data are collected and evaluated. Near-nozzle imaging shows distinct changes in spray morphology during the initial microseconds of spray formation. Sprays injected under conditions further into the supercritical regime exhibit increased spray angle and vapor-to-liquid ratio. Spray penetration is found to decrease with increasing temperature. A jump in vapor-to-liquid ratio is observed in the vicinity of 568 K, indicating a transition in spray behaviour trending towards more rapid fuel/air mixing across the transcritical region. Changes in the micro-scale structure of the spray during the initial microseconds of spray formation exhibit this same narrow transition region. A significantly greater fraction of the spray plume is observed to be in a vapor or vapor-like state at a given time after injection initiation as the injection conditions are advanced into the supercritical state. These findings indicate that injection fuel as a supercritical fluid has the potential to improve the mixing of a fuel/air charge, and thus, improve combustion quality.