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1. Experimental Characterization of Transcritical Spray with Varying Fuel Temperature and Injection Pressure

   Nonavinakere Vinod, Kaushik; Kempin, Robert; Fang, Tiegang (March 2023, Proceedings of the 13th United States National Combustion Meeting)

   The spray characteristics of fuels when sprayed under superheated and elevated fuel pressure are markedly different than traditional fuel injection sprays. Studying fuel sprays under these conditions will help us understand the complex behaviors that may provide us with information to optimize future applications of certain technologies like supercritical spray combustion. In this work optical diagnostics are used to study the behavior of Jet A-1 under subcritical, transcritical, and supercritical sprays into open air test chambers. The experimental setup includes a high-pressure air driven pump to create the required high fuel pressure and a special heated injector to increase the temperature of the fuel inside the injector before injection to the required temperatures. Optical techniques like Schlieren and backlit shadowgraph are used to capture and study the sprays from a single hole high pressure diesel injector. A combination of 4 different temperatures and 4 different pressures are tested and the resultant images are processed to obtain quantitative measurements such as spray penetrations, spray cone angle, and spray optical density for each case. Moreover, the spray plume structure transition with changing parameters from subcritical, transcritical, and supercritical states for the fuel are also studied. The results show that with the fuel being in a transcritical state before injection there is a measurable variation in the spray cone formation and penetration for any fixed pressure. At this state the spray cone shows a bimodal spray angle distribution with increasing penetration. An increase in vapor turbulence is also observed indicating the occurrence of flash boiling of the fuel. With the fuels pushed to a supercritical state, the spray shows a thinner spray jet near the injector with a reduced overall penetration and reduced optical density near nozzle. The transition between the three different states as shown in this study gives us an interesting relationship between the spray penetration, spray cone angle and the spray optical density. This can be used as an indicator in understanding spray atomization of the fuels under supercritical spray conditions. 

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2. Experimental Spray Characterization for Jet-A1 Under Temperature Controlled Subcritical, Transcritical, and Supercritical Spray Conditions

   Nonavinakere Vinod, Kaushik; Kempin, Robert; Fang, Tiegang (May 2023, Proceedings of 14th Asia-Pacific Conference on Combustion)

   Fuels when sprayed under superheated and elevated fuel pressure show different behavior than traditional fuel injection sprays. In this work optical diagnostics were used to study the behavior of Jet A-1 under subcritical, transcritical, and supercritical sprays into open air ambience. Five different temperatures were tested, and the resultant spray images were processed to obtain quantitative measurements such as spray penetrations, and spray cone angle for each case. The spray structure transition with changing parameters from subcritical, transcritical, and supercritical states were also studied. The transition between the three different states are shown in this study and the resulting spray cone angles and penetrations are compared for the fuel. The results show that a transcritical spray has a measurable variation in the spray cone formation and penetration process for a fixed injection pressure. At this state the spray cone shows a bimodal spray angle relationship with increasing penetration. Flash boiling of the fuel is observed near the nozzle of the injector. Increasing the temperature further into the supercritical regime, the spray plume shows a thinning of the jet near the nozzle with a reduced overall penetration compared to lower temperatures. 

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3. Transcritical and Supercritical Fuel Sprays in Subcritical Environments

   Kempin, Robert; Vinod, Kaushik N; Morris, Owen; Fang, Tiegang (June 2024, Proceedings of ICLASS 2024 the 16th Triennial International Conference on Liquid Atomization and Spray 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. 

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4. Decadal impacts of wildfire fuel reduction treatments on ecosystem characteristics and fire behavior in alaskan boreal forests

   https://doi.org/10.1016/j.foreco.2023.121347  

   Boyd, Melissa A.; Walker, Xanthe J.; Barnes, Jennifer; Celis, Gerardo; Goetz, Scott J.; Johnstone, Jill F.; Link, Nicholas T.; Melvin, April M.; Saperstein, Lisa; Schuur, Edward A.G.; et al (October 2023, Forest Ecology and Management)

   Wildfire activity is increasing in boreal forests as climate warms and dries, increasing risks to rural and urban communities. In black spruce forests of Interior Alaska, fuel reduction treatments are used to create a defensible space for fire suppression and slow fire spread. These treatments introduce novel disturbance characteristics, making longer-term outcomes on ecosystem structure and wildfire risk reduction uncertain. We remeasured a network of sites where fuels were reduced through hand thinning or mechanical shearblading in Interior Alaska to assess how successional trajectories of tree dominance, understory composition, and permafrost change over ∼ 20 years after treatment. We also assessed if these fuel reduction treatments reduce modeled surface rate of fire spread (ROS), flame length, and fireline intensity relative to an untreated black spruce stand, and if surface fire behavior changes over time. In thinned areas, soil organic layer (SOL) disturbance promoted tree seedling recruitment but did not change over time. In shearbladed sites, by contrast, both conifer and broad-leaved deciduous seedling density increased over time and deciduous seedlings were 20 times more abundant than spruce. Thaw depth increased over time in both treatments and was greatest in shearbladed sites with a thin SOL. Understory composition was not altered by thinning but in shearbladed treatments shifted from forbs and horsetail to tall deciduous shrubs and grasses over time. Modeled surface fire behavior was constant in shearbladed sites. This finding is inconsistent with expert opinion, highlighting the need for additional fuels-specific data to capture the changing vegetation structure. Treatment effectiveness at reducing modeled surface ROS, flame length, and fireline intensity depended on the fuel model used for an untreated black spruce stand, pointing to uncertainties about the efficacy of these treatments at mitigating surface fire behavior. Overall, we show that fuel reduction treatments can promote low flammability, deciduous tree dominated successional trajectories, and that shearblading has strong effects on understory composition and permafrost degradation that persist for nearly two decades after disturbance. Such factors need to be considered to enhance the design, management, and predictions of fire behavior in these treatments. 

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5. Micro-reactor design optimization and manufacturing for studying high temperature unimolecular decomposition of large molecules

   Jatinder Sampathkumar, Tianzhu Fan (March 2019, 11th U. S. National Combustion Meeting)

   Low residence time flow reactors (≤100 μs), when combined with photoionization mass spectrometry or matrix isolation/infrared spectroscopy, have the capability to directly probe the elementary pyrolytic reaction mechanism for various fuels. A qualitative analysis of flow inside these reactors (straight tubes approximately 1mm inner diameter and 3cm in length made of SiC) suggests pressure and velocity significantly change along the length of the reactor. This presents a challenge in determining actual flow conditions within the reactor making it complicated to establish conditions at which pyrolytic chemistry occurs. Computational and experimental testing have been used to optimize a new reactor geometry to control the flow profiles by adding a constriction at one end of the reactor. The nozzle has been shown computationally to stabilize fluid flow within the reactor. A review of available manufacturing processes and coating techniques to incorporate this constriction are presented along with the final processes selected. Further computational and experimental evaluations are discussed to highlight the performance of this new nozzle. The novel design of the micro-reactor will assist researchers in carrying out fundamental kinetic measurements of short residence time pyrolytic reactions of fuels influencing the creation of more efficient fuels and in sustainable engine technology. 

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