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1. Automated Kinetic Models to Predict the Flame Speeds of Halocarbons

   Khalil, Nora; Harris, Sevy; West, Richard H (March 2023, 13th U. S. National Combustion Meeting)

   Previous generations of halocarbon refrigerants and flame suppressants cause intolerable levels of ozone depletion and global warming, motivating the search for environmentally-friendly alternatives, but the complex flammability of proposed halocarbon compounds has proven to be a challenge. Because the combustion chemistry of these greener halocarbon refrigerants and suppressants is very condition-dependent, the flammability of potential alternatives need to be screened under a variety of operational conditions prior to marketing and further product development. To facilitate this screening, kinetic models can be generated automatically in Reaction Mechanism Generator (RMG) to predict the flammability. RMG, a software package that automates the generation of detailed reaction mechanisms, has recently been extended to predict the chemical kinetics involved in halocarbon combustion. Full kinetic mechanisms with kinetic, thermodynamic, and transport properties generated from RMG are then evaluated with Cantera to predict laminar flame speeds under different reacting conditions. Recent work developed an RMG model of flame suppressant 2-BTP (CH2=CBrCF3) in methane flames. Current work has advanced RMG’s 2-BTP model to achieve improved quantitative agreement with experimental flame speeds. We also use RMG to generate models of binary halocarbon blends to determine the importance of “cross reactions” that are not accounted for through simple concatenation of individual combustion mechanisms. Flame speeds of RMG-generated blend models and blend models comprised of concatenated mechanisms are compared, along with experimental flame speeds found in literature. As demonstrated in current and previous work, automating the generation of full halocarbon kinetic models through RMG expedites screening for the next generation of environmentally-friendly refrigerants and suppressants, a task that would be both time- and cost-intensive if conducted without automation. 

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2. Laminar flame speed regime at elevated temperature and pressure

   https://doi.org/10.1016/j.fuel.2024.131672  

   Klein, Jacob; Samimi-Abianeh, Omid (July 2024, Fuel)

   Elevated temperature and pressure laminar flame speed measurements of propane and n-heptane fuel blends were conducted using a Rapid Compression Machine-Flame (RCM-Flame) apparatus. Herein, the lack of experimental flame speed data at simultaneously high temperatures and pressures akin to practical combustion conditions is addressed. The RCM-Flame apparatus is validated against a larger constant volume combustion chamber (CVCC) and simulations using a propane-nitrogen-oxygen mixture at ambient temperature and different pressures, demonstrating high fidelity. Further experiments with an n-heptane-nitrogen-helium-oxygen mixture reveal agreement between experimental and simulated flame speeds at semi-elevated, post-compression conditions. Trials with a propane-helium-oxygen mixture over varied temperatures and pressures demonstrate measured flame speeds falling between two kinetic mechanism simulations, maintaining the general trend. A power-law model correlating laminar flame speeds with elevated temperatures and pressures is developed for propane-helium-oxygen flames at a unity equivalence ratio. Overall, the kinetic mechanisms are shown to be able to predict flame speeds at elevated temperatures and pressures providing validation at conditions not yet explored in literature, optimistically advancing combustion research for practical applications. 

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3. Extensive High-Accuracy Thermochemistry and Group Additivity Values for Automated Generation of Halocarbon Combustion Models

   Farina Jr., David; Sirumalla, Sai Krishna; West, Richard H. (May 2021, 12th U.S. National Combustion Meeting)

   null (Ed.)

   Standard enthalpies, entropies, and heat capacities are calculated for more than 14,000 halogenated species using a high-fidelity automated thermochemistry workflow. This workflow generates conformers at density functional tight binding (DFTB) level, optimizes geometries, calculates harmonic frequencies, and performs 1D hindered rotor scans at DFT level, and computes electronic energies at G4 level. The computed enthalpies of formation for 400 molecules show good agreement with literature references, but the majority of the calculated species have no reference in the literature. Thus, this work presents the most accurate thermochemistry for many halogenated hydrocarbons to date. This new dataset is used to train an extensive ensemble of group additivity values (GAV) and hydrogen bond increment groups (HBI) within the Reaction Mechanism Generator (RMG) framework. On average, the new group values estimate standard enthalpies for halogenated hydrocarbons within 3 kcal/mol of their G4 values. To demonstrate the significance of RMG’s improved halogen thermochemistry, a model for C3H2F3Br (2-BTP) is generated, and flame speeds are compared to a literature mechanism. A significant contribution towards the automation of detailed modeling of halogenated hydrocarbon combustion, this research provides thermochemical data for thousands of novel halogenated species and presents a comprehensive set of halogen group additivity values. 

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4. Experimental Investigation of Slowly Propagating Flames in Microgravity

   Mathew, Joel; Jayachandran, Jagannath (March 2024, Spring Techical Meeting of the Eastern States Section of the Combustion Institute)

   Accurate measurements of the laminar flame speed are useful to constrain the uncertainty of chemical models. However, for slowly propagating flames, buoyancy distorts the flame and measuring flame speeds accurately becomes challenging. This is relevant for novel hydrofluorocarbon refrigerants, preventing an accurate assessment of their flammability. Additionally, nitrogenated chemistry could be investigated by measurements of ammonia/air flames but the low laminar flame speeds also lead to similar issues. The only way to circumvent buoyancy-induced effects is to gather measurements in microgravity. In this study, an image processing technique was developed to accurately extract the radius of the spherical flame. This is required as the projection of a sphere on a plane leads to measurement error. A lab-scale drop tower was designed and built to achieve approximately 500 ms of free fall time. The direct imaging technique was combined with the drop tower to gather flame measurements in free fall. The methodology was applied to obtain the laminar flame speed of a lean, high-pressure methane/air flame. 

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5. Laminar Flame Speed Measurements of Primary Reference Fuels at Extreme Temperatures

   https://doi.org/10.1115/ICEF2022-90501  

   Susa, Adam J.; Zheng, Lingzhi; Nygaard, Zach D.; Ferris, Alison M.; Hanson, Ronald K. (October 2022, ASME 2022 ICE Forward Conference (ICEF2022))

   Abstract Experimentally measured values of the laminar flame speed (SL) are reported for the primary reference fuels over a range of unburned-gas temperatures (Tu) spanning from room temperature to above 1,000 K, providing the highest-temperature SL measurements ever reported for gasoline-relevant fuels. Measurements were performed using expanding flames ignited within a shock tube and recorded using side-wall schlieren imaging. The recently introduced area-averaged linear curvature (AA-LC) model is used to extrapolate stretch-free flame speeds from the aspherical flames. High-temperature SL measurements are compared to values simulated using different kinetic mechanisms and are used to assess three functional forms of empirical SL–Tu relationships: the ubiquitous power-law model, an exponential relation, and a non-Arrhenius form. This work demonstrates the significantly enhanced capability of the shock-tube flame speed method to provide engine-relevant SL measurements with the potential to meaningfully improve accuracy and reduce uncertainty of kinetic mechanisms when used to predict global combustion behaviors most relevant to practical engine applications. 

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