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1. Effects of radiation heat loss on planar and spherical hydrofluorocarbon/air flames

   https://doi.org/10.1016/j.combustflame.2023.113067  

   Tavares, Justin K.; Gururajan, Vyaas; Jayachandran, Jagannath (December 2023, Combustion and Flame)

   Hydrofluorocarbons (HFC), which are mildly flammable and pose potential fire risks, have received greater attention as a viable low global warming potential alternative to traditional refrigerant and fire-suppressant compounds. Therefore, there is a demand to accurately quantify their flammability and reactivity to establish proper safety metrics. This study investigates the effects of radiation heat loss on slowlypropagating HFC/air laminar flames. Planar 1-D simulations of R-32/air and R-1234yf/air flames show significant reductions in laminar flame speed due to radiative heat losses from the flame zone. Simulations of spherically expanding flames (SEF) revealed that the radiation-induced flow needs to be considered when interpreting data from experiments. To this end, a Spherical-flame RADiation-Induced Flow (SRADIF) model was developed to estimate the burned gas inward flow velocities in constant-pressure SEFs, utilizing the optically thin limit assumption to model radiation heat loss. The model was validated against results from detailed numerical simulations of SEFs, from which radiation-induced inward flow was derived using a new formulation considering both the radiation heat loss and convective flow effects. Results show that SRADIF accurately predicts the inward flow velocity for R-32/air mixtures over a range of conditions and performs significantly better compared to existing analytical models. However, the model was unable to accurately predict flow velocities for R-1234yf/air flames and the reason for this is discussed. 

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2. Radiation effects in hydrofluorocarbon/air flames: analysis and development of a spherical flame radiation model

   Tavares, Justin; Gururajan, Vyaas; Jayachandran, Jagannath (April 2023, 13th U. S. National Combustion Meeting Organized by the Central States Section of the Combustion Institute)

   Hydrofluorocarbons (HFC), which are mildly flammable and pose potential fire risks, have received greater attention as a viable low global warming potential alternative to traditional refrigerant and fire-suppressant compounds. Therefore, there is a demand to accurately quantify their flammability and reactivity to establish proper safety metrics. This study investigates the effects of radiation heat loss on slowly-propagating HFC/air laminar flames. Simulations of spherically expanding flames (SEF) revealed that the radiation-induced flow needs to be considered when interpreting data from experiments. To this end, a new spherical-flame radiation model was developed to circumvent the effects of radiation-induced inward flow in constant-pressure (CON-P) SEF experimental measurements, accounting for radiation heat loss using the optically thin limit model. The developed spherical-flame radiation model was validated against results from transient SEF simulations. 

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3. Instability growth rates in hydrofluorocarbon/air flames: analysis of the Rayleigh-Taylor instability

   Tavares, Justin K; Jayachandran, Jagannath (March 2024, Spring Techical Meeting of the Eastern States Section of the Combustion Institute)

   Hydrofluorocarbons (HFC), which are mildly flammable and pose potential fire risks, have received greater attention as a viable low global warming potential alternative to traditional refrigerant and fire-suppressant compounds. However, the reactivity of these compounds can be exacerbated under certain conditions, with buoyancy-induced instability growth promoting flame acceleration and substantially increasing flame speeds of HFC/oxidizer deflagrations. Therefore, the flame acceleration of HFC/oxidizer deflagrations must be investigated to properly assess the flammability characteristics of these compounds. This study investigates the effect of the Rayleigh-Taylor instability on instability growth rates during the linear regime. To this end, simulations were performed tracking the growth of instabilities caused by an initial disturbance in the flame front, from which dispersion relations were derived for R-32/air mixtures varying the gravitational acceleration. 

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4. Low-Mach-Number Simulation of Diffusion Flames with the Chemical-Diffusive Model

   https://doi.org/10.2514/6.2019-2169  

   Chung, Joseph D.; Zhang, Xiao; Kaplan, Carolyn R.; Oran, Elaine S. (January 2019, AIAA Scitech 2019 Forum)

   This work describes and tests the calibration process of the chemical-diffusive model (CDM) for the simulation of non-premixed diffusion flames. The CDM is an alternative, simplified approach for incorporating the effects of combustion in a fluid simulation, based on the ideas of regulating the rate of energy release such that the properties of combustion waves (e.g. flames and detonations) are reproduced. Past implementations of the CDM have considered single-stoichiometry fuel-air mixtures or mixtures with variable stoichiom- etry but with premixed modes of combustion. In this work, the CDM is tested and shown to work for non-premixed, low-Mach-number flames (i.e., diffusion flames) by incorporat- ing it into a numerical model which solves the reactive and compressible Navier-Stokes equations with the barely implicit correction (BIC) algorithm, which removes the acoustic limit on the integration time-step size. Simulations of one-dimensional premixed laminar flames reproduce the required premixed laminar flame speed, thickness, and temperature. A two-dimensional, steady-state, laminar coflow diffusion flame is computed, and the result demonstrates the ability of the algorithm to compute a non-premixed flame. Lastly, a two- dimensional simulation of two opposing jets of fuel and air show that the CDM approach can compute the structure of a counter-flow diffusion flame. 

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5. 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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