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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 modeling

   Tavares, Justin; Levi, Elijah; Gururajan, Vyaas; Jayachandran, Jagannath (April 2022, Spring Technical Meeting, Eastern States Section of the Combustion Institute, March 6-9, 2022)

   Hydrofluorocarbons (HFC), which are mildly flammable and pose potential fire risks, have received greater attention as a viable low global warming potential (GWP) 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 in slow-propagating 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 revealed that the radiation-induced flow needs to be considered when interpreting data from experiments. To this end, a 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. Validation of the radiation model is currently underway, but preliminary results show that the model better predicts the inward flow velocity for most conditions compared to existing analytical models. 

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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. 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. Effects of spacing on flaming and smoldering firebrands in wildland–urban interface fires

   https://doi.org/10.1177/07349041221081998  

   Kwon, Byoungchul; Liao, Ya-Ting_T (March 2022, Journal of Fire Sciences)

   Firebrand (ember) attack has been shown to be one of the key mechanisms of wildfire spread into wildland–urban interface communities. After the firebrands land on a substrate material, the ignition propensity of the material depends on not only the attributes (e.g. shape, size, and numbers) but also the distribution of the firebrands. To help characterize this process, this study aims to investigate the effects of gap spacing on the burning behaviors of a group of wooden samples. Experiments are conducted using nine wooden cubes, 19 mm on each side. These samples are arranged in a 3 × 3 square pattern on suspension wires and are ignited by hot coils from the bottom surface. The gap spacing (s) between the samples varies in each test (ranging from 0 to 30 mm). After ignition, the samples are left to burn to completion. The burning process is recorded using video cameras. Sample mass loss and temperatures are monitored during the flaming and smoldering processes. The results show that the flame height and the sample mass loss rate have non-monotonic dependencies on the gap spacing. When the gap spacing reduces, the flame height and the mass loss rate first increase due to enhanced heat input from the adjacent flames to each sample. When s ≤ 10 mm, flames from individual samples are observed to merge into a single large fire. As s further decreases, the air entrainment at the flame bottom decreases and the flame lift-off distance at the flame center increases, resulting in an increased flame height, decreased flame heat feedback to the solid samples, and a decreased mass loss rate. The decreased mass loss rate eventually leads to a decrease in the flame height as well. The gaseous flame height is correlated to the solid burning rate. The correlation generally follows previous empirical equations for continuous fire sources. For the smoldering combustion, compared to a single burning sample, the smoldering temperature and duration significantly increase due to the thermal interactions between adjacent burning samples. To help interpret the results of the burning experiments, thermogravimetric analysis is also performed in air and nitrogen, resulting in heating rates ranging from 10 to 100 K/min. 

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