| Summary: | This study investigated the effect of CO<sub>2</sub> on the burning behavior and radiative properties of a single ethanol droplet flame in microgravity. Measurements of the droplet burning rate, the flame size and temperature, and the radiative emissions were performed, under microgravity conditions for ethanol droplets burning in N<sub>2</sub> and CO<sub>2</sub> environments, using the 1.5 s drop tower facilities at the Korea Maritime and Ocean University (KMOU). The non-monotonic sooting behaviors (caused by the elevated O<sub>2</sub> concentrations) were found to have a significant influence on radiative heat losses in N<sub>2</sub> environments, resulting in non-linear droplet burning behaviors with O<sub>2</sub> concentrations. Due to the unique nature of CO<sub>2</sub> in microgravity, which absorbs radiative energy from the flame and raises the temperatures of the surrounding gases, the CO<sub>2</sub> environments suppressed the radiative heat losses from the flame, regardless of the non-monotonic sooting behavior observed at the higher O<sub>2</sub> concentrations. These experimental findings highlight the complicated physics of CO<sub>2</sub> gas radiation in microgravity, which has not been quantitatively explored.
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