The effect of pressure on radiative losses in hot, warm, and cool microgravity spherical diffusion flames

Abstract

This work investigates the effects of ambient pressure on radiative heat losses in hot, warm, and cool microgravity spherical diffusion flames using a numerical model and compares the results to observations from microgravity experiments. The experimental data was obtained during droplet combustion experiments, and the numerical model simulates burner-supported flames. The similarities in the gas phase between the two configurations allow these simulations to provide accurate insights into the experimental results. The model uses a statistical narrow-band model that includes emission and reabsorption. The effect as pressure was varied between 0.5 and 10 atm on the relative radiation contributions from CO₂ and H₂O, the primary radiating species, was examined. As the pressure increased H₂O contributed more to radiative loss than CO₂ in both the experiments and computations. This was determined to be caused by increased absorption coefficients leading to increased reabsorption at higher pressures. Calculations performed without reabsorption did not see this effect, confirming it is due to the optically thick nature of the system. Reabsorption was higher for CO₂ due to the prominence of the absorption band at 4.3 µm, which became saturated at higher pressures. These results demonstrated the importance of using the optically thick assumption to accurately account for radiative losses in these flames, which are crucial to capture flame behaviours such as radiative extinction and reignitions.