The effect of non-luminous thermal radiation in microgravity droplet combustion

The effect of radiative heat loss from isolated droplet flames is usually assumed to be negligible. For the small droplet sizes studied in most isolated droplet combustion experiments conducted to date (< 1.0 mm), this assumption has been shown to be reasonable. For example, by neglecting radiation, a detailed numerical model accurately predicts the burning rate, flame position and extinction diameter for 1 millimeter-sized methanol/water droplets. However, recent space-based 3 to 5 millimeter methanol/water droplet combustion data show an increase in extinction diameter and a decrease in burning rate with increasing initial diameter. These results suggest that, at larger initial droplet diameters, the effect of radiative heat loss cannot be neglected. By including a radiation sub-model, the modified numerical model predicts that at droplet diameters greater than about 1 mm the effect of radiation results in a decrease in burning rate and a non-linear increase in extinction diameter with increasing initial diameter. At very large initial diameters (> 6 mm in atmospheric air), the model predicts that the droplet flame will ignite, but quickly self-extinguish due to excessive radiative heat loss.