Effects of forced convection and surface tension during methanol droplet combustion

A numerical investigation of surface tension and forced convection effects on moving and suspended methanol droplets burning in a zero-gravity, low-pressure air environment is presented. Simulations were conducted using a predictive, transient, axisymmetric model for an initial droplet diameter of 0.5 mm, an ambient temperature of 1200 K, and initial Reynolds numbers (Re ₀) in the range of 1-100. Results indicate that, for moving droplets, due to the presence of an envelope flame at some stage during the droplet lifetime, surface tension is important over the range of Re₀ considered; the extinction diameter decreases with increasing Re₀. For suspended droplets, when transition or envelope flame is present (Re ₀ less than approximately 15), surface tension is important; when an envelope flame is present (Re₀ less than approximately 10), the extinction diameter increases with Re₀. Both for suspended and moving droplets, the droplet lifetime is weakly sensitive to surface tension. The variation of droplet lifetimes with Re₀ is much stronger for suspended droplets than for moving droplets. Depending on the Reynolds number, results on methanol droplet lifetimes and extinction diameters measured through suspended droplet experiments may not be applicable to moving droplets.