Combustion of moving droplets and suspended droplets: Transient numerical results

A numerical investigation of unsteady liquid fuel droplet combustion within an airstream at atmospheric pressure and under zero-gravity condition is presented. Combustion is modeled using one-step finite rate kinetics. A new multicomponent formulation (appropriate for the finite volume method) is used to describe mass diffusion in the gas-phase accurately. Results obtained for both suspended droplets (constant relative velocity) and for moving droplets shown that the flame configurations are a function of the time histories of both the Reynolds number and the Damköhler number. For a moving droplet, the Reynolds number decreases with time (due to both relative velocity and droplet size reduction), but the Damköhler number increases with time. For a suspended droplet, both the Reynolds number and the Damköhler number decrease with time due to the droplet size reduction. Consequently, for the same initial Reynolds number, suspended droplets may demonstrate different burning behavior than moving droplets. Within the range of initial Reynolds numbers considered (6,8, and 50), a moving droplet tends to develop an envelope flame at some stage during its lifetime, whereas a suspended droplet develops an envelope flame only for low initial Reynolds numbers. The flame configurations present during droplet burning are critical in determining droplet lifetime.