Combustion and microexplosion of freely falling multicomponent droplets

The combustion characteristics of isolated, low-Reynolds number, multicomponent droplets freely falling in a hot, oxidizing gas flow are studied. Experimental results on two-component fuels substantiate a three-staged combustion behavior, with diffusion being the dominant liquid-phase transport mechanism: that is, there exists an initial period during which the volatile component in the surface layer is preferentially gasified while the droplet temperature is relatively cold; a short, transition, period during which the droplet temperature increases rapidly, the burning rate is extremely low, the flame size shrinks, and the flame temperature diminishes; and a final, quasi-steady period during which the droplet interior concentration distribution remains constant, the surface region is more concentrated with the less volatile component, and the droplet temperature is relatively high. Experimental results on microexplosion show that its occurrence depends sensitively on the mixture concentration as well as the stability of the droplet generation mode, and that frequently nucleation appears to initiate in the vicinity of the droplet center.