Nonane droplet combustion with and withoutbuoyant convection: Flame structure, burning rateand extinction in air and helium

摘要

The burning and extinction characteristics of isolated small nonane droplets are examined in a buoyantconvective environment and in an environment with no external axial convection (as created by doing experimentsat low gravity) to promote spherical droplet flames. The ambience is air and a mixture of 30%O2/70%He to assess the influence of soot formation. The initial droplet diameter (Do) ranges from 0.4 to0.95 mm. Measurements are reported of the extinction diameter and time to extinction, and of the evolutionof droplet diameter, flame diameter, soot shell diameter, burning rate, and broadband radiative emissions.In a buoyancy-free environment for air larger droplets burn slower than smaller droplets for the rangeof Do examined, which is attributed to the influence of soot. In the presence of a buoyant flow in air, noinfluence of Do is observed on the burning rate while the buoyant flames are still heavily sooting. The effectof Do is believed to be due to a combination of dominance of the nonluminous, nonsooting, portion of thebuoyant flame around the forward half of the droplet on heat transport and the secondary role of the luminouswake portion of the flame. In a non-sooting helium inert at low gravity, no effect of Do is found on theevolution of droplet diameter.Flame extinction is observed only in the 30%O2/70%He ambience. For all of the observations, extinctionappears to occur before the disappearance of the droplet which is then followed by a period of evaporation.The extinction diameter and time to extinction increases with Do and an empirical correlation ispresented for these two variables.