Effects of the ambient temperature and initial diameter in droplet combustion

Results are presented from transient numerical simulations of combustion of an isolated heptane (n-C7H16C7H16) droplet in nearly quiescent ambient air. The focus is on the effects of environment temperature and droplet size on the combustion characteristics. Droplet sizes range from 10 μm to 1 mm, environment temperatures range from 800 K to 1600 K, and pressure is 1 atm.The results show that ignition delay times have a very moderate dependence on droplet diameter. With an increase in droplet diameter, the ignition delay time first decreases near the ignitable diameter and then increases far from the ignitable diameter. The ignition stand-off ratio always decreases with an increase in droplet diameter. A strong but short-lived convection, which is generated following the ignition, is demonstrated for the first time. The convection is caused by gas expansion which ‘pushes’ the droplet toward the incoming flow. At lower temperatures, the transition from the kinetically controlled regime to the diffusion controlled regime is shifted to larger droplet diameters, and a larger range of droplet sizes is affected by the transition. Flame stand-off ratios are higher, and flame temperatures are lower, for small droplets. The transient evolution of heat release rate in droplet flames is presented for the first time. The heat release rate first exhibits thermal runaway (ignition), then decreases once the diffusion flame is established, and finally increases precipitously towards the end of the droplet lifetime due to an increase in strain rate. The heat release rates exhibit a strong dependence on droplet diameter.Finally, a comparison of the combustion characteristics of methanol and heptane droplets is presented, which establishes the differences in the phenomena reported in this paper and those reported for methanol. The increase in flame stand-off ratios and fuel vapor accumulation effects as droplet size is reduced, are qualitatively the same as those reported for methanol. The gasification rates and droplet surface temperatures for heptane exhibit less variation. Heptane flame locations lie in the fuel-rich zone whereas methanol flames are located at near stoichiometric mixture fractions.