A Theoretical and Experimental Investigation of the Ignition of Fuel Droplets

Abstract

The influence on the self-ignition of single fuel droplets of ambient temperature and oxygen concentration, droplet relative velocity, droplet size, and fuel type was investigated both experimentally and theoretically. Using a unique droplet combustion experimental system, measurements of droplet ignition delay times were obtained for two pure fuels having widely different properties (furfuryl alcohol and butyl alcohol) and two different droplet diameters (200μ and 300μ nominal diameter) for representative droplet combustion conditions. A mathematical model describing the development of profiles for fuel vapor, temperature, and oxygen in the boundary layer surrounding a single fuel droplet moving relative to a hot oxidizing atmosphere was used to correlate the observed ignition delay times. Ignition delay times are found to increase substantially with decreasing ambient temperature and to a somewhat lesser degree with increasing droplet diameter and decreasing fuel volatility. A modest tendency for decreasing ignition delay times with increasing droplet relative velocity is noted, but this effect of relative velocity diminishes rapidly with increasing fuel volatility or decreasing droplet size. While the measured effect of ambient oxygen weight fraction on ignition delay times is shown to be negligible, the theoretical correlation shows a decrease in ignition times with decreasing oxygen weight fraction. This lack of agreement is ascribed to an oversimplification in the ignition criterion used in the mathematical model. Differences in predicted ignition delay times using mathematical models based on uniform and nonuniform internal droplet heating are found to be so small that no conclusions can be made regarding the actual mode of droplet heating during the preignition period.