Steady State Vaporization and Ignition of Liquid Spheres

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Abstract
A 0.83 cm porous sphere, coated with a thin film of liquid fuel, was used for the measurement of preignition vaporization rates and incipient ignition temperatures for three fuels: n-octane, n-decane, and ethanol. The vaporization rate was measured using a feed line to the center of the sphere, supplied with fuel from a variable flow ratesyringe pump. Measurements were carried out in a specially constructed furnace where the temperature and ambient oxygen concentration could be adjusted for a wide range of values. Pre-ignition evaporation rates could be predicted with reasonable accuracy for temperatures that were near the ignition point using analysis that does not include chemical reactions. The calculations accounted for the influence of free convection and radiation. Data showing the influence of ambient oxygen concentration on the measured incipient ignition temperature agreed qualitatively with previous theoretical predictions. The influence of wall radiation on ignition temperature was relatively small for the sphere size tested and appears to be negligible for spray sized droplets. However, ignition data obtained from large spheres, of the size used in the present work, can only be used as a lower limit for the ignition point of small droplets. The present data also differs from previous results which were obtained using fuel droplets impinging on a hot surface.