Study on the Burning of a Fuel Drop in Heated and Pressurized Air Stream : 3rd Report, Empirical Formula for the Flame Transition Air Velocity

Abstract

When the relative velocity between air stream and burning fuel droplet is raised, the flame shape changes suddenly from an envelope type to a wake type and the burning rate decreases remarkably at a specified velocity U_c. In this report, the experimental data on this flame transition air velocity, which were shown in the 1st Report, are summarized into a formula under the theoretical consideration. Although a few interpretations on the mechanism of flame transition have been proposed, they do not explain the experimental results so well. So the authors assume that the transition occurs when the rate of heat lost to the surroundings exceeds the rate of heat generated by chemical reaction in a region of frontal surface of flame. Under this assumption, the critical condition for transition is described quantitatively. Combining the governing factors, which are found from this condition, the empirical formula for U_c is set up such as to satisfy the experimental results.