Ignition and flame spreading over a solid fuel:Non-similar theory for a hot oxidizing boundary layer

Abstract

A more accurate theoretical study of ignition of a solid polymeric fuel in a hot oxidizing flow field is carried out as an extension of the previous work reported in the Thirteenth Combustion Symposium. In the present work, the complete non-similar calculation is carried out, eliminating the previously used local similarity approximation and the integral approximation to solve the condensed-phase energy equation. Comparison of the predictions based on the non-similar calculations indicates that both results predict similar trends of ignition behavior. However, the ignition delay time and the amount of downstream shift in the previous local similarity calculation differ significantly from those in the present non-similar calculation. This comparison has implications for other otheries of boundary layer combustion in the literature that utilize the self-similarity approximation. The flame-spreading behavior is also predicted as an extension of the ignition process. In agreement with the experimental observations reported in the previous work, results of the non-similar calculation predict that, in the case of high freestream oxygen concentration, the flame spreading downstream from the point of first ignition is slow. Reducing the oxygen concentration increases the flame-spreading speed in both directions (upstream and downstream). This aggreement between experiment and theory indicates that, when the flame spreads downstream, convective heat transfer is the dominant process in bringing the local surface element to vigorous pyrolysis, rather than conduction, and when the flame spreads upstream in the low freestream oxygen level case, the surface temperature ahead of the flame front is already high enough to cause spontaneous ignition, and the appearance of flame awaits the evolution of an abundant supply of fuel. Ignition and flame spreading are shown theoretically to depend sensitively on the thermal and transient properties of the diluent gas (e.g., helium, argon, nitrogen).