A Simple Phenomenological Model of an Organic Gel Spray Diffusion Flame

Abstract

Recent experimental evidence has revealed the unusual behavior of organic gellant-based fuel droplets that, under appropriate ambient thermal conditions, evaporate and burn in an oscillatory fashion. This peculiar phenomenon of the droplets in the gel spray is incorporated in a model of a two-dimensional spray flame. A combined analytical/numerical solution of the governing equations is utilized to examine the way in which the frequency of oscillatory evaporation (that is likely to be dependent on such factors as the droplet size, and the make-up of the droplet (i.e., gellant versus liquid fuel content)) influences the combustion field. It is shown that the frequency of evaporation of the burning gel droplets can have a profound impact on the thermal field downstream of the homogeneous spray diffusion flame front. Hot spots of individual (or clusters of) burning droplets can be created and under certain operating conditions can lead to hotter temperatures than experienced in the main homogeneous flame. This can be extremely important in realistic combustion settings in which hot spots in undesirable regions can damage the structural integrity of the chamber. Other computed results demonstrate that, in relation to the spray diffusion flames obtained using an equivalent purely liquid fuel spray, the use of a gel fuel spray can lead, under certain operating conditions, to a reduction in flame height and temperature. The latter effect is critical when considering flame extinction. These effects highlight the fact that even though gel fuel sprays may have a distinct advantage over liquid sprays in terms of their safety features it is crucial that the correct operating conditions be employed in order not to detract from attaining the desired combustion performance.

Keywords:

• Diffusion flame
• Gel
• Oscillatory combustion
• Sprays

Acknowledgments

JBG gratefully acknowledges the partial support of the Lady Davis Chair in Aerospace Engineering and the Technion Fund for the Promotion of Research.