Ignition of a Binary-fuel (Solvent-Solute) Cluster of Drops

Abstract

Evaporation and ignition of a binary-fuel cluster of drops is described by models under the assumptions that the volatile compound has infinite volatility with respect to the solvent and that the chemistries of the two compounds are independent. A Damköhler number criterion developed for use in sprays is utilized to determine the ignition time. Another criterion is used to determine the ignition location which can be either around individual drops, or around groups of drops inside the cluster, or around the entire cluster. Results show that except for very dilute situations where the initial liquid mass fraction of the volatile is very small, ignition always occurs around the entire cluster. Otherwise, ignition occurs around groups of drops inside the cluster but never around individual drops even though the ratio of the distance between the centers of two adjacent drops by the drop diameter is greater than thirty five. Studies performed by varying the air/fuel mass ratio for a variety of parametric combinations show that: (1) At typical gas temperatures for combustion devices, the ignition of very dense and very dilute clusters of drops is evaporation-controlled for identical chemistries; it is strongly-controlled by solvent ignition in the very dense cluster regime, it is strongly-controlled by ignition of the volatile in the very dilute regime. In the intermediary regime, ignition is controlled by the relative ignition chemistries of the compounds. These conclusions are independent of the amount of volatile initially present in the liquid. (2) The concept of volatile is more strongly associated with the latent heat of evaporation in the dense regime, and more strongly associated with the saturation pressure curve in the very dilute regime. (3) By increasing the surrounding gas temperature one gradually gains control of ignition in the dense and dilute regimes through the evaporation of solvent and volatile respectively. (4) The initial slip velocity between phases affects ignition only in the very dilute regime. (5) Changes in the cluster size affect the ignition time only in the very dense regime. Conclusions (3) and (4) are valid under the assumption of identical kinetics for the two compounds; when different kinetics are considered, it turns out that kinetic effects overwhelmingly dominate ignition.