Dilute laminar spray diffusion flames near the transition from group combustion to individual droplet burning

Abstract

A dilute laminar spray flame of heptane was experimentally studied in a coflow configuration. The spray flame was characterized by using phase Doppler interferometric techniques and thermocouples. When burning in air, the flame had a candle-like appearance. It was naturally anchored a few millimeters above the burner through a stabilization annular flame. A droplet vaporization region could be visually identified above this annulus, as a relatively dark inner core. The spray burned in an internal or partial group combustion mode, with an inner core surrounded by a common flame and some droplets at the outer periphery burning individually. Thanks to the relatively narrow droplet size distribution in the spray, some characteristic parameters of group combustion theory could be computed from the experimental measurements reliably. In the region bound by the common flame, the ratio of interdroplet distance to droplet diameter ranged from 16 to an upper limit at least as large as 50, which shows the remarkable efficiency of droplet clouds at inhibiting oxidizer penetration within the core. A group combustion number, that was determined using the flame radius as cloud length scale, was found to be approximately constant in the lower part of the combustion region. This part of the flame was found to set the stage for the burning mode throughout the bulk of the spray flame. The value of this characteristic number was also consistent with predictions of group combustion theory for spherically symmetric spray flames burning in internal group combustion mode. Another aspect of the theory that found experimental confirmation was the transition from group combustion to individual droplet burning that was promoted via oxygen enrichment of the oxidizer stream. Such an enrichment caused two effects. The group combustion number decreased via a decrease of the flame radius. At the same time, the critical group combustion number G cr for the transition to individual droplet burning, which is a function of the oxygen concentration in the free stream, increased. At sufficiently large oxygen concentration, the condition G < G cr was eventually reached, resulting in individual droplet burning. Experimentally, as the oxygen concentration was raised, the percentage of droplets experiencing individual droplet combustion progressively increased. At a molar oxygen concentration of 0.62, the transition to individual droplet burning seemed complete, as manifested by the flame appearance as a collection of chemiluminescent streaks radiating from a common origin.