The effect of fuel droplets on the burning velocity of strained laminar acetone/air flames

Abstract

The effect of fuel droplets on the burning velocity of strained laminar premixed flames is investigated via experimentation and simulation. The twin counterflow configuration was used to obtain reference flame speeds as a function of strain rate for a prevaporized flame and a dilute spray flame simultaneously, both composed of acetone and air. The mixtures were varied with respect to nominal equivalence ratio (0.8–1.4) and strain rate (200–600 s−1). Gas velocities were measured using particle image velocimetry. Droplet size, velocity, and concentration were measured using phase Doppler anemometry: non-reacting flow measurements were taken at a position upstream of the stagnation plane; reacting flow measurements were taken along the stagnation streamline. The droplet Sauter mean diameter ranged between 65–75 μm, and the estimated fuel liquid fraction varied between 6–22%, increasing with nominal equivalence ratio. The results show that the reference flame speed of the spray flame decreases slightly relative to that of the prevaporized flame in the case of lean mixtures, but appears unchanged in the case of stoichiometric and rich mixtures. Gas velocity profiles and droplet measurements along the stagnation streamline suggest that the spray flame vaporization is incomplete, such that the reference flame speed corresponds to that of a lower equivalence ratio. Conversely, the opposing flame is affected by the fraction of droplets that do not vaporize in the spray flame and either penetrate the flame front, causing fuel enrichment, or evaporate near the stagnation plane, reducing the adiabatic flame temperature. The effect appears primarily for rich mixtures with higher liquid fraction, as evidenced by the lower reference flame speed and lower axial velocity rise across the flame. This study provides a systematic framework to examine the influence of fuel droplets on laminar flame propagation using a single-component fuel in the counterflow configuration.