Influence of fuel injection frequency on droplet dispersion in ethanol pulsed spray flames

The optimized use of ethanol in current and new vehicles requires detailed information on atomization, evaporation, mixing and combustion processes of this fuel. In this work, laser diagnostic tools are applied to analyze ethanol combustion in pulsed spray flames. Fuel droplet diameter and velocity distributions are measured by Phase Doppler Interferometry (PDI) across the flame. The evolution of droplets dispersion at early stages of the injection event is visualized in a diametral plane at exit of the injector by high repetition rate Mie scattering. The burner is formed by a swirler mixer and an automotive port fuel injector both positioned at the top exit of a vertical cylinder. Three injection frequencies (100 Hz, 250 Hz, and 400 Hz) are analyzed in depth. The increase in the injection frequency induced a transition from higher to lower density spray patterns impacting the combustion process of the droplets. The higher density spray presented droplet dispersion similar to a cloud in group combustion mode, promoting a mono-modal droplet diameter distribution across the flame centerline and produced an elongated flame with a visible reaction zone down to the injection plane of the burner. The lower density spray suggests droplets in the internal group combustion mode, being highly susceptible to swirling flow structures and promoting a bi-modal diameter distribution at the centerline, mainly produced by residual droplets with high axial velocity and preferential evaporation of small droplets. Furthermore, the lowest density spray led to a shorter flame presenting similarities to a lifted flame.