Atomization and Combustion of Hybrid Electrohydrodynamic-Air-Assisted Sprays

This thesis presents an extensive study of the atomization and combustion of dielectric liquids using a hybrid air-blast electrostatic atomizer. While airblast atomization relies on the shear stresses generated at the liquid-air interface, electrostatic atomization introduces an electric charge into the bulk liquid, and the resulted Coulombic repulsive force facilitates the fragmentation process. The atomizer introduced in this contribution is specifically designed to operate in either a single (air-blast or electrostatic) or hybrid mode to enable the delivery of a charged and/or air-assisted spray for combustion applications. The aim is to understand the effect of adding electric charge to a liquid jet which is subject to break up in a co-flowing air stream. In addition to analysis of atomization processes, the influence of charge on flame structure is also analyzed. Laser diagnostics are utilized for measurements and the results obtained for the atomizer in hybrid mode (air-blast + electrostatic) are compared with the pure air-blast mode. Firstly, a high-speed microscopic shadowgraphy technique is implemented to examine near-field spray structure. Diesel is used as a dielectric liquid to create various sprays that cover a range of non-dimensional numbers. The effect of charge on liquid jet unsteadiness and on the probability distribution of wavelength and amplitude of instabilities is discussed. The influence of charge on droplet and ligament size and their population is also analyzed. The findings show that the application of charge makes the liquid jet more unstable and the instabilities forming on the liquid core exhibit a shift to a shorter wavelength with a broadening in the probability distribution of wave amplitude. In addition, a droplet and ligament size reduction along with an increase in droplet count is observed with the addition of charge. The thesis then progresses to discussing results from reacting sprays stabilized on a pilot where kerosene is chosen as the liquid. A premixed pilot flame is used to provide a steady heat source for stabilizing the hybrid atomized sprays. Flame stability characteristics, in terms of blow-off velocity, are presented as a function of controlling parameters, without and with charge. Downstream droplet statistics and flow field for both non-reacting and reacting sprays are shown using laser Doppler velocimetry/phase Doppler anemometry (LDV/PDA) revealing key features in the droplet fields from this burner. Due to relatively low spray specific charge for the aerodynamic Weber numbers investigated, the droplet size and velocity remained largely unaffected through the addition of charge, however, a rise in particle concentration at the center of the spray was noted. Finally, high-speed hydroxyl planar laser induced fluorescence (OH-PLIF) is used to locate reaction zones and comment on the morphology of the reaction zone structures. In a hybrid mode, the charge was seen to push reaction zones radially outward and assisted in stabilizing the flame by keeping OH islands more connected when compared to a pure air-blast mode. This observation was also consistent with a slight improvement in flame stability with the addition of charge.