High Speed Visualization and PIV Measurements in the Near Field of Spray Produced by Flow-Blurring Atomization

Abstract

Clean and stable combustion of alternative liquid fuels, such as biodiesel, straight vegetable oil and glycerol has been achieved in past studies by using a novel flow blurring (FB) injector without fuel pre-processing or combustor hardware modification. FB injector employs aerodynamic principle to form two phase flow immediately upstream of the injector exit. Explosion of air bubbles at the injector exit atomizes the liquid into a fine spray. In this study, the flow field in the near field of the FB injector is investigated by high-speed visualization and time-resolved Particle Image Velocimetry (PIV) techniques. Experiments are performed using water and air for air to liquid mass ratio (ALR) of 2. Flow visualization at the injector exit focused on field of view with the dimension of 2.3 mm × 1.4 mm, spatial resolution of 7.16 μm per pixel, exposure time of 1 μs, and image acquisition rate of 100 k frames per second (fps). Image sequence illustrates fine spray of FB atomization in the near field and the break-down process of larger droplets appearing occasionally. Time-resolved PIV technique is applied to quantify the injector near field. Plots of instantaneous, mean, root-mean-square velocities and turbulence kinetic energy are presented to reveal the droplet characteristics and secondary atomization process. Results show that the majority of the liquid is atomized into fine droplets at the injector exit. The droplet velocity increases in the flow direction and decreases from the center to the periphery of the spray. This result is consistent with the size of the droplets, i.e., the larger droplets move slowly while the finer droplets move faster as they follow the atomizing air flow.