Simultaneous Measurements of Droplet Size, Velocity and Temperature of “In-flight” Droplets in a Spray Flow by using a Molecular Tagging Technique

In the present study, a molecular tagging technique was developed to achieve simultaneous measurements of droplet size, flying velocity and temperature of “in-flight” droplets in a spray flow. Phosphorescent molecules, which can be turned into long-lasting glowing marks upon excitation by photons of appropriate wavelength, were premixed in the water droplets and served as the tracers for the quantitative flow measurements. A pulsed laser was used to ‘tag’ the phosphorescence tracer molecules, and the tagged phosphorescent molecules were imaged at two successive times within the phosphorescent lifetime of the tracer molecules. While the sizes of the water droplets were determined quantitatively based on the acquired droplet images with a predetermined scale ratio between the image plane and the object plane, the displacements of the water droplets between the two acquired images were used to estimate the flying/moving velocity of the droplets. The temperature of the “in-flight” water droplets were derived simultaneously by taking advantage of the temperature dependence of phosphorescence lifetime, which is estimated from the phosphorescence intensity ratio of the droplets in the two interrogations. The molecular tagging technique was used to examine the dynamic and thermodynamic behaviors of “inflight” micro-sized water droplets exhausted from a piezoelectric droplet generator. During the experiments, while the ambient temperature was kept constant at 22°C, the initial temperature of the water droplet was changed from 11°C to 18°C at the exit of the droplet generator. The temperature of the in-flight droplets were measured as a function of the initial tempetureature and the flying distance away from the exit of the droplet generator. The measured temperature data was compared with the theoretical analysis results quantatively to validate the measurement results.