Simultaneous measurements of droplet size, flying velocity and transient temperature of in-flight droplets by using a molecular tagging technique

In the present study, a molecular tagging technique is introduced to achieve simultaneous measurements of droplet size, flying velocity and transient temperature of in-flight liquid droplets in a spray flow. For the molecular tagging measurements, a pulsed laser is used to “tag” phosphorescent 1-BrNp·Mβ-CD·ROH triplex molecules premixed within liquid droplets. After the same laser excitation pulse, long-lived laser-induced phosphorescence is imaged at two successive times within the phosphorescence lifetime of the tagged phosphorescent triplex molecules. While the sizes of the droplets are determined quantitatively based on the acquired droplet images with a precalibrated scale ratio between the image plane and the object plane, the displacement vectors of the in-flight droplets between the two image acquisitions are used to estimate the flying velocities of the droplets. The simultaneous measurements of the transient temperatures of the in-flight droplets are achieved by taking advantage of the temperature dependence of phosphorescence lifetime, which is estimated from the intensity ratio of the acquired phosphorescence image pair of the inflight droplets. The feasibility and implementation of the molecular tagging technique are demonstrated by conducting simultaneous measurements of droplet size, flying velocity and transient temperature of micro-sized water droplets exhausted from a piezoelectric droplet generator into ambient air at different test conditions in order to characterize the dynamic and thermodynamic behaviors of the micro-sized in-flight droplets. The unsteady heat transfer process between the in-flight droplets and the ambient air is also analyzed theoretically by using a lumped capacitance method to predict the temperature changes of the in-flight water droplets along their flight trajectories. The measured temperature data are compared with the theoretical analysis results quantitatively, and the discrepancies between measurement results and the theoretical predictions are found to be within 0.80 °C.