The use of single-point measurements to characterize dynamic behavior in sprays

Abstract

In an effort to derive dynamic information from a single-particle counting device such as the phase-Doppler interferometer (PDI), a one-dimensional motion model and a sequential sorting algorithm have been developed to calculate the distance and velocity difference between consecutive droplets in sprays, as well as the number of droplets in close proximity (grouplets) and the frequency of grouplet formation. By focusing on sprays that are primarily uni-directional, this model can be used to highlight dynamic behavior and provide information that may indicate tendencies for clustering, collision, coalescence or evaporation among the droplets – information that is not available when considering the histograms of droplet size and velocity normally produced by PDIs. The motion model and sorting algorithm are validated for a mono-sized droplet stream generated by piezo-electric excitation, and are then applied to two other sprays to determine dynamic behavior. For a pressurized-liquid nozzle emitting water, the expected trends of droplet deceleration and spatial dispersion are found moving away from the exit of the nozzle, and significant in velocity differences between closely-spaced droplet pairs are observed. No dominant frequencies are present in the grouping of the droplets. For a droplet-laden air jet excited at a known frequency, similar results are calculated for droplet spacing, grouping and velocity differences, and the driving frequency is also obtained from analysis of the sequentially-sorted PDI data. Although the models developed assume an idealized PDI system, real operating characteristics of PDI systems place limits on the accuracy of these analyses.