Effect of viscosities on structure and instability of sprays from a swirl atomizer

The breakup phenomena of liquid spray sheets and jets are encountered in daily life as well as in various industrial applications. The onset of instability has a direct impact on the downstream spray development, and thus it is necessary to investigate the effect of liquid physical properties on this instability. The present paper focuses on the effects of liquid viscosity on the spray characteristics and instability during the transient operation of a swirl atomizer in a trigger sprayer via experiments. In the experiments, water–glycerol mixtures were used to simulate fluids with a wide range of viscosities. The transient displacement of the dispensing piston was measured. A high-speed digital video camera was used to visualize the near-nozzle spray structure. The digital images of the sprays were further processed in order to analyze the spray angle and the surface waves on the spray cone. By using the spatiotemporal diagrams, the surface wave temporal frequency was also analyzed for fluids with different viscosities. From the experimental results, it is seen that the fluid viscosity plays a critical role in controlling the liquid sheet breakup and atomization. More viscous fluids result in smaller average spray cone angles. The spray cone developed very fast during the early stage for less viscous fluids, while for the most viscous liquid, a very small cone angle with little breakup and atomization was observed. As the liquid viscosity increases, the cone collapses earlier with larger droplets in the end stage. In addition to the spray cone angles, the surface wave temporal frequency was also calculated from the recorded images. The results show that the surface wave temporal frequency depends on the dispensing time and the fluid viscosity, and is almost independent of the location in the near nozzle liquid cone. The results also indicate that the surface wave temporal frequency decreases with time.