The breakup length of a pressure-swirl spray was simply estimated using two analytical models and the measured spray angle due to the difficulty in determining the breakup length either numerically or experimentally. A film model, derived from the balance of forces acting on the liquid film, was employed to understand the initial film flow development before breakup. The film divergent angle, which represents the radial penetration of film before breakup, was obtained using this film model at different axial locations. The input parameters for the film model, such as liquid film thickness, flow angle, and Weber number (We), were experimentally obtained using a microscopic imaging system. A droplet trajectory model was employed to analyze the droplet motion after breakup. This model assumed that the droplet moves freely after breakup with its initial axial and tangential velocity. The droplet divergent angle was defined to analyze the increase in the axial and radial distances of a droplet compared to that of the liquid film. By analyzing the droplet trajectory model, it was found that the asymptotic value of the droplet divergent angle was the same as the flow angle at the breakup location. Therefore, the spray angle was determined by adding the flow angle at the breakup location with the film divergent angle at the breakup location. By linking the film model and droplet trajectory model, the breakup length was estimated using the measured spray angle obtained by the macroscopic spray images. The estimated breakup length showed a similar value to that estimated from the linear instability analysis.
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