Drop multimode breakup is more complex than the bag and shear breakup regimes, and modeling drop breakup in the multimode breakup regime is less reported. In this article, a theoretical model is developed to predict the drop breakup properties in the multimode regime in a more accurate manner. In the present model, the process of the drop deformation and breakup are regarded as the movement of the center of the mass along the cross-stream and streamwise directions, respectively. The displacement of the center of the mass is obtained by solving a pressure balance equation and a mechanical analog model equation. Drop breakup occurs when the displacement of the mass center is out of the scale defined by the threshold function of Weber number. The characteristics of the proposed model as well as the breakup properties of the drops in the multimode regime are addressed by comparing the predicted results with the experimental data in the open published literature.

The film formation during the spray/wall impingement has attracted more attention. The present study investigated the film formation characteristics of the pressure-swirl nozzle by applying the contact-free optical method. According to experimental results, the impingement distance had a slightly more significant effect on the actual spray angle than the mass flow rate, and the maximum changing value was 34.6°. The bulge at the center of the surface became insignificant with the impingement distance. The liquid film was divided into the raised zone, annular zone, and free flow zone. The maximum time-average thickness at the central position was 2.84 mm, and correlations for predicting the time-average thickness and surface roughness were fitted. The time-average thickness of the annular zone was 0.38–0.59 mm, relatively thinner than other zones. When the impingement distance was lower than 10 mm, the time-average film thickness and surface roughness in the annular zone and free flow zone decreased first and then increased with the impingement distance. However, effects of mass flow rates and impingement distance on the liquid film were negligible when the impingement distance was higher than 10 mm. The experimental findings are helpful to fundamentally understand the film formation during the spray/wall impingement.