Coupled VOF–DPM numerical simulation of pressure-swirl sprays with an improved transition criterion

In this work, a coupled VOF–DPM numerical model was developed to simulate droplet–liquid-film distribution in pressure-swirl solid-cone spray within the OpenFOAM framework. A novel VOF-to-DPM transition criterion was proposed for droplet detection, which integrates dual connected-component labelling (CCL) with a watershed segmentation to accurately capture the transition process. The proposed model was validated using visualization spray experiments with a high-speed camera under multiple operating conditions. The coupled approach enables more accurate reproduction of the spatially interwoven droplet–film structures compared with the single-VOF model. In addition, the new transition criterion outperforms the single CCL method in predicting SMD and VMD, with the relative errors decreasing from 6.5% to 0.7% for SMD and from 10.3% to 1.5% for VMD. The simulation revealed three distinct axial regions—film-dominated region, film–droplet mixed region, and droplet-dominated region, consistent with experimental observations. The simulated spray cone angle and droplet size distribution agreed well with measurements, with maximum deviations of 4% across the tested pressure range (88–208 kPa). This study provides a practical method for predicting the spray performance and associated droplet–liquid-film behavior in pressure-swirl sprays, thereby facilitating improved spray-system design and operation.