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The spray produced by pressure-swirl atomizers strongly depends on the character of their internal flow. Flow field inside a swirl chamber of small pressure-swirl spill-return atomizers was examined using high-speed imaging with image post processing using an in-house Matlab code. The dimensions of the production atomizers did not allow direct visualization of their internal flow, so a scaled modular, transparent Plexiglas model was used. Its flow characteristics were matched with the originally sized atomizer using dimensionless numbers (Reynolds, Swirl, Froude numbers). The test conditions were limited to the inlet overpressure of 5 kPa and spill-to-feed ratio, SFR = 0–0.75. Various spill-return configurations were compared in terms of the spatial and temporal behaviour of the internal air-core, and liquid sheet thickness and its perturbations. The only difference among the tested configurations was the geometrical arrangement of the spill-line (SL) orifice through which the liquid is spilled away. The results show that the presence of the SL orifice affects the internal flow characteristics even when the SL is closed. An axially placed SL orifice causes a decay of the internal air-core. The off-axial SL orifices stabilize the air-core, which is vital for regular formation of the liquid sheet and high-quality spray. However, the turn-down ratio and spray stability were found to be dependent on the distance of the SL orifices from the swirl chamber centreline. The results allow to determine the optimum SL configuration for given application.
