To obtain a comprehensive understanding of the primary breakup mechanism of dual-layer, rotating conical liquid sheets, the spray field of a dual-orifice, pressure-swirl atomizer is observed using high-speed shadowgraphy. The influences of the pressure drop in the primary and pilot flow channels and of the merger of the dual-layer liquid sheet on spray morphology, spray cone angle, and liquid sheet surface fluctuations are investigated on the basis of an analysis of spray field images. Attention is focused on the mechanisms underlying the behavior of the disturbance waves generated during the merger of the dual-layer liquid sheet. The results of this study reveal that the pressure drops in the primary and pilot flow channels affect the spray pattern, spray cone angle, and liquid sheet surface fluctuations. It is found that the influence of the pressure drop in the primary channel is dominant, and the changes in the liquid sheet surface fluctuations are related to the spray pattern. During liquid sheet merger (after the inner sheet has reached the expected spray cone angle), the spray cone angle of the outer sheet decreases, and only after the sheets are in contact with each other does the amplitude of the surface fluctuations become significantly larger and generate more medium- and high-frequency disturbance waves.
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