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The influence of the ambient pressure on the breakup process is investigatedby means of PIV and shadowgraphy in the configuration of a planar prefilmingairblast atomizer. The ambient pressure is varied from 1 to 8 bar. Otherinvestigated parameters are the gas velocity and the film loading. Fromsingle-phase PIV measurements, it is found that the gas velocity in thevicinity of the prefilmer partly matches the analytical profile from thenear-wake theory. The characteristics of the liquid accumulation are extractedfrom the shadowgraphy images of the liquid phase directly downstream of theprefilmer. Two different characteristic lengths, as well as the ligamentvelocity and a breakup frequency are determined. In addition, the dropletsgenerated directly downstream of the liquid accumulation are captured. Hence,the spray Sauter Mean Diameter (SMD) and the mean droplet velocity are givenfor each operating point. The novelty of this study is that a scaling law ofthese quantities with regard to ambient pressure is derived. A correlation isobserved between the characteristic length of the accumulation and the SMD,thus reinforcing the idea that the liquid accumulation determines the primaryspray characteristics. In this paper, a threshold to distinguish the zonesbetween primary and secondary breakup is proposed based on an objectivecriterion. It is also shown that taking non-spherical droplets into accountsignificantly modifies the shape of the dropsize distribution, thus stressingthe need to use shadowgraphy when investigating primary breakup. Additionally,the ambient pressure and the velocity are varied accordingly to keep theaerodynamic stress $\rho_g U_g^2$ constant. This leads to almost identicalliquid accumulation and spray characteristics.

Influence of the ambient pressure on the liquid accumulation and on the primary spray in prefilming airblast atomization