  The influence of the ambient pressure on the breakup process is investigated
by means of PIV and shadowgraphy in the configuration of a planar prefilming
airblast atomizer. The ambient pressure is varied from 1 to 8 bar. Other
investigated parameters are the gas velocity and the film loading. From
single-phase PIV measurements, it is found that the gas velocity in the
vicinity of the prefilmer partly matches the analytical profile from the
near-wake theory. The characteristics of the liquid accumulation are extracted
from the shadowgraphy images of the liquid phase directly downstream of the
prefilmer. Two different characteristic lengths, as well as the ligament
velocity and a breakup frequency are determined. In addition, the droplets
generated directly downstream of the liquid accumulation are captured. Hence,
the spray Sauter Mean Diameter (SMD) and the mean droplet velocity are given
for each operating point. The novelty of this study is that a scaling law of
these quantities with regard to ambient pressure is derived. A correlation is
observed between the characteristic length of the accumulation and the SMD,
thus reinforcing the idea that the liquid accumulation determines the primary
spray characteristics. In this paper, a threshold to distinguish the zones
between primary and secondary breakup is proposed based on an objective
criterion. It is also shown that taking non-spherical droplets into account
significantly modifies the shape of the dropsize distribution, thus stressing
the need to use shadowgraphy when investigating primary breakup. Additionally,
the ambient pressure and the velocity are varied accordingly to keep the
aerodynamic stress $\rho_g U_g^2$ constant. This leads to almost identical
liquid accumulation and spray characteristics.
