We present an experimental study of the flapping instability which appears when a coaxial liquid jet is atomized by a cocurrent fast gas stream. When primary atomization does not lead to a total break-up of the liquid jet, it undergoes a large-wavelength instability, characterized by very large amplitude oscillations, and can break into large liquid fragments whose typical size is the jet diameter. These large liquid fragments, and consequently the flapping instability, are to be avoided in applications related to combustion where liquid droplets need to be as small as possible. We carried out experiments with air and water coaxial jets, with a gas/liquid velocity ratio of order 50. We studied the consequence of the flapping instability on the break-up of the liquid jet. Measurements of the frequency of the instability were carried out. We suggest a mechanism where the flapping instability could be triggered by non axisymmetrical KH modes.
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