Unsteady Breakup of Liquid Jets in Coaxial Airblast Atomisers

The breakup process of coaxial airblast atomisers, comprising an annular axial air stream with a central liquid jet, was examined with high speed photography over a range of exit Weber numbers from 0 to 2300, air-to-liquid momentum and energy ratios from 0 to 130 and 0 to 11,700 respectively, liquid jet Reynolds number from 3000 to 46,000 and nozzles with air jet diameter 23, 15 and 9 mm and liquid jets with 2.3 and 1.2 mm. The photographs showed the deterministic unsteadiness of the breakup process leading in the formation of clusters in a nearly periodic manner and quantified the breakup length, frequency, cluster length and velocity, wavelength of the wave responsible for the breakup and the energy transfer from the air to the liquid jet. A simplified breakup model was based on surface waves due to Kelvin-Helmholtz instability, which grow through non-linear processes to form waves with long wavelength relative to the liquid jet diameter.