Combined effervescent and airblast atomization of a liquid jet

Effervescent atomization shows great promise towards the production of small droplet sizes, but it can suffer from substantial instabilities. Adding a coaxial shear flow to a central two-phase bubbly flow is a simple extension of effervescent atomization, however, the characteristics of a combined air-blasting shear flow and effervescent mode of fragmentation have not been well described in the literature. By making use of LDA/PDA measurements, high speed microscopic imaging of the atomization zone, and advanced image processing techniques, quantities such as the axial velocity fluctuations, pulsation frequencies, ligament sizes and liquid area fractions are measured and analysed with respect to the relative mass of effervescent air and air-blast air. The work shows that the coaxial air blast flow does not change the frequency of the effervescent core pulsations but can act to dampen fluctuations whilst simultaneously improving dispersion characteristics. For this hybrid atomization mechanism, the measured axial velocity fluctuations are now a combined result of the instability of the effervescent spray core as well as mixing from the surrounding air flow. Analysis suggests that frequencies associated with the effervescent atomization process can occur on similar scales as the surrounding mixing frequencies. Furthermore, sinusoidal instabilities from the coaxial air flow are seen as superimposed onto the effervescent core indicating that a complex coupling can occur between the two modes of atomization.