Atomization Dynamics in Swirl Injector Systems

Atomization in swirl injector systems encompasses the transformation of a liquid stream into a spray of fine droplets through the imposition of rotational motion. As fluid enters a tangential inlet, centrifugal forces drive the liquid into a thin conical film along the nozzle wall, from which instabilities precipitate primary breakup. A central recirculation zone often forms, stabilising the sheet and influencing the onset of ligament formation. Ligaments detach from the rim under surface tension and aerodynamic stresses, undergoing secondary fragmentation into droplets. The resultant droplet size distribution, frequently characterised by the Sauter mean diameter, dictates evaporation rates, mixing efficiency and subsequent combustion or drying performance. Computational studies employing large-eddy simulation and volume-of-fluid methods have elucidated the roles of turbulence, capillary waves and nozzle geometry in governing sheet instability modes. Experimental investigations, utilising high-speed imaging, phase-Doppler anemometry and particle imaging velocimetry, have quantified the influence of liquid viscosity, injection pressure and swirl chamber dimensions on spray angle, breakup length and droplet velocity. Advances in empirical correlations, informed by detailed measurements and big-data evaluations of drop-size distributions, have enabled more accurate prediction of spray behaviour across scales. These developments carry global significance for fuel efficiency, emission reduction in gas turbines and internal combustion engines, as well as for spray drying in the food and pharmaceutical industries.

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Technical terms

Swirl injector: A device that imparts rotational motion to a liquid, producing a conical film and enabling controlled breakup into droplets.

Recirculation zone: A region of reverse or swirling flow near the injector centre that stabilises the liquid film and influences atomisation onset.

Ligament: A slender thread of liquid formed from sheet detachment, which subsequently fragments into droplets under capillary and aerodynamic forces.

Hollow cone spray: A spray pattern in which most liquid is carried in a conical shell, creating an empty core beneath the injector exit.

Sauter mean diameter (D32): The diameter of a sphere having the same volume-to-surface-area ratio as the entire droplet ensemble, widely used to characterise spray fineness.

References

1. Numerical Investigation on Primary Atomization Mechanism of Hollow Cone Swirling Sprays. International Journal of Rotating Machinery (2016).
2. Oil droplet breakup during pressure swirl atomization of food emulsions: Influence of atomization pressure and initial oil droplet size. Journal of Food Process Engineering (2020).
3. The Two-Phase Conical Swirl Atomizers: Spray Characteristics. Energies (2020).
4. Application of big data analysis technique on high-velocity airblast atomization: Searching for optimum probability density function. Fuel (2020).