Effect of the air-to-liquid mass ratio on the internal flow and near-field spray characteristics of a two-phase swirl burst injector

Abstract

Previous studies have demonstrated that the two-phase swirl burst (SB) injector incurred ultra-fast and fine atomization, rather than a breaking jet core/film of conventional air-assisted atomizers. Thus, it has enabled clean, stable, and complete combustion of even highly viscous oils without fuel preheating nor hardware modification. The current study investigates the effect of the air-to-liquid mass ratio (ALR) on the internal bubbly flow formation that triggers the primary atomization by bubble bursting, and on the secondary atomization by shear layer instabilities. An SB atomizer with a diameter, D, of 1.5 mm, is investigated using high-spatial-resolution Shadowgraph and Particle Image Velocimetry. Results show that the bubbly flow commenced in the internal liquid tube at an ALR of 0.94 to initiate the SB atomization. The bubble zone elongated at higher ALRs and peaked at an ALR of 1.63. Further increase in ALR resulted in unstable sprays, indicating that 1.63 is the upper limit ALR for a stable water-spray of the current setup. An increase in ALR enhanced internal and external two-phase atomization, yielding finer droplets and decreased atomization completion length, which was about 1.67D downstream of the atomizer exit at the highest ALR. From the ALR of 0.94–1.63, i.e., the entire effective ALR ranges tested, the final Sauter Mean Diameter (SMD) after the atomization completion dropped from ∼50 μm to 30 μm. Weber number estimate suggests bag breakup dominates the secondary atomization in the SB spray for all the ALRs. Estimate of the overall atomization efficiency suggests that for an SB injector, optimal atomization performance with the balanced efficiency and spray fineness for various spray applications might be achievable at an ALR lower than the maximum value of 1.63.