Instability of an annular liquid sheet surrounded by swirling airstreams

A theoretical model to predict the instability of an annular liquid sheet s ubjected to coaxial swirling airstreams is developed. The model incorporate s essential features of a liquid sheet downstream of a prefilming airblast atomizer such as three-dimensional disturbances, inner and outer air swirl, finite film thickness, and finite surface curvature, Effects of Row condit ions, fluid properties, and film geometry on the instability of the liquid sheet are investigated. It is observed that the relative axial velocity bet ween the liquid and the gas phases enhances the interfacial aerodynamic ins tability by increasing the growth rate and the most unstable wave number. A t low velocities, a combination of inner and outer airstreams is more effec tive in disintegrating the liquid sheet than only the inner or only the out er airstream. Also, the inner air is more effective than the outer air in p romoting disintegration. Swirl not only increases the growth rate and the r ange of unstable wave numbers but also shifts the dominant mode from the ax isymmetric mode to a helical mode. With the presence of air swirl, the most unstable wave number and the maximum growth rate are higher than their no- swirl counterparts. Inner air swirl increases the most unstable wave number more effectively than outer air swirl, and both airstreams swirling togeth er Leads to higher values of the maximum growth rate than do only inner or outer air swirl.