Instability of an annular liquid sheet surrounded by swirling airstreams

A theoretical model to predict the instability of an annular liquid sheet subjected to coaxial swirling airstreams is developed. The model incorporates 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 conditions, fluid properties, and film geometry on the instability of the liquid sheet are investigated. It is observed that the relative axial velocity between the liquid and the gas phases enhances the interfacial aerodynamic instability by increasing the growth rate and the most unstable wave number. At low velocities, a combination of inner and outer airstreams is more effective in disintegrating the liquid sheet than only the inner or only the outer airstream. Also, the inner air is more effective than the outer air in promoting disintegration. Swirl not only increases the growth rate and the range of unstable wave numbers but also shifts the dominant mode from the axisymmetric 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 together Leads to higher values of the maximum growth rate than do only inner or outer air swirl.