A temporal linear stability analysis has been carried out to predict the instability
of a viscous liquid jet surrounded by a swirling air stream with three-dimensional
disturbances. The effects of flow conditions and fluid properties on the instability of
the liquid jet are investigated via a parametric study by varying axial Weber number
axial velocity ratio of the gas to liquid phase, swirl Weber numbers, density ratio
and the Ohnesorge number. It is observed that the relative axial velocity between the
liquid and gas phases promotes the interfacial instability. As the axial Weber number
increases, the growth rates of unstable waves, the most unstable wavenumber and
the unstable range of wavenumbers increase. Meanwhile, the increasing importance
of helical modes compared to the axisymmetric mode switches the breakup regime
from the Rayleigh regime to the first wind-induced regime and on to the second
wind-induced regime. The predicted range of wavenumbers in which the first helical
mode has higher growth rates than the axisymmetric mode agrees very well with
experimental data. Results show that the destabilizing effects of the density ratio
and the axial Weber number are nearly the same. Liquid viscosity inhibits the
disintegration process of the liquid jet by reducing the growth rate of disturbances
and by shifting the most unstable wavenumber to a lower value. Moreover, it damps
higher helical modes more significantly than the axisymmetric mode. Air swirl has
a stabilizing effect on the liquid jet. As air swirl strength increases, the growth rates
of helical modes are reduced more significantly than that of the axisymmetric mode.
The air swirl profile is found to have a significant effect on the instability of the liquid
jet. The global, as well as local, effects of the swirl profile are examined in detail.