Liquid jet instability and atomization in a coaxial gas stream

An overview of the near and far-field breakup and atomization of a liquid j et by a high speed annular gas jet is presented. The various regimes of liq uid jet breakup are discussed in the parameter space of the liquid Reynolds number, the aerodynamic Weber number, and the ratio of the momentum fluxes between the gas and the liquid streams. Recent measurements of the gas-liq uid interfacial instabilities are reviewed and used to analyze the underlyi ng physical mechanisms involved in the primary breakup of the liquid jet. T his process is shown to consist of the periodic stripping of liquid sheets, or ligaments, which subsequently break up into smaller lumps or drops. Mod els to predict the liquid shedding frequency, as well as the global paramet ers of the spray such as the liquid core length and spray spreading angle a re discussed and compared with the experiments. The role of the secondary l iquid breakup on the far-field atomization of the liquid jet is also consid ered, and an attempt is made to apply the classical turbulent breakup conce pts to explain qualitatively the measurement of the far-field droplet size distribution and its dependence on the liquid to gas mass and momentum flux ratios. Models for the droplet breakup frequency in the far-field region o f the jet, and for the daughter-size probability density function, which ac count for the effect of the liquid loading on the local turbulent dissipati on rate in the gas, are discussed in the context of the statistical descrip tion of the spray in the far field. The striking effect of the addition of swirl in the gas stream is also examined.