Nonlinear breakup of a coaxial liquid jet in a swirling gas stream

Nonlinear asymmetric breakup of a liquid jet exposed to a swirling gas stream is investigated by a perturbation expansion technique with the initial amplitude of the disturbance as the perturbation parameter. The effects of gas-to-liquid axial velocity ratio and gas swirl number on the liquid jet instability and breakup length have been studied. The breakup length predictions show good agreement with the available empirical correlations for liquid jet breakup in still gas as well as jet breakup in a co-flowing gas stream. Earlier linear analyses have predicted that the gas swirl has a stabilizing influence on the jet and that the axisymmetric disturbance is the most unstable disturbance compared to the helical modes. In contrast, experimental studies report that a swirl imparted on the surrounding gas makes the jet unstable, and at high gas swirl the jet disintegrates through an explosive breakup. The present nonlinear temporal analysis correctly captures the destabilizing effect of the gas swirl. With increasing gas swirl number, the helical modes become dominant and the transition to subsequent higher helical mode is achieved with smaller increments in the gas swirl number. The gas swirl number for transition to a highly asymmetric breakup with a high circumferential wave number (n=5) is found to vary as the inverse of the square root of the gas-to-liquid momentum ratio when the gas-to-liquid momentum ratio is less than 1. The jet breakup length decreases with an increase in the gas-to-liquid axial velocity ratio and the gas swirl number. (c) 2006 American Institute of Physics.