Formation and characteristics of sprays from annular viscous liquid jet breakup




Shen, Jihua

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The formation process and characteristics of sprays from annular liquid jet breakup in moving gas streams have been investigated. In the first part of the thesis, a linear instability analysis is carried out for the instability and breakup of annular liquid jets. A dispersion relation has been derived and solved numerically by using Muller's method. Temporal instability analysis shows that two independent unstable modes, para-sinuous and para-varicose, exist for the annular jet instability. The para-sinuous mode outgrows the para-varicose one at relatively low gas-liquid density ratios and large Weber numbers as typically encountered in the twin-fluid atomization. The curvature of the annular jet promotes the jet instability and may not be neglected for the breakup processes of annular liquid jets. Not only the velocity difference across each interface but also the absolute velocity of each fluid is important for the jet instability. Co-flowing gas at high velocities is found to significantly improve atomization performance. A mesh-searching method has been developed to determine absolute mode of instability. The numerical results indicate that both absolute and convective instability exist for para-sinuous and para-varicose modes under certain flow conditions. Para-sinuous unstable waves outgrow para-varicose ones, and hence dominate the jet instability according to both absolute and convective instability analysis. The liquid viscosity has a simple stabilizing effect on the jet instability while the gas inertial force shows fairly complex influence on the absolute instability of the jet. The convective growth rates for various inner gas velocities indicate that not only the velocity difference between, but also the absolute velocity of the liquid and gas, determine the jet breakup process. In the second part of this thesis, experimental investigations have been conducted for the breakup process of annular water jets exposed to an inner air stream by photographic technique, and the characteristics of the resultant sprays by Phase Doppler Particle Analyzer. Two annular nozzles of the same structure but different dimensions are designed and constructed especially to provide smooth contraction for the liquid flow. The test apparatus is constructed to produce the annular liquid sheets or sprays of good quality. Flow visualization reveals that there exist three regimes. i.e., bubble formation, annular jet formation and atomization regime for the jet breakup process. Within the bubble formation regime, the jet breakup characteristics measured from the photographs taken under various liquid and gas velocities show that uniform bubbles are observed for various air-to-water velocity ratios. The jet breakup and wave lengths decrease with the air-to-water velocity ratio. The measurements are compared with the predictions by the linear instability analysis, and fair agreement is obtained. Spray characteristics measured by a Phase Doppler Particle Analyzer indicate that using atomizing air enhances the jet breakup process and improves the atomization performance by producing fine sizes of droplets and increasing the uniformity of drop sizes. The drop axial velocity has a jet-type distribution in the radial direction, and decreases monotonically along the spray axis. Increase in the water and air velocities results in higher drop axial velocity. The droplet size described by its Sauter mean diameter (SMD) reaches a minimum value at the central region of the spray and increases towards the spray edge. The SMD has a complex variation along the spray axis.



Jets, Fluid dynamics, Fluid dynamics