Dispersed-phase structure of pressure-atomized sprays at various gas densities

Abstract

The dispersed-phase structure of the dense-spray region of pressure-atom ized sprays was studied for atomization breakup conditions, considering large-scale (9.5 mm initial diameter) water jets in still-air at ambient pressures of 1, 2, and 4 atm, with both fully developed turbulent pipe flow and nonturbulent slug flow at the jet exit. Drop sizes and velocities and liquid-volume fractions and fluxes were measured using holography. Measurements were compared with predictions based on the locally homogeneous flow (LHF) approximation as well as recent correlations of drop sizes after primary breakup of turbulent and nonturbulent liquids. The dispersed-flow region beyond the liquid surface was relatively dilute (liquid-volume fractions less than 0.1%), with significant separated-flow effects throughout, and evidence of near-limit secondary breakup and drop deformation near the liquid surface. Turbulent primary breakup predictions were satisfactory at atmospheric pressure, where the correlation was developed, but failed to predict observed trends of decreasing drop sizes with increasing gas density due to aerodynamic effects; in contrast, the laminar primary breakup predictions successfully treated the relatively small effects of gas density for this breakup mechanism. Effects of liquid turbulence at the jet exit were qualitatively similar to single-phase flows, yielding faster mixing rates with increased turbulence levels even though drop sizes tended to increase as well. LHF predictions within the dispersed-flow region were only qualitatively correct due to significant separated-flow effects, but tended to improve as the ambient pressure and the distance from the jet exit increased.