Structure and mixing properties of the near-injector region of nonevaporating pressure-atomized sprays.

A theoretical and experimental study of the dense-spray region of pressure-atomized nonevaporating sprays is described, emphasizing flows in the wind-induced and atomization breakup regimes. Large scale (9.5 and 19.1 mm exit diameter) water jets were injected vertically downward into still air at atmospheric pressure. Injectors producing fully-developed turbulent pipe flow and slug flow exit conditions were considered. The applicability of the locally-homogeneous flow (LHF) approximation to predict flow properties in the near-injector region was also evaluated. It was found that mixing was strongly influenced by the degree of flow development at the injector exit as well as the breakup regime: fully-developed injector flow and atomization breakup yielded the fastest mixing rates. The structure of the flow in the atomization breakup regime consisted of a liquid core surrounded by a drop-containing mixing layer with the inner portion of the mixing layer containing large irregularly-shaped liquid elements and drops. Even though the mean liquid volume fraction was high near the axis, the gas-containing region is relatively dilute at each instant, suggesting that drop formation and secondary drop breakup dominate the process in the mixing layer--not drop collisions. The velocities of large drops are generally much larger than small drops and the gas throughout the mixing layer providing direct evidence of significant separated flow effects. Conditions required for secondary drop breakup were found to exist mainly near the liquid surface. The LHF analysis gave encouraging predictions of dense-spray properties in the near-injector region for atomization breakup, including representation of flow effects wherever liquid volume fractions were greater than approximately 0.2. It was effective at high liquid volume fractions because in these regions, the small drops and the gas contribute very little to the dynamics of the flow due to their small mass fractions, not because the assumptions of the LHF approximation are formally satisfied. The LHF analysis underestimates the velocity of the larger drops in the mixing layer and overestimates the rate of development of the spray in the more dilute spray regions.Ph.D.Aerospace engineeringApplied SciencesMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/128535/2/9023631.pd