Liquid atomization for aerosol applications
Date
1997
Authors
Chen, Tingbao
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Abstract
Liquid atomization from pressure-swirl atomizers has been investigated experimentally in order to establish the possibility of using pure water solution as a substitute for hydrocarbon propellants for aerosol applications, because of environmental concerns and poor spray performance of the present products.
It is shown that the pressure-swirl nozzle alone is not adequate in meeting the atomization requirements for aerosol applications at low injection pressures. Studies have also been carried out for external acoustic excitations exerted on the baseline pressure-swirl nozzle. It is found that the acoustic effect on the disintegration of the conical liquid sheet is observable by flow visualization techniques and measurable by phase Doppler particle analyzer (PDPA), however, the reduction of droplet sizes from PDPA measurements is not significant. Furthermore, considerable variations of droplet diameters along the radial and axial directions have been measured.
To achieve better performance such as spray symmetry, spray penetration and smaller droplets, modified pressure-swirl atomizers with different insert geometry have been designed and the effect of external acoustic disturbances have also been investigated. More symmetrical spray pattern and some reduction of droplet sizes are achieved and more visible disturbance waves on the liquid sheet are observed. Investigations on both the baseline pressure-swirl nozzle and the modified pressure-swirl atomizers still show difficulties in meeting Johnson's droplet size requirement at low injection pressures without chemical additives.
A new-concept atomization technique has been developed for aerosol applications and a dramatic reduction in droplet sizes and almost complete spatial uniformity of mean droplet diameters has been achieved. The line-averaged mass mean diameter of droplets ( which is the key parameter that Johnson expects) in the spray produced by the new-concept impaction atomizer is about 45 µm, a value meeting Johnson's specification of 50 µm or less. The maximum variation of the droplet mass mean diameter in the whole spray is less than 5 µm, under the specified injection pressure of 0.4 MPa (60 psi). On the other hand, the variations of the droplet mass mean diameter are more than 20 µm for sprays produced by both the baseline and the modified pressure-swirl nozzles measured at the axial locations of 203.2 mm (8 inches) downstream of the nozzle. However, the impaction atomization results in significant amount of liquid dripping-off, which is not atomized. Preliminary analysis indicates that either using the Venturi tube or simply the hand-pump pressurization system, the dripping fluid of the new-concept impaction atomizer can be recirculated or reutilized completely. A prototype of the impaction atomizer has been designed and built in house, that fits onto the existing aerosol products. It is foreseen that the impaction atomizer could become a new atomization system for aerosol applications with quality spray performance but no negative environmental impact.