Size shifts in measurements of droplets with the aerodynamic particle sizer and the aerosizer.
Baron-P; Deye-GJ; Martinez-AB; Jones-EN; Bennett-JS
Aerosol Sci Tech 2008 Mar; 42(3):201-209
Observations of the size of liquid droplets using the Aerodynamic Particle Sizer (APS) and the Aerosizer indicated that the measured size was significantly different from the aerodynamic diameter as calculated by measuring droplet settling velocity. The size shifts appeared to be caused by droplet distortion in the detector flow field for the Aerosizer. However, for the APS, droplet sizing was further affected by droplet impaction on the upstream side of the focusing nozzle. It is suggested that liquid accumulated in and constricted the nozzle, resulting in a particle velocity increase at the sensor. The size shift can occur with the deposition of <0.5 microL liquid onto the nozzle; the size shift can occur in 1-10 minutes even at concentrations of 1000 particles/L; and the size shift can disappear after cessation of liquid aerosol sampling. CFD calculations provided information about the amount of velocity increase at the APS sensor for a selected constriction. Both solid and liquid particles are affected by the nozzle constriction, which produces approximately the same percentage size shift throughout the measurement range. The size shifts (delta) were related to droplet aerodynamic diameter (microm), viscosity (Pa-s), and surface tension (N/m) by the following empirical equation: delta=-a diameter(b)/(surface tension(c) X viscosity(e)). The value of b was arbitrarily set to two. The values for a, c, and e for the APS (including both droplet distortion and nozzle constriction) and for the Aerosizer were determined by a regression analysis of the available data.
Mathematical-models; Models; Particle-aerodynamics; Aerosols; Aerosol-particles; Aerosol-dispensers; Aerosol-generators
Paul Baron, NIOSH, R3, 4676 Columbia Parkway, Cincinnati, OH 45226, USA
Work Environment and Workforce: Emerging Technologies; Manufacturing
Aerosol Science and Technology