Generation and droplet size distribution of tracer particles for PIV measurements in air, using propylene glycol/water solution

Abstract

This work measures the size distribution of the particles exiting a simple-design seeding generator. A simple model for the prediction of the droplet size mean diameter is checked against these measurements, giving good agreement. The main fields of use of these generators include tracer particles based optical measurements like Particle Image Velocimetry (PIV). This investigation provides results against which the performance of different seeding generators, liquids, etc., could be compared, at the exit of the aerosol generator. The developed generic seeding generator is thus suggested as a starting point for a basis for a future scalable standard seeding generator for other researchers investigating the production of aerosols for PIV application.The proposed generator is able to seed flows up to 0.2kg/s with ∼10particles/mm3, under normal operation conditions. Air is injected inside a PG-water (propylene glycol – water) mixture through discharging orifices bubbling in the solution. This liquid presents the advantage of being food-grade and apparently harmless to health. In addition, according to practice it seems that the characteristic time of evaporation of the droplet deposited on surfaces is small, so that there is a cleaning effect that does not exist with other fluids like oils. Droplet size distribution at the outlet of the seeding generator is performed by means of a contemporary laser diffraction analyzer Malvern Spraytech®. Measurements reveal narrow-band, single distribution droplet sizes in the order of 1.5μm; suitable for seeding highly turbulent air flows.The use of dry air to generate particles of PG-water solution is proposed as a way of reducing the final particle diameter by means of water evaporation. Solutions with 60% mass fraction of PG reach sub-micrometric average diameters around 0.9μm at 6barg feeding gauge pressure.The proposed prediction model generalizes the results in an attempt to extrapolate them to other designs or fluids. It is based on the turbulent intensity downstream the nozzle exit and indicates that the discharging orifices depth is secondary in respect to the mean droplet diameter obtained.The study also shows that the volumetric concentration of the tracer particles in the generated aerosol is not much sensitive to feeding air pressure. Once the mean droplet size has been stablished with the previously commented method, this offers the possibility of estimating the number of particles generated and, thus, establishing design rules or fluid selection criteria for seeding generators or other similar devices. For feeding pressures from 2 to 6barg, particle quantity generation is in the order of 108particles/s per ∅ 0.5mm nozzle.