Generation and Control of Tracer Particles for Optical Flow Investigation in Air

Kurzfassung

The production of mono-disperse tracer particles with suitable properties for optical flow investigations, such as small size, spherical shape, smooth surface, appropriate density and diffraction index, non-evaporating and agglomerating, electrically neutral, non-toxic and easily removable, is a challenging task due to the sensitivity of the particle size distribution to the boundary conditions. In order to obtain general design and operating rules for atomizers which are mostly applied for air flows, this dependence is investigated here. It is shown that high concentrations of narrow band particle size distributions, with a mean diameter below 1 lm, can easily be generated by means of multi-hole nozzles under overcritical pressure conditions, when the kinetic energy, transferred into the seeding liquid through the nozzle exits, is well balanced with the liquid volume inside the atomizer. In addition, flow visualization pictures are presented which permit a useful assessment of the functioning of the nozzles and reveal operating features of the atomizers which were not previously known. In particular, it is shown that existing explanations of the importance of certain design features of the Laskin nozzle are of minor importance for the generation of appropriate tracer particles.