Laser-doppler-velocimetry

In various chemical or physical and technical processes particles of the order of 1 μm up to a few mm in size appear, whose size distribution is decisive or important for the process. There are many technical examples like in food production, pharmacy, process engineering, fuel burning in turbines, motors etc., slurry burning, and cosmetics. The laser diffraction particle sizer is capable to measure a particle size distribution from a collective of particles like a spray and to appropriately process, describe, and present it. The particle material may be solid or liquid in a gas or gas like air bubbles in a liquid. For the measurement it is only important, that the optical properties of the continuous and the dispersed phase are different. The laser diffraction particle sizer offers the advantage of a fast and non invasive measurement in a wide range of particle sizes. Especially in case of sprays or suspensions this method has become a standard measurement tool.

Particle size measurements are important in various applied sciences and in engineering, for example in fuel burning or in cavitation research as well as in particle size control. In all these applications it necessary to have knowledge about the particle dynamics. To gain that knowledge one has to measure the particle size and its velocity if possible in a noninvasive way with high local and temporal resolutions. A device which makes such measurements possible is the so called PhaseDopplerAnemometer (PDA), which has several advantages. First, it exhibits a large dynamic range from micrometer particles up to sizes in the millimetre range. Then it is very precise and no calibration procedure is necessary. Moreover, it is very robust against optical distortions. In the following the working principle of this flow and particle measurement technique will be explained.

For the generation and production of a spray the knowledge of information like the desired droplet size and its distribution is of fundamental importance. In order to characterise a spray correctly, one has to know the droplet size distribution of the droplets, which were generated out of a given liquid. In many applications it is desired to produce very small droplets. For the design and lay out of inhalers for example, it is necessary to achieve droplet sizes not larger than approximately 10 μm to be absorbed by the lungs. When one designs an atomisation nozzle in view of the desired droplet size distribution, one has to make sure via measurements, that the nozzle is really able the produce the expected droplet size distribution within an acceptable uncertainty range. Besides the droplet size distribution, the droplet velocity is an important parameter since it is a measure of the droplet momentum.

The particle image velocimetry (PIV) is an optical noninvasive working flow velocity measurement method. The flow field is made visible by introducing into the fluid small buoyancy neutral particles – the so called tracers. The particle flow then is illuminated with the aid of a light sheet and recorded at two defined different times within a small time interval with a digital camera. Because the particle move within this time interval with the flow, one can calculate the particles velocity by knowing its path between the two moments of illumination. The result is a velocity vector field of the flow. Here, we assume that the particles are small and follow the flow without any slip. Compared to local methods like LaserDopplerVelocimetry (LDV) with the ParticleImageVelocimetry one can reconstruct a whole flow region An extension of ParticleImageVelocimetry is the combination of PIV with thermometry (PIV/T).