Phase Doppler Anemometry

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 non-invasive way with high local and temporal resolutions. A device which makes such measurements possible is the so called Phase-Doppler-Anemometer (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. The PDA uses the additional information which is included in the phase position of the scattered light which in turn is a measure of the particle size. The Phase-Doppler-Anemometer is an extension of the classical Laser-Doppler-Velocimeter (LDV), with which one can measure the local flow velocity in a non-invasive manner. Similar to a LDV one can explain the working principle with the aid of a simple interference model. The LDV technique uses only one photo detector for catching the optical signal send by the tracer particle. In contrast to that, a PDA uses several photo detectors called photo multiplier. The figure below shows the situation of a particle in a measurement volume of crossing laser beams in the presence of two angularly shifted detectors. The frequency of both signals is equal, while the phase position of the signals is different. So the detectors collect Doppler signals of equal frequency but different phase position when the particle traverses the test volume, because relatively to each other they are differently positioned. While the measured frequency of the signals contains the velocity information of the particle, the phase positions contain the information of the particle size.

The optical/mechanical system of the PDA consists of a laser, the sending and receiving units, and an optical bench. The laser type one applies depends on the particle size range one expects to measure, the velocity range, and on the distance to the measurement location and its accessibility. The receiving optics consists of an integral set up and usually includes three photo multiplier. The signal processor is based on the application of a correlation function allowing a highly precise measurement of the frequency and phase of Doppler signals.

In our technical department, we apply PDA-techniques for the measurement of velocity fields and particle sizes of cyclone flows and similar separators, in order to obtain an optimal separator design. Also, the Phase-Doppler-Anemometer is applied to environmental measurements and gas cleaning processes for optimisation purposes. A further typical application is the analysis of atomisation and spray processes and the optimisation of spraying systems.