In a limited measuring volume two laser beams intersect (figure 1, left) forming an interference pattern (figure 1, center). A particle crossing the measuring volume scatters light with the Doppler frequency f_D which is proportional to the velocity of the particle. Velocity u can be calculated as the product of Doppler frequency and the distance of the fringes of the interference pattern (δ_f): u = δ_f⋅f_D If the frequency of the two intersecting laser beams is equal, a stationary interference pattern is formed. If two particles equal in the amount of speed but with opposing directions of flight cross the measuring volume scatterd light equal in Doppler frequency is detected. It is not possible to determine the direction of flight. Technically this problem is solved by a frequency shift between the laser beams. This results in a moving interference pattern. Thereby light scattered by a particle has a frequency f of Δf ± f_D depending of the direction of flight. Besides determination of the flow direction measurement of small flow rates becomes possible. The droplet size determination is based on one of the Doppler signals (figure 1, right) detected on three spatially separated detectors. The separation of the detectors causes a phase shift of the signal which correlates with the curvature radius of the droplets. The knowledge of the curvature radius allows the determination of the radius of the droplets. Furter information an brochures can be found at TSI Inc..

Services: measurement and testing

The PDPA allows for example the determination of flow rates and particles sizes of sprays, the evaluation of droplet separators and the measurement of velocity fields behind duct installations. The following figure as an example shows the flow profile of a flat suction hood.

By the help of this data you are able to benchmark or improve the performance of your products and evaluate the results of your flow simulations. If you have got other flow problems we would also like supporting you solving them.