AbstractudA Phase Doppler Anemometry system has been set up to characterize the behaviour of differentudarrangements of static impactors that are used as drift eliminators to intercept and remove residual water dropletsudentrained in the hot air flow released by an evaporative tower. The investigated evaporative tower has a squareudsection of 60x60 cm, that is the standard size for modular separation elements; it was tested while working withudor without the droplet separators; for each test condition, the residual water droplets entrained by the air flow andudexpulsed by the tower are detected few centimetres above the tower exhaust and are characterized by the PDAudsystem that can measure the velocity and diameter of each droplet. Many parameters describing the dropletudpopulation over the whole exit section, (number, mean velocity and diameter) can be obtained. The distributionudof the droplets is calculated as a function of their diameter, and represented as a percentage of their total numberudand of their total volume. The velocity-diameter plot of the same droplets shows other aspect of the population.udDroplets with same diameter show a spread velocity distribution due to the high turbulence of the exhaust airudflow. Droplets with larger diameters have smaller mean velocity, since the gravitational downward force is notudnegligible compared to the aerodynamic upward drag from the air flow. Few very large droplets have evenudnegative velocity: they are interpreted as droplets that, after the expulsion, are falling back downward. The use ofudan LDV-PDA system allows to detect such droplets and to discard them from the separation efficiencyudcalculations. The efficiency can be calculated by direct comparison of the number of water droplets that areuddetected in the exhaust air flow, both globally or for any specific class of droplet size. Global results can beudcalculated on the basis of the number or of the volume of the droplets. The accuracy of the efficiency estimationudis also studied. Two main aspects are considered. The first is that the PDA measurement volume dimensionudvariation with the detected droplet size: it has negligible effect on the result per classes of diameters, but theudeffect on the global efficiency is present and will be discussed. The second aspect is the presence of droplets thatudare falling downward in the upward air flow: their presence should be considered and corrected for. The resultsudof this paper are useful when comparing them to other measurements obtained with techniques that are not ableudto detect the droplet velocity.
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