Measurements of the droplet behavior near the wall in a vertical rectangular duct were conducted by a phase Doppler particle analyzer (PDPA). The test Reynolds number and drop size range is from 18,500 to 89,300 and from 5 μm to 110 μm, respectively. Results show that the negative slip-velocity of the drops near the free-stream region normally results in the reversed slip-velocity phenomenon in the boundary layer region. No negative slip-velocity of all drops are discovered for Reynolds number less than 38,300. This indicates no reversed slip-velocity phenomenon for the test drop size range under low Reynolds number conditions. However, when the Reynolds number is over 38,300, the free-stream slip-velocity of the bigger drops becomes negative. It is found that the negative slip-velocity and, hence, the reversed slip-velocity phenomenon may take place for drop size larger than 52 μm to 90 μm depending on the flow Reynolds number.

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