Near-wall spray dynamics: what actually deposits?

Droplet–wall interactions set the efficiency of spray cooling, fuel injection, and spray coating, but they occur in the first millimetre above the surface, closer than a conventional perpendicular Phase Doppler anemometry (PDA) setup can measure. Using an angled PDA arrangement, this study resolves droplet size and velocity as close as 0.1 mm above the substrate. Two atomizers were compared, a conventional airbrush (AB) and an in-house ultrasonic (US) nebulizer-based device, over a range of pressures and cylindrical substrates 10 to 30 mm in diameter. Most of the droplet volume kept Stokes numbers above unity and stayed ballistic to the wall, so entrainment was a minor loss. Near the wall the impact angle changed sharply. The radially weighted mean droplet angle stayed near 10° from 40 down to 5 mm, then rose to 38–72° at 0.1 mm, lowering the normal Weber number into the rebound range. Droplet fate therefore depended on where it was evaluated. At 1.6 mm most volume counted as lost, with miss reaching 85–88% for the smallest substrate, while at 0.1 mm it counted mostly as deposited, above 75% for larger substrates at low pressure. The predicted rebound volume changed by more than 100% with the Weber-number criterion used. The two atomizers lost volume differently. The AB lost mostly to miss, worst on the smallest substrate, and the high-pressure US device mostly to splashing. Neither was better overall. Deposition was highest for the largest substrate at the lowest pressure, which limited miss and splash together.