A HOLOGRAPHIC INVESTIGATION OF THE NEAR-NOZZLE STRUCTURE OF AN EFFERVESCENT ATOMIZER-PRODUCED SPRAY

An investigation of the near-nozzle structure of art effervescent atomizer-produced spray was conducted utilizing focused-image holography. This study builds upon previous work that focused on relating Sauter mean diameter (SMD) to operating conditions, fluid physical properties, and nozzle geometry, but did not address the details of the near-nozzle structure. Two areas of particular interest were how viscosity and surface tension influence the near-nozzle structure and the ultimate SMD. Therefore, holograms of the spray at the final exit orifice were made for fluids having three different viscosities and two different surface tensions, all at a liquid mass flow rate of 5 g/s and air-liquid ratios from 1 to 10%. The holograms show that increasing the air-liquid ratio resulted in a complex evolutionary breakup process. At air-liquid ratios of less than 2%, the breakup process is governed by individual bubble expansions. A cylinder of liquid(a ''trunk'') is observed exiting the final orifice and then exploding into ligaments and drops due to rapidly expanding bubbles. At air-liquid ratios greater than 5%, the length of the trunk is substantially reduced and sometimes even eliminated. Portions of the trunk are replaced by a ring of limbs, which forms a ''tree.'' As the air-liquid ratio is increased, the trunk decreases in length and the ring of limbs evolves from a small number of large limbs to a higher number of thinner limbs and branches. The holograms also show that an increase in viscosity increases the diameters of the fluid structures, although the effect is minimal at low viscosities. In addition, a decrease in surface tension was observed to decrease the diameter of the fluid structures. Finally, holographic results were compared to spray drop size data acquired with a Malvern spray analyzer and show that the ''knee'' in the curve of Sauter mean diameter versus air-liquid ratio corresponds to the transition in near-nozzle structure. The flat part of the curve corresponds to the high air-liquid ratio tree regime, while the steep part of the curve corresponds to the single-bubble regime. The drop size data also indicate that a decrease in surface tension yields a slight increase in SMD.