Abstract
The present study compares two twin-fluid atomizer concepts based on the airflow (shock waves) pattern obtained through shadowgraph imaging for atomization of water with a low air/water pressure supply. The research work was conducted using the backlight imaging technique for converging (sonic) and converging–diverging (supersonic) air-assist atomizers with a 3.0 mm (throat) diameter. An annular sheet of thicknesses 70 µm and 280 µm with a high-speed air-core was employed to study the breakup dynamics for different water mass flow rates (100–350 kg/h) and air mass flow rates (5–35 kg/h). Different sheet breakup patterns were identified as the function of the ALR ratio (air-to-liquid mass flow), liquid Weber number (WeL), and Reynolds number (Reg). Different breakup modes extend from canonical Rayleigh bubble breakup, ligament-type breakup, to the pure pulsating breakup via annular sheet disintegration. The sheet breakup dynamics were studied in terms of spray angle and breakup length. With higher ALR values, breakup length showed a decreasing trend, while spray angle showed an increasing trend in the converging and converging–diverging (CD) air-assist atomizers, respectively, owing to the drastic difference in the jet flow dynamics.
Highlights
Twin-fluid atomization is widely used, especially for heavy Newtonian fluids or non-Newtonian fluids
For higher flow rates (350 kg/h), the spray angle was narrower for the 70 μm sheet than for the 280 μm sheet because of the larger liquid axial momentum; spray diverges more for the 280 μm sheet because of the relatively lower velocity of the sheet, resulting in a more intense aerodynamic interaction of the high-speed air jet with the liquid sheet
Various modes were obtained from canonical Rayleigh bubble formation at very low air-to-liquid ratio (ALR) values, the annular sheet disintegration, and the ligament-type breakup at very high ALR values
Summary
Twin-fluid atomization is widely used, especially for heavy (viscous) Newtonian fluids or non-Newtonian fluids. Based on the inner/outer air velocity or momentum, many modes or breakup patterns were identified. Kawano et al [4] investigated the sheet breakup and found two modes based on a critical air velocity—liquid lump and liquid film. Leboucher et al [3,9] thoroughly studied the breakup based on air–liquid momentum and found modes such as Rayleigh, bubble, christmas tree, and pure pulsating. The main objective is to examine the effect of the sonic (converging) or supersonic (CD) air-assist atomizer on the annular sheet breakup and the resulting spray pattern. The CD nozzle goes through overexpansion (Pexit < Pambient), resulting in the initial formation of oblique shock waves Both configurations belong to a unique class that may result in entirely different breakup characteristics for the novel atomizer. Airflow rate 1 Water flow rate 1 Air-to-liquid ratio (ALR) Liquid Weber number (Wel) Air Reynolds number (Reg)
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