Spray and mixing characteristics of liquid jet in a tubular gas–liquid atomization mixer

Abstract

For the design and optimization of a tubular gas–liquid atomization mixer, the atomization and mixing characteristics of liquid jet breakup in the limited tube space is a key problem. In this study, the primary breakup process of liquid jet column was analyzed by high-speed camera, then the droplet size and velocity distribution of atomized droplets were measured by Phase-Doppler anemometry (PDA). The hydrodynamic characteristics of gas flow in tubular gas–liquid atomization mixer were analyzed by computational fluid dynamics (CFD) numerical simulation. The results indicate that the liquid flow rate has little effect on the atomization droplet size and atomization pressure drop, and the gas flow rate is the main influence parameter. Under all experimental gas flow conditions, the liquid jet column undergoes a primary breakup process, forming larger liquid blocks and droplets. When the gas flow rate ( Q g ) is less than 127 m 3 ·h −1 , the secondary breakup of large liquid blocks and droplets does not occur in venturi throat region. The Sauter mean diameter (SMD) of droplets measured at the outlet is more than 140 μm, and the distribution is uneven. When Q g > 127 m 3 ·h −1 , the large liquid blocks and droplets have secondary breakup process at the throat region. The SMD of droplets measured at the outlet is less than 140 μm, and the distribution is uniform. When 127 < Q g < 162 m 3 ·h −1 , the secondary breakup mode of droplets is bag breakup or pouch breakup. When 181 < Q g < 216 m 3 ·h −1 , the secondary breakup mode of droplets is shear breakup or catastrophic breakup. In order to ensure efficient atomization and mixing, the throat gas velocity of the tubular atomization mixer should be designed to be about 51 m·s −1 under the lowest operating flow rate. The pressure drop of the tubular atomization mixer increases linearly with the square of gas velocity, and the resistance coefficient is about 2.55 in single-phase flow condition and 2.73 in gas–liquid atomization condition. The breakup morphologies of liquid jet with different liquid and gas flow rates are photographed by a high-speed camera respectively. The liquid jet column swings and breaks up into large droplets and liquid blocks in the air flow. When flow through the Venturi throat, these large droplets and liquid blocks break up into smaller droplets. The particle size and velocity distribution of atomized droplets were measured by Phase-Doppler anemometry (PDA). Under the same gas flow rate condition, the droplet sizes at different measured points are basically the same, and the droplet size decreases gradually with the increase of gas velocity. With the increase of gas velocity, the droplet size distribution of each measured point is closer to a straight line, and the uniformity of atomized droplet distribution in the tube is better. With the value of the gas velocity 72 m 3 ·h −1 , the average droplet size is about 200 μm, and with the value of the gas velocity 216 m 3 ·h −1 , the average droplet size is about 50 μm. • Experiments were performed on droplet size and velocity in tubular atomizing mixer. • Breakup morphologies of liquid jet under different flow conditions were obtained. • The relationship between gas flow rate and droplet size was obtained. • Gas–liquid atomizing and mixing characteristics in tubular mixer were analyzed.