Analysis and experiment on atomizing characteristics of supersonic nozzle based on Laval effect

Abstract: Atomizing nozzle is a crucial part of spray device. And it is widely applied in many fields such as agricultural spraying, plant cooling and dust suppression. Moreover, the atomizing performance of nozzle has a great influence on the size and velocity of droplet, and the flow distribution. In order to improve the atomizing performance of the supersonic atomizing nozzle, atomizing principle of the supersonic atomizing nozzle is analyzed in this paper, and it is found that the velocity difference of air phase and water phase has a great effect on the atomizing performance. The spool structure of the atomizing nozzle is changed according to the supersonic principle of Laval nozzle when it is tapered before. The key size of Laval-style structure is important to its accelerated performance, the air inlet diameter is 4.5 mm, the throat diameter is 1.5 mm, the spool outlet diameter is 3.5 mm, the water inlet diameter is 0.9 mm, the subsonic contraction period length is 3.3 mm, the supersonic expansion period length is 6.2 mm, and the expansion period cone angle is 10°. A three-dimensional (3D) geometry model of Laval-style spool is built, supersonic can be achieved while the air pass through the nozzle, and velocity distributions law of the nozzle flow field is obtained by using computational fluid dynamics software Fluent. Furthermore, a test bench is built for atomization and the atomizing testing is carried out. The atomizing effect of nozzle is analyzed and compared before and after the spool structure being changed, and the atomizing performance influence rule of Laval-style atomizing nozzle is studied under different operational parameters. The numerical simulation results show that the velocity within most parts of the nozzle can reach supersonic and the velocity difference of air phase and water phase is significantly increased. In addition, the results of atomization testing indicate that the change of spool structure and the operational parameters have a great influence on atomizing performance, and the Laval-style atomizing nozzle is better than the tapered atomizing nozzle. Compared to the tapered atomizing nozzle, the droplet sprayed from Laval-style atomizing nozzle is smaller, the spray angle is larger, the droplet is more uniformly distributed in the flow field, and the droplet suspending time in the air is longer. In the test, the operational parameters of Laval-style atomizing nozzle include air pressure, water pressure and air-water pressure ratio. The air pressure is within the range of 0.3-0.6 MPa, the water pressure is within the range of 0.1-0.4 MPa, and the air-water pressure ratio is within the range of 0-6. The testing results reveal that it is beneficial to decrease the droplet size by increasing the air pressure or the air-water pressure ratio, but the effect will be the opposite by increasing the water pressure. The droplet size will increase when the values of air pressure and water pressure are almost the same or the droplet flow filed is split. However, the droplet size will decrease when the shrill crash appears during the spray process. Overall, with the air-water pressure ratio increasing in the range of 0-6, the droplet size shows an obvious trend toward decreasing. When the air-water pressure ratio is within the range of 0-3, the droplet size decreases rapidly by an amplitude that is as high as 90.56%, but when the air-water pressure ratio is within the range of 3-6, the droplet size displays a slow decrease. The Sauter mean diameter of droplet can be as small as 18.52 μm when the air-water pressure ratio is 6. The research results of the paper will provide a reference for the further research on the supersonic atomizing nozzle and the design of new atomizing device.