January 10, 2022

Theoretical Analysis of Nozzle Atomization

Studies have shown that the nozzle atomization process is mainly controlled by four forces, namely aerodynamic drag, viscous force, liquid surface tension and inertial force. The interaction between these four forces causes the continuous liquid injection to split and break. Generally, the nozzle atomization process is divided into jet atomization process and liquid film atomization process.

Rayleigh analyzed the jet breaking mechanism in 1876. He used the small disturbance method to analyze the conditions required for the breaking of the low-speed jet, and believed that only when the wavelength of the symmetrical disturbance wave was comparable to the diameter of the jet could the jet break. Tyler studied the relationship between jet breakage and the wavelength of the disturbance wave by measuring the frequency of jet breaking, which verified Rayleigh’s theoretical analysis.

Weber developed a more general low-speed viscous jet breaking theory, proposed that there is an optimal disturbance wave wavelength for jet breaking, and gave its expression. By analyzing the interaction of various forces in the process of liquid atomization, he believes that the frictional effect of aerodynamic forces on the liquid and the high-speed flow inertia of the liquid itself are important reasons that cause the droplets to break. When the effect of aerodynamic resistance is greater than the surface tension, the liquid will be atomized, and the liquid droplets will peel off on the surface of the liquid. Based on this, he proposed a dimensionless constant-the Weber number, and gave important index parameters such as the critical Weber number and the critical liquid velocity for the occurrence of atomization.

Haenlein verified Weber’s conclusions through experiments and divided the liquid jet atomization into four types of processes: droplet formation without air influence, droplet formation with air influence, droplet formation caused by jet fluctuations, and jet flow It is atomized when it is completely broken. Ohnesorge organizes the data according to the importance of the force of the jet, and introduces the dimensionless number Ohnesorge number to divide the jet breaking process into 3 stages:

(1) Low Reynolds number section, at this time Rayleigh mechanism controls the crushing process:
(2) In the middle Reynolds segment, the jet breaking is controlled by jet disturbance;
(3) High Reynolds number segment, the atomization process is completed within a short distance from the nozzle outlet.

This category is widely cited. Recently, in order to solve the problem of uncertain states in the Ohnesorge classification chart, Reitz analyzed the test data of diesel engine spray and proposed the following 4 types of crushing states: Rayleigh-shaped crushing; primary wind-induced crushing; secondary wind-induced crushing and atomization.

Fraser and Eisenklam defined three methods of liquid film breaking: edge shedding, surface fluctuation and liquid film perforation. They believe that when the liquid film is broken, it first transforms into a liquid zone, and then continues to break into liquid droplets. The droplets formed by the edge shedding still move in the direction before breaking. The droplets formed by the liquid film perforation method have good uniformity, while the size of the droplets formed by the surface wave method varies greatly. For nozzles where the liquid film is atomized, the three breaking methods may occur at the same time. In the 1950s, Dombrowski and Fraser conducted in-depth studies on the breaking process of liquid membranes through a large number of experiments. They found that the liquid band is mainly caused by the perforation of the liquid film. If the hole is caused by air friction, the liquid band will break very quickly; and if the hole is caused by the turbulence in the nozzle, the liquid band will break very slowly. They concluded that: the liquid film with high surface tension and high viscosity is the most difficult to break: the density of the liquid hardly affects the breaking of the liquid film. York et al. conducted theoretical and experimental studies on the breaking mechanism of the flat liquid film, and concluded that the instability between the continuous phase and the discrete phase interface and the formation of surface waves are the main factors affecting the breaking of the liquid film into droplets.

It can be seen that the above-mentioned mechanism analysis is inseparable from the support of experiments. Due to the complexity of the atomization process, almost all theoretical research results so far are empirical and semi-empirical.

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