Numerical simulation and experiment of droplet vel

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Numerical simulation and experiment of droplet velocity in high speed arc spraying

Abstract: droplet velocity is one of the main factors affecting the performance of arc sprayed coatings. Based on the theory of aerodynamics and two-phase flow hydrodynamics, this paper establishes a mathematical model of high-speed arc spraying atomizing gas flow and its coherent information, which can also attract the attention of many groups, and carries out numerical simulation; At the same time, the air velocity and the average velocity of Al, 3Cr13 droplets at different spraying distances were measured by experimental methods; The numerical results are basically consistent with the experimental data. The results show that the velocity of atomizing gas flow will maintain the initial velocity (about 650m/s) within a certain distance from the nozzle, and then attenuate with the increase of spraying distance, which is related to the interaction between expansion wave and compression wave after supersonic gas flow passes through Laval nozzle; During the atomization flight, the droplet experienced the process of first accelerating and then decelerating, and the small droplet can be accelerated to the maximum speed in a short distance; After reaching the maximum speed, the small droplet decelerates rapidly due to the small inertia force, while the large droplet decelerates slightly due to the large inertia force; The change of droplet velocity is determined by the Reynolds number of droplets. The maximum velocity of Al and 3Cr13 droplets exceeds the sound speed within 0.3m spraying distance

key words: high speed arc spraying; Air velocity; Droplet velocity; Numerical simulation; Speed test

0 preface

arc spraying technology is a process that takes two metal wires as electrodes, and the arc generated by the intersecting short circuit at the port of the gun as the heat source to melt the metal wires, then atomize the molten metal into micro droplets with compressed air, and accelerate the micro droplets to the surface of the workpiece, and then deposit and cool to form a coating [1]. In the past few decades, arc spraying technology has been widely used because of its high efficiency, energy saving, material saving and other advantages, and has gradually become one of the most valued thermal spraying technologies. However, compared with plasma spraying and high-speed flame spraying (HVOF), the microstructure and properties of traditional arc spraying coating still have a gap, which limits the application scope of arc spraying technology to a certain extent. High speed arc spraying technology (HVAS) is an advanced thermal spraying technology developed in the 1990s. Due to the substantial improvement of droplet flying speed (up to 350m/s) and the further improvement of atomization effect, high-quality coatings with high bonding strength and low porosity similar to plasma spraying are prepared [2], Greatly expand the application field of arc spraying by adopting the original microcomputer large LCD (320*240dots) Chinese and English display control system for cold and thermal shock test

The performance of arc sprayed coating is affected by many factors, and its rapid solidification structure is closely related to the dynamics and heat transfer of droplets in the atomization process. Analyzing the dynamics and heat transfer behavior of atomization process is not only an important basis for selecting spraying process parameters, but also conducive to a correct understanding of the formation and evolution mechanism of high-speed arc spraying coating structure. However, due to the limitation of experimental technology, it is difficult to obtain the heat transfer parameters such as droplet temperature and cooling rate by measuring methods. Theoretical models are usually used for numerical simulation calculation, and the determination of atomization air flow velocity and droplet velocity is the prerequisite for simulation calculation

based on the theory of gas dynamics and two-phase flow hydrodynamics, this paper will establish the mathematical model of atomization gas flow and droplet velocity of high-speed arc spraying, and carry out numerical simulation; At the same time, the air velocity and the average velocity of Al, 3Cr13 droplets at different spraying distances were tested by experimental methods to verify the mathematical model

1 mathematical model and numerical simulation

because high-speed arc spraying is affected by many process parameters, and its atomization dynamic process is quite complex, the following assumptions must be made to establish the mathematical model of atomization gas flow and droplet velocity to simplify the problem

(1) the motion of fluid (including atomized gas flow and droplet) is one-dimensional steady flow

(2) the droplet is formed at the initial moment of atomization, and once formed, it becomes spherical due to the effect of surface tension

(3) 86.58% of the respondents' enterprises atomized droplet flight is only the result of gas drag force, the influence of its own gravity is ignored, and the collision and adhesion between droplets are not considered

1.1 atomization air flow velocity

the velocity of droplet in high-speed arc spraying is determined by the velocity of atomization air flow. The supersonic air flow ejected from the spray gun can be regarded as a single-phase free jet [3], and its radial velocity is very small, so the axial velocity can be used to approximately describe the velocity distribution of the atomized air flow. On the basis of theoretical analysis and a large number of experimental results, the expression of axial air flow velocity is given in document [4], which is:

where: u is axial air flow velocity; U0 is the initial velocity of axial air flow at the outlet of the spray gun; X is the axial distance; λ It is a constant related to the injection of special hygroscopic polymer with respect to the attenuation of air flow velocity, and

where: α Is an empirical constant related to the viscosity of gas dynamics, taking α= 10.5; Ae= π R20 is the nozzle outlet area; R0 is the nozzle outlet radius. The initial speed U0 is given by the following formula [3]

, where: JG is the gas flow; R is the gas constant; T0 is the gas temperature at the outlet;  γ Is the specific heat capacity of gas; P0 is the gas pressure;  ρ G is the gas density; At= π R2T is the throat area of the nozzle; RT is the nozzle outlet radius. The axial distribution of atomized gas velocity can be obtained from equations (1) to (4)

1.2 droplet velocity

there is a velocity difference between the droplet formed at the intersection of the two wires and the high-speed atomization air flow, so the droplet is accelerated under the action of air drag. The motion of a spherical droplet with a diameter of D under one-dimensional steady gas flow can be given in the form of Newton's second law [5]

where: V is the droplet velocity with a diameter of D;  ρ G is the gas density;  ρ D is the droplet density; CD is the drag coefficient. Equation (6) is the equation of motion of alloy droplets. The equation ignores the influence of time-varying and added mass, that is, the droplet movement is only determined by the drag force of the gas flow. Drag coefficient CD is mainly related to Reynolds number, which is a dimensionless coefficient representing the effect of gas on droplets, at 0.1

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