CN110592404A - A Numerical Simulation Method of Bubble Movement and Mass Transfer in Aluminum Melt Dehydrogenation - Google Patents

Abstract

The invention discloses a method for simulating bubble motion and mass transfer numerical simulation of aluminum melt dehydrogenation, and relates to the technical field of aluminum melt; At the same time, the purification bubbles in the fluid are the research object. Considering the basset force, the mass transfer model of the bubble movement is obtained, and the main influencing factors on the transient movement and mass transfer of the purification bubbles in the composite field are obtained by using the MATLAB numerical analysis method. At the same time, water simulation is used for experimental verification. The present invention uses VOF to analyze the flow field of the bubble movement process, and analyzes the influence of rotation and ultrasonic effects through the changes in the velocity potential, pressure field, and bubble shape around the purified bubble during the process; The phase flow model can analyze the gas holdup in the liquid phase.

Description

A Numerical Simulation Method of Bubble Movement and Mass Transfer in Aluminum Melt Dehydrogenation

technical field

The invention belongs to the technical field of aluminum melts, and in particular relates to a numerical simulation method for bubble movement and mass transfer of aluminum melts under the composite action of rotating field and ultrasonic waves for dehydrogenation.

Background technique

In recent years, aluminum and its alloys have been widely used in electrical materials, aerospace materials, high-speed train underframes, automobile bodies and other fields due to their excellent properties such as low density, high strength, and formability. The related aluminum production and consumption With the expansion of the industrial scale, it increases sharply, but in the actual production process of aluminum and its alloys, it is very easy to react with water vapor in the air to generate hydrogen, and at the same time, the melt and the oxides in the furnace lining produce replacement reactions to form inclusions These gases and impurities form pores, white spots, porosity and other defects during the solidification process of the casting. In addition, when the hydrogen content is too high, the second type of hydrogen embrittlement will occur, which will affect the mechanical properties of the casting, casting performance, etc., which is very important for aluminum. Occasions with strict performance requirements will bring fatal consequences, because it not only destroys the continuity inside the aluminum material, but also reduces the effective cross-sectional area of the aluminum material. These pores are very easy to become crack sources during use. To obtain high-quality products, the purity of aluminum melt must be improved. Purification technology is the key to obtain high quality aluminum melt. Moreover, there is a mutual restrictive relationship between the inclusions and hydrogen in the aluminum melt. In the case where the complete removal of the inclusions cannot be guaranteed, degassing is the most direct method for purification, and the progress of purification technology depends on the development of purification theory. Therefore, it is very important to study the mechanism of degassing purification process of aluminum melt.

At present, among the degassing purification methods in aluminum melt, the most widely used method is the rotary blowing method in the bubble flotation method. Because of its simple operation, good degassing effect, and no pollution hazard caused by refining agents, it has been widely used. Applications, both purification equipment and methods have been in-depth research. However, the blowing method can only remove the hydrogen in the melt to a limited extent. During the rotary blowing purification process, the purity and properties of the introduced gas, the uniformity and residence time of the bubbles in the aluminum alloy melt Factors such as size and size are the determinants of the degassing effect. It is known that when the rotation speed of the rotary nozzle gradually increases, the bubbles are broken into smaller bubbles by the shear force generated by the rotation. The more uniform the distribution of the bubbles, the smaller the bubbles, the slower the floating speed, and the longer the contact time with the melt, the more effective the degassing. However, excessive rotation speed will cause defects such as air entrainment and inclusion, and will also hinder the uniform distribution of air bubbles, which seriously limits the improvement of purification efficiency. In order to further improve the efficiency of degassing and purification, it is necessary to promote the diffusion process of hydrogen between the gas-liquid two-phase, and it is necessary to study new degassing and purification technologies. As a new type of clean energy, power ultrasound has attracted widespread attention because it can refine grains, improve the performance of aluminum melts, and has obvious effects on impurity removal and can promote mass transfer between two phases. However, limited by the ultrasonic power, its range of action is small, and it cannot be used in mass production, so it can only be used in the experimental research process. It is not difficult to see from this that a single purification method has different effects. If you want to further improve the efficiency and quality of aluminum melt purification, it is necessary to study a composite degassing purification method.

Research status and analysis at home and abroad:

1. Current status of research on impurity removal methods and equipment: There are many classification standards for the methods of purifying aluminum alloy melts. At present, the more common classifications are divided into three categories according to the working principles of each method, namely adsorption purification method, non-adsorption purification method and Compound purification method.

1.1. Adsorption purification method: Adsorption purification refers to the direct physical, chemical or mechanical combination of adsorbents and gas and solid impurities in the melt to achieve the goal of purifying aluminum melts. It is relatively mature and widely used The purification methods of aluminum alloy melt mainly include blowing method, flux method, filtration method and so on.

The blowing purification method, also known as the bubble floating method, has developed a single-tube blowing method, a porous nozzle blowing method, a fixed blowing method and a rotating blowing method. Among them, in recent years, it is considered that the degassing effect is better than the rotary injection technology, and there are many researches and improvements on its nozzle structure. Foreign representatives mainly include SNIF method, ALPUR method, MINT method, RDU method and LARS method rotors, etc.; domestically, Jiang Haiyan and others proposed the pulse-inlet rotary injection technology, which can obtain smaller bubbles than ordinary rotary injection. , and when increasing the gas flow rate or increasing the rotor speed, it can prolong the lag time of the vortex on the liquid surface. Su Zhifu and others developed the upper and lower double-layer nozzle rotors. At the same time, in order to continuously improve the degassing efficiency, some researchers have also carried out water simulation and computer simulation of the internal flow field of the melt, bubble movement and degassing process, etc., which has certain reference significance for the optimization of rotor structure and process parameters. At the same time, due to the problem that the size of the bubbles in the rotating jet is difficult to reduce, WURZ et al. developed a new type of spray purification equipment. Canada STAS Company has designed two multi-nozzle rotary degassing structures, namely box type and trough type online degassing device, which usually contain 2-3 nozzles according to the flow rate of aluminum water. Alkali (earth) metals, hydrogen and inclusions in the melt.

The flux method refers to the addition of solid refining flux to the interior of the molten pool during the refining of the aluminum melt, and through a series of physical and chemical actions, the purpose of removing gas, solid inclusions, and impurity elements is achieved. In recent years, new methods of flux injection and rotary injection have emerged, which can improve the purification effect of flux, but the equipment pipeline is easy to block. Since then, Han Jiande and others proposed a new type of flux purification equipment to solve the above problems. At the same time, in order to protect the environment and meet the requirements of various melt processing, the research of multifunctional and environment-friendly composite fluxes is a hotspot. For example, Chen Faqin's NUC series composite fluxes and Tang Xiaolong's new rare earth composite fluxes have good purification effects in experiments. Zeng Jianming and others proposed to use the pressure difference to spray the flux into the melt and mix it fully and evenly with the melt to achieve efficient purification of the flux adding new technology, and the experiment proved that its purification effect is remarkable. At the same time, the research on the purification of aluminum melt by electroslag remelting technology is gradually increasing, and this technology is considered to have an obvious purification effect.

The filtration purification method refers to passing the aluminum melt through a neutral or active filter medium to separate the solid inclusions suspended in the melt. At present, the excellent filtration effect of ceramic foam filter has been affirmed by most researchers or engineers and technicians. It can filter out fine impurities below 20 microns, and can even filter out liquid flux impurities that are generally extremely difficult to filter out. The ceramic filter plates of Select, Vesuvius and Fuzhou Maitexin High Temperature Materials Co., Ltd. have advanced production technology and excellent product performance. Yin Zhidong developed a centrifugal filter device using multi-layer gradient filter series foam ceramics. The filtration flow rate can reach 200kg/min, which can meet the large flow demand of the production line. G. P. Martins et al. studied the interaction between metal melt and inclusion particles in ceramic foam filters. Through the study of the surface tension of the liquid phase of the melt and the critical velocity of the inclusions penetrating the liquid film, it is concluded that whether the inclusions can be adsorbed by the filter surface largely depends on the surface force of the liquid phase of the metal melt. In recent years, some researchers have proposed a new method to improve the filtration capacity of foam ceramics. Under the action of an electromagnetic field, the molten metal can pass through the foam ceramic filter smoothly. Experiments have proved that when the metal liquid flows through the foam ceramic filter, electromagnetic Loren Zili can make the molten metal "drain start" in the filter of 80ppi, and the pressure drop is equal to or lower than the pressure drop when the molten metal passes through 30ppi. Finally, multi-stage combined filtration represented by the same filtration method, different filtration stages, combination of different filtration methods and pore gradient filters will be the development direction of high-efficiency filtration and purification technology in the future.

1.2. Non-adsorption purification method:

Non-adsorption purification refers to a purification method that directly changes the equilibrium state of the two systems of aluminum-gas and aluminum-inclusions through certain physical effects such as ultrasonic waves, vacuuming, and density differences, and separates gas and solid impurities from the melt. , mainly including vacuum purification, ultrasonic purification, electromagnetic purification, etc.

The main theoretical basis of vacuum purification technology is Siward's law, which is considered to be the most effective method to reduce the hydrogen content in aluminum alloys. There are two types of vacuum purification, static and dynamic. The degassing efficiency of the static method is low; while the dynamic method accelerates the mass transfer in the melt through the flow of the melt to improve the degassing efficiency. Relevant experiments have confirmed that the effect of 5 minutes of dynamic degassing is better than that of 20 minutes of static degassing. However, this type of technology still has problems such as complex equipment, large investment and low processing capacity. Therefore, ZENGJM et al. developed a new type of vacuum degassing device, which generates a vacuum state in the suction head without a vacuum chamber, and can make the hydrogen content in 100g Al melt as low as 0.1mL.

Ultrasonic purification is a green and fast method of purifying aluminum alloys, and its research began in the former Soviet Union. In the 1930s, when studying the effect of ultrasonic waves on liquids and low-melting point melts, it was found that ultrasonic vibrations have the effect of degassing. In the early days, due to the constraints of technology and materials, ultrasonic purification research mainly focused on low melting point melts. In the 1990s, domestic and foreign research on ultrasonic degassing of magnesium and aluminum alloys with higher melting points began to increase. In terms of purification principle, researchers believe that its degassing mechanism is related to ultrasonic cavitation. In recent years, research on the influence of different process parameters on ultrasonic degassing has been common. The main conclusions are: ultrasonic vibration has a better degassing effect on small-volume melts, ultrasonic power and action time are key factors for degassing, and process parameters The generation and existence of acoustic cavitation bubbles should be promoted; Li Xiaoqian et al. believed that the lower the ultrasonic frequency, the better the degassing. In the research of new ultrasonic purification technology, LIJW et al. proposed to introduce a new technology of bottom strong cooling in the ultrasonic degassing device; foreign countries have begun to pay attention to the research of composite purification technology combining ultrasonic purification with other types of purification.

Electromagnetic purification technology is a rapidly developing new material processing technology that efficiently removes non-metallic inclusions in liquid metals. The principle of its removal is electromagnetic separation. In recent years, electromagnetic purification technology has become a research hotspot. In the field of theoretical research, TERADAT et al. performed theoretical calculations on the separation of impurities in aluminum melts in superconducting magnetic fields and verified the accuracy of calculations through experiments. It is worth mentioning that researchers have done many studies on the influence of process parameters on impurity removal in different electromagnetic fields. A more obvious purification effect has been achieved. In terms of computer simulation, AFSHARMR et al. simulated the electromagnetic purification process of the melt in a constant magnetic field orthogonal DC current environment, and at the same time proved the accuracy of the optimal parameters of the simulation through experiments. Recently, a new type of purification technology has emerged. The online electromagnetic purification technology is a kind of online filtration outside the furnace. Because the aluminum melt is conductive, but the alumina is not conductive. Under the action of the electromagnetic field, the aluminum melt tends to one side and crowds out the alumina The trend of biasing towards the other side makes the alumina segregated and pushed to the adsorbed ceramic tube, thereby realizing the purification of the aluminum melt.

1.3. Compound purification method:

Composite purification mainly refers to a new purification method that uses two or more purification technologies at the same time and complements the advantages of various technologies to significantly improve the purification effect and efficiency of aluminum melt. At present, it mainly includes compound degassing and degassing-filtration compound purification.

At present, the main research direction and hotspot in foreign countries in compound degassing is the compound degassing technology based on ultrasonic purification combined with other methods. XUHB et al. studied the effect of ultrasonic vibration and vacuum composite purification, and found that this type of technology can significantly improve the degassing efficiency, and the degassing efficiency is still high when the vacuum degree is not too large. HAGHAYEGHIR et al. combined the blowing method with ultrasonic degassing, and developed a new type of composite degassing equipment that enhances the effect of ultrasonic degassing by argon blowing, which can effectively solve the problem that ultrasonic degassing cannot handle large-capacity melts. PUGAH et al. have developed a new device for enhanced ultrasonic degassing by low-frequency mechanical vibration, which can stir the melt through low-frequency mechanical vibration, which can minimize the porosity in the alloy without increasing the processing time. The degassing-filtration compound purification method is a mature compound purification technology, which has dual functions of degassing and impurity removal, and has been widely used in major aluminum and aluminum alloy production enterprises in the world. At present, degassing-filtering composite purification devices are widely used in online and continuous purification, and the commonly used combination methods are generally two-stage degassing and multi-stage gradient filtration. In terms of degassing, it is mainly the blowing method, in which the multi-hole head blowing and rotary blowing are the main ones. The filtration technology is mainly based on flux filtration and gradient filtration of filters of different materials, and ceramic foam filters are generally used as filters.

It is foreseeable that the multifunctional composite flux and its addition technology, ultrasonic purification, electromagnetic purification and composite purification methods will be the focus and hotspot of research on the purification of aluminum alloy melts in the future. The development of simple, efficient, cheap and pollution-free purification technology and devices is the general development trend of aluminum alloy purification technology.

2. Research status of bubble motion equation and mass transfer:

Research status of bubble motion characteristics under rotation: the rotary injection method mainly generates a swirling field in the fluid through the agitation of the rotor. A large number of related literatures have proved that the swirling field has a great influence on the motion of the bubbles, and the bubble movement process will affect the flow rate of the bubbles in the melt. Efficiency of degassing in the body. In the two-phase and multi-phase fluid dynamics book edited by Guo Liejin, the stress of particles in different phases is analyzed. Cao Zhongwen and others analyzed the radial force of the dispersed phase particles in the rotating field, and determined the motion equation of the particles in the process through Newton's second law. Li Xuebin and others analyzed the lateral force of particles in the liquid phase, while Tian Hengdou and others analyzed the axial force of bubbles in the liquid. The research method of the above literature is to study the force of bubbles under the condition of rotating air blowing Provided ideas.

Current status of mass transfer research: The principle of introducing inert gas into the aluminum melt to remove hydrogen is: the initial pressure of hydrogen in the bubbles is zero, and there is a pressure difference of hydrogen at the interface between the bubbles and the molten aluminum, so that the hydrogen dissolved in the molten aluminum Hydrogen is continuously sucked into the bubbles, and this suction process will not stop until the pressure of hydrogen in the bubbles is equal to the pressure of hydrogen in the aluminum liquid. After the bubbles rise above the liquid surface, the hydrogen in the bubbles escapes and enters the atmosphere.

In order to improve the degassing efficiency, the number of bubbles should be increased as much as possible, the diameter of the bubbles should be reduced, and the distance of the bubbles floating in the aluminum liquid should be prolonged, and the rotary injection method can effectively enhance the above goals, so the hydrogen removal effect of the rotary injection method better. According to the hydrogen partial pressure theory, the influencing conditions of the melt purification process can be qualitatively analyzed, but the purification effect cannot be directly reflected. To further improve the purification rate of the rotary blowing process, it is necessary to conduct a quantitative analysis of the process to obtain certain influencing parameters.

The power ultrasonic degassing purification mechanism is mainly the ultrasonic cavitation mechanism, which uses the cavitation effect of ultrasonic cavitation bubbles generated under the action of ultrasonic field and the direct action of ultrasonic to change the pressure field and temperature field inside the aluminum melt, thereby affecting the internal transmission of the melt. qualitatively. Zhao Zhiping introduced the promotion of mass transfer by the four major effects of ultrasonic cavitation (mechanical effect, thermal effect, light effect and activation effect) through the double-membrane model. Wang Jie et al. used numerical analysis methods to analyze the movement process of cavitation bubbles, and the results showed that the bubbles would collapse rapidly after exceeding a certain size. H. Puga combined ultrasound with molten metal agitation, applied water simulation tests, and monitored the ultrasonic pressure in different directions of the container. The test results show that after combining the two methods, the degassing effect of the alloy material has been significantly improved, and the material performance can be improved.

Contents of the invention

In order to solve the problems of the prior art; the purpose of the present invention is to provide a method for numerical simulation of bubble movement and mass transfer in aluminum melt dehydrogenation.

A method for bubble movement and mass transfer numerical simulation of aluminum melt dehydrogenation of the present invention, its method is as follows: on the basis of diffusion mass transfer theory and Rayleigh equation, purify bubbles in the fluid under the simultaneous action of rotation and ultrasonic field As the research object, the bubble movement mass transfer model is obtained considering the basset force, and the main influencing factors on the transient movement and mass transfer of the purification bubbles in the compound field are obtained by using the MATLAB numerical analysis method. At the same time, the water simulation is used for experimental verification. The specific method is as follows:

1. Analyze the mass transfer during the movement of the inert bubbles under the action of the composite field, analyze the main force of the purification bubbles in the flow field, establish the transient motion and mass transfer equations of the purification bubbles in the composite field, and establish the corresponding mathematical coupling model;

2. Use the Runge-Kutta method in MATLAB to perform numerical calculations, study the trajectory of the purification bubbles, the relationship between radius change and time, and the mass transfer curve, and compare and analyze it with the single rotation condition;

3. On the basis of numerical analysis, the water simulation method is used to verify the above calculation results and determine the optimal process parameters of the compound purification method.

Compared with prior art, the beneficial effect of the present invention is:

VOF can be used to analyze the flow field of the bubble movement process, and the influence of rotation and ultrasonic action can be analyzed through the change of velocity potential, pressure field and bubble shape around the purified bubble during the process; the multiphase flow model can be used to analyze the gas in the liquid phase. content analysis. Numerical calculation using MATLAB can directly obtain the change curve of the purification bubble radius and the pressure field around the bubble with respect to time, and further analyze the change of mass transfer during the bubble movement.

Detailed ways

This specific embodiment adopts the following technical solutions:

Because the dynamic viscosities of water at 20 °C and aluminum alloy melt at 730 °C are approximately equal, the two conform to the triangular similarity rule of motion similarity, geometric similarity and dynamic similarity, so this subject uses water simulation experiments: Supersaturated oxygen is introduced into the water to simulate the dissolved hydrogen in the aluminum melt, and an inert gas is introduced to simulate the process of bubble dehydrogenation. Use a dissolved oxygen meter to measure the concentration of oxygen in the purified water to determine the purification effect. While feeding the inert gas, the trajectory of the bubble movement and the distribution of the bubbles are observed through a high-speed camera, and finally compared with the numerical calculation results.

The experimental plan is as follows:

1. Experimental equipment: rotary blowing device system, ultrasonic equipment, high-speed camera, dissolved oxygen meter and data acquisition software;

2. Experimental parameters: the main influencing parameters are rotating nozzle speed, degassing time, air flow, ultrasonic power;

The experimental parameters are as follows:

Table 1 Rotary injection assignment table

Table 2 Ultrasound assignment table

The rotary jet orthogonal experiment scheme is shown in Table 3:

Table 3 Orthogonal experiment table of rotary injection

The ultrasonic degassing orthogonal test scheme is shown in Table 4:

Table 4 Orthogonal experiment table of ultrasonic degassing process

The composite degassing orthogonal experiment scheme is shown in Table 5:

Table 5 Composite degassing orthogonal experiment table

This embodiment is based on the flow field simulation of the VOF calculation method, the main application software is Fluent, OpenFOAM, etc., and the MATLAB numerical analysis based on the Runge-Kutta calculation method. VOF can be used to analyze the flow field of the bubble movement process, and the influence of rotation and ultrasonic action can be analyzed through the change of velocity potential, pressure field and bubble shape around the purified bubble during the process; the multiphase flow model can be used to analyze the gas in the liquid phase. content analysis. Numerical calculation using MATLAB can directly obtain the change curve of the purification bubble radius and the pressure field around the bubble with respect to time, and further analyze the change of mass transfer during the bubble movement.

The compound degassing purification is not a simple combination of methods. If you want to improve the purification efficiency of aluminum melt, you must study the mechanism. To this end, the superiority of the rotary air blowing method and power ultrasonic purification is comprehensively used, and a composite aluminum melt purification method based on rotary air blowing and power ultrasonic purification is proposed. This method mainly purifies the aluminum melt by purifying bubbles, and promotes the diffusion process of hydrogen in the melt from the liquid phase to the gas phase through ultrasonic action, in which the size of the bubbles and the distribution of the bubbles have a great influence on the purification effect.

It will be apparent to those skilled in the art that the invention is not limited to the details of the above-described exemplary embodiments, but that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Accordingly, the embodiments should be regarded in all points of view as exemplary and not restrictive, the scope of the invention being defined by the appended claims rather than the foregoing description, and it is therefore intended that the scope of the invention be defined by the appended claims rather than by the foregoing description. All changes within the meaning and range of equivalents of the elements are embraced in the present invention.

In addition, it should be understood that although this specification is described according to implementation modes, not each implementation mode only includes an independent technical solution, and this description in the specification is only for clarity, and those skilled in the art should take the specification as a whole , the technical solutions in the various embodiments can also be properly combined to form other implementations that can be understood by those skilled in the art.

Claims (1)

1. A bubble movement and mass transfer numerical simulation method for aluminum melt dehydrogenation is characterized by comprising the following steps: the method comprises the following steps: on the basis of a diffusion mass transfer theory and a Rayleigh equation, a bubble motion mass transfer model is obtained under the condition that the basset force is considered by taking purified bubbles in fluid under the simultaneous action of a rotating field and an ultrasonic field as a research object, main influence factors on transient motion and mass transfer of the purified bubbles in a composite field are obtained by using an MATLAB numerical analysis means, and experimental verification is carried out by using water simulation, wherein the specific method comprises the following steps:

analyzing mass transfer in the movement process of inert bubbles under the action of a composite field, analyzing the main stress of purified bubbles in a flow field, establishing transient movement and mass transfer equations of the purified bubbles in the composite field, and establishing a corresponding mathematical coupling model;

secondly, numerical calculation is carried out by using a Longge Kuta method in MATLAB, the relation between the motion track and the radius change of the purified bubbles and the time and the mass transfer change curve are researched, and comparative analysis is carried out on the purified bubbles and the purified bubbles under the condition of single rotation;

and thirdly, on the basis of numerical analysis, performing experimental verification on the calculation result by using a water simulation method, and determining the optimal process parameters of the composite purification method.