CN110592404A - Bubble movement and mass transfer numerical simulation method for aluminum melt dehydrogenation - Google Patents

Abstract

The invention discloses a bubble movement and mass transfer numerical simulation method for aluminum melt dehydrogenation, and relates to the technical field of aluminum melts; the method comprises the following steps: on the basis of a diffusion mass transfer theory and a Rayleigh equation, purified bubbles in fluid under the simultaneous action of a rotating field and an ultrasonic field are taken as research objects, a bubble motion mass transfer model is obtained under the condition of considering the basset force, 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 meanwhile, experimental verification is carried out by using water simulation; the gas content in the liquid phase can be analyzed by using a multiphase flow model.

Description

Bubble movement and mass transfer numerical simulation method for aluminum melt dehydrogenation

Technical Field

The invention belongs to the technical field of aluminum melts, and particularly relates to a bubble movement and mass transfer numerical simulation method for aluminum melt dehydrogenation under the combined action of a rotating field and ultrasound.

Background

In recent years, aluminum and its alloys have been widely used in the fields of electrical materials, aerospace materials, underframe of high-speed trains, car bodies and the like due to their excellent properties such as low density, high strength, formability and the like, and the production and consumption of aluminum related thereto have been increased sharply with the enlargement of industrial scale, but aluminum and its alloys are very likely to react with water vapor in the air during the actual production process to generate hydrogen, and at the same time, the melt and oxides in the lining have a displacement reaction to form inclusions, and these gases and impurities have defects such as pores, white spots, looseness and the like during the solidification of castings, and in addition, when the hydrogen content is too high, a second type of hydrogen embrittlement is generated to affect the mechanical properties, casting properties and the like of castings, which may bring fatal consequences in the case of severe requirements for the properties of aluminum materials because it not only destroys the continuity inside the aluminum materials, but also reduces the effective sectional area of aluminum materials, these pores are highly likely to be sources of cracks during use. For this reason, the purity of the aluminum melt must be increased to obtain a high-quality product. The purification technology is the key to obtaining high quality aluminum melt. In addition, the mutual restriction relationship exists between the inclusions in the aluminum melt and hydrogen, and under the condition that the complete removal of the inclusions cannot be guaranteed, the degassing is the most direct method for purification, and the progress of the purification technology depends on the development of the purification theory, so that the research on the mechanism of the degassing and purification process of the aluminum melt is very important.

At present, the rotary blowing method in the bubble floating method is the most widely applied method in the degassing purification method of the aluminum melt, and the rotary blowing method is widely applied due to simple and convenient operation, good degassing effect and no pollution hazard brought by a refining agent, and both the purification equipment and the purification method are deeply researched. However, the blowing method can only remove hydrogen in the melt to a limited extent, and in the rotary blowing purification process, factors such as the purity and the property of the introduced gas, the uniformity, the residence time and the size of bubbles in the aluminum alloy melt are the determining factors of the degassing effect. It is known that when the rotating speed of the rotary nozzle is increased gradually, bubbles are broken into finer bubbles by shearing force generated by rotation, the more uniform the distribution of the bubbles is, the more fine the bubbles are, the slower the floating speed is, and the longer the contact time with the melt is, the more effective the degassing is, but an excessive rotating speed causes defects such as entrainment and inclusion, and also hinders the uniform distribution of the bubbles, which severely limits the improvement of the purification efficiency. In order to further improve the efficiency of degassing and purification, it is necessary to promote the diffusion of hydrogen between the gas phase and the liquid phase, and to develop a new degassing and purification technique. The power ultrasonic is a novel clean energy, and can refine crystal grains, improve the performance of the aluminum melt, have obvious effect on impurity removal, and promote mass transfer between two phases, so that the power ultrasonic is widely concerned. However, limited by the ultrasonic power, the ultrasonic probe has a small action range and cannot be applied to mass production, so that the ultrasonic probe can only be applied to an experimental research process. Therefore, it is obvious that the single purification method has different functions and effects, and the research on the combined degassing and purifying method is very necessary if the purification efficiency and quality of the aluminum melt are further improved.

The current research situation and analysis at home and abroad:

1. the current research situation of impurity removal method and equipment is as follows: the method for purifying the aluminum alloy melt has a plurality of classification standards, and the current common classification is divided into three types according to the working principle of each method, namely an adsorption purification method, a non-adsorption purification method and a composite purification method.

1.1. An adsorption purification method: the adsorption purification refers to a mode of directly combining an adsorbent with gas and solid impurities in a melt physically, chemically or mechanically to achieve the purpose of purifying an aluminum melt, and is a relatively mature and widely used aluminum alloy melt purification method which mainly comprises a blowing method, a flux method, a filtering method and the like.

The air blowing purification method, also called a bubble floating method, has been developed as a single tube air blowing method, a multi-hole nozzle air blowing method, a fixed blowing method, a rotary blowing method, and the like. Among them, the rotary blowing technique, which is considered to be superior in degassing effect in recent years, has been studied and improved in many cases. Foreign representatives include rotors of SNIF method, ALPUR method, MINT method, RDU method and LARS method; in China, Jianghai Yangyi and the like propose a pulse air-intake rotary blowing technology, bubbles smaller than those obtained by ordinary rotary blowing can be obtained, the lag time of liquid level vortex can be prolonged when the gas flow is increased or the rotating speed of a rotor is increased, and an upper-layer and lower-layer nozzle rotor is developed by Su Shi Pao and the like. Meanwhile, in order to continuously improve the degassing efficiency, researchers also perform water simulation and computer simulation on the internal flow field of the melt, bubble movement, degassing process and the like, and have certain reference significance on rotor structure and process parameter optimization. Meanwhile, due to the problem that the size of the rotary blowing bubbles is difficult to reduce, WURZ and the like develop novel atomization purification equipment. Canadian STAS company designs two multi-nozzle rotary degassing structures, namely a box-type online degassing device and a runner-type online degassing device, and the box-type online degassing device often contains 2-3 nozzles according to the difference of the flow rate of aluminum water, and after Ar is introduced, alkali (earth) metal, hydrogen and impurities in aluminum melt can be efficiently removed.

The flux method is that when the aluminum melt is refined, solid refining flux is added into a molten pool, and the purpose of removing gas, solid inclusions and impurity elements is achieved through a series of physical and chemical actions. In recent years, new methods of flux injection and rotary injection have been developed, which can improve the flux purification effect, but the facility piping is easily clogged. Thereafter, korean mond et al proposed a new flux purification apparatus to solve the above problems. Meanwhile, in order to protect the environment and meet the requirements of various melt processing, the research of the multifunctional and environment-friendly composite flux is a hotspot, such as the old and frequent NUC series composite flux, the novel rare earth composite flux of the Tangxiaolong and the like, and the experimental purification effect is better. The present invention proposes that flux is sprayed into melt by means of pressure difference and mixed with the melt homogeneously to realize efficient purification. Meanwhile, researches on purifying the aluminum melt by the electroslag remelting technology are gradually increased, and the technology is considered to have an obvious purification effect.

Filtration purification is a process in which the aluminum melt is passed through a neutral or active filter medium to separate solid inclusions suspended in the melt. Currently, the excellent filtering effect of the ceramic foam filter is confirmed by most researchers or engineers, and the ceramic foam filter can filter out fine impurities below 20 microns and can filter out liquid flux type impurities which are generally extremely difficult to filter out. The ceramic filter plates of the Saikte company, the Wiusowei company and the New high temperature materials of Mitsu, Fuzhou have advanced production technology and excellent product performance. Yi Shi Dong develops a centrifugal filter device which adopts multilayer gradient filtration stage foamed ceramics, the filtration flow can reach 200kg/min, and the requirement of large flow of a production line can be met. G.p. Martins et al investigated the interaction of metal melts with inclusion particles in ceramic foam filters. By studying the liquid phase surface tension of the melt and the critical speed of the inclusions penetrating the liquid film, it is found that whether the inclusions can be adsorbed by the surface of the filter depends on the surface force of the liquid phase of the metal melt to a large extent. In recent years, some researchers have proposed a new method for improving the filtering capacity of the foamed ceramic, so that the molten metal can smoothly pass through the foamed ceramic filter under the action of an electromagnetic field, and experiments prove that when the molten metal flows through the foamed ceramic filter, the electromagnetic Lorentz force is applied to enable the molten metal to be in 'drainage starting' in the 80ppi filter, and the pressure drop is equal to or lower than the pressure drop when the molten metal passes through 30 ppi. Finally, the multi-stage combined filtration represented by the same filtration mode, the filtration combinations of different filtration stages, the combination of different filtration modes, the pore gradient filter and the like is a development direction of the future high-efficiency filtration and purification technology.

1.2. Non-adsorption purification method:

non-adsorption purification refers to a purification method for separating gas and solid impurities from a melt by directly changing the equilibrium state of two systems of aluminum-gas and aluminum-inclusion through certain physical actions such as ultrasonic wave, vacuum pumping, density difference and the like, and mainly comprises vacuum purification, ultrasonic purification, electromagnetic purification and the like.

The main theoretical basis of vacuum purification technology is the specific law of Xihua, which is considered to be the most effective method for reducing the hydrogen content in the aluminum alloy, and the vacuum purification technology comprises static vacuum purification and dynamic vacuum purification. The static method has lower degassing efficiency; the dynamic method accelerates mass transfer in the melt through melt flow so as to improve degassing efficiency. Relevant experiments prove that the effect of 5min dynamic degassing is better than the static degassing effect of 20 min. However, the technology still has the problems of complex equipment, large investment, low processing capacity and the like. Therefore, ZENGJM et al developed a novel vacuum degassing apparatus that generated a vacuum state in the suction head, had no vacuum chamber, and could reduce the hydrogen content in a 100gAl melt to as low as 0.1 mL.

Ultrasonic purification is a green and rapid aluminum alloy purification means, and the research of the ultrasonic purification is started in the former Soviet Union at the earliest. In the 30 s of the 20 th century, when the action of ultrasonic waves on liquid and low-melting-point melt is studied, ultrasonic vibration is found to have the effect of degassing. In the early days, ultrasonic purification research mainly focuses on low-melting-point melts due to the restriction of technologies, materials and the like. In the 90 s of the 20 th century, the research on ultrasonic degassing of magnesium and aluminum alloys with higher melting points is gradually increasing at home and abroad. In terms of the purification principle, researchers believe that its outgassing mechanism is related to ultrasonic cavitation. In recent years, the research on the influence of different process parameters on ultrasonic degassing is common, and the main conclusions are as follows: the ultrasonic vibration has better degassing effect on small-volume melt, the ultrasonic power and action time are key factors for degassing, and the technological parameters are used for promoting the generation and existence of cavitation bubbles; li am think that the lower the ultrasonic frequency is better for degassing. In the research of a novel ultrasonic purification technology, LIJW and the like propose a new technology of introducing strong cold at the bottom into an ultrasonic degassing device; abroad, the research on the composite purification technology combining ultrasonic purification and other kinds of purification has been focused.

The electromagnetic purification technology is a new material processing technology which is rapidly developed and can efficiently remove non-metal impurities in liquid metal, and the impurity removal principle is an electromagnetic separation phenomenon. In recent years, electromagnetic purification technology has become a hot point of research. In the field of theoretical research, TERADAT and the like carry out theoretical calculation on the impurity separation condition of an aluminum melt in a superconducting magnetic field and verify the calculation accuracy through experiments. It is worth mentioning that researchers have more researches on the influence of process parameters in different electromagnetic fields on impurity removal, and the process of dawn quick and the like and the process of GUOQT and the like respectively adopt different parameter combinations to carry out purification tests in a constant magnetic field, a direct current electric field and a high-frequency magnetic field, so that the relatively obvious purification effect is achieved. In the aspect of computer simulation, AFSHAMRR and the like simulate the electromagnetic purification process of the melt in the environment of constant magnetic field positive alternating current and direct current, and the accuracy of the simulated optimal parameters is proved through test verification. Recently, a novel purification technology emerges, and belongs to online filtering outside a furnace, wherein the aluminum melt is conductive, the aluminum oxide is non-conductive, and under the action of an electromagnetic field, the aluminum melt tends to deviate to one side, and the extruded aluminum oxide tends to deviate to the other side, so that the aluminum oxide is deviated and pushed to an adsorbed ceramic tube, and the purification of the aluminum melt is realized.

1.3. A composite purification method:

the composite purification mainly refers to a new purification method which uses two or more purification technologies at the same time and obviously improves the purification effect and efficiency of the aluminum melt through the complementary advantages of various technologies. At present, composite degassing and degassing-filtering composite purification are mainly included.

Currently, the main research direction and hot spot in the field of composite degassing abroad is composite degassing technology which is based on ultrasonic purification and is combined with other methods. The effect of ultrasonic vibration and vacuum composite purification is researched by XUHB and the like, and the technology is found to be capable of obviously improving the degassing efficiency, and when the vacuum degree is not too large, the degassing efficiency is still high. HAGHAYEGHIR, etc. combines the blowing method with the ultrasonic degassing, develops a novel composite degassing device for blowing argon to enhance the ultrasonic degassing effect, and can effectively solve the problem that the ultrasonic degassing can not process large-capacity melt. PUGAH and the like develop a novel device for enhancing ultrasonic degassing by low-frequency mechanical vibration, and the porosity in the alloy can be reduced to the maximum extent under the condition of not increasing the treatment time by stirring the melt through the low-frequency mechanical vibration. The degassing-filtering composite purification method is a mature composite purification technology, has the double functions of degassing and removing impurities, and has been widely applied to main aluminum and aluminum alloy production enterprises in the world. Currently, a degassing-filtering composite purification device is generally applied to online and continuous purification, and a common combination mode generally mainly comprises two-stage degassing and multi-stage gradient filtering. In terms of degassing, an air blowing method is mainly used, in which blowing with a porous head and rotary blowing are used as main bodies. The filtration technology mainly uses flux filtration and gradient filtration of filters made of different materials, and the filters generally adopt foamed ceramic filters.

It is expected that the multifunctional composite flux and the adding technology thereof, the ultrasonic purification, the electromagnetic purification and the composite purification method will be the focus and hot point of the research on the purification of the aluminum alloy melt in a period of time in the future. The development of simple, efficient, cheap and pollution-free purification technology and device is the general development trend of aluminum alloy purification technology.

2. Bubble equation of motion and current research situation of mass transfer:

the current research situation of the motion characteristics of the bubbles under the action of rotation is as follows: the rotational blowing method mainly generates a rotating field in a fluid through the stirring action of a rotor, and a large number of related documents prove that the rotating field has great influence on the movement of bubbles, and the moving process of the bubbles influences the efficiency of degassing in a melt. The force conditions of the particles in the different phases were analyzed in the two-phase and multiphase flow mechanics books of the guo-brocade compilation. Cao Chong et al analyzed the radial force of the dispersed phase particles in the rotating field and determined the equation of motion of the particles in the process by Newton's second law. The plum bin and the like analyze the transverse stress condition of particles in a liquid phase, the field constant funnel and the like analyze the axial stress condition of bubbles in the liquid, and the research methods of the documents provide ideas for researching the stress condition of the bubbles under the condition of rotary blowing.

Current research situation of mass transfer: the principle of introducing inert gas into the aluminum melt to remove hydrogen is as follows: the initial pressure of hydrogen in the bubbles is zero, and the pressure difference of hydrogen exists on the interface of the bubbles and the aluminum liquid, so that the hydrogen dissolved in the aluminum liquid is continuously sucked into the bubbles, and the suction process is stopped until the pressure of the hydrogen in the bubbles is equal to that of the hydrogen in the aluminum liquid. After the bubbles float out of the liquid surface, hydrogen in the bubbles escapes and enters the atmosphere.

In order to improve the degassing efficiency, the number of the bubbles is increased as much as possible, the diameter of the bubbles is reduced, and the floating distance of the bubbles in the molten aluminum is prolonged. The influence conditions of the melt purification process can be qualitatively analyzed according to the hydrogen partial pressure theory, but the purification effect cannot be directly reflected, and quantitative analysis is needed to be carried out on the process to obtain determined influence parameters in order to further improve the purification rate of the rotary blowing process.

The power ultrasonic degassing and purifying mechanism mainly adopts an ultrasonic cavitation mechanism, and changes a pressure field and a temperature field in the aluminum melt by utilizing the cavitation action of ultrasonic cavitation bubbles generated under the action of an ultrasonic field and the direct action of ultrasonic, thereby influencing the proceeding of mass transfer in the melt. Zhao's passage through the two-membrane model introduces four major effects of ultrasonic cavitation (mechanical, thermal, optical and activation effects) to the acceleration of the transport. Wangjie et al analyzed the motion process of cavitation bubbles by numerical analysis, and the results showed that the bubbles rapidly collapsed after exceeding a certain size. H. Puga combines ultrasound with molten metal agitation, and applies a water simulation test to monitor the ultrasonic pressure at different orientations of the vessel. Test results show that the degassing effect of the alloy material is obviously improved and the material performance can be improved after the two methods are combined.

Disclosure of Invention

In order to solve the problems of the prior art; the invention aims to provide a method for simulating bubble movement and mass transfer values in aluminum melt dehydrogenation.

The invention relates to a bubble movement and mass transfer numerical simulation method for aluminum melt dehydrogenation, which 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.

Compared with the prior art, the invention has the beneficial effects that:

the VOF can be used for analyzing a flow field in the bubble movement process, and the influence of rotation and ultrasonic action is analyzed through the change of the velocity potential, the pressure field and the bubble form around the purified bubbles in the process; the gas content in the liquid phase can be analyzed by using a multiphase flow model. The MATLAB is used for numerical calculation, so that change curves of the radius of the purified bubbles and the pressure field around the bubbles with respect to time can be directly obtained, and the change condition of mass transfer in the bubble movement process is further analyzed.

Detailed Description

The specific implementation mode adopts the following technical scheme:

because the dynamic viscosity of the aluminum alloy melt under the conditions of 20 ℃ and 730 ℃ is approximately equal, the two methods conform to the triangular similarity law of similar movement, geometric similarity and dynamic similarity, and therefore the subject adopts a water simulation experiment: supersaturated oxygen is introduced into the water to simulate the hydrogen dissolved in the aluminum melt, and inert gas is introduced to simulate the process of removing hydrogen by bubbles. And measuring the concentration of oxygen in the purified water by using an oxygen dissolving instrument, thereby determining the purification effect. And observing the movement track and the distribution condition of the bubbles by a high-speed camera while introducing the inert gas, and finally comparing the movement track and the distribution condition of the bubbles with a numerical calculation result.

The experimental protocol was as follows:

1. experimental equipment: the system comprises a rotary blowing device system, ultrasonic equipment, a high-speed camera, an oxygen dissolving instrument and data acquisition software;

2. experimental parameters: the main influencing parameters are the rotating speed of the rotary nozzle, degassing time, gas flow and ultrasonic power;

the experimental parameters are as follows:

TABLE 1 Rotary blowing value-assigning table

TABLE 2 ultrasonic value assignment Table

The orthogonal experimental protocol for rotary blowing is shown in table 3:

TABLE 3 spin blow orthogonal Experimental Table

The ultrasonic degassing orthogonal test protocol is shown in table 4:

TABLE 4 orthogonal experimental table for ultrasonic degassing process

The composite degassing orthogonal experimental protocol is shown in table 5:

TABLE 5 composite degassing orthogonal Experimental Table

In this embodiment, flow field simulation based on the VOF calculation method is mainly applied to Fluent, OpenFOAM, and the like, and is based on MATLAB numerical analysis of the loggerstota calculation method. The VOF can be used for analyzing a flow field in the bubble movement process, and the influence of rotation and ultrasonic action is analyzed through the change of the velocity potential, the pressure field and the bubble form around the purified bubbles in the process; the gas content in the liquid phase can be analyzed by using a multiphase flow model. The MATLAB is used for numerical calculation, so that change curves of the radius of the purified bubbles and the pressure field around the bubbles with respect to time can be directly obtained, and the change condition of mass transfer in the bubble movement process is further analyzed.

The combined degassing and purifying method is not a simple method combination, and the mechanical research is needed if the purifying efficiency of the aluminum melt is to be improved. Therefore, the rotary blowing and power ultrasonic combined type aluminum melt purification method is provided by comprehensively utilizing the advantages of the rotary blowing method and the power ultrasonic purification. The method mainly purifies the aluminum melt through purifying bubbles, and promotes the diffusion process of hydrogen in the melt from a liquid phase to a gas phase through an ultrasonic effect, wherein the size of the bubbles, the distribution condition of the bubbles and the like have great influence on the purification effect.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Furthermore, it should be understood that although the present description refers to embodiments, not every embodiment may contain only a single embodiment, and such description is for clarity only, and those skilled in the art should integrate the description, and the embodiments may be combined as appropriate to form other embodiments 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.