Numerical analysis of melting and holding furnaces in secondary aluminum production
- Autores: Mauricio Yilmer Carmona García
- Directores de la Tesis: Cristóbal Cortés Gracia (dir. tes.)
- Lectura: En la Universidad de Zaragoza ( España ) en 2014
- Idioma: español
- Tribunal Calificador de la Tesis: Assensi Oliva Llena (presid.), Javier Pallarés Ranz (secret.), Pedro Acisclo Rodríguez Aumente (voc.)
- Materias:
- Enlaces
- Tesis en acceso abierto en: Zaguán
- Resumen
INTRODUCTION Aluminum is a metal used in a wide variety of applications through all sectors due to their good properties. The secondary aluminum production is a key factor in the global market of this metal. Long product lifetimes and growing markets mean that the future demand will continue to be met from both primary and recycled sources.
The transformation of scrap into recycled aluminum alloys requires approximately only 5% of the energy input needed to produce primary ingot from bauxite. In secondary aluminum production, energy is consumed throughout the refining process, but the key equipments for the metallurgical processing are furnaces. The aluminum secondary industry focuses its efforts in develop a method for recycling the metal that be economically competitive and satisfies the requirements of the market. Several furnace types are developed for the different applications and conditions. Nevertheless, traditional design of melting and holding furnaces has been based on semi-empirical methods due to the complexity of the phenomena involved, which has limited the use of computational techniques based on first principles physics. In addition, the inherent difficulty and shortcomings of experimental measurements do not allow a comprehensive description of the operation. Accordingly, a great deal of information has been missing in these kinds of devices.
Some companies have developed some simplified numerical models of their units. Nevertheless, the use of numerical techniques is not a common practice in the development process of industrial furnaces. In scientific literature, only few works has been published concerning to numerical simulations in melting and holding furnaces. In melting applications, most works about numerical simulations did not consider the molten load in the furnace and the interactions between heating space and load; others consider the solid¿liquid region of the load as a conducting solid with constant thermal properties (i.e. the convection effects are not considered) and neglect the heat losses from the walls. In holding applications, the works that consider the interaction with the molten load have been developed under steady state assumption, others simulations also neglect the conjugated problem, the radiation losses, the three dimensional geometries or the convection effects. According to the literature reviews of scientific papers and industrial devices, it is possible to point out that comprehensive simulations with realistic models are still undeveloped for these kinds of devices.
THEORIST DEVELOPMENT In this thesis, two different aluminum furnaces are analyzed numerically: 1) a new prototype of melting furnace heated by a plasma torch; 2) a new prototype of holding furnace heated by electrical resistances. These kind of devices are a key factor in secondary aluminum production. Models used take into account heat conduction in solid parts, convection in air and molten aluminum, interactions between gas-liquid-solid zones, phase change of the load and radiation heat transfer. With the objective to develop a calculating tool to assist in the design and scaling-up of industrial furnaces, several calculation strategies are tested concerning their computational economy and their accuracy in computing different key parameters. Estimations of temperatures in furnaces are compared with experimental measurements taken in real prototypes in typical operation cycles.
The aim of the simulations is to develop a calculating tool to assist in the design, scaling-up and control of industrial furnaces used in aluminum secondary production with good compromise of the computational resources and accuracy. It is also proposed that the use of CFD can provide accurate information about thermal variables, many of them very difficult to determine experimentally, such as load and refractory temperatures and their time evolution, patterns of movement inside the molten load and distribution of heat losses. These results can be used in order to understand the physics involved in the process.
Chapter 2 is an introduction of the aluminum features, production and utilization. After that, the role of the secondary aluminum production in the global market is presented. Also, the stages of aluminum secondary production are presented, including details of the melting and holding furnaces used, and their present-day technologies. Finally, a motivation of the current thesis is presented.
Since the core of such a simulation must include a model for the phase change of the load material, an overview of the numerical methods used in phase change problems and the state of the art of the related simulations are presented in Chapter 3. The governing equations and the radiation heat transfer model used for the numerical simulations in the present thesis are also presented in this chapter.
In chapter 4, simulations of a new aluminum melting furnace heated by a plasma torch are presented; these simulations reproduce an experimental test carried out in a real prototype. Models used are 2D axisymmetric and take into account heat conduction in solid parts, convection in air and molten aluminum, interactions between gas-liquid-solid zones and radiation heat transfer. Several calculation strategies are tested concerning their computational economy and their accuracy in computing different key parameters. Results show that interactions gas-liquid-solid have an important effect. Firstly, a proper account of heat transfer and losses requires solving the conjugated problem comprising refractory walls and heated load. Secondly, thermal interaction with air cavities seems to determine the convective movement of the molten load and therefore inner-load temperature patterns and their time evolution.
An experimental prototype of holding furnace heated by an electrical resistances system is numerically simulated in Chapter 5. Models used take into account the same complexities presented in the simulations of the melting furnace, but in this case the models are 3D. As a first approach, the symmetry of the geometry is used to reduce the computational resources and only a quarter of domain is initially simulated. Nevertheless the predicted velocities for the flow in the molten load suggest that the patterns induced by the convection effects have a three dimensional behavior. For this reason a new simulation with the full domain is carried out. Both simulations predict almost the same temperatures and energy distributions with different patterns of flow in the molten load.
Finally, a summary, general discussion on the major results and conclusions are presented in Chapter 6. New contributions and recommendations for continuing research are also included.
CONCLUSIONS Numerical studies of the thermal performance of two different furnaces have been completed: 1) a new prototype of aluminum melting furnace; 2) a new prototype of aluminum holding furnace. These devices are a key factor in secondary aluminum production. Models used takes into account heat conduction in solid parts, convection in air and molten aluminum, interactions between gas-liquid-solid zones, phase change of the load and radiation heat transfer. With the objective to develop a calculating tool to assist in the design and scaling-up of industrial furnaces, several calculation strategies are tested concerning their computational economy and their accuracy in computing different key parameters. Estimations of temperatures in furnaces are compared with experimental measurements taken in real prototypes in typical operation cycles. The models employed show the capability to estimate the most important parameters in the operations of the furnaces; many of them are difficult to obtain with the current measurement techniques, e.g. the melting rate (melting furnace), power utilization, load and refractory temperatures and their time evolution, flow patterns in the molten bath and distribution of heat losses. Furthermore, results obtained in this work present the potential of numerical approaches, to support the experimental procedures to understand the different phenomena involved in melting and holding furnaces. The predicted values appear to be satisfactory considering the complexity of the simulations.
CONCLUSIONS: Melting furnace Numerical simulations of a new prototype of aluminum melting furnace have been discussed in detail, including the assumptions taken into account, the methodology followed in the numerical analysis and results. Taking advantage of the symmetry of the furnace, the models have been simplified to a 2D axisymmetric domain.
In order to determine the reduction in both computational expense and accuracy of the simulations, five computational cases using successively simplified models have been studied to simulate the losses from the load to the interior cavity of the furnace by radiation and convection. Case 1: thermal radiation modeled with the S2S radiation model and air convection fully modeled. Case 2: radiation no modeled and air convection fully modeled. Case 3: purely diffusive transfer in the gas, with an augmented, effective thermal conductivity. Case 4: air in cavity not modeled. Case 5: air in cavity not modeled and load heated by pure conduction.
Cases 1-4, showthe same behavior in the prediction of the melting rate and their differences are negligible. Nevertheless, cases 3 and 4 cannot predict the full convection effects shown by cases 1 and 2. Predictions of the temperature for the domain are similar for the cases 1 to 4. All those results seem very reasonable and in agreement with empirical observations and analytical models. Results obtained by the pure conduction approach (case 5) show that in industrial applications, the simplification of no dependency with temperature for the density leads to overestimates in temperature, and thus to unrealistic results.
Case 1 uses 1.6 times of the computational resources than case 2; case 2 uses 2.25 times of the computational resources than the case 3; case 3 uses 27.52 times of the computational resources than case 4; finally, case 4 uses 21.8 times of the computational resources than the case 5. With the assumptions taken in cases 3 and 4, it is possible to perform initial estimates of the process (e.g. melting time and molten temperature), but neglecting the full convection effects between the air and load, that may be important for a comprehensive analysis. The main drawback of the most comprehensive approach (case 1) is their high computational cost, but it can be compensated with the current computer resources, which allow the analysis of a case in few days.
The computational advantage of simplified models (cases 2-5 vs. case 1) must be considered for each specific application. Difference in computational times is very significant, so that purpose and number of simulations required should be always considered. Predictions and cost of the simplified simulations have been discussed and compared, which could assist in this respect.
CONCLUSIONS: Holding furnace Three dimensional unsteady state simulations have been presented with the purpose to estimate the thermal behavior in a holding furnace heated by an electrical resistances system. Assumptions, followed methodology and a discussion of the main results obtained have been presented at length.
As a first approach, the symmetry of the geometry has been used to reduce the computational resources and only a quarter of domain has been initially simulated. Nevertheless, the predicted velocities for the flow in the molten load suggest that the patterns induced by the convection effects have a three dimensional behavior. For this reason a new simulation with the full domain has been performed.
The numerical simulations show that the holding furnace can keep the temperature of the molten load above the melting point, in a very homogeneous temperature distribution in the load and in an almost constant operational condition. The temperatures predicted seem very reasonable and agree well with empirical observations taken in a real prototype for a typical operation cycle.
Temperature distribution and energy fluxes predicted by the two cases computed are almost the same. Nevertheless, the flow patterns in molten aluminum differ greatly between both approaches. Benchmarking of the holding furnace with simplified problems of natural convection, allows to pointing out that the expected flow pattern in the molten load, should looks like that obtained with the full domain; therefore, a main conclusion is that the symmetry condition prevents the full develop of the flow patterns in furnaces with quasi-steady state heating systems. The works of flows induced by natural convection considers simplified domains in order to use smaller mesh sizes and achieve DNS conditions, but the requirements to solve all the turbulence scales is prohibitively expensive for the current industrial purposes.
Even for the homogeneous temperatures presented in the furnace, the models show the capacity to estimate complex patterns of movement inside the molten load caused by the convection effects. These behaviors are usually observed in natural convection systems as a result of their intrinsic instability. Many publications have been presented in metals with fusion points close to ambient temperature, simplified geometries and controlled boundary conditions, but, due to the difficulty in measurements in industrial aluminum furnaces, currently it is not feasible a detailed characterization of the internal flow, and there is no bibliographical reference about this subject for this kind of devices.
In practical terms, the homogeneity of the temperature inside the load makes that the current approaches are good enough for industrial purposes. Convection effect does not appear to have a major impact on the global energy balance, but it is necessary take into account the movement in the molten load to avoid over prediction in the temperature. Likewise, the unstable patterns preclude obtaining a steady state solution.
PERSPECTIVES FOR FUTURE WORK Considering the results of the present thesis, a natural next step is the validation of the methodologies employed with comprehensive experimental data. Nevertheless, a rigorous experimental validation for the numerical simulations of melting and holding furnaces must include the accurate measurements of velocities and temperatures in the molten load, but these kinds of experimental rigs are not currently available for metal industrial furnaces and of course, it exceed the scopes and objectives of the present work or derived research lines.
Simplifications considered for this thesis must be reduced as a future work. Firstly, more complex models will be included in the solution, i.e. turbulence models, three dimensional and not simplified geometries, or more refined grids. These problems will be solved as the computational resources increase. Secondly, several phenomena can be incorporated for a more complete analysis, e.g. chemical reactions for interactions with alloy materials and oxidation of the load, the influence of the scrap sources (composition, shape and size) and evolution of metallurgical effects; to this end, several submodels must be coupled within the CFD solution. Finally, a combination of different physics can be added to the present solution; as an example, currently the volume expansion of the metal is typically solved with the VOF method, but this technique is very difficult to implement in conjugated problems and it is not available in commercial solvers; multiphysics codes present a growing opportunity to compute this and other kind of phenomena combinations.
Actually, future work will be mainly centered on capture the essential information required in industrial furnaces. The compromise between the economy of the models and the quality of the prediction must be judiciously analyzed for any particular application. While the computational resources and the commercial codes become enough comprehensive, fast and accessible to not qualified personnel, the numerical simulations will be used in order to develop simplified reduced models, which allow a quick evaluation of key parameters in industrial furnaces.
