1 Ab initio modelling of thermal plasma gas discharges (electric arcs) Von der Gemeinsamen Naturwissenschaftlichen Fakultät der Technischen Universität Carolo-Wilhelmina zu Braunschweig zur Erlangung des Grades eines Doktors der Naturwissenschaften (Dr.rer.nat.) genehmigte Dissertation von Jens Wendelstorf aus Tübingen
2 ii 1. Referent: Univ.-Prof. Dr. U. Motschmann 2. Referent: Univ.-Prof. Dr. M. Kock 3. Referent: Univ.-Prof. Dr. J. Mentel eingereicht am: 6. Juli 2000 mündliche Prüfung (Disputation) am: 20. Dezember
3 iii Teilergebnisse aus dieser Arbeit wurden mit Genehmigung der Gemeinsamen Naturwissenschaftlichen Fakultät, vertreten durch den Mentor der Arbeit, in folgenden Beiträgen vorab veröffentlicht: 1. J. Wendelstorf: Two-temperature, two-dimensional modeling of cathode-plasma interaction in electric arcs. In P. Pisarczyk, T. Pisarczyk and J. Wolowski (Ed.): Proceedings of the XXIV International Conference on Phenomena in Ionized Gases, Warschau, Polen, July 1999 (ISBN ), Bd.II, S (best computational poster award) 2. J. Wendelstorf, H. Wohlfahrt und G. Simon: Two-temperature, two-dimensional modeling of cathode hot spot formation including sheath effects application to high pressure xenon arc lamps. In: C. Deeney (Ed.): 1999 IEEE Int. Conf. on Plasma Science, Monterey, California, USA, IEEE Cat. No. 99CH36297 (ISBN ), S.240 (5P03) 3. J. Wendelstorf, I. Decker, H. Wohlfahrt und G. Simon: Modeling of arc-electrode systems - preliminary results for high pressure xenon arc lamps. 8th Int. Symposium on the Science & Technology of Light Sources (LS-8), Greifswald, (ISBN ), S J. Wendelstorf, I. Decker, H. Wohlfahrt und G. Simon: Investigation of cathode spot behavior of atmospheric argon arcs by mathematical modeling. In: G. Babucke (ed.), XII Int. Conf. Gas Discharges and their Applications, Greifswald, (ISBN ), Bd.I (1997), S I. Decker, O. Voß, J. Wendelstorf und H. Wohlfahrt: Nutzen numerischer Simulationen für den Einsatz realer Schweißprozesse. DVS-Berichte, Bd.186 (1997), S J. Wendelstorf, I. Decker, H. Wohlfahrt und G. Simon: TIG and plasma arc modelling. In: H. Cerjak (ed.), Mathematical Modelling of Weld Phenomena 3 (ISBN X), The Institute of Materials, London, 1997, S B. Rethfeld, J. Wendelstorf, T. Klein und G. Simon: A self-consistent model for the cathode fall region of an electric arc. J. Phys. D: Appl. Phys., Bd.29 (1996), S J. Wendelstorf, I. Decker, H. Wohlfahrt und G. Simon: Berechnung anwendungsrelevanter Zielgrößen von thermischen Lichtbögen: Kathodenfallmodelle und deren Integration in die Lichtbogenmodellierung. 8te Bundesdeutsche Fachtagung Plasmatechnologie, Dresden, , S J. Wendelstorf, G. Simon, I. Decker und H. Wohlfahrt: Untersuchungen zur Berechenbarkeit technologischer Parameter thermischer Lichtbögen am Beispiel des atmosphärischen Argonbogens. Verhandl. DPG (VI), Bd.32 (1997), S J. Wendelstorf, I. Decker, H. Wohlfahrt und G. Simon: Modellierung thermischer Lichtbögen zur Optimierung von technologischen Prozeßparametern. 7te Bundesdeutsche Fachtagung Plasmatechnologie, Bochum , S J. Wendelstorf, I. Decker, H. Wohlfahrt und G. Simon: Modellierung des Kathodenfalls zur Simulation thermischer Lichtbögen. 7te Bundesdeutsche Fachtagung Plasmatechnologie, Bochum , S B. Rethfeld, T. Klein, C. Tix, J. Wendelstorf und G. Simon: Modellierung der katodennahen Plasmarandschichten eines WIG-Lichtbogens. Verhandl. DPG (VI) Bd.30 (1995), S.243 Braunschweig, 6. Juli 2000
4 iv Copyright c Jens Wendelstorf 2000 Alle Rechte, auch das des auszugsweisen Nachdruckes, der auszugsweisen oder vollständigen Wiedergabe, der Speicherung in Datenverarbeitungsanlagen und der Übersetzung, vorbehalten. Jens Wendelstorf Internet:
5 v dedicated to Prof. Dr. Gerhard Simon
6 vi Kurzfassung: Thermische Plasmen wurden in Form von Lichtbogenentladungen Anfang des neunzehnten Jahrhunderts entdeckt. Die fundamentalen physikalischen Mechanismen dieser Gasentladungen wurden bereits im Jahre 1903 von J. Stark beschrieben. Während die grundlegende Physik dieses Phänomens somit weitgehend bekannt ist, war es bis heute praktisch kaum möglich, wesentliche Entladungsparameter quantitativ vorauszuberechnen. Ein fundamentales Problem der Modellierung lag in der Beschreibung der Nichtgleichgewichtsgebiete (Plasmaschichten) vor den Elektroden. Das andere war die Tatsache, daß es sich bei diesen elektrischen Entladungen um dissipative, selbstorganisierende Systeme handelt, deren Eigenschaften sich erst aus der komplexen Wechselwirkung ihrer Teilsysteme ergeben (Emergenz). Diese Probleme werden im Rahmen dieser Arbeit diskutiert und weitgehend gelöst. Basierend auf der Motivation des Themas und der Geschichte seiner Untersuchung wird zunächst ein Konzept zur vollständigen quantitativen Vorausberechnung dieses Typs von Gasentladungen entwickelt. Es folgt eine physikalische Beschreibung der Lichtbogenteilsysteme Elektrodenfestkörper, Elektrodenoberfläche, Raumladungs- und Ionisationsschichten und der Plasmasäule. Die zur Lösung notwendigen Plasmaparameter und Transportkoeffizienten werden für den Fall des partiellen lokalen thermodynamischen Gleichgewichts (pltg) in Abhängigkeit von Elektronen- und Schwerteilchentemperatur und Entladungsdruck berechnet. Mit diesem ab initio Gesamtmodell werden dann zunächst die wesentlichen Eigenschaften der Entladung im Detail berechnet. Eine Variation der Entladungsparameter Gas, Druck, Strom und Katodendurchmesser weist im folgenden die quantitative Berechenbarkeit der fundamentalen Verhaltensweisen von Lichtbögen im Entladungsdruckbereich von 0.1 bis 8 MPa und Strömen oberhalb von 1A für Argon, Xenon und Quecksilber als Plasmamedium nach. Durch das rechnerische Ausschalten einzelner physikalischer Effekte wird deren konkreter Einfluß auf das Verhalten der Entladung untersucht. Diese Analyse gestattet zusammen mit einer im Bezug auf die benötigten Stoßquerschnitte durchgeführen Sensitivitätsanalyse eine Bewertung des Vergleichs mit anderen Modellen und den wenigen quantitativen Messwerten, die für derartige Hochdruckbogenentladungen bisher publiziert wurden. Für sehr unterschiedliche Entladungsparameter wird ein Vergleich mit dem Experiment vorgenommen, welcher insbesondere die hohe Genauigkeit des entwickelten Kathodenfallmodells nachweist und erste Validierungsaussagen liefert. Die Zusammenfassung stellt die wesentlichen Neuerungen des Berechnungskonzeptes und die grundlegenden physikalischen Prozesse kurz dar und beschreibt die große Zahl der möglichen Anwendungsbereiche des Modells. Es werden zudem Detailverbesserungen angeregt und Hinweise zur Durchführung von weiteren Validierungsexperimenten gegeben. Bei dem vorliegenden Berechnungsverfahren wurde weitgehend auf ungerechtfertigte Vereinfachungen verzichtet und durch Kopplung der modellmäßigen Erfassung der Teilsysteme erstmalig die weitgehend vollständige quantitative Vorausberechenbarkeit dieses Typs von Gasentladungen nachgewiesen.
7 vii Abstract: Thermal plasma gas discharges (electric arcs) were discovered at the beginning nineteenth century. The fundamental physical mechanisms of such gas discharges were described first by J. Stark in While the basic physical laws governing this phenomenon are mostly well known, up to now, a quantitative prediction of the fundamental discharge parameters was practically impossible. One fundamental modelling problem was the description of the non equilibrium layers (plasma sheaths) in front of the electrodes. Additionally, such electric discharges are dissipative self organizing systems. Their properties emerge from the complex interaction of their parts. These problem will be addressed and solved by this work. Based on a motivation of the objective and the history of its investigation, a concept of a complete self consistent quantitative prediction of such electric arcs was be developed. A physical description of the physical partial systems electrode solid, electrode surface, space charge and ionization sheaths and plasma column is provided. For the case of partial local thermodynamic equilibrium (plte), the plasma parameters and transport coefficients as a function of electron- and heavy particles temperature are calculated. Using this ab initio model, the properties of arc discharges are computed. A variation of the discharge parameters gas, pressure, current and cathode diameter establishes the quantitative predictability of the fundamental arc behaviour for a discharge pressure range of 0.1 to 8 MPa and arc currents above 1 A for argon, xenon and mercury fillings. By computational disableing individual physical effects, their specific influence on the discharge behaviour will be investigated. Together with the sensitivity analysis performed with respect to the required cross section data, this analysis allows for an assessment of the comparison with other models and the few available experimental results, actually published for such high pressure arc discharges. For a number of very different discharge parameters an experimental validation is provided. Especially the high precision of the cathode layer model becomes evident leading to a first positive model assessment. Finally, the main innovations of the computational concept and the basic physical processes together with the broad range of application areas of the model are summarized. Some enhancements are proposed and the realization of validation experiments will be discussed. The present computational scheme cuts out most unjustified simplifications and, by a numerical coupling of the partial systems models, a complete quantitave predictability of this type of electric gas discharges is established.
9 Contents 1 Introduction A short history of thermal plasma gas discharges Fields of model applications A practical definition of ab initio modelling Outline of this work Conceptual framework for modelling electric arc discharges 9.1 Introduction to complex adaptive systems Complexity measures Emergence: self generated complexity Is the arc discharge a complex system? The overall behaviour of arc discharges Numerical modelling concepts The transfer function concept Summaryoftheconcept Physics of the electric arc discharge Introduction Basicdischargefeatures Modelling tasks Physical processes inside the electrodes and at the electrode surface The cathode plasma transition layers Electron emission of the cathode surface Spacechargelayer(sheath) Energy balance and current continuity at the sheath edge Ionization layer (pre-sheath) Asimplepresheathcontrolmodel Sheath-surface boundary conditions Presheath-plasma boundary conditions Theanode plasmatransitionlayers The thermal arc plasma Modelling the flow of the conducting plasma fluid Modelling the energy transport within the plasma Modelling the electric current transport Diffusion and other transport phenomena Summaryofthearcdischargephysics ix
10 x CONTENTS 4 Plasma properties and transport coefficients Introduction Plasma composition and thermodynamic properties The condition of quasineutrality Daltons law Saha-Eggert equation Mass density Enthalpy Specific heat Particle collision integrals Neutral particle interaction Charged particle interaction Ion atom interaction Electron atom interaction Summary of particle collision modelling Energy exchange term for the plte model Elastic energy exchange rate Inelastic energy exchange rate Diffusities Viscosity Heat conductivity Electron heat conductivity Heavy particles translational heat conductivity Heavy particles internal heat conductivity Heavy particles reactive heat conductivity Electric conductivity Simple treatment of radiation effects Simple treatment of ionization non equlibrium Sensitivity analysis of the transport coefficients and their influence on the resultingdischargeparameters Modelling results, validation and sensitivity analysis Introduction Detailed properties computed for a representative arc configuration (model lamp) Properties of the overall discharge Properties of the cathode hot spot Parameter variations Physical sensitivity analysis Model validation Low current atmospheric argon arc Low current high pressure xenon model lamp HighpressurexenonarcsafterBauerandSchulz The high pressure xenon short arc lamp The high pressure mercury discharge The TIG welding arc Comparison with other modelling results Summary of the modelling results and validation
11 CONTENTS xi 6 Summary, applications, outlook and conclusions Majorachievements Conceptualprogress Applications Futuredevelopments Conclusions Bibliography 122
12 xii CONTENTS
13 Unser Kopf ist rund, damit das Denken die Richtung wechseln kann. Francis Picabia Chapter 1 Introduction Conversion of electric power to high heat intensities or light fluxes can be efficiently achieved by High Intensity gas Discharges (HID). During the long history of commercial applications in this area, progress is often based on the trial and error approach. Due to their complexity and the number of possible parameter combinations, the development process relies on the practical experience of the individual developer but not directly on the existing physical knowledge about these discharges. From the viewpoint of a scientist, atmospheric pressure gas discharges are very easy to obtain and thus understanding and modelling is a matter of scientific interest since the first detection of the arcing phenomenon itself in the nineteenth century. During the last decades, modern computing hardware and advanced software provides the opportunity to compute the overall discharge behaviour in advance and in a quantitative way. This work is dedicated to contribute to the understanding of the overall discharge properties by using these new modelling tools. This objective is reached by the development and application of accurate and validated modelling software. The investigations are applicable to the so-called thermal plasmas, which are distinguished from other types of plasmas by the following characteristics: All species have Maxwellian velocity distribution. If they are all according to the same temperature the plasma is named to be in Local Thermodynamic Equilibrium (LTE), otherwise the equilibrium is stated to be partial (plte). The excited energy states are populated to a Boltzmann term. Composition can be derived from Local Chemical Equilibrium (LCE) or demixing effects have to be calculated from an additional set of diffusion equations. Temperatures are less than a few ev (1eV=11604K). The plasma is not confined by an external magnetic field. The discharge is stabilized by its own magnetic field, by external gas flow, the electrodes or the surrounding solid or liquid walls. 1
14 2 Chapter 1 In the American and European literature these plasmas are often called hot plasmas (the heavy particles are hot compared to low pressure gas discharges). In the Russian literature and in Germany they are often called low temperature plasmas, because their electron temperature is small compared to nuclear fusion plasmas. During the last decades it became increasingly clear that the existence of LTE in a plasma is rather an exception than a rule. The excited states population often deviates from Boltzmann equilibrium. This is of major importance for spectroscopical plasma diagnostics, but for most practical applications at least the partial Local Thermodynamic Equilibrium (plte) concept can be used for the following reasons: The contribution of the excited states to the total plasma enthalpy is very small. The local energy balance of the discharge is dominated by Joule heating, radiative, convective and conductive source terms. The plasma composition and transport coefficients mainly depends on the excitation (electron) temperature and the fundamental gas properties. There is no direct impact of specific excited states populations on global arc discharge properties. Large errors in the calculation of the excited states density highly affect experimental methods, were these quantities are required for data interpretation, but in the self consistent system calculation, the effect is limited to very small changes in the calculated self-consistent plasma temperature distribution. Typical discharge parameters are listed in table 1.1. The values are reflecting the typical application areas of these arc discharges: lighting and material processing, synthesis and destruction. In the following section, a short historical and application overview is provided. This is followed by the definition of ab initio modelling, the objective of the following chapters, as summarized in section 1.4. Quantity Range Comparable to max. power density J/m 3 chemical energy storage max. current density A/m 2 critical current density for type II superconductors electric field V/m lightning discharge pressure MPa atmospheres discharge power W airplanes discharge power density W/m 2 electron beams light intensity cd/m 2 sunlight light flux lm sunlight lighting efficacies up to 300 lm/w low pressure discharge lamps Table 1.1: Parameters typical for thermal plasmas (for the technical term efficacy see e.g. [Way71]).
15 Section A short history of thermal plasma gas discharges High intensity gas discharges are also called electric arcs, because the first discharge observed in detail was a carbon arc burning in horizontal position in the ambient atmosphere and thus bent by natural convection. In table 1.2, a small collection of the milestones in the 200 years history of the arcing phenomenon is sketched. The arc discharge not only forms a basis for efficient discharge lamps, it is also an initiator for the development of plasma physics itself in the 1920 s. From discovery of a certain phenomenon (e.g. the use of metal halides for lighting purposes) to it s industrial use it can take more than 50 years and up to now, most practically important discharges cannot be computed in advance and thus may not be regarded as fully understood. Year Contributor Contribution 1804 Davy carbon arc lamp 1860 Berthelot electric arc methane to acetylene conversion 1878 Siemens arc melting 1890 Moissant, Heroult arc furnaces 1901 Marconi arc radio transmitter 1902 Ayrton monograph The electric arc [Ayr02] 1905 Birkeland, Schönherr nitrogen fixation 1911 Steinmetz principle of metal halide arc lamps 1920 Eggert, Saha equilibrium composition in plasmas 1923 Compton low pressure sodium lamp 1930 Pirani 300 lm/w low pressure sodium lamp 1933 welding with carbon arcs 1935 Elenbaas high pressure mercury lamp 1950 Maecker segmented arc for fundamental research 1951 inductrial plasma torches 1960 USAF/NASA multi MW wind tunnels for space reentry simulation 1962 commercial metal halide arc lamps 1963 Schmidt high pressure sodium lamps 1965 Paton transferred arc plasmatron for stainless steel 1977 Liu multidimensional model of the arc column 1981 Hsu 1st attempt of a 2-D plte model for the arc column 1984 arc synthesis of fullerenes 1987 Fischer 1st 2-D plasma electrode model for high pressure lamps 1988 Palacin 2-D model of the xenon short arc lamp column 1989 Tsai, Kou column model with boundary fitted coordinates 1990 Delalondre integrated 2-D column-cathode model 1991 low power high pressure mercury discharge car headlights 1995 plasma- and cathode surface temperature measurements for the tungsten inert gas (TIG) welding arc are state of the art D plasma-electrode models including convection are available for a number of discharge configurations 1998 commercial ultra high pressure (UHP)-mercury lamp (200bar) Table 1.2: History of arc discharge science and technology.
16 4 Chapter 1 level of understanding try and error experimental qualitative plausible understanding explanations quantitative experimental modelling & modelling computation quantitative in advance understanding ~1900 ~1950 ~2000? technical development method Figure 1.1: Development of HID science and technology. discharge power [W] arc furnaces circuit breakers thermal plasma materials processing, chemistry, deposition & waste destruction cutting & welding arcs high intensity discharge lamps (mercury, sodium, metal halides, rare gases,..) discharge pressure [MPa] Figure 1.2: Applications of high intensity gas discharges.
17 Section Concluding from this long lasting history of the subject, every fifty years, there was a qualitative advance in the overall development paradigma (see figure 1.1). This work is dedicated to the actual change from trial & error supported by a first quantitative understanding to a computation of specific discharge features in advance. 1.2 Fields of model applications As demonstrated in figure 1.2, discharge powers range from a few W to several MW and discharge pressures are atmospheric in most cases. For lighting and (underwater) materials processing purposes, hyperbaric applications became more and more important in the last decades. This work is dedicated to the computation and quantitative understanding of arc discharges. As a first step, the models will be applicable to stationary discharges showing cylindrical symmetry: Arc lamps, at low frequency alternating current (AC) or direct current (DC), especially for the optimization of electrode geometry and erosion, global energy balances, radiative properties. Welding arcs (see [WDWS97b]) optimized arc configurations will permit higher processing quality and performance, the impact of new gas mixtures and electrode materials can be determined in advance. Arc furnaces (steel and high melting point materials synthesis) optimized high power torches will allow to replace conventional carbon electrodes. Plasma torches for enhanced or even new applications: waste destruction, gas heating for reentry simulation and space propulsion, synthesis of nanostructure materials (fullerenes), plasma chemistry and materials deposition (plasma spraying) or removal (plasma etching). Modelling of AC or pulsed operation and 3-D modelling is actually possible, but requiring large computing resources.
18 6 Chapter A practical definition of ab initio modelling Specification: Computation: Results: Verification: basic theory measurement material & plasma properties (e.g. conductivity) measurement (e.g. spectroscopy) discharge configuration (e.g. gas, electrodes, geometry & current) transport theory (mass, momentum, energy & radiation) spatial distributions of discharge parameters (e.g. temperature) model validation sensitivity analysis Figure 1.3: Information flow and subtasks of ab initio modelling. The common meaning of ab initio modelling is computation based on first principles and fundamental constants. The practical definition used for this work is sketched in figure 1.3: An ab initio model is based on fundamental physics and any material, gas- or plasma property determined by a discharge configuration independent method. The model should not contain any parameter or input value which has to be fitted to experimental discharge data for every individual configuration under investigation. A practical ab initio model should allow for the computation of important discharge characteristics with an accuracy better than experimental error or sufficient for automatic optimization of discharge configurations. The underlying motivation for model development with such high accuracy is the extrapolation capability these models should have. Additionally the strong nonlinear character of the arc discharge implies a need for relative high computation accuracy. The model validation by quantitative comparison with experimental data has to be supplemented by a sensitivity analysis: Some experimental quanitities are rather insensitive to changes in the arc configuration (e.g. doubling the current often implies only a few percent change in plasma temperature), while other are easy to determine (e.g. voltage-current characteristics) and also sensitive to important nonlinearity effects like mode transitions. Sensitivity analysis has to be undertaken within all fields important for the development of the models and their validation: Modelling accuracy can be restricted in principle, by the numerical techniques applied and by the available computing resources. Validateability of the physical effects included will require detailed knowledge of the transport coefficients involved or the need to compute critical effects like mode transitions. The validation of physical details may be completely hidden by configuration- or measurement-inaccuracies. Analysis of the numerical schemes and their parameters allows to increase computation speed as well as to obtain important information on the principle accuracy of the concept.
19 Section As a result, some of the involved numerical schemes may be less accurate (10 3 ), while other have to solve stiff problems at high accuracy (< 10 8 ). One of the most important experience from modelling such systems can be summarized in one sentence: The validity of the physical description of the system can be justified aposteriori(afterwards), but not apriori(in advance). Another is the importance of a sensitivity analysis in order to obtain information about the role of a specific input parameter (e.g. electrode work function) on the overall or local discharge behaviour. The modelling approach is therefore relying on the comparison with selected reproduceable experimental data of known accuracy. The answers of this validation and sensitivity analysis are not always positive: The specific experimental data may be not accurate enough to validate a specific modelling approach, e.g. a validation of a cathode layer model by plasma temperature measurements may require unaffordable spatial resolutions and accuracies cathode layer models are better justified by an electrical determination of the cathode fall voltage. The physical detail may be hidden by a material property not known with sufficient accuracy (e.g. cathode material work function). The physical or application detail may be superimposed by the dependency of the results on numerical parameters (e.g. grid size). The computing resources may not allow for a sensitivity analysis at all. The behaviour of the discharge system itself can be out of the range of the specific modelling approach (e.g. time dependent and 3-D, chaotic or stochastic). The model will enhance the understanding of the arc phenomenon itself, but the overall objective is quantitative prediction of specific discharge configurations. Some sort of understanding of such complex systems can be obtained by many different plausible explanations or models with internal fit parameters. But these will fail if new discharge configurations are to be computed in advance.
20 8 Chapter Outline of this work Chapter 1: Chapter 2: Chapter 3: Chapter 4: Chapter 5: Chapter 6: Introduction. Conceptual framework for modelling electric arc discharges. Physics of the electric arc discharge: Plasma column. Electrodes. Electrode layers. Plasma properties and transport coefficients. Modelling results, validation and sensitivity analysis. Summary, outlook and conclusions. Table 1.3: Outline of this work. Mathematical or physical modelling conventionally consists of a derivation of the equations describing the system and then solving them. This requires making all assumptions and approximations in advance. As the systems under investigation become more complex, this procedure becomes inapplicable in terms of effort and due to the nonlinearity of nature. Initially made assumptions need to be controlled afterwards and the modelling work becomes an iterative process itself. Most of all important arc phenomena occur as an emerging result of the interaction of different physical processes and discharge regions. The accurate modelling of the individual parts of the discharge is a prerequisite, but the iterative linkage of these models is of the same importance. As sketched in table 1.3, the overall modelling approach is discussed first in chapter 2. After sectioning the discharge into regions governed by different physical processes, the global iteration algorithms needed for the complete discharge description are presented. Within chapter 3, the physical models of the plasma column, the electrodes and the boundary layers are presented. The computation of the basic plasma properties and transport coefficients is summarized in chapter 4. In chapter 5, the model proves its ability to reproduce all major discharge features. Initially, an easy computable arc configuration is selected, and the detailed results available from the modelling data are provided. Additionally, the impact and origin of the flow pattern formation are discussed. For this model lamp, the general arc behaviour is computed for a large range of external parameters. The rest of the chapter is dedicated to the validation of the model by comparison with available experimental data. The last chapter (6) gives a summary of the achievements, it applications and the possible future enhancements of the model.
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