INTERNATIONAL DESIGN CONFERENCE - DESIGN 2002 Dubrovnk, May 14-17, 2002. DESIGN AND MODIFICATION OF WATER- STABILIZED PLASMA GENERATOR J. Moravec, M. Hrabovsky and R. Matas Keywords: Transformaton process, CAD model, nnovatve engneerng, CFD software, desgn, smulaton parameters, stablsed arc 1. Introducton Plasma torches wth -stablzed arcs provde an alternatve to the commonly used sources of thermal plasmas based on gas stablzed arcs or RF dscharges. The oxygen-hydrogen plasma jet s produced wth extremely hgh plasma enthalpy and flow velocty. Such plasma torches wth stablzed arc provde specal performance characterstcs n some plasma processng applcatons such as plasma sprayng or waste treatment. The sprayng rates wth plasma torches are almost one order hgher than the rates acheved n commonly used gas plasma torches. tangental nlet swrl I.E m arc cathode vapour zone anode vortex Fgure 1. Schema of the -stablzed arc Q outlet ext nozzle The arc chamber of the torch s dvded nto several sectons by baffles wth central holes. Water s njected tangentally nto the sectons where the vortex s created. The nner dameter of the vortex s determned by the dameter of the holes n the baffles. Water s exhausted at two postons along the arc chamber. The cathode s created ether by a graphte rod, whch s automatcally moved nto the chamber to compensate for eroson, or by a tungsten tp protected from contact wth plasma by a stream of nert gas. An anode made of a copper dsc wth nternal coolng s located outsde the arc chamber downstream of the nozzle ext. The scheme of the chamber s shown n Fg. 1; a more detaled descrpton of the equpment has been publshed e.g. n [Jensta 1999] and [Hrabovsky 1998]. All plasma and arc parameters as well as the stablty of the torch operaton are strongly nfluenced by the flow of the stablzng. Ths paper s devoted to the modellng of flow n an arc-stablzng chamber. TECHNICAL SYSTEMS 1079
2. Desgn of the chamber geometry and computatonal mesh To acheve an mproved desgn of the -stablsed plasma generator were use connectons among the Theory of Techncal Systems and other methods and tools for nnovatve engneerng desgn e.g. Desgn for X, FMEA, IMLab (TIPS) etc. The lnkage of the Theory of Techncal Systems wth these methods and tools creates a strong potental for the nnovaton of exstng solutons [Hubka 2001]. Here we used Desgn Scence (Theory of Techncal Systems) aded by the IMLab tools. Inventon Machne Lab (IMLab) s an ntellgent SW problem solver for engneers, engneerng desgners, technologst, nventors, scentsts, teachers all those who are professonally engaged n technology, engneerng and /or engneerng desgn. Envronment : Space, Tme Human System Techncal Systems Informaton System Management & Goal System Transformaton System TrS HuS TS IS M& GS Operand Od1 n exstng state Transformaton Process TrP Feedback Operand Od2 n desred state Fgure 2. Transformaton process n the Theory of Techncal Systems IMLab s based on the Theory of Inventve Problem Solvng (TIPS), whch was created by accumulatng and generalsng the nventve deas from the world s patent collecton. It has been establshed that the best nventve deas comply wth a relatvely small number of unversal prncples and rules. Our understandng of ths makes the process of problem solvng much more effcent and predcts the development of techncal systems and technologcal processes [Fg. 1]. In ths case we frst used IMLab n the process of the analyss of propertes. Here we created foundatons for solvng new problems and prncples. After an analysng of propertes and formulatons of the task we used IMLab n the process of creatng a desgn varant of a plasma torch for fndng solutons of prncples. The next step was to mprove the desgn varant of ths plasma generator for computer testng. computed area cathode anode Fgure 3. CAD model of the plasma chamber geometry 1080 TECHNICAL SYSTEMS
A 3D model of the geometry of the -stablzed plasma generator was prepared wth several modfcatons usng CAD software I-DEAS MS8. The full parametrc model was used to enable smple and rapd modfcatons of geometry. A model of the nternal space of plasma chamber (selected part of the overall geometry model) together wth the 3D geometry model was created to solve two-phase flow feld [Fg.3]. Ths model was parttoned for quck desgn of the mesh. The models n several modfcatons n IGES format were read nto pre-processor GAMBIT, whch had been used to generate the computatonal meshes. 3. Mathematcal model for the multphase soluton The flow n the plasma torch of a plasma generator s a very complcated physcal phenomenon and the avalable computng systems do not allow to solve the problem wthout some smplfcaton, but the qualty of the wall s a man factor n the stablzaton. The system FLUENT 5 for flud flow and heat transfer computatons based on the fnte volume method was used to solve partal dfferental equatons. The basc equatons of flud flow follow: The mass conservaton equaton ρ t + ( ρu ) = 0 (1) and the momentum conservaton equaton (Naver-Stokes equaton) t j ( ρ u ) + ( ρu u ) = + + ρg j j p τ j, (2) where τ j s the stress tensor. The segregated solver was used to solve the multphase models, the dscretzaton schemes of the frst and the second order were appled. The turbulent flow was consdered, the rng k-ε two equaton s turbulence model was used to close the equatons system. The ncompressble flud was consdered. The Volume of Flud (VOF) model was appled to solve the mult-phases flow feld. The VOF formulaton reles on the fact that two or more fluds (or phases) do not nterpenetrate. In each control volume, the volume fractons of all phases sum to unty. The trackng of the nterface between the phases s accomplshed by the soluton of a contnuty equaton for the volume fracton of one (or more) phases. For the q-th phase, ths equaton has the followng form: α t q + u α q = 0. (3) The volume fracton equaton s not solved for the chosen phase (so-called prmary phase); the prmary-phase volume fracton s based on the followng constrant: n q= 1 α. (4) q = 1 A sngle momentum equaton s solved throughout the doman, and the resultng velocty feld s shared among the phases. The momentum equaton results from the volume fractons of all phases dependng upon the propertes ρ (densty) and µ (vscosty). The mplct method was used to solve the multphase flow. TECHNICAL SYSTEMS 1081
4. Performed calculatons The descrbed model and ts parameters were tested successfully on a smple model [Matas 2001]. Hexahedral computatonal mesh was used for the base model. Optmsaton of the mathematcal model led up to the model wth hybrd mesh combnng hexahedral and tetrahedral elements [Fg.4]. The number of cells was about 1 Mo. cells for all meshes. ext base computed area wth mesh optmsed computed Fgure 4. Example of computatonal meshes The followng boundary condtons were used to complete the task: the velocty on nlet 26, 28 and 30 ms -1, the pressure on outlet from 70 to 100 kpa and the pressure of gas on nozzle ext 100 kpa. Several geometrcal modfcatons of the exhaust channel were compared. The wdth and the shape of the channel play very mportant roles n the gas sucton [Hrabovsky 2001]. The range of the computed area also plays a very sgnfcant role. The computed results gve good basc nformaton about the multphase flow n the torch chamber wthout arc. The computed flow felds show the nfluence of the asymmetrc locaton of nlets on the dstrbuton of flow parameters (pressure, velocty and volume fracton of phases) n the centre of the chamber. Examples of results are n fgures 5 and 6. Fg. 5 demonstrates the unsutable structure of flow n the outlet area for the older chamber desgn wth keen edges (left part of the pcture) and much better flow structure for the new desgn wth rounded edges (rght part). Fgure 5. Velocty vectors colored by volume fracton of ar on the nput to the sucton system 1082 TECHNICAL SYSTEMS
Fg. 6 presents the contours of the dstrbuton of the phases - the -ar dstrbuton. It depends on the computatonal model used and geometry. On Fgure 7 the velocty of n the vortex on slce at the velocty nlets plane s depcted, the vortex speed s about 3000 rpm. Fgure 6. Contours of fracton on slces through the computatonal doman for two dfferent ranges of computed area The comparson wth expermental results shows a good agreement of phase nterface. It seems that the -gas nterface s stable for varous pressures and geometrc condtons, the unsteady smulaton shows major nstabltes only on the nput of the sucton system. The shape of the gas nterface s not optmal, the optmsaton of the geometry nlet dameters would do for more unform dstrbuton of phases. Fgure 7. Velocty magntude of n vortex on slce through velocty nlets The value between 0.5 and 0.7 was computed for the volume fracton of ar (argon) on the outlet n the base models, the optmsed models gve the value between 0.35 and 0.4 whle the expermentally measured value s about 0.3. It has been proved that the computatons are very senstve to the settng of boundary condtons and the range of the computatonal doman. 5. Conclusons The am of the work has been the optmsaton of the torch chamber geometry. The new desgn of the chamber can reduce the sucton of the gas (ar, argon) by the outlet and mprove the stablty of -gas nterface. The performed computatons confrmed possblty of the smulaton of the multphase flow n arc chamber optmsaton. The computed flow feld shows the phase dstrbuton of and ar. The defnton of pressure boundary condtons proved to be very mportant for the calculatons and TECHNICAL SYSTEMS 1083
the sze of the computatonal doman has a very sgnfcant nfluence on the computed mass-flow rate of gas. The results show the sgnfcant nfluence of modfcatons of the sucton system geometry on beng flow feld dstrbuton. Future computatons wth mproved models of the output system are prepared. They should gve more accurate results of the phase dstrbuton and gas flow rate. Further actvtes wll be drected towards a model wth an ncluded heat source burnng electrc arc. Acknowledgement Ths paper s based upon work sponsored by the Mnstry of Educaton of the Czech Republc under research and development project LN00B084. References Jensta J., Hrabovsky M., Kopecky V., Effect of Vortex Moton of Stablzng Lqud Wall on Propertes of Arc n Water Plasma Torch, Annals New York Academy of Scences, Vol. 891, Heat and Mass transfer under Plasma Condtons (ed. P. Fauchas, J. van den Mullen, J. Heberlen), New York 1999, 64-71. Hrabovsky M., Water-Stablzed Plasma Generators, Pure & Appled Chemstry 70, 1998, No. 6, pp 1157 1162. Hubka, V., Eder, W.E., Hosnedl, S., Manual of Engneerng Desgn, Preprnt. Heursta Zürch CH, 2001. Matas, R., Frst Experences wth Multphase Modellng of Flow n Arc Chamber of Water Stablzed Plasma Generator, Proceedngs of UWB, Vol 4/2000, Plzen 2001, pp 117-121. Hrabovsky M., Matas, R., Moravec J., Desgn and Modfcaton of Water-Stablzed Plasma Generator and Multphase Modellng of Flow n Arc Chamber, Proceedngs FSO 2001, Brno, 2001, pp 69-72. Ing. Jan Moravec New Technologes Research Centre, Unversty of West Bohema Unverztn 8, Plzen, 306 14, Czech Republc Phone: ++420/19/749 15 50 Emal: moravec@ntc.zcu.cz 1084 TECHNICAL SYSTEMS