Numerical Simulation and Experimental Validation of Electrode Life in Plasma Cutting Torch for Different Injection Angles.

Transcription

1 Numerical Simulation and Experimental Validation of Electrode Life in Plasma Cutting Torch for Different Injection Angles. 1 M. Senthil Kumar, Dept of Mechanical Engineering, JNTU Kakinada, India, senthilwelcomes@gmail.com 2 B.Dhanasekar, General Motors, Bangalore, India, ersekar@gmail.com 3 G. Ranga Janardhana, Principal, JNTU vizianagaram, India, ranga.janardhana@gmail.com 4 K.S.Jayakumar 4 Asst.Prof, Dept of Mechanical Engineering, S.S.N College of Engg, Chennai, India, Abstract Plasma arc cutting technology has wide range of applications in diverse fields including non engineering sectors. The understandings of generation of plasma and its behavior is more ambiguous. This work is aimed to study the behavior of plasma due to the swirling action produced by swirler or baffles inside the plasma cutting torch. The swirling action produced in plasma cutting torch has predominant effect over the cut quality of the material being cut as well as life of the consumables used. In this paper, the effects of injection of plasma gas at different swirl angles are studied. The injection angles of 30, 45 and 60 through swirler are considered for this numerical analysis. The numerical simulations of plasma behavior are carried out by using Computational Fluid Dynamics (CFD) software FLUENT. A 2D axisymmetric model of plasma torch is considered for the analysis. A comparative study of the plasma jet coming out of the torch is made for the components of pressure, velocity, swirl velocity and temperature profiles for the three different swirl angles considered. The simulated results are validated by experimental trials. 1. Introduction Keywords: Numerical Simulation, Swirlers, Swirl Angles. Plasma jets produced by plasma cutting torch have been used for processing industries, manufacturing, ship building, aerospace, bio-chemical, fabrication, paper, mining, petro chemical industries etc. In this research effects due to swirl of the plasma are studied. The swirling of plasma gas has following advantages, which improves cut quality, the arc will attach to the leading edge of the cut, the arc is evenly distributed along the side of the cut and able to produce stabilized arc. The plasma behavior and swirling effects are understood in a better way from the following papers. The performance of nontransferred plasma torches is significantly depended on jet flow a characteristic out of the nozzle is described in [1]. The swirler induces vorticity, which stabilizes the arc in the center of the anode throat, provides convective cooling. Higher the swirl causes more vacuum to be created in front of the electrode and also tells the strength of the vortex. It causes the arc to rotate and allow hot spots of the anode to cool, reducing the electrode erosion is illustrated in [2]. The effects of swirl were studied numerically to design an optimal plasma torch is given in [3]. The swirling of gas increases the intensity of plasma energy, but the optimized swirl is required for better cut quality and life of electrode. In gas turbine combustor, increased air swirl reduces the formation of NO and spray angle has no influence over the NO concentration is explored in [4]. The unsteady swirling flow, 3D numerical simulation of conical diffuser is explained in [5]. The effect of swirler causing the flash back in burner of gas combustor turbine is described in [6]. The effects of the swirl flow injection on the characteristics of an atmospheric ICP generator were investigated in [7]. The capability of axisymmetric swirling flow models to correctly reproduce the flow features in the draft tube cone of a pump turbine was investigated in [8]. In casting molds, the presence of swirling flow reduce flow unevenness during filling is elucidated in [9]. The effects of supersonic nozzles, throat length, and injection angle on torch performance were investigated in [10]. The injection angle of the particles in plasma spraying is investigated in [11], it is found that upstream inclined injection leads to higher International Journal of Intelligent Information Processing(IJIIP) Volume3. Number2. June doi: /IJIIP.vol3.issue

2 levels of heating and acceleration of the particles. The flow patterns produced by baffles and formation of eddies have the significant effect over the electrode life. In this paper, numerical simulations are performed to study the plasma characteristics due to different injection angles and their swirl behavior. Qianlong Zhou, et al [12] studied the effect of plasma gas swirl in detail. Freton et al explained theoretical and experimental generation of plasma inside the plasma torch in [13]. 2. Mathematical model and assumptions 2.1 Physical domain A simplified model of the plasma cutting torch is developed and its schematic is shown in fig.1. The geometry is considered as 2D axi symmetric swirl. The symmetric geometry of the torch can reduce the working memory of the solvers. The inlet gas path profile is calculated from the equivalent area of baffle with 8 holes. Actually in 2D, it is not possible to show the real profile of the gas injection path from swirler, hence the equivalent area of number of baffle holes is calculated for the injection path area. In the domain considered, the radius of the electron emitter portion hafnium is 1mm and the radius of nozzle is 1.1mm. The distance between the torch end and work plate is 6.2mm as a cutting configuration. The length of the plenum chamber is 5.7mm. 2.2 Computational grid Figure.1. Geometry of the torch. The torch geometry was modeled and meshed in Gambit, which is the preprocessing session of the CFD Analysis. The fig.2 shows the meshed geometry of the torch. The mesh consists of quadrilateral cells, where the wall of the torch have closely spaced grids in order to observe the flow characteristics such as swirl, turbulence, vortex generation etc, for better accuracy. The meshed grid consists of 9689 cells. The further refinement of the mesh will not have any effect on the results. 2.3 Numerical methods and solver Figure.2. Meshed model of the Torch The numerical methods used to solve is described here. The commercial CFD software Fluent is used for this analysis which is based on the finite volume method. The torch geometry comprises of hafnium which is high electron emitter (cathode), nozzle, work piece (anode). The pressure based solver with implicit formulation is selected with absolute velocity formulation for axisymmetric conditions. Though, the computing time for implicit scheme is more than explicit, it has been used for numerical stability. The flow inside the torch is turbulent after inlet, then laminar inside the nozzle and 78

3 again turbulent at the exit. Hence the cut gas flow field through the torch is considered turbulence k-ε realizable model with standard wall functions, where k is the turbulent kinetic energy and ε is dissipation rate of kinetic energy. The Semi-Implicit method for pressure linked equations (SIMPLE) method is chosen for pressure velocity coupling. For the pressure discretization, pressure staggering option (PRESTO) is used and the second order upwind schemes were selected for momentum and energy. 3. The governing equations and boundary conditions The governing equations of plasma are similar to the compressible fluid with mass, momentum and energy equations. The plasma flow is considered as fluid flow and it follows the Navier-Stokes equation. The equations are the general form of Patankar s equation. In this paper, it is assumed the energy term is only due to electrons. To solve the problem considered, it is necessary to impose certain boundary conditions. For the numerical simulation oxygen is considered as the working fluid. The cut gas, oxygen will become as plasma gas, which flows around electrode, through nozzle and enters work piece with very high pressure and velocity. The inlet condition is specified as pressure inlet boundary, the cut gas enters with Pa gauge pressure of atmospheric temperature with axial and tangential velocity vectors of the flow direction. For turbulence, the default values k and epsilon are selected for specification method, as well as for turbulence kinetic energy and turbulence dissipation rate for both the cases of inlet and outlet boundary conditions. For the case of the occurrence of the swirl, the numerical analysis was performed for the velocity vectors of baffles for the different inlet angles of gas. The inlet gas is with 30 angle, the directions cosines of axial velocity is -0.5 which is calculated as cosine (90+30) and the directions cosines of tangential velocity is calculated as which is calculated as cosine (90+60). The above values are applied at the pressure inlet velocity components. Figure.3. Torch specification. The outlet condition is specified as pressure outlet boundary with the values of zero gauge pressure. The computational analysis is carried out at atmospheric pressure operating conditions. Where, ρ is mass density, v is velocity having axial vz, radial vr and azimuthal components vθ. The scalar quantity is φ, the diffusivity term Γφ and the source term Sφ are given for each governing equations listed in the table 1. The terms h, p and v are enthalpy, static pressure and electric potential. The components jr and jz are the radial and axial components of current density. The magnetic field component is calculated from the radial Ar and Az axial components of magnetic vector potential. μe and ke are the effective viscosity and effective thermal conductivity. Cp and σ are the specific heat capacity and electric conductivity respectively. 79

4 Equation φ Γφ Sφ Table 1. Two Dimensional Governing Equations Mass continuity Axial momentum v z μ e p 1 rμ e v r z r r z μ e υ z z z 2 μ e ( 3 z.υ) j r B θ Azimuthal momentum v θ μ e ρv r v θ μ e v θ v θ μ e r r 2 r r Energy h k e 5 k j z h j r h c p 2 e c p z c p r Electron potential V σ 0 Axial potential vector A z 1 μ 0 j z Radial potential vector A r 1 μ 0 j r A r r 2 Table 2. Boundary conditions Pressure Vz Vr V θ T V A z A r INLET (Pa) k 0 Az = 0 Ar = 0 (DE) CD k V = 0 Az = 0 Ar = 0 BC k 0 Az = 0 Ar = 0 EF k V = 0 Az = 0 Ar = 0 FG k 0 Az = 0 Ar = 0 GH k 0 Az = 0 Ar = 0 HI k 0 Az = 0 Ar = 0 JK k V = 0 Az = 0 Ar = 0 KL k 0 Az = 0 Ar = 0 IJ k V = 0 Az = 0 Ar = 0 LM k 0 Az = 0 Ar = 0 4. Results and Discussions 4.1 Case-1 For analyzing the effect of swirl and other behaviors of plasma for different injection angles, the plasma torch domain considered is same for all the cases. The cut gas is directed into the plasma torch with swirling velocity components by the holes presented in the baffles. For the axial and tangential velocity components, the angles considered in first case is 30 and 60 respectively. This swirling velocity component ensures the stabilization of plasma arc in the torch. The fig.4 (a) shows the pressure distribution contours. The pressure remains constant inside the plenum chamber and decreases. 80

5 (a) Pressure distribution contours (b) Temperature distribution contours (c) Swirl velocity contours (d) velocity contours 4.2 Case-2 (e) Stream line velocity contours Figure.4. Case -1 Contours of plasma flow for 30 and 60 velocity components. (a) Pressure, (b) Temperature, (c) Swirl velocity, (d) velocity and (e) stream lines plot. The axial and tangential velocity components, for the second case are 45 and 45 respectively. The fig.5 (a) shows the pressure contours. The minimum back pressure obtained is Pa. The pressure distribution is more or less similar to the above case. The fig.5(b) shows the temperature distribution of plasma inside the torch and to the work piece. It has maximum temperature of k, proximity to the hafnium surface and then at the nozzle region. The maximum velocity magnitude observed at the nozzle exit is 3690 m/s, and is shown in fig.5(c). The maximum swirl velocity 19.3 m/s, is observed along the nozzle walls which is higher side than the 30 injection path is shown in fig.5 (d). The flow pattern of plasma is showed as stream lines of velocity. The fig.5 (e) shows the stream lines of gas flow. The eddy formation in front face of the cathode is more compared to the earlier case. The formed eddies will affect the molten hafnium part of the electrode. This will produce vortex break down, which will influence the electrode life comparably. 81

6 (b) Pressure distribution contours (b) Temperature distribution contours (c) Swirl velocity contours (d) velocity contours 4.3 Case-3 (e) Stream line velocity contours Figure.5. Case -1 Contours of plasma flow for 45 and 45 velocity components. (a) Pressure, (b) Temperature, (c) Swirl velocity, (d) velocity and (e) stream lines plot. For the last case, the axial and tangential velocity components, considered are 60 and 30 respectively. The fig.6 (a) shows the contours of pressure distribution. The back pressure in this case is around Pa. The pressure distribution is more or less similar to the second case. The fig.6 (b) shows the temperature distribution of plasma inside the torch. The maximum temperature of k, is proximity to the hafnium surface and the next higher temperature is at the nozzle convergent area. The maximum velocity magnitude is observed at the nozzle exit which is around 3690 m/s, and the velocity contours are shown in fig.6(c). The maximum swirl velocity 19.3 m/s, is along the nozzle walls which is higher than both first and second case of injection shown in fig.6(d). The flow pattern of plasma observed is showed as stream lines of velocity in fig.6 (e). There is no eddy formation in front face of the cathode, but the gas flow is not uniform. This type of flow will not produce much energy, as well as flow pattern is disturbed. This will not give better electrode life compared to earlier two cases. 82

7 (a) Pressure distribution contours (b) Temperature distribution contours (c ) Swirl velocity contours (d) velocity contours (e) Stream line velocity contours Figure.6. Case -1 Contours of plasma flow for 60 and 30 velocity components. (a) Pressure, (b) Temperature, (c) Swirl velocity, (d) velocity and (e) stream lines plot. 5. Experimental Results The experimental setup consists of plasma cutting CNC machine, cutter carriage with cutting torch. The plasma torch primarily consists of nozzle, electrode holder, electrode, baffle, shield cup, etc. The X, Y movements of the truck is controlled by vision52 controller. The cutting process parameters can be fed through the man machine interface (MMI). The closed loop coolant circulator system consists of heat exchanger, fan, pump, motor and other accessories which ensure the proper cooling of the consumables inside the torch. To study the effect of injection angles of cut gas through the swirlers over the life of the electrode three different injection angles 30, 45 and 60 of cut gas through the swirlers are considered. The experimental results depict the same outcome as the numerical simulation results. 83

8 Figure.7. Electrode wear for 30 & 60, 45 & 45, and 60 & 30 velocity components, Figure.8. Electrode life comparison chart The life test result of sample worn out electrode pictures are shown in fig.7 for the velocity components combinations of 30 and 60, 45 and 45 and 60 and 30 arrived from the experimental trials. The graph in fig.8 shows the comparative plot of electrode life versus time for the above three cases specified. From the plot it is obvious, the 30 and 60 velocity components, series 1 have longer life with lesser wear compared to the other combinations. The 60 injection angle has the worst life with quicker electrode wear. Though the 45 and 45 angles combination have comparable life with 30 and 60 angles, but the quality of the cut is not so good which is not explained in detail. 6. Conclusion The swirling effect will increases the intensity of plasma energy coming out of the torch. This paper is focused to simulate the behavior of plasma gas swirl for different injection angles of inlet cut gas through swirler. The presented work depicts the simulation results of plasma behavior for the three different injection angles with combination of 30 and 60, 45 and 45 and 60 and 30, velocity components of the inlet gas respectively. The simulated results of pressure, temperature, velocity, swirl velocity contours are presented here. These results give us better understandings of swirl due to different injection angles. From the above work, it is observed that, the injection angles for the velocity components angles of 30 and 60 is having the better and smooth velocity stream lines without any obstruction. It is also noticed that, the swirl velocity increases, if the injection angle is increased. It has been concluded that, these 30 and 60 combination will give more consumable life than other two cases, due to the non eddies formation in front face of the electrode as well as in the gas path trajectory. This study, has given better idea about the effect of injection angle of cut gas for the plasma torch design concerns. The experimental results validate that the 30 and 60 injection angles combination will give more consumable life. There is a good agreement between numerical simulation and experimental results. 84

9 7. References [1] You-Jae Kim, J.-G. Han and Youn J. Kim, Numerical Analysis of Flow Characteristics of An Atmospheric Plasma Torch, Sungkyunkwan University, KOREA, [2] Scott D. Gallimore, Operation of a High-Pressure Uncooled Plasma. Torch with Hydrocarbon Feedstocks, August 1998, Blacks burg,virginia. [3] J.-H. Moon, J.-G. Han, Youn J. Kim, Performance of an atmospheric plasma torch with various inlet angles, Surface & Coatings Technology 193 (2005) [4] D Chatterjee et al. Effects of inlet air swirl and spray cone angle on combustion and emission performance of a liquid fuel spray in a gas turbine combustor Seventeenth National Convention of Aerospace Engineers, Ranchi, November 3-5, [5] Sebastian Muntean, et al. 3D numerical analysis of the unsteady turbulent swirling flow in a conical diffuser using fluent and open foam, 3rd IAHR International Meeting of the workgroup on cavitation and Dynamic Problems in Hydraulic Machinery and Systems, October 14-16, [6] Frank Kiesewetter et al. Two-dimensional flashback simulation in strongly swirling flows, Proceedings of ASME Turbo Expo 2003 Power for Land, Sea, and Air, June 16 19, 2003, Atlanta, Georgia, USA. [7] Takayoshi Inoue et al. Effects of swirl flow on an atmospheric inductively coupled plasma, 36th AIAA Plasma dynamics and Lasers Conference, 6-9 June 2005, Toronto, Ontario Canada. [8] Oliver Kirschner et al. swirling flow in a straight cone draft tube: Axi-symmetric flow analysis and comparison with circumferentially averaged piv measurements, 2nd IAHR International Meeting of the Workgroup on Cavitation and Dynamic Problems in Hydraulic Machinery and Systems Timisoara, Romania, October 24-26, [9] Line Hallgren et al. Effect of nozzle swirl Blade on flow pattern in runner during uphill teeming, ISIJ International, Vol. 46 (2006), No. 11, pp [10] Scott D. Gallimore, A Study of Plasma Ignition Enhancement for Aeroramp Injectors in Supersonic Combustion Applications, May 2001 Blacksburg, Virginia. [11] D. Khelfi et al Modeling of a 3D plasma thermal spraying and the effect of the particle injection angle, Review of Renewable Energy CISM08 Oum El Bouaghi (2008) [12] Qianhong Zhou et al, The effect of plasma-gas swirl flow on a highly constricted plasma cutting arc, Phys. D: Appl. Phys. 42 (2009) (9pp) [13] P Freton et al, Numerical and experimental study of a plasma cutting torch, J. Phys. D: Appl. Phys. 35 (2002) [14] Asad A.Saleem, Numerical Simulation of Fluid Flow and Heat Transfer in a Plasma Cutting Proceedings of the 2006 WSEAS/IASME International Conference on Fluid Mechanics, Miami, Florida, USA, January 18-20, 2006 (pp19-24). [15] D. Cook and J. Start, Electrode Wear in Air & Oxygen Plasma How to tell good electrode wear from bad and improve system performance, May/June [16] M.Senthil Kumar et al, Numerical Simulation and Experimental Verification of Electrode Life for different coolants and flow in plasma cutting torch, IEEE, Dec [17] Different Coolants and Its Flow in Plasma Cutting TorchParker, Optimizing consumable life in mechanized plasma cutting, [18] A.K.Das, Arc root dynamics in high power plasma torches-evidence of chaotic behavior J.Phys., Vol.55, Nos 5&6, Dec