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Journal of Materials Processing Technology 211 (2011) 1620– 1628

Contents lists available at ScienceDirect

Journal of Materials Processing Technology

j o ur nal ho me p age : www.elsev ier .com/ locate / jmatprotec

umerical and experimental determination of plasma temperature during airlasma spraying with a multiple cathodes torch

. Bobzin, N. Bagcivan, I. Petkovic ∗

urface Engineering Institute, RWTH Aachen University, Aachen, Germany

r t i c l e i n f o

rticle history:eceived 16 December 2010eceived in revised form 28 April 2011ccepted 1 May 2011vailable online 8 May 2011

a b s t r a c t

A numerical model for the calculation of hydro and thermo dynamical gas flow in a three electric arctorch, considering electromagnetic influences on gas flow behaviour was described and developed. Themodelling results were compared with experimentally determined temperatures, obtained by computertomography. A characteristic threefold symmetrical distribution of temperature on the torch nozzle outletwas obtained numerically as well as experimentally. Calculated and tomographic temperature contours

eywords:ir plasma sprayingomputational fluid dynamicsomputer tomographylectric arc

and their rotations were compared and showed good agreement. In the second step, a model for the freejet was used for the evolution of thermal conditions into plasma jet outside the torch. The comparison ofthe calculated temperature distributions and those determined by tomography exhibit good agreement.

© 2011 Elsevier B.V. All rights reserved.

lasma temperature

. Introduction

Air plasma spraying (APS) is a widely used coating technol-gy for processing high temperature metallic, non-metallic anderamic materials. Characteristic low costs and high flexibility ofhis technology led to a spread of the technology in many indus-rial areas for coating production of wear, erosion, corrosion andhermal protection coatings (Fauchais, 2004). Key parts of the pro-ess are plasma generation inside the torch and expansion in freeet outside the torch. Required energy is provided by electric arcnside the torch. Outside the torch, the plasma expands in a highpeed jet. With most APS torches powdery feedstock is injectedadially into the plasma jet, where the powder is heated to aelted or partially melted state and accelerated towards the part

o be coated. The dynamical behaviour of the electric arc insidehe torch nozzle produces unstable properties of the plasma inree jet. This induces non-continual impulse transfer from plasmao powder particles as well as an inhomogeneous heat treatment,ffecting adversely quality and reproducibility of plasma depositedoatings.

Fluctuations of plasma properties in the free jet are present inost of the conventional torch designs (Dzulko et al., 2005). The

implest torch design with one rode-shaped cathode surroundedy hollow anode exhibits especially a high fluctuation ratio ofhe plasma free jet. But, because of simple construction and a

∗ Corresponding author. Tel.: +49 2418095550.E-mail address: petkovic@iot.rwth-aachen.de (I. Petkovic).

924-0136/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2011.05.001

well known operation field, this kind of torch is the most-usedin the industrial production of plasma sprayed coatings (Fauchaiset al., 2006). Main disadvantages are limited quality of the coat-ings and low process efficiency. In order to improve the coatingquality, much research work was related to the development ofnew torch designs with a decreased fluctuation ratio of the plasmajet. Recently, new torch designs, characterized by multiple elec-trodes, emerged on the market with significant improvements. Thetorches with multiple electric arcs show especially great promisefor the reduction of plasma fluctuation and improvement of sprayefficiency. One of such systems is TriplexProTM-200 (Bobzin et al.,2008). This torch generates three electric arcs between three cath-odes and one anode ring at the end of the torch nozzle. Due to afixed distance between the three cathodes and the anode ring, thearcs movement is restricted and thus the significant stabilisationof plasma can be achieved. The threefold auxiliary conception ofthe torch induces the characteristic plasma with three auxiliarysymmetrical temperature and velocity distributions in the jet crosssection near the torch nozzle exit. The intensity and position ofthe triangular distribution are determined with process parame-ters such as gas flow rate, electric current and proportion in thegas mixture. The threefold symmetry of jet cross section offers apossibility to optimize the powder injection as well as to heat andmelt the coating material uniformly, respectively. This is knownas “cage” effect and is based on the injection of powder in the

three cold or warm zones with different viscosity ratios, keepingthe powder concentrated in the centre of the plasma jet and allow-ing deposition in such concentrated form on the substrate (Scheinet al., 2008).

ocessi

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2

2

tis

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�

S = S + S (15)

K. Bobzin et al. / Journal of Materials Pr

Plasma torches have been objects of intensive theoretical andxperimental research for decades. Because of the lower costshan for experimental methods and significant growth of comput-ng power computer simulation became a standard method fornalysis of the torch. Most numerical models have been devel-ped to describe the behaviour of electric arcs in the simplest oneathode–one anode torch system, primary to comprehension of arcynamic. Due to modelling achieved knowledge and experienceuilds a fundamental basis for improvements of torch conceptsnd coating processes. Excellent modelling work on the dynamic oflectric arc was done in the study from Trelles et al. (2007) whichs based on the computational fluid dynamic (CFD) method. Spe-ial part in this work was the modelling of the arc reattachmentssuming that the plasma is in the local thermodynamic equilib-ium (LTE) state. This model was later extended for the plasma inon-local thermodynamic equilibrium (NLTE) (Trelles et al., 2009).

n the study from Selvan et al. (2010) a three dimensional numericalodel was used for the calculation of the velocity and temperature

istribution at the torch exit, considering the plasma fluctuation.he multiple electrodes torches have been not sufficiently inves-igated numerically or experimentally. Recently, there are onlytudies from Muggli et al. (2007) and Molz et al. (2007) related to theumerical analysis of torch generating three electric arcs. Withinhe frame of these studies numerical results were not sufficientxperimentally verified.

In this work a three-dimensional mathematical model forlasma torch TriplexProTM-200 was developed in order to pre-ict the temperature distribution at the torch exit and within thelasma jet in the zones of powder injection. The model consistsf two parts. The first one considers, beside thermo-dynamical gasow also the electro-dynamic effects in the torch. Only the geom-try of the torch nozzle was used for modelling. The calculatedonditions at the nozzle exit, representing the torch exit, wereorrelated with experimental results and used for the second partf the model as start condition. The second model represents theree plasma jet. The reason for splitting of model into two parts isuture usage of free jet model for the investigation of powder injec-ion. The torch model takes into account electromagnetic effectsesides fluid dynamic effects. Therefore, the model is very complexnd requires a large number of mesh elements followed with longalculation time. Furthermore, the presence of discrete phase forowder can lead to extra instabilities in the convergence of numer-

cal solution in one part of the model. Hence, separate calculation ofhe plasma torch and plasma free jet is in this case reasonable. Forhe verification of the simulation results the plasma temperatureistribution was obtained experimentally due to reconstruction ofomographic images of plasma jet. For this task, the measurementystem equipped with three CCD cameras for recording of plasmaas developed.

. Mathematical model and boundary condition

.1. Plasma torch

The mathematical model for the plasma torch is based on con-inuums approach hence the plasma was assumed as compressible,deal gas in the LTE state. The model can be expressed by followingystem of equations:

∂� ( )

∂t

+ ∇ · ��u = 0 (1)

(∂�u∂t

+ �u · ∇�u)

= ��g + ∇ · �� − ∇p + �fL (2)

ng Technology 211 (2011) 1620– 1628 1621

∂�h

∂t+ ∇ ·

(� h �u

)= −p∇ · �u + �� : ∇�u + ∇ ·

(�

cp∇h

)

+ SJoule + SNet (3)

∇ × �E + ∂�B∂t

= 0 (4)

∇ × �H − ∂ �D∂t

= �j (5)

∇ · �B = 0 (6)

∇ · �D = �f (7)

where t is the time, � gas density, �u gas velocity, p pressure, �g grav-itation, �� stress tensor, h gas enthalpy, � thermal conductivity, cp

specific heat at constant pressure, �E electric field, �B magnetic induc-tion, �H magnetic field, �D electric displacement,�j current density and�f electric charge density. There are two subgroups of equations inthis system. The first three equations are conservation equation ofmass (1), impulse (2) and energy (3), describing the thermo dynam-ical fluid flow in the torch nozzle. Equations (4)–(7) are Maxwell’sequations related to the evaluation of electromagnetic fields. TheMaxwell’s equations of electromagnetism determine the couplingbetween the electric and magnetic fields and the fluid. The above-mentioned set of equations can be completed with expression toelectric (8) and magnetic vector potential (9) as well as currentdensity (10)

�E = −∇� − ∂�A∂t

(8)

�B = ∇ × �A (9)

j = �(�E + �u × �B) (10)

Here is � electric potential, �A magnetic vector potential and �electric conductivity. Using potential equations the Maxwell’sequations can be replaced by charge conservation (11) and mag-netic induction equation (12) and expressed as:

∇(�∇�) = 0 (11)

∇2 �A = −�0�j (12)

where �0 is magnetic permeability.The influence of electromagnetic fields on the fluid flow dynam-

ics is described by the Lorenz force:

�fL = �j × �B (13)

and is implemented as a term in the impulse conservation equation(2). The Joule heating:

SJoule = �j ·(�E + �u × �B

)(14)

provides resistive gas heating and completes the energy conser-vation equation (3). Because of the very high temperatures ofthe plasma in the torch, radiation losses cannot be neglected.But the reflection at the nozzle walls is responsible for energyre-absorption. Therefore, an advanced radiation model with there-absorption of energy has to be realized. This is expressed as atemperature dependant net energy term in conservation equation(Eq. (3)) with:

Net Rad Abs

where the SRad is radiations and SAbs reabsorption term, respec-tively. The data used for these terms were taken from Speckhofer(1995). For calculations pure argon as plasma forming gas wasconsidered. Temperature dependent thermodynamic andtransport

1622 K. Bobzin et al. / Journal of Materials Processing Technology 211 (2011) 1620– 1628

Fig. 1. Geometry of the TriplexProTM-200 interior.

F

pwm

wptnr

atops

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TB

ig. 2. Calculation domain and boundary conditions used in plasma torch model.

roperties as well as specific electro-magnetic properties of argonere taken from Murphy and Arundelli (1994). The turbulenceodel is based on the standard K–ε model (Li and Chen, 2001).In Fig. 1 the interior geometry of the TriplexProTM-200 torch

ith 9 mm nozzle diameter used for calculation is presented. Thelasma gas enters trough 24 orifices into the convergent part ofhe torch nozzle where the three cathodes are placed. Behind theozzle throat there is a long section with neutrode rings. The anodeing is located near the torch outlet.

The geometry of the torch nozzle, presented in Fig. 1, shows threefold axial symmetry. This allows reduction of the domaino 1/3 of torch interior with only one cathode by usage of peri-dic boundary conditions (Fig. 2). The periodic boundary as cuttinglanes with 120◦ angle between them were defined on the periodicides 1 and 2.

The conditions of the boundaries showed in Fig. 2 are listed inable 1. The boundary condition at the inlet is defined by the massow rate (mi), varied equally to different process parameter. Onhe walls of the nozzle interior the heat transfer by overall convec-ive heat transfer coefficient of 105 W/m2 K was defined, accordingo the turbulent fluid flow in a pipe. Exceptions are cathode andnode walls, which were considered as adiabatic. This means that

here is no difference between cathode and anode and the charac-eristic sheath regions at the electrodes surfaces are not consideredor electric arcs in this approach. This simplification allows usagef coarse calculations mesh and significant decrease of calculation

able 1oundary conditions used in the simulation setup.

Boundary p [Pa] �u [ms−1] T [K] � [V] �A [V s m−1]

Inlet p(mi) �u(mi) 300 ∂�∂n

= 0 ∂Ai∂n

= 0

Internal wall ∂p∂n

= 0 �u = 0 500 ∂�∂n

= 0 ∂Ai∂n

= 0

External wall ∂p∂n

= 0 �u = 0 500 ∂�∂n

= 0 ∂Ai∂n

= 0

Cathode wall ∂p∂n

= 0 �u = 0 500 � = 0 ∂Ai∂n

= 0

Anode wall ∂p∂n

= 0 �u = 0 500 j(A) ∂Ai∂n

= 0

Outlet ∂p∂n

= 0 ∂�u∂n

= 0 ∂T∂n

= 0 ∂�∂n

= 0 0.0

Fig. 3. Calculation domain and boundary conditions used for the free jet model.

time respectively. The total amount of electric current is integratedin the model by current flow j(A) perpendicular to anode surface.

The calculation domain presented in Fig. 2 was meshed bytetrahedral elements. Zones near the electrode surfaces had to bemeshed very fine because of higher temperature gradients. The ele-ment size ratio was increased adaptively from electrode zones torest of calculation domain. However, for the complete domain inFig. 2 over 6 million elements were used resulting in a calcula-tion time of about 5 days per case until a converged solution wasachieved. The calculation was carried out parallel on a four proces-sors workstation with the commercial CFD software package ANSYSCFX 12.

2.2. Plasma free jet

The model for the free jet outside the torch is a classical CFDmodel including equations (1)–(3) without electromagnetic phe-nomena (�fL , SJoule, SNet). All material data are the same as in the torchmodel. The setup of boundary conditions is presented in Fig. 3. Themost important boundary is the nozzle exit determined by the cal-culated field parameters at the nozzle outlet in the torch model.As in the torch model, here a periodic boundary by periodic sides1 and 2 was used. Free boundary at the surface of the calculationdomain represents the plasma free jet behaviour in the infinity ofthe atmospheric ambient. Detailed description of the model can befound in work from Herzog et al. (2006).

The domain is 200 mm long with a 25 mm radius and includesabout 3.5 million tetrahedral elements. Calculations were carriedout with same software as for the calculation of the torch model.Calculation time per case was about ca. 6 h on the same workstationas above.

3. Tomographic investigation of plasma jet

The temperature distribution within the plasma produced byelectric arcs in the torch can be reconstructed by means of theemission computer tomography (CT) 0. The tomographic recordwas carried out by three CCD cameras mounted at 0◦, 90◦ and 225◦

on a single turning unit, Fig. 4. The measurement system graphi-cally presented in Fig. 4 was used for recording of plasma producedwith TriplexProTM-200 torch.

Each camera is equipped with a narrow-band interference filterof 10 nm spectral width. Since within the frame of these experi-ments pure argon was used as plasma gas, the selected interferencefilters are centred at 694, 776 and 830 nm, including the main linesof non-ionised argon. The cameras are rotated in 3◦-steps 180◦

around the plasma jet to determine the emission at 60 differentangles. This provides the data for three-dimensional reconstruction

of emissivity distribution of the plasma for each selected nar-row spectral range. The reconstruction requires a nearly stationaryplasma jet during the scanning time (about 2–3 min). This require-ment is assumed as fulfilled, because of the low fluctuation level of

K. Bobzin et al. / Journal of Materials Processing Technology 211 (2011) 1620– 1628 1623

Fi

ttaoo

vrttfa

Fc

ig. 4. Experimental setup using three CCD cameras with different narrow-bandnterference filters.

he TriplexProTM-200 plasma torch. The tomography reconstruc-ion requires the plasma to be optically thin. If LTE of plasma isssumed, then the emissivity of the plasma gas is a known functionf the gas temperature according to a standard calculation basedn the Saha equation (Marqués et al., 2009).

Following effects have to be taken into account for the intensityalues measured by each CCD camera: a geometry factor incorpo-ating the influence of the distance between jet and detector andhe transmission function of interference filter and camera. Lat-

er effect is corrected by determining the mentioned transmissionunction, using a tungsten band reference lamp of known temper-ture and emission. The first effect is cancelled out by considering,

ig. 5. Current density, represented by stream lines between anode ring and threeathodes for cases 50 (top) and 70 SLPM (bottom).

Fig. 6. Hottest areas, with temperatures above 13 350 K in the torch representedwith iso-surfaces for cases 50 (top) and 70 SLPM (bottom). These areas representthe cores of the electric arcs.

in analogy to the two-color pyrometry, the quotient between twointegrated emissivities in each of the narrow spectral windows,selected to contain those wavelengths for a maximum emissivityof non-ionised argon. Comparing the ratios with those theoreticallycalculated for the central region of the jet, where the temperatureand thus the emissivity is the highest, the maximum tempera-ture is identified. With the direct correlation between temperatureand measured intensities at the identified location, the completetemperature distribution within the jet can be subsequently deter-mined.

Five different operating setups, presented in Table 2 were con-sidered for the calculations and tomographical measurements. Theelectric power was fixed to 500 A and the gas flow was increasedfrom 50 up to 70 SLPM (standard litre per minute) in steps of 5SLPM.

The presence of the three powder injectors which are positioned

close to the torch outlet prevent recording of the plasma in thisregion directly. The first unobstructed cross section of the plasmajet to be tomographically resolved is located at a distance of 12 mmdownstream from the torch outlet. In order to estimate the tem-

Fig. 7. Complex pattern of gas vortex into a torch presented by velocity streamlinesfor cases 50 (top) and 70 SLPM (bottom).

1624 K. Bobzin et al. / Journal of Materials Processi

F

pwtg

TD

ig. 8. Calculated (left) and measured (right) temperature distributions at torch exit.

erature values and distributions at the torch exit for correlationith modelling results, tomographically determined reconstruc-

ions have to be re-adjusted by usage of effective torch power andas flow rate. The temperature distribution (16) and corresponding

able 2etails of the experimental cases.

Current [A] Input electricpower [kW]

Effective power[kW]

Plasma gas[SLPM]

500 49.4 30.0 50500 50.2 28.5 55500 51.2 30.7 60500 52.1 32.1 65500 52.9 35.0 70

ng Technology 211 (2011) 1620– 1628

velocity distribution (17) within the jet at the torch outlet can beformulated by polar coordinates (r, ) as:

T(r, ) = TmaxfT (r, ) (16)

v(r, ) = vmaxfv(r, ) (17)

where Tmax is maximum temperature value, vmax maximum veloc-ity value and accordingly relative distributions fT(r, ), fv(r, )of temperature and velocity respectively. The relative distributionprofile fT(r, ) is obtained directly from the tomographic recon-struction of the temperature at the first section behind the injectorring. In a first approximation, the corresponding relative distribu-tion profile for velocity is assumed to be nearly equal to the relativetemperature profile,

fv(r, ) ≈ fT (r, ) (18)

This approximation requires that the transport coefficient (viscos-ity) responsible for the radial dissipation of velocity scale with thetemperature in the same way than the transport coefficient (ther-mal conductivity) for the radial smoothing out to temperature. Thisis fulfilled for a nearly laminar flow. For the gas flow within a down-stream distance in the order of magnitude of the torch diameter(9 mm for the TriplexProTM-200 torch), nevertheless, the effect ofturbulences on the jet can still be neglected, even more since theexcentric arrangement of the three separated arcs protects the cen-tral jet region against turbulent entrainment of cold air. Hence thelaminar flow condition can be assumed in a first approximation andrelation (15) holds and by definition, the mass flow and effectivepower at the torch exit section are given by the following integrals:

m =∫

section

�(T(r, ))V(r, )drd (19)

Weff =∫

section

�(T(r, ))h(T(r, ))V(r, )drd (20)

with �(T) and h(T) respectively, are the temperature dependentmass and enthalpy density of argon, taken from the calculatedtables in [16].

Nevertheless, for the TriplexProTM-200 torch, the first transver-sal section of the gas jet which can be tomographically measured islocated after the injector ring, at a distance of 12 mm to the torchexit. For that jet region shadowed by the opaque injector ring theemissivity distribution at the torch outlet is obtained by extrapo-lating back the first unobstructed tomographic section, assumingthat the dissipation in the first 12 mm of the gas jet is negligible.Such extrapolation requires the determination of the jet rotationrate. This is achieved by estimating the angular orientation of eachtransversal section of the tomographic reconstruction. For that allconnected pixels in the central region of each section for whichthe relative intensity (relative to the absolute maximum of all sec-tions) is higher that a certain level (20%) are selected as the regionto be analyzed. Their centre of mass and their total area are calcu-lated and the region’s contour edge is determined as a function ofthe local radial extension r as function of the polar angle �. Subse-quently, the discrete Fourier transform for r(�) is evaluated

r(�) ≈N−1∑k=0

(ak cos(k�) + bk sin(k�)) (21)

with N the number of different points along the contour. The peri-odicity k > 0 with the largest amplitude

√a2

k+ b2

kis selected. If this

periodicity corresponds to k = 3 the contour maintains a triangularshape and its corresponding phase = − arctan(b3/a3) contains theinformation about the local angular orientation of the triangularcontour. This calculation is carried out at each transversal section

K. Bobzin et al. / Journal of Materials Processing Technology 211 (2011) 1620– 1628 1625

tours

asrtct

4

ab5shpeh

csI

Fig. 9. Simulated (left) and experimentally obtained (right) temperature con

nd thus the rotation rate of the gas jet can be followed in down-tream direction. Since for all considered cases that rotation rateemains constant along the measured jet, the upstream segment ofhe gas flow shadowed by the injector ring can be extrapolated byorrespondingly rotating the first unobstructed tomographic sec-ion back to the torch exit.

. Results and discussion

First, the numerical results of electric arcs from the torch modelre analyzed. In Fig. 5 stream lines, representing current paths,etween the anode ring and three cathodes for the simulation cases0 and 70 SLPM of argon flow and 500 A of electric current arehown. It can be seen, that the high current density region in theottest areas of the plasma is present, Fig. 6. These areas with tem-erature values above 13 350 K in Fig. 6 represent the cores of threelectric arcs. Because of the lower flow rate, the larger zone of theigh temperature zones in the case of 50 SLPM was observed.

Gas entrance direction induces an intensive fluid swirl into theonvergent part of the torch chamber, presented in Fig. 7 with thetreamlines from inlet surfaces to nozzle outflow for the same case.n this part of the torch the electric arcs are dragged along by the

of plasma free jet at 6, 15 and 26 mm from torch exit. Isosurface for 12 000 K.

vortex gas flow. In Fig. 6 it can be identified that the vortex influenceratio is more pronounced in case of the lower flow rate (50 SLPM)and the arcs zone exhibits higher rotation relatively to cathodes.Behind the nozzle throat, the gas swirl intensity was significantlyreduced because of increased axially component of velocity.

In Fig. 8 calculated and measured temperature distributions arecompared. For numerical as well as experimental results the threecathodes tips are depicted to allow an estimation of the displace-ment and the arc rotation angle. Regarding the reconstruction ofthe tomographic measurements discussed above, the form of thetemperature contours at the torch outlet correlates well with thenumerical results.

However, some qualitative differences between the numericallyand experimentally obtained results can be observed. The simula-tions show a clear triangular structure of the three plasma coresfor the high flow rates cases (65 and 70 SLPM) and a smoothedout contour for the lowest flow rates (50 SLPM). The tomographicresults, display a somewhat opposite trend: the triangular struc-

ture is determined in all cases but is more pronounced in the lowgas flow case (50 SLPM), where even the finger structure of thethree cores can still be identified. Several reasons may be responsi-ble for this discrepancy. The tomographic reconstruction of the gas

1626 K. Bobzin et al. / Journal of Materials Processing Technology 211 (2011) 1620– 1628

ure di

jtapflatf7tifsaaHrrfcp

fgc5c

the contour rotation angle obtained from the numerical simulationand from the tomographic reconstruction are very similar to eachother.

Table 3Maximum gas temperatures and angular location of the three plasma cores, com-pared between numerical simulation and the tomographic measurements.

Argon flow rate[SLPM]

Simulation Tomography

Tmax [K] Angle [◦] Tmax [K] Angle [◦]

50 13 330 −117, +3, +123 14 843 −123, −3, +117

Fig. 10. Calculated (left) and experimental (right) temperat

et segment obstructed by the injector ring is extrapolated from aransversal section 12 mm downstream from the torch exit underssumption of laminar and stationary plasma flow. But the reallasma flow exhibits a certain fluctuation and is turbulent. Fasterow in the real process leads to a more turbulent dissipation andlters first unobstructed section. Therefore, the determination ofhe rotation rate to extrapolate the jet back to the torch outlet there-ore can be more imprecise for cases with higher flow rates (65 and0 SLPM), leading to the deviation in the angular orientation ofhe temperature distribution reconstructed at that outlet observedn Fig. 8. On the other hand, the numerical modelling assumes alsoully steady state of the plasma flow. The numerical model takes thetandard K–ε model into account which could not be fully appropri-te for plasma flow because of lower turbulent diffusion. This can belso one of the reasons for results mismatching on the fastest flow.ence, the correction in the turbulence model and tomographical

ecord of unobstructed plasma and not re-adjusted of tomographyespectively, could make the simulation and tomography resultsully converge. However, the correlation of the results is satisfyingonsidering the complexity of numerical as well as experimentallasma analysis.

Furthermore, the maximum gas temperature at the torch exitor all cases resulting from both the modelling as well as the tomo-

raphic reconstruction, are compared in Table 3, showing goodorrelation with a deviation below 10% for all cases except for0 SLPM, where the deviation is slightly higher (11.5%). All cal-ulated and measured temperature distributions rotate clockwise

stribution in free jet in x–y plane including the torch axis y.

in downstream direction due to the gas inlet within the torch.Both, simulated and tomographic contours show a decrease of therotation with increasing flow rate, since the axial downstream com-ponent of the gas velocity accordingly grows with the higher gasflow rate. The results of the angle locations of the plasma cores atthe torch outlet, measured from the left horizontal axis and taken aspositive in clockwise direction, are also listed in Table 3. In the caseof the tomographic sections, this angle corresponds to the phase ofthe third harmonic of a Fourier transform carried out on the contourof 20% emissivity, described with Eq. (21) and, as discussed above,the higher turbulence of the high flow cases (65 and 70 SLPM) leadsto more imprecise determination of such phase. It can be seen that

55 13 301 −123, −3, +117 14 200 −135, −15, +10560 13 299 −132, −12, +108 14 000 −144, −24, +9665 13 215 −149, −29, +91 14 000 −147, −27, +9370 13 168 −151, −31, +89 14 018 −156, −36, +84

K. Bobzin et al. / Journal of Materials Processing Technology 211 (2011) 1620– 1628 1627

ure di

e2Dgdteadasct

paeotioTjida

Fig. 11. Calculated (left) and experimental (right) temperat

For the analysis of the plasma jet downstream from the torchxit, calculated results from the free jet model described in Section.2 with re-adjusted tomographic reconstruction of jet were used.istance of 35 mm from torch exit was tomographically investi-ated. In Fig. 9 the calculated and tomographically determinedistributions in the free jet are presented. Beside the contours ofhe torch exit contours at distance of 6, 15 and 26 mm from thexit are depicted in order to enable an estimation of the temper-ture conditions in the jet. Both, numerically and experimentallyetermined contours in the free jet behind the torch outlet exhibit

good similarity to contour at the torch exit. But these contours aremoother and the high temperature zones are shrinking. The cal-ulated results exhibit lower temperatures as in case of contours athe torch exit.

Furthermore, isosurfaces for temperatures of 12 000 K are dis-layed for all cases. The surfaces in modelling results exhibit almost

cone shape. Surfaces determined by CT show a somewhat differ-nt form with still expressed triangular cross section. Also lengthsf the numerical and experimental surfaces show some oppositerend. The length of the surfaces in the modelling results is increas-ng proportionally to the flow rate, whereas the tomographicallybtained surfaces show inversely behaviour and become shorter.hese differences can be related to a stronger dissipation of the fast

et outside the torch responsible for the effect that the tomograph-cally measured is clearly shorter than for lower gas flows aboveiscussed re-adjusting of tomographic records, possibly not fullyppropriate turbulence model in previous torch model as well as

stribution in free jet in y–z plane including the torch axis y.

fluctuation of plasma in real process. The assumption of fully steadystate can also contribute to discrepancy.

In Figs. 10 and 11 the temperature distributions in the free jetplanes are presented in x–y and y–z respectively. For both cases,modelling and tomographic results areas with temperatures below10 000 K were screened off. Only the areas between 10 000 and15 850 K are presented. Because of limited tomographically investi-gated space of 35 mm in y direction fully presentation of measureddistribution is not possible but the investigated space is sufficientfor correlation with modelling results. Except the case with 50 SLPMwhere the higher temperature zone was tomographically obtainedas in the simulated ones, the modelling und experimental distri-butions have showed very satisfying agreement of temperaturedistribution at the both planes.

5. Summary and conclusions

A three dimensional numerical model for the simulation ofhydrodynamic and electromagnetic phenomena in the three cath-ode TriplexProTM-200 plasma torch was described and developed.The model does not take into account the sheath regions at theelectrodes surfaces. The second model was used for estimation of

plasma conditions in the free jet. The modelling results were veri-fied by experimental temperature distributions in free jet, achievedby complex equipment based on the computer tomography ofargon emission. The temperature contours at the torch exit and

1 rocessi

sp

•

•

•

•

•

•

•

•

A

Gn1

628 K. Bobzin et al. / Journal of Materials P

everal distances from torch exit were correlated as well as of tem-erature isosurfaces in the free jet.

Following conclusions can be drawn from present study:

The achieved results show that the process modelling cancorrectly predict the plasma parameters at the torch outlet, par-ticularly the temperature values but also the distribution.Both, simulation and experimental results showed a triangularcouture of the hot cores at the torch exit and partially in thefree jet as well as dependence between flow rate and the contourrotation ratio.The rotation trend of the experimental and calculated tempera-ture contours is same-increase of gas flow rate induces decreasingof contours rotation angle.In the cases of lower and higher gas flow the maximum temper-ature agrees but the triangular structure of the hot cores as wellas the rotation rate begin to deviate.The best correlation was shown for the case at intermediate gasflow (60 SLPM).Deviations of results were related to the assumption in numeri-cal model, position extrapolation of tomographic records back totorch exit and low but present fluctuation ratio of the plasma inthe real process.The results contribute to comprehension of physical processesprimarily in the plasma torch and show influences of differentprocess parameters on the free jet characteristics.The tomography is difficult but very promising experimentalmethod for thermal analysis of jets in the plasma spraying pro-cess.

cknowledgements

The authors gratefully acknowledge the financial support of theerman Research Foundation (DFG) within the project “Homoge-ization of Coating Properties in Atmospheric Plasma Spraying” (Bo979/7-1). Furthermore, the authors appreciate the contribution to

ng Technology 211 (2011) 1620– 1628

tomographical investigation of plasma performed by Laboratory forPlasma Technology, Universität der Bundeswehr, München.

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