22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Stability of an arc generated by a TIG plasma torch
K. Takeda and K. Maruyama
Faculty of System Science and Technology, Akita Prefectural University, Yurihonjo, Japan
Abstract: The transitions from a laminar arc to an unstable one were investigated.
Experimental observations revealed that the transition occurred at the Reynolds number of
100 - 200. Plasma gas temperatures in various experimental conditions were estimated,
measuring the arc motion driven by an imposed magnetic field.
Keywords: transferred arc, stability, Reynolds number, temperature, electromagnetic force
1. Introduction
Transferred arcs driven by an alternating magnetic field
have been used to apply for various heat treatments of
metals [1, 2].
Schematic arrangement for the
magnetically driven arc is illustrated in Fig. 1, where a
long arc is generated using a TIG (Tungsten Inert Gas)
plasma torch. An alternating magnetic field is imposed
perpendicularly to the arc. The electromagnetic force
induces the oscillatory motion of the arc. Heat flux
distribution on the anode material can be controlled by
adjusting the magnetic field. The formation of a laminar
arc is required for precise processing. However, few
studies have been done on the stability of the arc [3]. In
this work, the conditions to maintain the arc stable were
studied.
plasma forming gas
Qo
arc torch
z
power
supply
y
The arc columns were observed by a camera, the shutter
speed of which was fixed at 1/4000 sec. The arc became
unstable with the increase of the gas flow rate as shown in
Fig. 2.
The fluctuation of the arc voltage was
concomitant with the unstable arc column as shown in
Fig. 3.
Fig. 2. Variation of arc profiles with the increase of gas
flow rate for d = 5 mm and I a = 60 A.
x
vo
magnetic field
B(t)=Bocos(ω t )
Ia
Fig. 3. Fluctuation of arc voltage for various gas flow
rates.
anode
Fig. 1. Schematic illustration of magnetically driven arc.
2. Experiment
The stabilities of the arc were mainly investigated under
no imposed magnetic field. Experimental conditions are
summarized in Table 1.
Table 1. Experimental conditions.
parameter
arc current, I a
arc distance, L
plasma forming gas
gas flow rate, Q
nozzle diameter, d
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quantity
40 - 80
[A]
50
[mm]
Argon
1 - 12
[NL/min]
3.2, 4, 5 and 6
[mm]
The variation of voltage fluctuation was shown in Fig. 4.
Critical gas flow rate (Q*) was defined as the gas flow
rate at which the arc became unstable.
Q*
Fig. 4. Definition of the critical gas flow rate; d = 5 mm
and I a = 60 A.
1
The critical flow rate depended not only on the nozzle
diameter of the torch but also on the arc current. The
variation of the critical flow rates with the nozzle
diameters and that with the arc currents were plotted in
Figs. 5 and 6, respectively.
From previous work on the magnetically driven arc [5], it
was known that plasma gas velocity was expressed as:
v=
I a Bo Ya 2 + L2
(
)
Q
2Ya
(4)
Considering the equation of state for gas, T p is given by
Tp =
Mpo pd 2 v pMpo I a Bo d 2 Ya 2 + L2
(
)
=
R
4 Q
4R
2Ya
Q2
(5)
where M, R and p o represent mass per mole, gas constant
and atmospheric pressure, respectively.
One example to estimate the plasma temperature is
shown below. This experiment was carried out under the
following conditions; nozzle diameter d = 5mm, pressure
p 0 = 1 atmosphere, arc current I a = 80 A and gas flow rate
Q = 5 NL/min (Q = 1.5x10-4 kg/s). In these conditions, a
stable laminar arc was observed. Then alternating field
was imposed perpendicularly to the arc. The amplitude
(B o ) of the AC magnetic field was 3.7 x 10-3 T. The arc
was driven by electromagnetic force and oscillated as
shown in Fig. 7. Obtained amplitude of the arc motion
(Y a ) was 17x10-3 m. Substituting Y a = 17x10-3 m into
Eq. 5, plasma gas temperature was calculated as
T p = 9050 K.
Fig. 5. Variation of Q* with nozzle diameter.
d=5mm
Fig. 6. Variation of Q* with arc current.
3. Determination of Reynolds number
In the next section, critical Reynolds number (Re*) will
be calculated, corresponding to each critical gas flow rate
(Q*). Before head, general procedure to determine the
Reynolds number will be discussed in this section.
3.1. Relation between Re and Q
Using viscosity (µ), nozzle diameter (d) and gas
velocity (v), Reynolds number is expressed as
Re = ρvd / µ
(1)
While, gas flow rate is written as
Q = (πd 2 / 4) ρv .
(2)
Then Re is represented in the following form,
Re = (π / 4)Q / dµ
Fig. 7. Magnetically driven arc for the measurement of
plasma gas temperature.
3.3 Determination of viscosity and Reynolds numbe
Once the plasma gas temperature is known, the
viscosity can be evaluated consulting the known relation
between µ and T [4]. For argon plasma gas at T p = 9050
K, its viscosity is µ = 0.28x10-3 kg/(m s).
Using known parameters of Q, d and µ, Reynolds
number could be calculated as Re = 135 in this case.
(3)
In order to determine the numerical value of Re, it is
important to know the plasma gas temperature (T p ),
because viscosity strongly depends on the plasma gas
temperature [4].
3.2. Determination of plasma gas temperature
One of the easy ways to estimate the plasma gas
temperature is to measure the amplitude of the oscillatory
motion (Y a ) in the magnetically driven arc shown in Fig. 1.
2
Ya
4. Critical Reynolds number
As shown in Figs. 5 and 6, various critical gas flow
rates were obtained for various experimental conditions.
In similar procedures mentioned above, critical Reynolds
number (Re*) was calculated for each critical gas flow
rate (Q*). Obtained critical Reynolds numbers were
plotted in Fig. 8. It is well known that the transition from
a laminar flow to a turbulent one occurs at the Reynolds
number of R e= 2000 ~3000. Different from the
laminar/turbulent transition in ordinary fluid dynamics,
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he transition from stable arc to unstable one took place at
Re* = 100 ~200.
Fig. 8. Critical Reynolds number for arc stability.
5. Summary
Stability of an arc was investigated. Experimental
observations revealed the following results.
1. An arc changes its profile from a straight column to an
irregular one when a flow rate of plasma forming gas
exceeds a certain critical value of Q*.
2. Appearance of such unstable arc profile follows
unstable fluctuation in the arc voltage.
3. Critical gas flow rate of Q* depends on the nozzle
diameter of the plasma torch.
4. The increase of arc current results in the increase of Q*.
Stability of the arc was discussed in terms of Reynolds
number. With no information of plasma gas
temperature, Reynolds number cannot be calculated.
In order to estimate the temperature of the plasma gas,
a new method was developed, using magnetically
driven arc.
5. Transition from a stable arc to an unstable one was
governed by Reynolds number. The critical Reynolds
number is Re* = 100 ~200.
P-II-12-14
6. Acknowledgements
The authors would like to express our thanks to
Prof M. Sugimoto of Akita Prefectural University for his
kind help.
7. References
[1] T. Toh, J. Tanaka, J. Muraki, Y. Yamamoto and
K. Takeda. ISIJ Int., 45, 947-953 (2005)
[2] R. Akiho. M. Sugimoto, K. Takeda, Y. Noguchi and
T. Miura. Trans. Japan Soc. Mechanical Engineers
Ser. C, 79, 690-703 (2013)
[3] K. Maruyama, K. Takeda, M. Sugimoto and
Y. Noguchi. J. Phys. Conf. Ser., 550, 012009
(2014)
[4] P. Fauchai. Fundamental concepts. in: Int. Summer
School on Plasma Chemistry; (IUPAC) 50 (1987)
[5] K. Takeda, H. Okubo and M. Sugimoto. J. Phys.
Conf. Ser., 550, 012011 (2014)
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