011401-1 CHINESE OPTICS LETTERS / Vol. 9, No. 1 / January 10, 2011
Computational fluid dynamic modeling of gas flow
characteri stics of the high-power CW CO
2
laser
Hongyan Huang (
õõõ
) and Youqing Wang (
qqq
)
∗
Wuhan National Laboratory for Optoelectronics, School of Optoelectronic Science and Engineering,
Huazhong University of Science and Technology, Wuhan 430074, China
∗
Corresponding author: yqwang13@163.com
Received June 12, 2010; accepted July 10, 2010; posted online January 1, 2011
To increase the photoelectronic conversion efficiency of the single discharge tube and to meet the re-
quirements of the laser cutting system, optimization of the discharge tube structure and gas flow field
is necessary. We present a computational fluid dynamic model to predict the gas flow characteristics of
high-power fast-axial flow CO
2
laser. A set of differential equations is used to describe the operation of
the laser. Gas flow characteristics, are calculated. The effects of gas velocity and turbulence intensity on
discharge stability are studied. Computational results are compared with experimental values, and a good
agreement is observed. The method presented and the results obtained can make the design process more
efficient.
OCIS codes: 000.4430, 140.3425, 140.3470.
doi: 10.3788/COL201109.011401.
High-power fast-axial flow (FAF) CO
2
laser is a well-
established cutting tool in the manufacturing industry.
In large-format laser cutting systems, the laser generally
moves with the whole system along the guide rail; there-
fore, the structure of the laser has to be as compact and
light as possible. The key factor in achieving all these
characteristics is to raise the photo electronic conversion
efficiency of the single discharge tube. This technology
has been under continuous improvement. In the early
stage, a single discharge tube of cruciform structure can
reach a maximum output of merely 290 W with an op-
timized entrance nozzle structure. A maximum output
of 333 W can be achieved. We recently discovered that
the maximum output power of the single discharge tube
can reach 500 W. The co re theory of this prog ress is
the discharge tube structure a nd the gas flow field opti-
mization. However, there has been very little domestic
research on this subject.
Many studies on FAF CO
2
lasers have focused on the
modeling of laser processes in the laser medium
[1−9]
.
The effects of turbulence flow on the performance of the
FAF CO
2
laser have also been discussed
[10−13]
. These
studies have provided good theoretical bases for our re-
search. Computational fluid dynamic (CFD) method
has become a powerful approach to analyze the three-
dimensional (3D) flow in complicated domains. We can
make use of previous resear ches and the CFD method
to obtain further insight on the realistic gas flow of the
laser and make the design more efficient.
In our ear lier letter
[14]
, we presented a preliminary a t-
tempt on numerical investigation of a FAF CO
2
laser
using CFD method, and the results were encouraging.
However, much improvement is needed before the ac tua l
application. Our previous work simplified the discharge
cavity into a straight tube. With the further demand for
laser researches and a more ac c urate grasp of the internal
flow field, we established a realistic 3D discharge tube
model, including the turbulence generator and the anode
and cathode area. In our previous work, the effect of the
external electric field was only described via a given co n-
stant value, which was equal to the difference between
the input electric and lase r output power. This approach
was not sufficiently accurate becaus e the electric field
effects on particular regions inside the laser cavity are
different. The contribution of electrons of vibrational
states and rela xation of electrons from the asymmetric
stretch vibrational levels of CO
2
to the ground level also
need to be considered
[15]
. In this letter, we divide the
computational grid into four regions, and then se t the
source ter m.
An overall v iew of the grids used in the computations
is shown in Fig. 1. The size of the grid is approximately
129184 cells. From Fig. 1, the turbulence generator is
constituted by a cylindrical annular cavity located out-
side the ellipsoid chamb e r with a gas inflow opening.
The jet orifice connecting the cylindrical annular cavity
and ellipsoid chamber is opposite the initial gas inlet and
45
◦
away from the axial. The electrode pin is coaxially
disposed with the axis of the jet orifice. This structure
helps generate a vortex stree t and a high-turbulence gas
flow, according to the following numerical investigations.
The model consists of a set of differential equations
for numerical solution of discharge proce ss of the FAF
CO
2
laser. Before introducing the governing equa-
tions, we discuss the division of the computational grids,
which is the main improvement from our previous work.
The gases in the region of fluid inlet and the cylin-
drical annular cavity have not bee n excited; in other
words, the effect of the electric field and electric cur-
rent need not take the energy conservation equation into
account. We define this regio n as inflow area. The
region of intense electrical heating near anode pin is
the ellipsoid chamber. We define this region as the an-
ode area. The region inside the cylinder of the radius
19.15 mm between the axial distances of 20.42 a nd
216.3 mm is defined as the pos itive column area. The
axial distance between 216.3 and 224.3 mm is defined as
the cathode area. The related source terms will be set
1671-7694/2011/011401(4)
c
2011 Chinese Optics Letters
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