June 2013
Volume 2
Issue 3
Bengbu College Learning
ge
Jun. 2013
Vo1.2, No. 3
Numerical simulation study on gas flow field of laser cutting assisted composite nozzle
(Bengbu College
Department of Mechanical and Electronic Engineering, Anhui
Bengbu
)
: The conical nozzle flow field and the composite nozzle flow field were simulated and analyzed, and the impact of nozzle parameters on the nozzle gas flow field velocity and pressure distribution were analyzed.
ring. Through numerical simulation analysis, it was concluded that the composite nozzle reduces the fluctuation of air flow compared to the conical nozzle, ensures the stability of the processing process, and more
It is conducive to improving the quality of laser cutting.
Keywords: numerical simulation; nozzle; laser cutting; flow field analysis; fluid dynamics
Zhongcai Classification Number: TM561
Document identification code: A
Article number: (2013)03—0007—04
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Laser cutting uses the focused laser beam as the main heat source
The thermal cutting process method, the energy density is highly concentrated during processing
The laser beam moves relative to the workpiece. To get good
The cutting quality is often used to improve the processing strips with additional auxiliary gases.
. If oxygen-assisted cutting is used, it can promote oxidation of the metal surface.
Improve cutting efficiency and can also be cut by increasing the oxygen blowing pressure
The seams are reduced to prevent the molten material from being re-bonded¨
The nozzle serves as the discharge channel of the laser beam and auxiliary gas,
It is an important component for laser cutting. What are the structural parameters of the nozzle
Selection will affect the distribution of internal and external flow fields and control the air flow
And laser cutting efficiency and quality have a very important impact. Book
The paper selects conical nozzle and new composite nozzle as research objects.
And analyze the flow field formed by the composite nozzle and the conical nozzle
Comparison, according to the compressible two-dimensional axisymmetric N-S equation,
Software function to the free jet pressure and velocity field of nozzle
Received date: 2013-02-15
Author profile: Yang Li (1980), female, from Shou County, Anhui Province, lecturer, master's degree.
Perform numerical simulation and study the velocity and pressure distribution diagram along the axis
The impact of nozzle structure shape on the velocity field and pressure field of free jet
In order to demonstrate the impact of nozzles of different structures on laser cutting quality
(see Figure 1, Figure 2).
Figure 1
Ordinary conical nozzle
Yang Li
Numerical simulation study on gas flow field of laser cutting assisted composite nozzle
Figure 2
Schematic diagram of composite nozzle structure
Comparison of two nozzle structures
Ordinary conical nozzles have the advantages of simple structure and low cost.
However, shock waves are easily formed in the external flow field of the nozzle, which affects the cutting quality.
Structurally, there is no distinction between inner and outer nozzles
I4J. Its main parts include
Two segments, namely the shrinking segment and the stable segment, the main parameters are the cone angle of the shrinking segment
Only the length of the stable segment
, its structure is shown in Figure 1. Compound spray
The nozzle is provided with an external nozzle outside the conventional conical nozzle, and its structure
As shown in Figure 2. The inner nozzle and the outer nozzle outlet have a certain axial distance
The inner nozzle has no stable section length, and the outer nozzle part has a stable section.
During operation, the inner and outer nozzles supply two different air pressures, and finally,
Ejected along the outlet of the outer nozzle to form a composite airflow of different pressures.
In order to study the influence of nozzle structural parameters on gas flow field,
First, the article aims at common conical nozzles, under the same initial conditions,
That is, when the total import pressure is 6 atm, the shrinkage is changed separately
The length of the angle or the stabilization section is used to mold the gas flow field at the nozzle
Plot. Secondly, in order to reflect the influence of the nozzle structure on the cutting air flow,
Under the same initial conditions, the air of the composite nozzle and the conical nozzle is also applied.
The body flow field was simulated and analyzed and compared.
Control equations and turbulence model
When performing gas flow field simulation, you need to select appropriate controls
Equations and turbulence model.
The control equation is based on the Reynolds average continuity equation N-S
The turbulence model is used to numericalize the RNGk-￡ model.
simulation.
The standard k-s equation has a larger number when describing axisymmetric jet.
For deviation, this paper adopts the RNGk-8 two-equivalent equation model.
The RNG equation is:
P-alarm = go +G+G-P8-(3)
J"6
(j)
Go +G
(G+G3G
Pdt
)Rose J+
(+
o2
A G2P
One R
(4)
Where: G represents the turbulent kinetic energy caused by the average velocity gradient
Generate, G represents the generation of turbulent kinetic energy caused by the influence of buoyancy;
Indicates the effect of compressible turbulent pulsating expansion on the total dissipation rate.
Compared with the standard k-8 equation model, the RNGk-2 equation model
A condition is added to the e equation, which has a certain improvement in accuracy.
And considering turbulent vortex, it is widely used in axisymmetric nozzle injection
Calculation of streams.
Gas dynamics analysis and comparison
3.1
Establish a computing model
Two common nozzles and composite nozzles are established
Dimensionally calculate the model and divide the plane grid, as shown in Figures 3 and 4.
It mainly includes the internal air flow field of the nozzle and the external air flow field. Cone in the picture
The axial length of the shaped nozzle and the composite nozzle are the same, both of which are 15 mm.
The outlet radius of the nozzle is also equal, both of which are 2 mm, and the stability at the outlet
The lengths of the fixed sections are all 3 mm. Consider axial symmetry and ease of calculation
The calculation domain is arranged axially symmetrically
J.
Figure 3
Calculation model of ordinary conical nozzle
Only the N-S equation for Reynolds average is described below.
Continuous equation:
Figure 4
Composite nozzle calculation model
or
= trash ( )=0
(1) 3.
2 Setting of boundary conditions
Momentum equation:
For ease of comparison, two nozzle models were modeled during simulations
p police = old + poison
cc mei
3u
___L+ coupon
2 Coix,
+Poison
(1 P “ui)
(2)
The N —S equation describes the actual flow more accurately, and it is also
The most basic equation for calculations in fluid mechanics.
Set the same boundary conditions, i.e.:
(1) The inlet and outlet are the pressure inlet and pressure outlet conditions;
(2) The inner wall of the nozzle is in the form of “wall”;
(3) The fluid is set to compress the ideal gas;
(4) The surrounding borders in the outflow field are pressure far fields.
June 2013
Volume 2
Issue 3 (Total Issue 9)
In the composite nozzle, the gas pressure in the inner nozzle is set to 6
atm, the pressure inside the outer nozzle is set to 2 atm. Inside ordinary conical nozzle
The pressure of the part is set to 6 arm, and the turbulence model used is RNGk-￥
Model.
3.3
Simulation results
When the stable length is 3 mm, change the normal conical nozzle to collect
The shrink angle is 24.6. , one is 14.7. , compared its axis
The pressure and velocity distribution are shown in Figures 5 and 6. At the cone angle
24.6. When changing the length of the normal conical nozzle stabilization section from 3 mm
When it becomes 5 mm, the pressure and velocity distribution along the nozzle axis was observed.
Figure, as shown in Figure 7. Finally, the pressure and speed of the composite nozzle were analyzed.
degree distribution diagram, as shown in Figure 8.
1 o
2 0
3 0
40
5 0
6 0
7 0
1 o
2 0
3 0
4 O
5 0
6 O
7 0
Location/m
Location/tom
(8) The cone angle is 24.6
(b) The cone angle is 14.7
Figure 6
When the length of the stable segment is 3 mm. Variation diagram of the velocity of outlet jet along the axis at different shrinkage angles
Location/Am
Location/u
(8) Pressure distribution circle
(b) Speed distribution circle
Figure 7
The length of the stable segment is 5 trim and the cone angle is 24. 6. The pressure and velocity curve along the axis
3.4
Analysis of simulation results
Analyze and compare the above simulation results and can be concluded that
The following points:
As can be seen from Figure 5(a), the total pressure in the inlet is 6 atm, and the cone angle is
is 24.6. , when the stabilized section length is 3 mm, the gas of the conical nozzle
The outlet pressure is about 2.0 atm, and after the gas is sprayed out of the nozzle, the surrounding area is sprayed
The constraints on the air flow of the mouth are lost, and the gas continues to expand, about 2 from the outlet
At mm, the pressure quickly decays to 0.9 arm, that is, the lowest point, and the final pressure
The force tends to stabilize to ambient pressure.
As can be seen from Figure 5 (b), in the same case, due to the
The pressure at the location is higher than the ambient pressure, the gas continues to expand, and the speed increases.
Increase to 470 m/s and then lower. Finally, it tends to decrease in the outflow field
Weak, but slower trend. This phenomenon is also beneficial to the gas
Entering into the narrow cutting slit at a higher speed, taking away the chips during processing to ensure
The processing proceeded smoothly.
As can be seen from Figures 5(b) and 6(b) that the shrinkage angle is large
With the cone angle, the diameter at the nozzle outlet remains unchanged
The speed is reduced slightly, and the speed is increased slightly, and it can be seen that the speed at the highest point has reached
480 m/s, while the pressure at the outlet drops slightly, but the overall curve
The trend of change is basically the same.
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【hs ． Ⅲ/Silent Nitrogen
10
Yang Li
Numerical simulation study on gas flow field of laser cutting assisted composite nozzle
As can be seen from Figure 7, when changing the length of the stable segment,
The length of the stable section changes from 3 mm to 5 mm, the length of the overall nozzle
Change from 15 mm to 17 mnq, and the overall curve of pressure and speed
There is no obvious difference in the trend, and the highest speed drops slightly, at 465
m/s, but the nozzle outlet pressure is reduced, about 1.5 atm.
As can be seen from Figure 8, the air flow ejected from the composite nozzle is sprayed internally.
in conclusion
0o +0
0e + 0
0e + 0
0e + 0
0e + 0
0e + 0
0e + 0
0e + 0
00 + 0
0e + 0
The jet is affected by the jet of the external annular nozzle and eventually the jet is
The airflow from the mouth is constrained by the surrounding airflow, and the expansion is weakened, and it does not
There are fluctuations in pressure and velocity under a conical nozzle. With Figure 5,
Figure 6 compares the flow rate of air flow from the nozzle because it is affected by the cycle.
The influence of the surrounding low pressure reduces the speed compared to the conical nozzle.
Refined,
1 0
2 0
30
4 0
50
6 O
7 0
Position/rm
(b) Speed distribution diagram
Figure 8
The velocity and pressure distribution of the outlet jet of the composite nozzle along the axis
The simulation software used in this article can be used under experimental conditions
Under insufficient conditions, it can simulate the internal and outflow of the auxiliary nozzle well.
The flow condition of the field and the changes in parameters such as pressure and speed are
Designers provide reference.
During laser cutting, if oxygen is used as auxiliary gas,
The basic requirement for cutting air flow is that the air flow entering the cutout must be large and the speed
It is necessary to be high, in this case, so that the cutter material can be fully oxidized and progressed.
exothermic reaction; at the same time, there is enough momentum to molten metal
The material is sprayed and blown out to obtain good cutout quality.
Based on this goal, the above results can be analyzed and can be seen
When using a conical nozzle, the total pressure at the inlet remains unchanged and the diameter at the outlet is
When the change is unchanged, only the shrinkage angle or the length of the stable segment is changed, at this time
The pressure and speed change trend along the axis direction are roughly the same.
The overall shape of the curve does not change significantly. Observe the subtle points,
You can see that as the shrinkage angle increases, the airflow speed will decrease.
Low. The larger the shrinkage cone angle, the less sufficient the gas expansion inside the nozzle,
The pressure at the mouth is slightly higher and the speed is slightly lower, and this is slightly higher and the pressure is low
The speed has a certain hindering effect on the subsequent airflow, which affects the cutting
Mass, so large cone angles are not recommended. The stable length of the segment
The effect of change on speed is not so obvious, but it will cause the exit
The pressure at the time is reduced, and the longer stable segment will cause resistance along the way
Energy consumption such as loss, so the increase in the length of the stable segment is beneficial
Obtaining uniform air flow, but is not conducive to the cutting process, also not recommended
Use long stable segments.
Finally, compare the conventional conical nozzle and the composite nozzle, although from the figure
The above reflects the weakening of gas expansion when using the composite nozzle.
So that the maximum speed in the corresponding axis direction is lower than that of the tapered nozzle
The speed value on the axis, but due to the high speed area of the composite nozzle
Increasing concentration is conducive to the airflow entering the cut joint, so it promotes the regular processing
Conduct often. Moreover, due to the constraints of the outer nozzle, the air flow is reduced
The pressure and velocity fluctuate, making the airflow more stable and more suitable for cutting
Cutting processing.
