A visual observation of the air
flow pattern for the high speed nozzle applicable to high
power laser cutting and welding
☆
Chi-Shan Tseng
a, Chun-Ming Chen
b, Chi-Chuan Wang
a,⁎
a
Department of Mechanical Engineering, College of Engineering, National Chiao Tung University, Hsinchu 300, Taiwan
bLaser Application Technology Center, Industrial Technology Research Institute, Tainan 734, Taiwan
a b s t r a c t
a r t i c l e i n f o
Available online 6 November 2013 Keywords:
Visual observation Nozzle
Cutting and welding
In this study, theflow pattern and pressure variation of the nozzles applicable to high power laser cutting and welding were studied. A total offive nozzles are made and tested with nozzle diameter ranging from 0.8 to 4 mm. The depth of focus and the width of focus are measured based on theflow visualization. It is found that the depth of focus is increased with the rise of exit Mach number while the width of focus is increased with the nozzle diameter. The visual results indicate that the supersonic nozzle reveals a more concentrated jetflow pattern even theflow has passed through the depth of focus. On the other hand, appreciate deviation of the flow pattern is observed for the subsonic nozzle after the depth of focus. As far as more concentrated air flow pat-tern is concerned, a supersonic nozzle with an exit diameter less than 3.0 mm is recommended. The total pres-sure decreasing tremendously with the exit Mach number is encountered while moderate decline of the static and dynamic pressure for supersonic nozzles is seen. However, its corresponding dynamic pressure is still higher than that of the subsonic nozzle.
© 2013 Elsevier Ltd. All rights reserved.
1. Introduction
In high power laser cutting and welding processes, the shielding gas plays an essential role. In laser cutting, high pressure and high speed air flow are needed in order to blow out the melted materials to ensure high quality of the laser cutting. In laser welding, the shielding gas pro-tects optical lens from spatters and screens the solidifying metal from the surrounding ambient. The operational speed and pressure of the shielding gas for laser welding are generally much lower than that in laser cutting. In practical operation, the airflow speed is strongly related to the nozzle design. The nozzle affects the inlet stagnation pressure, tip to workpiece standoff distance, and width and thickness of the cut kerf on the behavior of the gas jet patterns inside the kerf[1].
There are a lot of experimental and numerical investigations on the effects of the high speed gas jet in laser cutting process. The influence of stand-off distance on laser cutting using supersonic nozzles is investi-gated by Chen et al.[2,3], Hu et al.[4]. Man et al.[5–7]through which supersonic jet inside laser cut kerf was experimentally and numerically examined. Supersonic gas dynamic characteristic is studied by Guo et al. [8]numerically. Supersonic impinging jet inside an inclined substrate was investigated by Mai and Lin[9]. The above researches[8,9]focused on the dynamic properties of supersonic gasflow and the associated in-fluence of impinging jet on the work piece subject to laser cutting. As pointed out by Chen et al.[2], a strong normal shock prevails in front
of the work piece and this normal shock is undesirable for Mach number after this normal shock would substantially decrease to subsonic level and it creates gigantic energy loss. Those researches focused on the in-fluence of stand-off distance to shock structure before or inside the cut kerf. Although there are many researches on supersonic gasflow in laser cutting, the depth of focus and width of focus of high Mach num-ber jet from supersonic nozzles had not been reported clearly. In this re-gard, it is the objective of this study to examine the influence of associated important parameters subject to various nozzle designs. 2. Experimental setup
A total of 5 nozzles were made and tested in the present study. Their detailed geometry is tabulated inTable 1. In thesefive nozzles, nozzles #1 to #4 are designed to attain at supersonic speed. Hence the nozzles have a divergent part in order to further accelerate the airflow to super-sonic speed. Nozzle #5 is designed at super-sonic speed because it does not have divergent part. Note that when the upstream reservoir pressure reaches 1.89 bar or higher, the massflow rate is choked at the throat of a nozzle and the Mach number at throat is exactly 1. The desired out-let Mach number is made by area ratio of throat to outout-let area. Outout-let Mach number and other parameters of each nozzle are listed in Table 1. Where Dois outlet diameter and Dtis throat diameter.θois the open angle of the divergent part of the nozzle and h is the distance between throat and outlet tip. The distance between throat part and outlet tip (h) for nozzle #1 and #2 are 1.5 mm and for nozzle #3 and #4 are 3 mm. By exploitation of the nozzles, various outlet Mach num-ber can be achieved. The supersonic speed is achieved by convergent–
☆ Communicated by W.J. Minkowycz.
⁎ Corresponding author at: EE474, 1001 University Road, Hsinchu 300, Taiwan. E-mail address:ccwang@mail.nctu.edu.tw(C.-C. Wang).
0735-1933/$– see front matter © 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.icheatmasstransfer.2013.10.007
Contents lists available atScienceDirect
International Communications in Heat and Mass Transfer
divergent nozzle if the upstream reservoir pressure is higher than 1.89 bar which is demonstrated by Man et al.[7]who mentioned that when the gas pressure Pois less than 1.89 kg/cm2(0.185 MPa) (abso-lute pressure), velocity V andflow Q increase with the increasing of inlet pressure Po, but when Poreaches 1.89 kg/cm2(0.185 MPa), V at-tains a maximum value. A further increase in Poalso increases the flow Q, but the gas velocity will remain unchanged.
The schematic of the test facility is shown inFig. 1which contains an air compressor capable of providing compressed air of 5–7 bar. The compressed air is then stored in a tank. A metering valve is then used to regulate the compressed air into the test nozzles. A hi-precision Pitot tube (Dwyer, model 167 series) is used to measure the outlet static and dynamic pressure of the incoming jetflow out of the nozzle. A dif-ferential pressure transducer (Yokogawa, model EJA110A, style:S1) with a full scale resolution of 0.1% and an absolute pressure transducer (Yokogawa, model FP101A-E11-L20A*B) having a calibrated accuracy of 0.2% is used to measure the dynamic and static pressure of the Pitot tube, respectively. The Pitot tube is placed 2 mm from the nozzle tip. In addition to the measurements of dynamic and static pressure, a high speed camera (Dongmao Instrument Scientific Co., SVSi Gigaview) is used to measure the depth of focus of the airflow from the design jet. Theflow visualization is made possible by a smoke generator with par-ticle size being around 1.3μm. The generated smoke is first collected in a bag, and was fed to the nozzle to mix with the compressed air from the air tank. The high speed camera is then used to record the smokeflow from the nozzle. Normally 1000–2000 pictures per second were taken with pixel resolution of 720 × 480. The major effort of theflow visuali-zation is to identify the depth of focus of the jetflow.
3. Results and discussion
Fig. 2depicts theflow visualization of the five nozzles. Since the depth of focus is an important parameter in laser cutting. This is because high power laser beam can only melt the material, and it would require the associated high speed gasflow to cut the material accordingly. In this sense, the depth of focus is quite imperative since it acts as the focal point of impingent airflow that provides sufficient momentum to cut the melting material. Definition of the depth of focus (DOF) is de-fined as the point where the air flow starts to diverge.
As illustrated in Fig. 2(a) and Table 1, the DOF is approximate 9.5 mm with a Mach number of 1.76, and the width of focus is approx-imately 1.91 mm. For nozzle #2, as shown inFig. 2(b), the correspond-ing DOF is approximately 12.4 mm whereas the Mach number equals to 2.3 and the width of focus is approximately 2.86 mm. It appears that the jet deviates much faster after the DOF and the width of focus becomes larger. This is because the outlet nozzle diameter is also larger. Analo-gously inFig. 2(c) for nozzle #3, an attained Mach number of 2.02 is achieved and the DOF is approximately 11.9 mm and the width of focus is approximately 2.39 mm. Similar toFig. 2(a), the jetflow does not show an appreciable deviation after DOF. This is attributed to its smaller outlet diameter of 2.55 mm. On the other hand, the largest
outlet diameter is 4.96 mm in nozzle #4. The DOF is considerably in-creased to 17.2 mm with the highest exit Mach number of 3.1, but a de-tectable deviation of the jetflow after DOF emerges. Conversely, nozzle #5 is designated with the smallest exit diameter of 0.82 mm. The DOF is reduced to 7.2 mm as expected but the airflow pattern deviates and di-verges pronouncedly after the DOF as shown inFig. 2(e). The results imply that the subsonic operation may not be appropriate for theflow pattern scatters notably after DOF. The visual results for the more concentrated jet flow stream under supersonic operation can be substantiated by the diamond structure as shown inFig. 3formed by an under-expanded jet impinging on to aflat plate[3,8]. The air flow is confined within the boundary and reveals a less scattering. In the meantime, to avoid a larger deviation offlow pattern after DOF for supersonic operation, an exit diameter less than 3.0 mm is more ap-propriate based on the present visualization. The results are in line with all the previous investigations since the nozzle diameters for all the studies falls between 0.8 and 1.7 mm (e.g. Man et al.[1,5–7]; Do= 0.8–1.5,1.6,1.7 mm; Chen et al.[3], Do= 1.35 mm; Hu et al.[4], Do= 1.6 mm; Mai and Lin[9], Do= 1.5 mm).
Summarization from the foregoing discussion, it is found that the DOF is strongly related to the exit Mach number. This is applicable for both supersonic nozzles (#1–#4) and subsonic nozzle (#5). Hence, efforts are made to correlate the associate relation of DOF and WOF (width of focus) against the exit Mach number and diame-ter.Fig. 4shows the relevant correlations of the observed DOF/WOF against the outlet Mach number or exit diameter. Generally, DOF in-creases linearly with the exit Mach number while the WOF is related to exit diameter rather than exit Mach number.
In this study, the outlet pressures and the Mach number of the nozzles are measured. Since the Mach numbers of these nozzles can be either su-personic or sonic, normal shock wave forms before the Pitot tube. The pressure and Mach number we measured from Pitot tube are behind the normal shock, and the relation of the exit Mach number and total pres-sure before and after the normal shock is given in Eqs.(1) and (2) [10]:
M22¼ 1þγ−12 M21 γM2 1− γ−1 2 ð1Þ Po2 Po1¼ 1þ γM2 1 1þ γM2 2 1þγ−12 M 2 2 1þγ−1 2 M 2 1 !γ γ−1 ð2Þ Accordingly the corresponding total pressure, static and dynamic pressures for the test nozzles before and after the normal shock are shown inFig. 5. The inlet reservoir is kept at 6 bar for all the nozzles. The results show that the total pressure decreases considerably with the outlet Mach number. This is associated with the huge energy loss after the normal shock. The static and dynamic pressure for supersonic nozzles (#1–#4) also shows moderate decline against the exit Mach number as compared to that of total pressure. However, its correspond-ing dynamic pressure is still higher than that of subsonic nozzle (#5). Despite high Mach number has a betterflow shape and longer depth of focus it also causes much larger energy loss.
4. Conclusions
This study examines theflow pattern and pressure variation of the nozzles applicable to high power laser cutting and welding. A total of five nozzles having diameter of 0.8–4.0 mm are used for flow visualiza-tion and pressure measurement. The depth of focus and the width of focus were measured based on theflow visualization. Based on the fore-going discussions, the following conclusions are made:
1. The depth of focus increases with the rise of exit Mach number while the width of focus increases with the nozzle diameter.
Nomenclature
Do nozzle outlet diameter Dt nozzle throat diameter θo divergent degree of nozzle
h distance between nozzle throat and tip Po1 total pressure before normal shock wave Po2 total pressure after normal shock wave M1 Mach number before normal shock wave M2 Mach number after normal shock wave γ specific ratio of air
2. The visual results indicate that the supersonic nozzle reveals a more concentrated jetflow even the flow has passed over the depth of focus. On the other hand, appreciate deviation offlow pattern is ob-served for the subsonic nozzle. This is because the airflow is confined
within the boundary and it reveals a less scattering for supersonic operation.
3. To avoid a larger deviation offlow pattern after DOF for supersonic operation, an exit diameter less than 3.0 mm is recommended.
Table 1
Geometrical configuration of the test nozzles.
Nozzle Configuration Domm θodegree h Outlet Mach number
1 2.03 10 1.5 1.76
2 3.2 30 1.5 2.3
3 2.55 10 3 2.02
4 4.96 30 3 3.1
4. The total pressure decreases tremendously with the outlet Mach number due to huge energy loss after the normal shock. The static and dynamic pressure for supersonic nozzles also shows appreciable decline against the exit Mach number. However, its corresponding dynamic pressure is still higher than that of subsonic nozzle. References
[1] H.C. Man, J. Duan, T.M. Vue, Analysis of the dynamic characteristics of gasflow inside
A cut kerf under high cut-assist gas pressure, J. Phys. D. Appl. Phys. 32 (1999) 1469–1477.
[2] K. Chen, Y.L. Yao, V. Modi, Gas dynamic effects on laser cut quality, J. Manuf. Process.
3 (2001) 38–49.
[3] K. Chen, Y.L. Yao, V. Modi, Gas jet–workpiece interactions in laser machining, J.
Manuf. Sci. Eng. 122 (2000) 429–438.
[4] J. Hu, Z. Zhang, J. Luo, X. Sheng, Simulation and experiment on standoff distance
af-fecting gasflow in laser cutting, Appl. Math. Model. 35 (2011) 895–902.
[5] H.C. Man, J. Duan, T.M. Vue, Modeling the laser fusion cutting process: II. distribution
of supersonic gasflow field inside the cut kerf, J. Phys. D. Appl. Phys. 34 (2001)
2135–2142.
[6] H.C. Man, J. Duan, T.M. Vue, Modeling the laser fusion cutting process: III. effects of
various process parameters in cut kerf quality, J. Phys. D. Appl. Phys. 34 (2001) (2001) 2143–2150.
[7] H.C. Man, J. Duan, T.M. Vue, Design and characteristic analysis of supersonic
noz-zles for high gas pressure laser cutting, J. Mater. Process. Technol. 63 (1997) 217–222.
[8] S. Guo, H. Jun, L. Lei, Z. Yao, Numerical analysis of supersonic gas-dynamic
character-istic in laser cutting, Opt. Lasers Eng. 47 (2009) 103–110.
[9] C.C. Mai, J. Lin, Flow structures around an inclined substrate subjected to a
superson-ic impinging jet in laser cutting, Opt. Laser Technol. 34 (2002) 479–486.
[10] J.D. Anderson, Modern Compressible Flow with Historical Perspective, McGraw-Hill,
2004.
(a) Nozzle #1
(b) Nozzle #2
(c) Nozzle #3
(d) Nozzle #4
(e) Nozzle #5
Fig. 2. Visualization of the airflow pattern of the test nozzles.
Fig. 3. Flow structure from a supersonic nozzle.
Mach number
0.5 1.0 1.5 2.0 2.5 3.0 3.5DOF (mm)
6 8 10 12 14 16 18Mach number ve DOF Linear prediction
(a)
Depth of Focus vs. Mach number
Diameter (mm)
0 1 2 3 4 5 6WOF (mm)
0.5 1.0 1.5 2.0 2.5 3.0 3.5Diameter vs Depth of focus y(x)=1.1509+1.355*ln(x)
(b)
Width of focus vs. nozzle diameter
Fig. 4. Depth of focus and width of focus vs. Mach number and nozzle diameter for the test nozzles.
Outlet Mach number
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Pressure (kPa)
0
100
200
300
400
500
600
700
Total pressure Static pressure Dynamic pressure Pressure Reservoir at 6 barFig. 5. Exit total pressure, static pressure, and dynamic pressure vs. exit Mach number for the test nozzles for an inlet pressure of 6 bar.
