Supersonic Nozzle Design for 1µm Laser Sources Ali Khan Bill O Neill Innovative Manufacturing Research Centre (IMRC) Centre for Industrial Photonics Institute for Manufacturing, Department of Engineering, University of Cambridge. Email: ahk31@cam.ac.uk Date 20 th Feb 2008
Contents Laser Material Processing system Overview Characteristics of sonic/conical nozzle Characteristics of supersonic nozzle Standard laser cutting (gas-jet interaction) Annulus gas jet Supersonic side-jet Side-jet cutting trials Kerf width and gas-jet relationship Standard laser cutting trials (2mm Mild Steel) Laser assisted Oxygen Cutting (LASOX) trials Laser drilling trials Entrance/Exit diameter with 1.5mm, 2.17mm sonic & 1.0mm supersonic nozzle. Average maximum recast/oxide analysis 1-D approach to supersonic nozzle design Axisymmetric approach to supersonic nozzle design Straight sonic line Curved sonic line Comparison with other commercially available code (case study) Conclusion Date 20 th Feb 2008 2
Overview Significant improvements Laser output Optical components Cutting head Very little improvement in nozzles Date 20 th Feb 2008 3
Sonic / Conical nozzle characteristics Free stream characteristics with inlet gas pressures Impinging characteristics with standoff distance Date 20 th Feb 2008 4
Supersonic nozzle characteristics Free stream characteristics with inlet gas pressures Impinging characteristics with standoff distance Gas Inlet Pressure 8 bar oxygen Date 20 th Feb 2008 5
Standard laser cutting (gas-jets interaction) Flow separation is a major problem Flow separation is less sever for supersonic nozzle Date 20 th Feb 2008 6
Annulus gas-jet Improvement in the flow separation Date 20 th Feb 2008 7
Supersonic side jet Flow separation eliminated But gas speed is less Date 20 th Feb 2008 8
Side jet cutting trials Good performance with Al Little affect on Stainless Steel Date 20 th Feb 2008 9
Gas jet and kerf relationship Flow separation minimised with smaller nozzle Smaller nozzles difficult to manufacture and align Date 20 th Feb 2008 1
Standard laser cutting trials Laser 200W SPI redpower Nozzles 1.0mm De-Laval, 1.0mm Sonic & 0.5mm De-Laval Gas oxygen, 2 12 bar Focal length 70mm Focal position - +1 Standoff - 1mm Material 2mm Mild Steel Date 20 th Feb 2008 1
Performance / Comparison (Nozzle type & Diameter) Optimum performance at 8bar 1.0mm & 0.5mm De-Laval designed for 8bar Date 20 th Feb 2008 1
Cut quality assessment Date 20 th Feb 2008 1
Laser assisted O 2 cutting (LASOX) Sonic nozzle does not work! Date 20 th Feb 2008 1
Laser drilling (Diameter) Laser PRIMA (JK704) single rod Beam diameter ~ 14mm 15mm Beam quality (M 2 ) ~ 12 13 Focal length = 200mm Gas = oxygen Pressure = 6.2 bar (90PSI) Material 2mm Ni Drilling angle 19 o to the surface Consistent performance from Supersonic Nozzle. Larger entrance/exit diameter: Higher drilling rate. Date 20 th Feb 2008 1
Laser drilling (Recast & Oxide) Supersonic nozzle produces lowest recast and oxide level Date 20 th Feb 2008 1
Date 20 th Feb 2008 1 1-D approach to Supersonic Nozzle Design. ( ) ( ) 1 1 2 2 1 1 1 2 1 + + + = γ γ γ γ r r r M M A A Quasi-One-Dimensional Theory Very difficult to obtain uniform flow
Families of Convergent-Divergent nozzles De-Laval nozzle:- smooth and gradual expansion Minimum length nozzle (MLN):- sudden expansion Date 20 th Feb 2008 1
d Axisymmetric approach to Supersonic Nozzle Design. Method of Characteristics dr dx ( θ ± υ) = tan( θ ± µ ) ± 1 2 ( M 1) ± cotθ Characteristic Equation dr = 0 Compatibility Equation r Date 20 th Feb 2008 1
Nozzle contours (straight sonic line) MLN Gas oxygen Pressure 8 bar Throat dia. 2mm Mach No. 2.04 Centred Prandtl expansion MLN No centre expansion Require sonic line extension De-Laval No centre expansion Final geometry dependent on the Throat radius curvature ratio Re Date 20 th Feb 2008 2
Curved sonic line Mixed supersonic and subsonic flow at the throat Sonic line position depends on the ratio (Re) of throat curvature to throat radius Cuffel et at (1969), Transonic Flowfield in a Supersonic Nozzle with small Throat Radius of Curvature, AIAA, vol,7, Dutton et al (1980), Transonic Flow in the Throat Region of Axisymmetric Nozzles, AIAA, vol 19 Date 20 th Feb 2008 2
Complete nozzle contour Subsonic Region: 1-D Ideal Gas Theory Transonic Region: Perturbation Velocity Potential Supersonic Region: Method of Characteristics Date 20 th Feb 2008 2
Computational Inlet Parameters. Choose Gas Type (10 gas species and their properties are in the data base). Customise gas properties with Boundary Layer Menu. Specify Nozzle inlet parameters. Date 20 th Feb 2008 2
Two supersonic nozzle designs De-Laval MLN or Bell Date 20 th Feb 2008 2
Design validation (CFD) Comparison with other available code 5 4 Nozzle type: MLN Nozzle Radius, [mm] 3 2 1 0 M = 1 M < 1 M > 1 MLN de-laval_bell Throat location -4-2 0 2 4 6 8 10 12 14 Inlet pressure: 12 bar Gas type: Nitrogen Inlet temp.: 298K Throat dia.: 5.3mm Exit Mach No. : 2.23 Axisymmetric Nozzle Length, [mm] Date 20 th Feb 2008 2
CFD validation (Mach No. Contour) Current MLN nozzle design is superior Date 20 th Feb 2008 2
Free stream gas jet performance 4.0 14 Centreline Mach Number, [-] 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 MLN Design Laval-Bell Design 0 20 40 60 80 100 120 Centreline Total Pressure, [bar] 12 10 8 6 4 2 0 MLN Design Laval-Bell Design 0 20 40 60 80 100 120 Axial distance from nozzle exit, [mm] Axial distance from nozzle exit, [mm] MLN under-expanded by 0.96% (accuracy of 99.04%). Laval Bell over-expanded by 57.7% (accuracy of 42.3%). Laval Bell design pressure 100% to 7mm, 80% to 80mm, and 30% to130mm. MLN design pressure 100% > 120mm (>23 nozzle diameters). Date 20 th Feb 2008 2
Conclusion. Supersonic nozzles can be beneficial in laser material processing Flow from supersonic nozzle is more stable than sonic nozzle Supersonic nozzle can provide better control than sonic nozzle Supersonic nozzle is operable to >12 nozzle diameter compared to just 2 from sonic nozzle Uniform expansion is only achieved with accurate nozzle design Nozzle inlet geometry depends on the inlet pressure Sonic line location depends on the radius of curvature Final nozzle geometry depends on sonic line position One supersonic nozzle for one pressure MLN shows greater promise for laser material processing Shorter length than De-Laval Easier to manufacture Cost the same as sonic / conical nozzle Difficult to design Smaller diameter nozzles may provide better process performance while minimising gas consumption. Date 20 th Feb 2008 2
Date 20 th Feb 2008 2