Design and Performance Evaluation of a Supersonic Nozzle used for Laser Cutting of Thick Carbon Steel

Abstract

Laser cutting of mild steel has been widely used in the manufacturing industries for many decades because of its accuracy and efficiency. The present work deals with design of a supersonic nozzle for laser cutting of thick carbon steel by a sub 1 kW laser system utilizing the heat generated from the oxidation process. In this case, most of the power required for cutting operation is contributed by the exothermic reaction and laser is used only for heating the material to facilitate oxidation. The critical part in the proposed approach is the design of a suitable supersonic nozzle, which is discussed in thispaper. An axi-symmetric, straight small supersonic nozzle has been designed.The nozzle profile is designed on the basis of Method of Characteristics (MOC) and its performance has been evaluated and compared with results obtained from FLUENT. The distribution of pressure, velocity and gas density are predicted and mapped. The behavior of the supersonic jet from the nozzle exit has been investigated using Computational Fluid Dynamics (CFD) and validated experimentally using shadowgraph techniques. The designed supersonic nozzle exhibits good gas dynamic characteristics under high operating pressure. Cutting trials have been conducted using the nozzle assembly with satisfactory performance. A Area [m 2 ] x, r, ζ Cartesian co-ordinates [m/s] Cle> Qf» C3ε, ^μ Constants Greek symbols * D Diameter [m] Ρ Density [Kg/m 3 ] G k Generation of turbulent kinetic energy due σ Μ & σ ε Turbulent Prandtl numbers for k and G k to the mean velocity gradients [kg/ms 3 ] σ Μ & σ ε ε respectively Generation of turbulent kinetic energy due Dissipation rate of turbulent kinetic to buoyancy [kg/ms 3 ] ε energy [m 2 /s 3 ] k Turbulent kinetic energy [m 2 /s 2 ] θ Streamline angle [rad] L t Length of initial expansion region [m/s] ν Prandtl-Meyer angle [rad] Μ Mach number Mmol Molecular viscosity [kg/ms] Μ, Turbulent Mach number Veff Effective viscosity [kg/ms] Ρ Pressure [bar] Μι Turbulent viscosity [kg/ms] R Gas constant [ J/kg mol-K] Μ Dynamic viscosity [kg/ms] s k User-defined source terms [kg/ms 3 ] Ύ Specific heat ratio