Nozzle design and simulation of gas flow for the laser cutting process

Abstract

In the laser cutting process the gas flow is of main importance to remove molten material from the cut kerf. Changes in nozzle geometry, gas supply pressure and nozzle workpiece stand-off distance have a strong influence on the cut quality. The aim of the investigation is the numerical simulation of supersonic gas flow. Supersonic nozzles for laser applications are designed which can be manufactured more easily and cost effective than conventionally used Laval nozzles. For the simulation wall friction is regarded and the flow field is calculated two dimensional and assumed turbulent. Normal jet impingement for the designed nozzles and underexpanded conic-cylindrical nozzles is compared at various nozzle stand-off distances and shows a more constant pressure on the workpiece for the designed nozzles. Furthermore supersonic effects in the cut kerf are investigated and compared with results from Schlieren photographs. The nonlinear pressure distribution inside the kerf indicates both the shock formation and detachment of the gas flow. In the presence of shock waves the existing equations in finite volume form have been modified, as they lead to wrong predictions both in the shock location and strength due to an incorrect inertia term in the momentum equations.