Analysis Of High Pressure Assisted Gas Flow In Laser Cutting And Improvement Of Nozzle Structure

Laser cutting has been used more and more widely as metallic heat cutting with high efficiency and quality. The realization of laser cutting results from the interaction between laser and assist gas. It provides a mechanical force to eject the melt from the cut zone and cools the cut zone by forced convection. Amongst the many process parameters in laser cutting, the cut-assist gas pressure and gas flow pattern are of prime importance to the cut edge quality. Reasonable assist gas type and flow pattern can not only increase the cutting ability, but also confine the cut zone in small range to ensure the cut quality.This dissertation is dependent on two great special projects of Shanghai science and technology committee. The current development and tendency of laser cutting is stated, and the high pressure assist gas flow structure in the process of laser cutting is presented in detail. Finally, the structure of supersonic nozzle is improved according to aerodynamic theory.In this thesis, common assist gas types and the function of assist gas in laser cutting is stated, the assist gas includes oxygen and non-oxygen gas. The analytical relationship and dynamic characteristic of assist gas in laser cutting is described in detail, and high pressure assist gas flow structure is analyzed at the same time. The mathematic relationship on status parameter and dynamics characteristic at two sides of shock waves is set up. Numerical methods in computational fluid dynamics are stated finally.High pressure assist gas free jet in laser cutting is studied to build the relationship between mach disk and exit process parameters. Through the simulation on free jet with the different exit pressure and different exit Mach number, the main feature and typical wave field of the flow is analyzed and depicted in detail. Numerical simulations are carried out using an explicit, coupled solution algorithm with solution-based mesh adaptation. The modeling of a supersonic turbulent jet impinging on a plate with a hole concentric with the jet is presented. It is completely shown that there are two states including contact and separation between the incident shock and the normal shock in gas jet-workpiece interaction. The effect of processing parameter on dynamic characteristic is analyzed, and this dissertation focus on setting up the relationship between machining capacity of assist gas and standoff distance to guide the optimization of cutting parameters.Finally, two improved supersonic nozzles are introduced, one is complex Laval nozzle, and the other is simple linear structure. In order to display the differences of two nozzles in dynamics, the pressure distribution and mach disk location in outflow field are analyzed to reveal the differences in dynamic characteristic.In conclusion, the research in this dissertation enriches the theory and method of the analysis of assist gas jet in laser cutting, and provides theoretical foundation and important reference to high-pressure and high-velocity laser cutting in thick metal plate.