Visualization of flow separation inside cut kerf during laser cutting of thick sections

The behavior of oxygen gas not only determines the oxidation heat input but also affects the removal of molten metal during laser-oxygen cutting. Flow separation inside the cut kerf is the major problem for the deterioration of cut ability and cut quality, especially for cutting thick steel sections, because it significantly weakens the shear stress on molten slag and changes the pressure gradient along the cut front. There have been some researches on flow separation using numerical simulation and model kerf. However, many experimental details and results are unclear, and the numerical simulation is questionable due to lack of experimental verification. In this paper, a model kerf is set up based on the previous results of observation and cutting trials. High resolution and high speed Schlieren visualization is used to observe the formation of flow separation in the model kerf. Stand-off distance, inlet pressure, cut front slope, nozzle displacement, kerf width, and nozzle tilting angle are used as variables for Schlieren visualization of flow separation for both a conical subsonic and a minimum length nozzle supersonic nozzle. The effect of the above parameters on the position and the pattern of flow separation is discussed based on the gas dynamic theory. It is found that nozzle displacement, kerf width, and nozzle tilting angle are the key factors for flow separation. Possible strategies for controlling flow separation are also proposed for both subsonic and supersonic nozzles. (C) 2016 Laser Institute of America