Evaluation and optimal design of supersonic nozzle for laser-assisted oxygen cutting of thick steel sections

Abstract

With the full use of combustion heat from Fe-O reaction, laser-assisted oxygen cutting (LASOX) is able to cut thick mild steel plates by a defocused laser of medium power (approximately 1 kW). The dynamic behavior of oxygen gas flow, determined by nozzle structure and cutting parameters, is the key factor affecting cut ability and quality. It is significant to understand the method to design a suitable supersonic nozzle and to evaluate its performance. A supersonic minimum length nozzle (MLN) was designed and made specially for LASOX. The configuration (D exit/D critical) of the designed nozzle was chosen based on two-dimensional isentropic calculation of dynamic gas pressure. The divergent section of the MLN was designed in accordance with the method of characteristics (MOC). Numerical simulation and Schlieren visualization were used to observe and compare the flow behavior of the free jet from the designed MLN nozzle and a commercial supersonic nozzle. The MLN nozzle generates a highly uniform and stable supersonic jet extending a longer distance along the axis of nozzle, which is around 1.4 times longer than that from the commercial supersonic nozzle. Mild steel plates in 20 mm thickness were cut to investigate the effect of cutting parameters on LASOX cut quality of two nozzles. The MLN significantly improved cutting quality due to the better performance of gas flow. The optimal cut quality is related to both oxygen pressure and cutting speed. The oxygen content was measured along the cut surface to explain the difference in cut quality.