Nozzle Stand-off Distance Research Articles

Gas Dynamic Effects on Laser Cut Quality

The presence of a gas jet plays an important role in laser cutting. Both the cutting efficiency and cut quality are very sensitive to gas pressure and nozzle standoff distance because of the complex nature of shock fronts and associated phenomena in a supersonic gas jet impinging on a workpiece. An idealized case is considered first, where the cut is assumed to be a circular hole directly underneath and concentric with the gas jet nozzle. A more realistic case of an axisymmetric nozzle impinging on a plate with a linear cut is considered next. Unlike the idealized case, the problem now is three-dimensional. Simple experiments to measure the through-kerf mass flow rate were carried out for both geometries. The two important forces exerted by the gas jet for melt ejection, namely, shear force and pressure gradient, show the same trend as that of the mass flow rate with varying gas pressure and standoff. The mass flow rate for the three-dimensional case shows the same behavior as that of the axisymmetric case, indicating the basic shock structures of the axisymmetric case are applicable to the real cutting cases. Laser cutting of mild steels under the corresponding conditions was performed, and the cut quality characterized by roughness, dross attachment, and recast layer thickness was analyzed. The deterioration of cut quality with the gas pressure and standoff is found to closely match reductions in through-kerf mass flow rate. It is thus verified that the shock structure of the gas jet and the associated mass flow rate have a direct impact on laser cutting as predicted.

Gas Jet–Workpiece Interactions in Laser Machining

Laser machining efficiency and quality are closely related to gas pressure, nozzle geometry, and standoff distance. Modeling studies of laser machining rarely incorporate gas effects in part because of the complex structure and turbulent nature of jet flow. In this paper, the interaction of a supersonic, turbulent axisymmetric jet with the workpiece is studied. Numerical simulations are carried out using an explicit, coupled solution algorithm with solution-based mesh adaptation. The model is able to make quantitative predictions of the pressure, mass flow rate as well as shear force at the machining front. Effect of gas pressure and nozzle standoff distance on structure of the supersonic shock pattern is studied. Experiments are carried out to study the effect of processing parameters such as gas pressure and standoff distance. The measured results are found to match and hence validate the simulations. The interaction of the oblique incident shock with the normal standoff shock is found to contribute to a large reduction in the total pressure at the machining front and when the nozzle pressure is increased beyond a certain point. The associated reduction in flow rate, fluctuations of pressure gradient and shear force at the machining front could lower the material removal capability of the gas jet and possibly result in a poorer surface finish. The laser cutting experiments show that the variation of cut quality are affected by shock structures and can be represented by the mass flow rate. [S1087-1357(00)01702-0]

Effect of Feed Rate During Comminution of Coal by High Energy Waterjet

Abstract High pressure waterjets attack, fragment and comminute coal. The purpose of this research is to determine the effect of selected waterjet parameters on the specific energy required to create particles less than 75 μm in size. The selected parameters are waterjet pressure, nozzle diameter, relative velocity between moving coal sample and stationary waterjet nozzle and nozzle standoff distance, defined as the distance between nozzle and target. This information is necessary as a starting point for the design of coal comminution equipment based on waterjet technology. Over 980 individual specimens of bituminous coal from Moberly, Missouri USA, were subjected to controlled attack by waterjet. Trials were executed with waterjet pressures up to 100 MPa, and with nozzle standoff distances between 0.80- to 44.5 × 10−3 m. Nozzle diameters of 1.14-, 0.81 -, and 0.41 × 10−3 m were used, and coal sample feedrates varied up to 0.127ms−1 relative to a stationary nozzle. The most important parameter was found t...

Theory of Formation Cutting Using the Sand Erosion Process

Abstract The process of sand erosion has been harnessed to perform a useful function-the directed perforating of oilfield tubular goods and formation rock. In this process the sand is carried by a liquid medium and the resulting slurry is forced through a nozzle at an extremely high velocity. Upon impingement against casing or formation, the fast-moving sand particles erode a perforation in the casing followed by cavity formation in the medium behind the casing. The many factors influencing the rate and extent of this penetration are inherently difficult to establish experimentally. Equations describing submerged jet behavior have been applied to these conditions. The following paper develops the equations for predicting the performance of the perforating jet. Introduction The cutting power of high-velocity sand has been well known in the petroleum industry for many years. Sand carried by a high-velocity gas stream causes severe erosion. This effect has been the cause of numerous production problems in certain areas. It was logical to expect that this phenomenon would one day be adapted to useful industrial applications. Development of mobile, high-pressure, high-horsepower pumping equipment in the last decade has made this possible. The sand erosion process involves pumping a fluid containing abrasive solids through a set of nozzles at a high differential pressure (2,000 to 4,000 psi). The pressure conversion into kinetic energy imparts high velocity to the sand particles which, upon impact with the formation face or casing wall, erode the material in an organized pattern. This paper attempts to establish the effects of the various parameters and to predict the performance and limitations of the process. The following factors will be considered: nozzle differential pressure, perforation size, sand concentration, nozzle stand-off distance and back-pressure. In general, this discussion is presented along theoretical lines and is substantiated by field results and data where reliable field data exists.