References on Cutting Heads for Abrasive Waterjets

1. ISO 5167 – “Methods of Measurement of Fluid Flow by Means of Orifice Plates, Nozzles, Venturi Tubes Inserted in Circular Cross-section Conduits Running Full”, International Standards Organization.

Of particular interest in this Standard are orifice inlet flow requirements. To understand the effects of inlet flow conditions on orifice discharge coefficients, orifice flow measurement facilities were built in many countries. International collaboration between national flow measurement facilities, extended to shipping complete orifice plate installations, including pipe work, between a flow measurement facilities.

Natural variability of flows, in what appear to be identical passageways, result in quite large tolerances having to be used for orifice discharge coefficients. Inlet flow conditions to AWJ cutting heads play a major role in cutting head performance and nozzle wear. It is important to record waterjet orifice inlet passage geometry.

Swirl can persist for over a hundred passage diameters and effect the quality of a waterjet. Any change in flow direction generates some swirl. Flow passage arrangements that give rise to strong swirl need to be avoided.

Waterjet orifices and the effects of inlet flow conditions will get a separate Web page in the future.

• Demonstrating that static pressures along a cutting nozzle can be measured
• Static pressure tapings were not located in the area of greatest interest at the start of the nozzle bore, where pressure conditions vary rapidly
• Static pressures in the cutting head inlet chamber were unusually close to atmospheric and did not decrease significantly with abrasive flow. Also the ratio of volumetric airflow to water flow ratio remained the same with and without abrasive flow, whereas other studies have shown a factor of 2 to 4 decrease in airflow when abrasive is flowing
• Measuring the reaction force on a cutting head and using the water, abrasive and air flow rates to calculate velocities along a cutting head resulted in erroneous answers. The authors made the assumption that flows in cutting nozzles are one-dimensional, whereas these flows are an extreme example of non one-dimensional flows. Calculated abrasive particle velocities were nonsensical
• The authors found support for their abrasive particle velocity calculations from the “virus paper” discussed in the next Section. If they had compared there calculated velocities against more reliable experimental measurements maybe they would have realized their assumption of one-dimensional flow was wrong.

The success of the authors in demonstrating that static pressures along a cutting nozzle can be measured is extremely important. Researchers now need to extend this work to better understand flow processes in cutting nozzles. Some of this work could be carried out without abrasive flow, thereby allowing metal rather than ceramic cutting nozzles to be used and thereby simplify static pressure tapping arrangements.

• A laser anemometer was used to measure velocities downstream of a cutting nozzle. The anemometer was of a type designed to measure the velocity of sub micron particles normally present in flowing fluids. Such particles are assumed to follow the motion of fluid immediately surrounding a particle
• The software for the type of anemometer used is normally designed to reject signals from particles over a few microns in diameter. These signals are assumed to be errors in measurement. It could, therefore, be expected that only signals from small abrasive particles formed by the breakup of larger particles would be treated as valid signals
• The “measured” velocities bear no logical relationship to operating water pressures and the authors provide no explanation – the authors could have expected some particle velocities to be nearly double the “measured” velocities
• Particle velocities were shown as increasing ten or more jet diameters downstream of a cutting nozzle. The authors make no attempt to explain how this was possible
• No measurements of airflow were made
• Details of the cutting head geometry are lacking.

Measuring particle velocities in an abrasive waterjet is probably the ultimate challenge in industrial velocity measurement. Several different methods have been tried but so far only one method shows real promise:

1. Roth, P., Looser, H., Heiniger, K.C., Buhler, S. (2005) “Determination of Abrasive Particle Velocity Using Laser-Induced Fluorescence and Particle Tracking Methods in Abrasive Waterjets”, 2005 WJTA American Waterjet Conference.

To be continued