The most effective way of improving cutting performance is to reduce the amount of material removed to make a cut. This means reducing cutting nozzle bore diameters for a given set of operating parameters - water flow, abrasive flow and water pressure. Reducing cutting nozzle bore diameter not only reduces the amount of material removed to make a cut it also increases the cutting energy density on to a workpiece.
Over ninety percent of an AWJ nozzle bore is occupied by the air that carries abrasive to a cutting head and into a nozzle bore. If a nozzle bore tapers from its inlet to its outlet, so as to produce a smaller diameter cutting jet, air pressure increases towards a bore outlet and air can arrive at a cutting nozzle outlet at above atmospheric pressure. If air is above atmospheric pressure at a cutting nozzle outlet it is said to be under expanded. Under expanded air at a nozzle outlet expands through an expansion wave system. This is highly undesirable as small particles are carried by the air and cause damage to workpiece surfaces and cut edges.
Ideally, air should be extracted from a nozzle bore once it has carried abrasive into a bore. Unfortunately, it is not possible to extract air once it is in a nozzle bore. If, however, steam replaces air as the carrier fluid most of the steam is condensed rapidly - dynamic events within a cutting nozzle bore always result in water vapour along a bore.
Reducing bore diameters and increasing energy density increase nozzle bore wear. The adverse effects of bore wear can offset gains in cutting performance from smaller diameter cutting jets. Super hard materials, such as diamond, with better wear characteristics than the best performing tungsten carbide cutting nozzles, cannot be economically used for AWJ mixing tubes. However, by using steam as the carrier fluid it may be possible to intensify the transfer of momentum from water to abrasive so as to be able to substantially reduce cutting nozzle lengths sufficiently for them to be economically made from diamond or other superhard material.
Condensing steam is a strong driving mechanism for inducing steam, with its abrasive load, into a cutting nozzle. This means flow conditions at a nozzle inlet are similar to those for IAWJs. It should be noted that it is usually necessary for steam to flow at high velocities towards a condensing zone to prevent a condensing front moving back along the steam flow passage. In the case of a cutting head, the momentum of a waterjet would prevent water flowing into the abrasive feed system.
Figure 5.1 shows a schematic drawing of a SAW cutting head. Inlet flow passage geometries are similar to those of IAWJ cutting heads. It is particularly important for a waterjet nozzle outlet to be located close to a cutting nozzle inlet in order to limit the amount of steam condensed prior to a nozzle bore.
Pre-heating with warm air will reduce condensation within the inlet chamber but some condensation of steam is inevitable. It is, therefore, necessary to have a compact inlet chamber with flows over surfaces to prevent wet abrasive accumulating.
Figure 5.2 shows an abrasive feed arrangement for a SAW cutting head.