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4. Improved AWJ Cutting Heads (IAWJs)

4.1. Introduction

In Section3 it is shown that AWJ cutting heads are unique fluid dynamic devices that involve very different flow phenomenon from that previously assumed. In the entryway to an AWJ cutting nozzle bore a disintegrating waterjet compresses air to provide the motive power that carries abrasive to a cutting head and into a nozzle bore. The air compression process varies markedly with the condition of a waterjet orifice and the extent of cutting nozzle wear.

Once it is appreciated that air is compressed as it enters a AWJ cutting nozzle bore, the question arises as to whether it would be better to raise the pressure of the abrasive carrier air so that waterjet induced air compression is not necessary. The benefits of pre pressurizing carrier air include:

  1. Acceleration of air by a pressure difference is orderly compared to the violently chaotic situation when water droplets are involved in accelerating air.
  2. Optimizing the amount of air entering a nozzle bore in order to minimize adverse effects of cutting nozzle wear.
  3. The capability to bleed off air from a cutting head. This allows optimum airflow for carrying abrasive to a cutting head and removes the need for a vacuum source, as used with AWJ cutting heads, to initiate abrasive flow towards a cutting head just prior to starting a waterjet.
  4. A waterjet orifice can be replaced by a waterjet nozzle that passes a reasonable amount of abrasive whilst continuing to produce a satisfactory water jet - the cost of a waterjet nozzle in super hard material being comparable to a diamond waterjet orifice.
  5. Inside a cutting head abrasive is only in contact with surfaces made from super hard materials.
  6. Reduced nozzle wear due to improved flow conditions.

4.2 IAWJ Cutting Head Flow Passages

An IAWJ cutting head is shown schematically in Figure 4. Abrasive is carried to an IAWJ cutting head by air pressurised to a bar or so above atmospheric pressure. An air pressure of a bar is sufficient to accelerate air to supersonic velocity as it enters a nozzle bore. Within a nozzle bore drag from water increases air velocities towards that of a waterjet - typically two to two and a half times the speed of sound in air.

Schematic arrangement of IAWJ cutting head

 

 

A particular feature of an IAWJ cutting head is the attachment of the waterjet nozzle to the downstream face of its carrier. This allows the nozzle outlet to be close to a cutting nozzle inlet. The outlet face of a waterjet nozzle forms a super hard wear roof to the inlet chamber. The manufacture of IAWJ waterjet nozzle assemblies is the subject of an International Patent Application.

 

Figure 4.1 IAWJ Cutting Head

 

4.3 IAWJ Abrasive Feed Arrangements

It is usual to have a mini abrasive hopper as close as practical to an AWJ cutting head and to use compressed air to automatically top up the mini hopper from a bulk abrasive hopper. An AWJ cutting head abrasive feed circuit, modified for feeding IAWJ cutting heads is shown in Figure 4.2.

Abrasive feed circuit

 

 

For an AWJ abrasive feed circuit that uses a restrictor in the base of a hopper to meter abrasive, atmospheric air is drawn in after the restrictor. With this arrangement the top of the hopper is at or near atmospheric pressure, as is the outlet from the abrasive metering restrictor - no air pressure difference. The feed circuit in Figure 3.2 the same air pressure in the top of the hopper and just downstream of the abrasive metering restrictor.

 

 

Figure 4.2 IAWJ Feed Circuit

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