Flows within AWJ cutting heads are considered in what may seem to be excessive detail. However, it should be borne in mind that the kinetic energy and complexity of flows in AWJ cutting heads are the most extreme encountered in industry.
Section 3.1 - Provides reasons why a new model for AWJ cutting head flows is needed.
Section 3.2 - Flows in waterjet orifices and their upstream passageways are described.
Section 3.3 - Flows in cutting head inlet chambers are described.
Section 3.4 - Flows in nozzle bores are described.
Section 3.5 - Interacting flows within complete cutting head are described.
Section 3.6 - The effects of component wear on cutting head performance are considered.
Section 3.7 - Discusses abrasive particle penetration upstream of waterjet orifices when water flow is stopped.Annex A - Considers the effects of varying cutting head non-dimensional geometric parameters.
Over the past twenty years plausible explanations of how AWJ cutting heads operate have been proposed. For reasons given below these explanations do not represent reality.
Explicitly or implicitly, published models assume that flow phenomena in AWJ cutting heads are similar to those in a jet pump. Two important non-dimensional geometric parameters for the two devices are shown in Figure 3.1.
In terms of non-dimensional geometric parameters, jet pumps are compact devices compared to AWJ cutting heads. The large differences in non-dimensional geometric parameters between the devices arise from their operating modes.
A jet pump has a concentrated momentum exchange zone where the driving fluid and the entrained fluid are turbulently mixed to give a homogeneous discharge flow. Typically energy efficiencies of jet pumps are below 25%. Clearly a jet pump is first and fore most an energy dissipater. It is, therefore, very important that an AWJ cutting head does not operate like a jet pump.
Momentum exchange in an AWJ cutting head occurs over two extended regions. The first region is the inlet chamber of a cutting head where momentum is predominately transferred from a waterjet to air. The second region is the cutting nozzle bore where momentum is transferred from water to abrasive particles.
Abrasive, water and air leave a cutting nozzle with large velocity differences within phases and between phases. Typically, abrasive particles in a cutting jet carry 60% of the theoretical kinetic energy that could be transferred to particles. Average kinetic energies of particles at a workpiece are equivalent to about 4% of a waterjet’s kinetic energy. Abrasive particles of the same mass have a 4 to 1 spread in kinetic energies on impact with a workpiece.
A jet in a jet pump can be distorted, with high levels of turbulence and swirl, and only be roughly aligned with a mixing section. In contrast, a waterjet in an AWJ cutting head needs to have a near one-dimensional velocity profile with a low turbulence level, absence of swirl and be accurately aligned along a cutting nozzle bore.
In the literature the assumption is made that, like a jet pump, mixing in AWJ cutting heads occurs at and shortly after flows enter a nozzle. A mean fluid density is then used to calculate the distance needed to accelerate abrasive particles to a particular velocity. There is no known flow process that could cause such mixing, as it requires the disintegration of a waterjet without the forces to cause such disintegration. Also, if a mean density existed, friction losses appropriate to the mean density would occur along a cutting nozzle bore. To overcome such friction losses a static pressure rise of over a 100bar would be necessary, leading to other implausible phenomena such as substantial air compression.
Air is the continuous fluid phase in a cutting nozzle and flows at supersonic velocities. With supersonic velocities in a cutting nozzle, information about conditions at the outlet of a cutting nozzle cannot propagate upstream. This means that static pressures within the inlet chamber of a cutting head are independent of the static pressure (usually atmospheric) at a cutting nozzle outlet. In the case of a jet pump a relationship exists between the static pressure in an inlet chamber and a pump outlet.
Without abrasive flow, measured air mass flows to AWJ cutting heads are such that static air pressures have to be above atmospheric in cutting nozzle bores, otherwise air velocities would be much higher than water velocities, which is not physically possible. This means that a mechanism exists to compress air as it enters a cutting nozzle bore. Air compression is also present when abrasive is flowing and is the key to understanding how AWJ cutting heads operate.
Clearly from the above there are no fluid dynamic reasons to relate flows in AWJ cutting heads to those in jet pumps. They belong to different fluid dynamic device families - Entrainment cutting head family and jet pump family.
Ultra high pressure water is discharged through an orifice to form a waterjet with a velocity typically above 800 m/s. Waterjet formation is a fluid dynamically simple, energy efficient and low loss process. However, the characteristics of a waterjet from an orifice or a nozzle depend on a number of factors including:
1. Bulk swirl - This is usually generated by a change of flow direction and may persists for well over 100 passage diameters. The configuration of AWJ cutting heads involves a 90 degree change in flow direction through a water shut off valve. This configuration has the potential to cause swirl.
2.Uniformity of inlet flow - It is usual to specify a passage length of 20 diameters or so before an orifice but if the flow is conditioned so that it has a near uniform velocity distribution with small scale turbulence this distance can be reduced.
3. Inlet flow turbulence - Absolute turbulence velocities increase through an orifice so it is essential to have a low turbulence at inlet, preferably with eddy diameters that are small relative to an inlet passage diameter
4. Flow stagnation - Orifices have a stagnation region on their upstream face that is effected by the approach flow velocity and turbulence distribution. Vortices and other flow disturbances can be shed from stagnation regions to adversely affect waterjet quality.
5. Obstructions - Obstructions mainly take the form of mineral deposits that form around an orifice and disrupt flow towards and through an orifice.
6. Orifice edge and form - Orifice edges may have a radius or a chamfer, provided the water flow springs clear at the start of the radius or chamfer and does not re-attach. An orifice edge should be free of faults.
A water flow contracts after separating from an orifice edge to form a jet with a diameter about 0.8 of the orifice diameter for a sharp edged orifice. Fluid dynamically, waterjet orifices are usually not sharp edged, so discharge coefficients are higher than theoretical coefficients for sharp edged orifices. Discharge coefficients change as an orifice edge wears. There is, therefore, considerable uncertainty over water flow rates so in experimental work water flows need to be checked by mass/volumetric or other means.
Air begins to be dragged along by a waterjet as soon as water springs clear of an orifice edge. Initially, the amount of air moving with a jet increases roughly linearly with distance but, as a jet’s internal turbulence begins to distort its surface, entrainment increases. A further increase in entrainment occurs as droplets begin to be ejected from a jet.
It should be born in mind that for the first 100 jet diameters or so the effects of air on a waterjet are relatively small in comparison with a jet’s internal turbulence. Shear effects prior to an orifice edge and microscopic edge imperfections also cause disturbances that are amplified with distance from an orifice to cause micron sized droplets to form.
Droplets slow down rapidly due to air drag. When looking at photographs of waterjets discharged into air at atmospheric pressure, it is useful to remember that most of the droplets in a photograph have come from a part of a jet that is not in the photograph. This means droplet hold up is high. As air pressure is reduced drag on droplets decreases and droplets tend to travel at the jet velocity. When this is the case, droplet hold up is low, but it does not necessarily mean that there is less jet break-up than when air pressure is higher and droplet hold up is high.
Air, carrying abrasive, enters a cutting head inlet chamber and circulates in a chamber until it exits via a cutting nozzle bore, dragging abrasive particles into a bore. Because of complex circulatory patterns within inlet chambers, the residence time of individual parcels of air within a chamber vary widely. Similarly the residence time of abrasive particles varies widely.
Abrasive particle inertia causes most particles entering an inlet chamber to impact on a chamber wall before being entrained into a chamber’s circulatory airflow. A percentage of particles entering an inlet chamber interact directly with a waterjet and it’s surrounding airflow.
Angling an abrasive inlet connection, so as to have a particle momentum component in the axial direction, does not prevent particles impacting on a chamber wall. Whether or not angling an air/abrasive inlet is beneficial depends on how well abrasive is redistributed in an inlet chamber so as to provide a reasonably uniform abrasive flow at inlet to a nozzle bore.
Self-reinforcing swirling flow tends to be set up when fluid enters a chamber at an appreciable angle to the direction fluid leaves a chamber. Such flows can be stable or unstable and occur in inlet chambers of AWJ cutting heads. Swirl is superimposed on the axial circulatory pattern within an inlet chamber.
The discussion that follows starts by considering airflow in inlet chambers without abrasive particles. Over 40 different cutting head designs are used in industry, with wide variations in non-dimensional geometric parameters. Some of the most important non-dimensional geometric parameters are given in Annex A and comments made on the effect of varying geometric parameters. Here a typical cutting head geometry is assumed.
Without abrasive, conditions and events in inlet chambers and cutting nozzle inlets include:
Turning on abrasive to a cutting head has a marked effect on conditions within an inlet chamber and cutting nozzle inlet:
With a waterjet accurately aligned along the centreline of a nozzle bore and no abrasive flow:
Turning on abrasive flow to a cutting head dramatically changes conditions from those without abrasive. The best way of visualizing events in a nozzle bore is to think of the air and abrasive initially flowing in an annulus between a waterjet and a cutting nozzle wall and interacting with a waterjet and nozzle wall. Assuming the following:
Abrasive particles traveling along a hypothetical annulus between a waterjet and nozzle wall are subjected to a number of forces:
Drag from air
Impact and drag from water
Impacts on a bore wall
Interaction with other particles
Abrasive particles enter a nozzle bore with a wide spread of velocities – mainly axial but with tangential and radial components. In the first few annulus widths mean particle axial velocities are only 5% or so of the water velocity. Abrasive hold is substantial, forcing air moving at sonic velocity to follow tortuous paths in an annulus
There is a substantial decrease in static pressure over the first 40 or so annulus widths resulting from the effects of abrasive hold up, chaotic abrasive movements, shock waves and other phenomena. With increasing distance from the bore inlet abrasive hold up decreases, abrasive particles velocities become predominately axial and static pressures stabilize and remain more or less constant to a nozzle outlet. Interactions between particles and bore wall become less energetic with distance along an annulus as particle velocities become mainly axial.
AWJ cutting heads are two stage fluid dynamic devices. The first stage is a conditioning stage in the inlet chamber and inlet to a cutting nozzle. The second stage is a momentum transfer stage that occurs in a cutting nozzle bore.
In the first stage a relatively small percentage of a waterjet’s energy is fed into a number of energy inefficient but very beneficial flow processes. The flow processes allow:
All commercial cutting nozzles have a simple bore of constant diameter when new. However, cutting nozzles wear as soon as abrasive flow starts. Wear changes conditions within a cutting nozzle bore and within an inlet chamber. The effect of wear is dealt with in Section 3.6
Supersonic airflow in cutting nozzle bores prevents information about ambient static pressures at a nozzle outlet propagating back to an inlet chamber. This means that the static pressure just before the outlet of a nozzle is determined by conditions in the inlet chamber and nozzle inlet. If conditions within the inlet chamber result in too high a static pressure along a nozzle bore airflow leaving a nozzle will be under expanded.
Under expanded air leaving a cutting nozzle results in:
Undesirably high static pressures towards the outlet of a cutting nozzle are most likely to occur because of a damaged orifice and/or worn cutting nozzle, see Section 3.6 below.
The flexibility provided by the flow processes in cutting head chambers is reflected in the wide variation in the design of inlet chambers of commercial cutting heads. This flexibility comes at a price in terms of less than optimum cutting performance and excessive wear of cutting head components. Because of the wide geometric variation between cutting heads from different manufactures it is not possible, at present to establish design rules for cutting heads. End users could benefit greatly by the development of design criteria for cutting heads of good performance.
Cutting head performance depends greatly on the abrasive feed system. All abrasive feed system should provides a steady, metered flow of abrasive from a hopper into air flowing to the inlet chamber of a cutting head. For abrasive to arrive at a cutting head in a stable manner:
Remarkable features of industrial abrasive feed systems are:
Clearly it would be beneficial if manufacturers provided information on the inlet static pressure range for a cutting head, including how it varies with waterjet orifice to cutting nozzle bore diameter ratio. Similarly manufacturers of abrasive feed systems and integrators of feed system components should provide systems that match particular cutting heads.
Flow behavior within an AWJ cutting head changes gradually with wear and abruptly because of damage. Unpredictable changes are the most troublesome, particularly when they cause the loss of a batch of items resulting in material being scrapped in addition to lost production time.
Problems that occur with waterjet orifices include:
Deposits, wear and damage can cause many undesirable effects through changes to a waterjet that increase air entrainment, distort the shape of a waterjet and move its centreline so that it is no longer aligned along the centerline of a cutting nozzle. Detrimental effects include:
To be continued
An abrasive particle can travel upstream of abrasive waterjet orifice to subsequently cause damage to an orifice. A mechanism for this unexpected behaviour is to found in a combination of events that occur every time a waterjet is stopped.
Air entrainment as soon as a water separates at an orifice edge. This entrainment air provides a mechanism whereby particles are carried up and into an orifice bore. Such particles are either violently expelled from an orifice bore by a waterjet, possibly damaging an orifice carrier in the process or, if a particle is small enough and wet, it may become attached to a bore wall.
When a shut off valve closes the following events occur:
If a particle settles on the front face of an orifice carrier, away from an orifice bore, on opening of a shut off valve, high water velocities may have been established before a particle reaches an orifice edge. If this is the case the risk of damage is much higher than if a particle had settled close to an orifice bore.
Abrasive particles passing through a waterjet orifice can occupy a significant part of an orifice bore. Large pressure differentials across a particle as it moves round an orifice edge cause high point loads. Point loads are probably the major cause of edge chipping.
A way of preventing particles reaching the vicinity of an orifice is to bleed air from atmosphere to an orifice outlet. However, this complicates the design of a cutting head and reduces the amount of air available to carry abrasive to a cutting head.
Appropriate dimensioning of passageways downstream of a waterjet orifice reduces the number of particles reaching an orifice. Very rapid closure of a valve over its last ten percent or so of movement could be expected to minimize the amount of air drawn through an orifice.
Fluid dynamics research is pre-eminently an experimental science. Flow phenomena are similar in a model and at full scale if non-dimensional geometric and flow parameters are the same for a model and full scale.
Flows in cutting heads involve too many non-dimensional flow parameters that scale modelling is not an option. This means experiments have to be carried out at “full” scale and with flow parameters typical of industrial cutting operations.
“Full” scale in the case of AWJ cutting heads involves geometric variations of 3:1 to cover typical cutting nozzle bore diameters between 0.4 and 1.2 mm. The practice is to maintain the same ratio of nozzle bore to waterjet diameter ratio when a cutting head’s nozzle diameter is changed, but leave other cutting head dimensions unchanged. As a result flow similarity is not maintained because relationships between non-dimensional geometric parameters are changed.
Changing key geometric parameters without corresponding changes in other geometric parameters can have adverse or beneficial consequences on flow behaviour. In practical terms, a cutting head may perform optimally with a particular cutting nozzle diameter and nozzle/orifice diameter ratio, but not optimally when the nozzle diameter is changed whilst maintaining the same nozzle/orifice diameter ratio.
Not maintaining geometric similarity adds to the problem of trying to understand cutting head flow behaviour. For end users it probably means they should be offered different cutting heads by manufacturers, depending on input power, rather than one cutting head for all power inputs.
Important non-dimensional geometric parameters include:
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