WATER JET CONFERENCE - Waterjet Technology Association

Through analysis and comparative studies of the energy consumption by various
types of jets under different working conditions, the swing-oscillation jet was selected as
the most favorable technology for use in coal drivage (1), (2).
From results obtained during the experimental study and field trials, the principal
conclusions derived for the swing-oscillation jet use in cutting coal are that:
1. Combining a high frequency oscillating motion perpendicular to the traverse direction
of the nozzle, with a high normal speed will produce a swing oscillation jet moving at
a high relative velocity (i. e., traverse speed (Figure 1). Figure 1 shows the curves
correlating resultant moving speed of nozzle exit (V ), amplitude (a), oscillation
frequency (F) and traverse speed (V). A review of Figure 1 reveals that by applying a
swing oscillation motion, the traverse speed of the jet will be increased to a level it
would be difficult to realize with conventional jets.
2. The area covered by the jet in unit time, using a swing-oscillation jet at high traverse
speed is remarkably greater than that of a conventional jet (3,4)
3. By adjusting the amplitude of motion of the swing-oscillation jet it is possible to spall
out the ridge left between slots when cutting. The water will also crack a zone around
the cut, which will require a lower energy input to be removed later.
4. A wider broken zone will substantially improve the operational conditions for jet
cutting. The depth of slot cut is linearly related to the number of traversing passes,
over the range that the jet is efficiently cutting. Thus wider and deep kerfs can be
created, i. e., a slot can be created of a certain width which can be cut to great depth.
5. In addition, any coal left between adjacent cutting paths can be broken down by
vertical impact. Thus a much lower energy consumption would be necessary.
6. The excavation pattern for each site should be determined based on local geological
conditions, and the mechanical and physical properties of the coal. Generally, deeply
pre-kerfing the bottom of the coal face if done first will create favorable conditions
for a large volume big amount of solid coal to fall out; as a result, the lowest energy
consumption will be achieved and the maximum potential of the water jets for coal
cutting would be achievable.
Figure 3 is a schematic diagram of a typical application, where the area
surrounded by dotted lines indicates the amount of fallen coal which resulted from the
extensive collapse of solid coal (4,5).
THE CUTTING ABILITY OF A SWING-OSCILLATION WATER JET AND THE
TECHNICAL MEANS USED TO ENHANCE ITS ABILITY FOR COAL CUTTING
The schematic diagram given in Figure 3 shows that effective coal drivage with a
swing-oscillation jet, requires that deep kerfs and surrounding fractured zones be created
at high speed. As stated previously, it is necessary to study the jet coal cutting ability
before studying the coal collapsing ability. With the aid of a Least Squares analysis of the
experimental data, an equation predicting the ability of a swing-oscillation jet to cut
material was derived and simplified. An appropriate equation is given as follows:
(EQUATION IS MISSING FROM THE MS)
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