WATER JET CONFERENCE - Waterjet Technology Association

"water hammer" pressure level on the target. In order to maintain this pressure level, the
frequency of impact has to be high enough to maintain a small amplitude of variation.
From a technical point of view the problem is quite difficult, because of the high
jet velocity and the aggressive cutting ability of the jet. Several authors have tried to
develop a working solution to this problem (Table 1). Any of these may produce an
interrupted jet, but the quality of the leading edge of the jet slugs produced, from a
geometric point of view, is not high enough to create the "water hammer" effect. Further,
it is very difficult to obtain a truly discretely segmented jet by method 2 and 3 before the
jet hits the sample. The reason is that the fluctuating velocity of the jet is not high enough
to form a clear separation of the jet. In order to develop a "water hammer" pressure level,
the break between segments has to be long enough to allow a segment to hit the surface
and then leave before the following segment impacts.
The method used by Summers and Lichtarowicz, in which rotating plate with
holes, cuts the jet, will create splashing and a deviation of the jet. The major problem,
however, is in the rapid wear of the interrupter blade and the high frequency noise it
generates.
PROPOSED SOLUTION
From this author's point of view, one practical solution to the problem, which would
eliminate the indicated weaknesses of existing designs, is that a laser beam used to break
the jet. The laser action on the water jet will evaporate the required portion of the jet
thereby forming a discontinuous water jet (Fig. 2).
Taking into consideration a water jet diameter d (m); laser beam diameter L (m);
ρ - density of the water (kg/m 3 ); C p - heat of water vaporization (5.4 x 105 cal/kg); C w ~
specific heat of the water (103 cal/kg deg); t the temperature difference between boiling
point and existing jet temperature (60 deg C); X - dissipate ratio (2); and A conversion
factor (0.24 cal/J), the necessary energy for evaporating such a segment of the jet can be
found from the equation:
E =
2
d
4 •l • • (C p + Δt • c 1
w ) • x •
A (J)
For a jet moving with a velocity v, the laser beam action must occur in a period of time
t = l
v
This means that the power consumption will be expressed in the following equation:
p = 3925 d 2 v , assuming l = d
The results of these calculations are presented in Figure 3, for different jet
diameters and where the jet is moving with a velocity in the range of 250 m/s to 3000
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