WATER JET CONFERENCE - Waterjet Technology Association

CLEANING AND CUTTING WITH SELF-RESONATING PULSED WATER JETS
Georges L. Chahine Andrew F. Conn
Virgil E. Johnson, Jr. Gary S. Frederick
HYDRONAUTICS, Incorporated
Laurel, Maryland 20707
ABSTRACT
A new type of pulsed water jet, which uses principles of self-resonance to create
pressure fluctuations, is now being developed. The advantages of pulsing to improve the
erosive action of a jet—by interrupting the flow, hence using the water hammer impact
stress created by each individual slug—have long been appreciated by the users of water
jets for cutting and cleaning applications. Of several self-resonating pulsed jet, or
''SERVOJET”, concepts, a design incorporating a Helmholtz resonating chamber, tuned
to drive an organ-pipe segment, has proven feasible for in-air interruption of a water jet.
Experimental results for several cleaning and cutting applications are described.
INTRODUCTION
Motivation
The increased erosivity afforded by causing a water jet to break up into a series of
water slugs has long been recognized. Such interrupted or pulsed jets offer the following
advantages, relative to steady-flowing jets, for either cleaning a substance from a
substrate or cutting into a bulk material such as a rock:
1. Larger impact stresses, due to the water hammer pressure, which enhance the local
erosive intensity,
2. Larger outflow velocities across the surface being cleaned (or cut) thus providing an
increased washing action, (or opening cracks and flaws in a bulk material for cutting
applications),
3. Greater ratio of impacted area per volume of jetted water, thus exposing larger areas
of the surface to the water hammer pressure,
4. Cycling of loading; this promotes unloading stresses which may enhance the process
of de-bonding the substance from the substrate, or fracturing the bulk material being
cut, and
5. Short duration loadings, which tend to minimize energy losses within either the
substrate being cleaned or the bulk material being cut, and hence increase the material
removed per input energy.
A variety of mechanical techniques have been used to interrupt water jets. There
have been external, rotating disks containing slots, holes, or sprockets (Summers (1975),
Lichtarowicz and Nwachukwu (1978), Erdmann-Jesnitzer, et al. (1980)); an internal,
spinning slotted rotor (Nebeker (1981); and an internal, piezoelectric transducer (Danel
and Guilloud (1974)). Although each of these methods produced a series of water
slugs—and improved erosion relative to steady jets was consistently observed—the
frequency of interruption was always well below optimum as discussed below. Also, the
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