Utilization of ultrasound to enhance high-speed water jet effects
Josef Foldyna
a,*
,Libor Sitek a ,Branislav  Svehla b , Stefan  Svehla
b
a
Institute of Geonics,Academy of Sciences of the Czech Republic,Studentsk  a 1768,Ostrava-Poruba 70800,Czech Republic
b
Ecoson Ltd.,Nov  e Mesto nad V  a hom,Slovak Republic
Abstract
Continuous high-speed water jets are presently used in many industrial applications such as cutting of various materials,cleaning
and removal of surface layers.However,there is still a need for further research to enhance the performance of pure water jets.An obvious method is to generate water jets at ultra-high pressures (currently up to 700MPa).An alternate approach is to eliminate the need for such high pressures by pulsing of the jet.This follows from the fact that the impact pressure on a target generated by a slug of water is considerably higher than the stagnation pressure of a corresponding continuous jet.
Ultrasonically forced modulation of a continuous stream of water represents the most promising method of pulsed jet generation because of its simplicity and practicality.A pulsed jet is generated by modulating a continuous stream of water by ultrasonic waves.A velocity transformer connected to a piezoelectric transducer is located axially inside a nozzle to induce longitudinal pulsations in the water.An extensive laboratory research program is in progress to understand the basic principles of the process and to optimize the nozzle design for several applications.
The results reported in this paper show that the performance of such a pulsed jet is far superior to that of a continuous jet operating at the same parameters.Experimental results obtained with the ultr
asonic vibration of a tip situated inside the nozzle indicate that using this technique one can achieve performance of the jet even order of magnitude higher in comparison to con-tinuous jet at the same hydraulic parameters.Performance of ultrasonically modulated jets in cutting of various materials was tested in laboratory conditions.In this paper,results of measurement of dynamic pressure in the nozzle and force effects of modulated jets are presented together with results obtained in cutting of various materials using ultrasonically modulated water jets.The results are compared with those obtained with continuous jets at the same operating parameters.Potential of forced modulation of the jet in applications of cleaning,paint and coating removal from surfaces and concrete cutting in the process of repair of concrete structures is mentioned.
Ó2004Elsevier B.V.All rights reserved.
PACS:43.35.+d;47.27.Wg
Keywords:Water jet;Modulated jet;Ultrasonic modulation;Dynamic pressure;Stagnation force of the jet
1.Introduction
High-speed water jet technology achieved significant progress during last decades in applications such as cutting of wide range of materials,surface cleaning and removal of surface layers,and repair of concrete struc-tures.Nowadays,number of commercial high-pressure systems is available on the market,some of them gen-erating pressures up to 400MPa,other delivering up to hundreds liters of water per minute.However,there is
still need of further improvement of the technology to-wards higher performance and economical advanta-geousness as well as its adaptation to constantly more and more demanding environmental requirements.
Generation of pulsed water jets represents one of possible approaches to achieve the improvement of the technology.This follows from the fact that impact pressure p i generated by the impact of slug of water on a target at the velocity v 0is considerably higher than corresponding stagnation pressure p s generated by a continuous jet under the same operating conditions.When a continuous liquid jet impinges normally on a flat rigid surface at the velocity of v 0,the maximum pressure at the point of impact is the stagnation pressure p s ,given by:
*
Corresponding author.Tel.:+420-596-979-111;fax:+420-596-919-452.
E-mail address:foldyna@ (J.Foldyna).1350-4177/$-see front matter Ó2004Elsevier B.V.All rights reserved.
doi:10.1016/j.ultsonch.2004.01.008
Ultrasonics Sonochemistry 11(2004)
131–137
p s¼1=2Ãq v2
½Pa ;ð1Þwhere q represents liquid density[kg mÀ3].For water (q¼998kg mÀ3),Eq.(1)can be written as follows:
p s¼499v2
½Pa :ð2ÞHowever,if a drop or a slug of liquid strikes the same target at the same velocity of v0,the initial impact pressure will be much higher.The impact pressure(so-called waterhammer pressure)developed by the initial impact of a liquid jet with aflat head on a rigid plane surface can be derived by a one-dimensional(Rankine–Hugoniot)analysis as:
p i¼q cv0½Pa ;ð3Þwhere c represents speed of sound in liquid[m sÀ1].For water(q¼998kg mÀ3and c¼1480m sÀ1),Eq.(3)can be simplified as:
p i¼1477040v0½Pa :ð4ÞThus pulsing the jet leads to an amplification of the impact pressure z¼p i=p s¼2960=v o.Since velocities of continuous jets currently used in outdoor applications do not exceed700m sÀ1,the impact pressure of pulsed jet will be at least four times higher at the same velocity. Therefore significant improvement in cutting and/or cleaning performance of such pulsed jets can be ex-pected.Also additional effects such as fatigue failure of target material due to cyclic loading can contribute to the higher performance of pulsed jets.Therefore, incessant attempts have been made for generating var-ious types of pulsed water jets in the last thirty years (refer to[1]for details).A particular method of gener-ating pulsed jets represents modulating a continuous stream of water.A modulated jet escapes from the nozzle as a continuous stream of liquid having unsteady velocity(cyclically modulated over time).Slow and fast portions of each cycle tend toflow together,forming a train of’’bunches’’in the free stream,which eventually separate[2,3].Attempts were made to modulate jets using internal mechanicalflow modulators[3],Helm-holtz oscillators[4],self-resonating jets[5],electrohy-draulic discharge[6],ultrasonic vibration of the nozzle body[7]and ultrasonic vibration of a tip situated inside the nozzle[8,9].
Ultrasonically forced modulation of a continuous stream of water represents the most promising method of pulsed jet generation because of its simplicity,via-bility,and practicality.In this case,a pulse
d jet is gen-erated by modulating a continuous stream of water by ultrasonic waves.A velocity transformer connected to a piezoelectric transducer is located axially inside a nozzle to induce longitudinal pulsations in the water.The ultrasonically modulated jet breaks-up into slugs of water at certain distance from the nozzle,and starts to act as a pulsed jet.
During the last decade the research of the ultrasonic modulation of continuous jets proved that the modula-tion has potential for further increasing of the efficiency of the technology.However,experimental results pre-sented so far were obtained using mostly empirical ap-proach([9,10]).Therefore,an extensive research program was started in2000in the Institute of Geonics in Ostrava to understand the basic principles of the process and to optimize the ultrasonic nozzle design for various applications.The program is oriented at the evaluation of potential of modulated jets in cutting of different materials as well as at the understanding of fundamental processes occurring both within and out-side the nozzle during ultrasonic modulation of the jet. Highly complex dynamics of the modulatedflow is studied both theoretically with aid of CFD methods and experimentally using methods of direct measurement of dynamic pressure inside the nozzle and impact forces generated by modulated jets.In this paper,results of measurement of dynamic pressure in the nozzle and force effects of modulated jets at various standoffdis-tances from the nozzle exit ar
e presented together with selected results obtained so far in cutting of various materials using ultrasonically modulated water jets. 2.Ultrasonic nozzle device
Some of possible concepts and configurations of the ultrasonic nozzle can be in[8,9,11,12].Basi-cally,ultrasonic modulation of the jet is produced by vibrations of ultrasonic tool located inside a nozzle.The vibration is generated by an ultrasonic transducer con-nected to the ultrasonic tool.
In our experiments,ultrasonic nozzle was driven by 20kHz piezoelectric transducer connected to ultrasonic generator with maximum output power of630W.Two configurations of the ultrasonic nozzle were used:(i) nozzle equipped with stepped ultrasonic tool with con-ical vibrating tip2.0mm in diameter(referred as nozzle I in the paper)and(ii)nozzle equipped with exponential ultrasonic tool with vibrating tip10.0mm in diameter (referred as nozzle II in the paper).
3.Experimental facility
The experimental facility consisted essentially of a high-pressure water supply system,an ultrasonic nozzle device(see above),X–Y table for traversing of the jet over testing samples and a PC-based measuring system.
High-pressure water was supplied to the nozzle by a plunger pump capable to deliver up to43l minÀ1of water at pressure up to120MPa.
Dynamic pressure in the nozzle was measured by a calibrated piezoelectric pressure sensor Kistler6211;
132J.Foldyna et al./Ultrasonics Sonochemistry11(2004)131–137
operating pressure was measured at the ultrasonic noz-zle inlet by a piezoresistive pressure sensor Kristal RA-G25A1000BC1H.Force effects of the jet were measured by the apparatus for the measurement of stagnation force of the jet,consisting of piezoelectric force sensor Kistler9301A and charge amplifier Kistler5007.The apparatus was developed at the Institute of Geonics in Ostrava(see[13,14]for more details on apparatus and measurement method).
Data acquisition and processing was performed using PC-based measuring system equipped with DAQ board NI PCI-MIO-16E-1and controlled by NI LabVIEW6.1.
The traverse speed of X–Y table could be varied from 0.01to8.00m minÀ1.
4.Results and discussion
4.1.Measurement of dynamic pressure generated in nozzles I and II by ultrasonic modulation
To investigate differences in dynamic pressure gen-erated in nozzles I and II by ultrasonic modulation, series of measurements of dynamic pressure in the nozzle was performed under following testing condi-tions:nozzle diameter was1.98mm,operating pressure was changed from0.5to40MPa,and output ultrasonic power was set to maximum value of630W.
The results do not indicate any significant differences in dynamic pressure developed by ultrasonic modulation in both nozzle configurations.Examples of time and
frequency domains of dynamic pressure measured in the nozzle can be seen in Fig.1for nozzle I and Fig.2for nozzle II.
4.2.Measurement of force effects of modulated jet
Series of measurement of force effects of the modu-lated jet was performed under following testing condi-tions:nozzle diameters were  1.19and  1.98mm, operating pressure was changed from0.5to50MPa, and output ultrasonic power was set to maximum value of630W.Standoffdistance was changed during the tests within the range from20to160mm.Position of the vibrating tip of ultraso
nic tool inside the distance from the exit plane of the nozzle to the end of the tip)was varied from6to12mm for nozzle I and from10to25mm for nozzle II.
Authors assumed that analysis of frequency domain of the measured signals oriented at the determination of maximum amplitude of the jet force will provide suffi-cient information about the jet performance under given conditions.However,results obtained from the analysis of frequency domain of measured signals were not al-ways in accordance with actual performance of modu-lated jet tested on samples.Therefore,the time domain signal of force effects of the jet was also analyzed to obtain pulse characteristics(such as amplitude,and high and low state levels of the pulse––see Fig.3)in addition to frequency analysis of the measured signal.
The results of the measurement of force effects indi-cate significant difference between effects of modulated jets produced by tested nozzle configurations.Whereas force effects of the modulated jet generated by the nozzle I are strongly influenced by the tip position inside the nozzle,tip position in the nozzle II influences force effects of the jet to a much lower degree.Moreover,force effects of the jet produced by the nozzle II seem to be higher in comparison with those produced by the nozzle I.
Examples of influences of the tip position and standoffdistance on high and low state levels of the pul
se are illustrated in Fig.4for nozzle I and in Fig.5 for nozzle II.Measurement of force effects of the mod-ulated high-speed water jet has shown that the ultra-sonic nozzle I should be‘‘tuned’’by changing the ultrasonic tool position inside the ultrasonic nozzle to maximize impact effects of the jet.On the other hand, the ultrasonic tool position does not play such an important role in the performance of the modulated jet generated by nozzle II.Thisfinding was verified also by cutting tests on metallic
samples.
Fig.1.Time and frequency domains of dynamic pressure in nozzle I (operating pressure40MPa).
J.Foldyna et al./Ultrasonics Sonochemistry11(2004)131–137133
4.3.Cutting of metals and rocks by modulated jet Selected results obtained during laboratory tests of cutting of metal and rock samples by modulated water jet are presented in following sections.
4.3.1.Cutting of metallic samples
Two types of materials were selected for the tests of cutting of metallic samples by modulated jet:mild steel Czech Standard No.11375of the low carbon range (C
max.0.20%,P max.0.05%,S max.0.05%,minimum tensile strength 363MPa,minimum yield strength 235MPa,density 7850kg m À3)and brass Czech Standard No.423223(58%Cu,40%Zn,2%Pb,minimum tensile strength 412MPa,density 8300kg m À3).Thin discs 40mm in diameter and 5mm thick were manufactured and used as test specimens.
Tests were conducted at pressure of 40MPa,nozzle diameter was 1.98mm,standoffdistance 140mm,t
ra-versing velocity 0.03m min À1,and ultrasonic power was set to maximum (630W).The standoffdistance of 140mm was found to be optimum for the operating parameters specified above in previous tests.In order to compare the performance of modulated jet with the continuous one,tests with the latter were performed using standard nozzle for continuous jet at the same values of operating parameters.Rate of mass loss D m s calculated using following equation was used as a measure of performance:D m s ¼
D m Áv tr
s
½mg min À1
ð5
Þ
Fig.2.Time and frequency domains of dynamic pressure in nozzle II (operating pressure 40MPa).
Fig.4.Influence of tip position and standoffdistance on the high and low state levels of the pulse (nozzle I);pressure 20MPa,nozzle dia 1.19mm.
Fig.5.Influence of tip position and standoffdistance on the high and low state levels of the pulse (nozzle II);pressure 20MPa,nozzle dia 1.19mm.
134J.Foldyna et al./Ultrasonics Sonochemistry 11(2004)131–137
where,D m ¼mass loss [mg],v tr ¼traversing velocity [m min À1],and s ¼length of the cut [m].
Experimental results are presented in Fig.6for mild steel samples and in Fig.7for brass samples.In bo
th cases,while the modulated jet formed an irregular slot both in shape and depth,the kerf produced by the continuous jet is hardly visible.The rate of mass loss obtained for steel sample was roughly 36-times higher and for brass sample even 673-times higher using mod-ulated jet compared to that obtained with continuous jet at the same parameters.
Laboratory tests of cutting of metal samples by modulated jet proved that modulated water jet is able to cut metal materials (such as mild steel and brass)at pressures as low as 40MPa,whereas continuous water jet can cut these materials only at pressures well above 500MPa.
4.3.2.Cutting of rock samples
Medium grained sandstone (locality R  a ztoka),med-ium grained granodiorite (locality  Zulov  a )and Carrara marble were selected as testing materials for tests of cutting of rock samples by modulated jet.Basic physical and mechanical properties of rock samples can be found in Table 1.
Tests in rock samples were performed at pressure of 50MPa,nozzle diameter was 1.19mm,traversing speed 1m min À1,and ultrasonic power was set to maximum (630W).The standoffdistance was set to 60mm for sandstone and granodiorite,40mm for marble.These standoffdistances were found to be an optimum for individual rock materials and operating parameters specified above in previous tests.
In order to compare the performance of modulated jet with the continuous jet,tests with the latter were performed using standard nozzle for continuous jet at the same values of operating parameters.Average depth of cut calculated from five arbitrary measurements taken along the path of the jet was used as a measure of per-formance of the jet.
Results are presented in Fig.8for sandstone,in Fig.9for granodiorite,and in Fig.10for marble samples.It can be seen that while modulated jet formed quite reg-ular slots both in sandstone and granodiorite samples,slots produced by continuous jet are irregular both in shape and depth,being formed mostly by breaking out of rock chips.Average depths of cut produced by modulated jet are 2.2–3.1-times higher in sandstone and 2.8–3-times higher in granodiorite compared to that produced by continuous jet at the same operating parameters.As can be seen in Fig.10,slots produced by both types of the jet in marble are similar.Again,slots created by continuous jet are more irregular;however,the difference between slots created by modulated and continuous jets is not very distinctive.Typical are smaller irregularities both in shape and depth
compared
Fig.6.Mild steel exposed to (a)modulated jet and (b)continuous jet.Rate of mass loss 705.7mg min À1(a)and 19.3mg min À1
(b).
Fig.7.Brass exposed to (a)modulated jet and (b)continuous jet.Rate of mass loss 1077.6mg min À1(a)and 1.6mg min À1(b).
Table 1
Basic physical and mechanical properties of rock samples Sandstone Granodiorite Marble Locality
Reka Zulov  a Carrara Compressive strength (MPa)115144.597Tensile strength (MPa)  5.811.37.8Young’s modulus (GPa)19.74539.2Density (kg m À3)265226482721Unit weight (kg m À3)249526102710Porosity (%)
5.8
1.41
0.42
J.Foldyna et al./Ultrasonics Sonochemistry 11(2004)131–137135
to that obtained in other tested materials and lower degree of chip breakage.Average depths of cut pro-duced by modulated jet in marble are only 1.2-times higher compared to that produced by continuous jet.Authors believe that the performance of modulated jet in rock cutting can be further improved by proper matching of operating parameters,such as operating pressure and traversing velocity.Investigations in these areas are still in progress in the laboratory.
5.Conclusions
Results of measurement of dynamic pressures gener-ated in nozzle configurations I and II do not indicate any significant differences in dynamic pressure devel-oped during ultrasonic modulation in both nozzle con-figurations.
The maximum force effects of modulated jets can be determined by the analysis of time domain of measured signal focused on determination of pulse characteristics.Thus,the measurement of stagnation force can be used to optimize configuration of the ultrasonic nozzle for maximum performance of the modulated jet.
Results obtained in cutting of metallic samples show clearly the potential of ultrasonically modulated jets.The performance of modulated jet was approximately 36-times higher in mild steel and even mor
e
than
Fig.8.Sandstone exposed to modulated (a)and continuous (b)jets.Average depth of cut:6.8mm (a)and 3.2mm
modulate(b).Fig.9.Granodiorite exposed to modulated (a)and continuous (b)jets.Average depth of cut:5.1mm (a)and 1.6mm
(b).
Fig.10.Marble exposed to modulated (a)and continuous (b)jets.Average depth of cut:4.8mm (a)and 4.1mm (b).
136J.Foldyna et al./Ultrasonics Sonochemistry 11(2004)131–137

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