ABRASIVE WATER JET CUTTING - TECHNIQUE ... - Revtn.ro

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adial and axial directions; hence the velocitydecreases up to value v(x,r) < v 0 and the density of thecore of the jet decreases;• Disintegration zone starts from the point where wehave not a core of the jet anymore.In figure 2 it is presented a water jet obtained on aexperimental water jet cutting equipment from ICTCM. Thecharacteristics of this water jet are: pressure 230 MPa, nozzlediameter 0.20 mm. In this picture, it can be observed the lengthof the principal zone of the jet, and the disintegration of the jet(dimmed white layer on outside part of the jet) due to injectionand friction with the air.and a water flow of 1.2 l/min, the length of principal zone (x p )is about 90 mm.Force(N)12,51211,51110,51011125153050709011,1111211,511,1 1111 11,2 10,8Distance from nozzle exit (mm)Figure 4. Variance of the water jet force withdistance from nozzle exit (nozzle diameter 0.2 mm)Figure 2. View of a high pressure water jet (230 MPa)Most important from application point of view, is the length ofprincipal zone (x p ), inside which are cut materials.In figure 3, the values experimentally determined arerepresented for water jet force at different pressures. Thevalues are determined using an experimentally device placedperpendicular on jet direction.1510Force (N)507,310 10,5 1190 160 175 200Pump pressure (MPa)Figure 3. Water jet forceExperimental results of a water jet force at different distancesfrom nozzle exit are presented in Figure 4. It can be observedthat, for a nozzle diameter of 0.2 mm, a pressure of 200 MPaThe length x p can be determined introducing the water jetlength quotient (k), expressed by the following formula:k = x p / d 0 (1)The authors experimentally determined values for the quotientk measuring the force of different water jets (e. g. in Figures 3and 4). Considering that at the end of principal zone the forceof the water jet decreases with 70%, the authors determined thefollowing experimental values for k quotient. Hence, forpressures in range 170 ... 230 MPa and nozzle diameters of0.15 ... 0.30 mm, the water jet length quotient can bedetermined with the following formula:k 170...230 = 250...520 (2)The velocity of a water jet, considering Bernoulli’s law, can bedetermined by the following formula:v 0= μ ⋅2Pρ( P) (3)- p 1 - pressure before the water exit the nozzle ;- v – water jet velocity at exit point from the nozzle;- ρ – density of the water at the pressure p 1;- µ - quotient counting energy looses at input.In Figure 5 it is represented variation of µ quotient withpressure for carbide water nozzles determined by the authorson an experimental water jet equipment presented in nextchapter.Press.[MPa]2502001501005000,75 0,71 0,78 0,93Figure 5. Values for µ quotient (for carbide nozzles)µ41
Another specific aspect of high pressure water jets is thedensity of the water. If, for low pressures (up to 100 MPa) thewater is incompressible, at pressures above 150 MPa the wateris compressible. The density of water can be calculated withthe following formula:ρ / ρ 0 = 1 / [1 – ( β ⋅ p)] (4)- p - pressure;- ρ – water density at current pressure;- ρ 0 – water density at atmosphericpressure (0,1 MPa) ;- β – water compressibility module.In formula 4 can be observed that there is a linear dependencebetween density and pressure, if we consider that watercompressibility module β is constant . In fact it is not constantit varies with pressure [2] .In Figure 4 it is presented variation of water density withpressure for two cases:• A gaseous phase represented by the air introduced influid due to method of introduction of abrasiveparticles in water jet: injection.The mechanism of abrasive water jet forming it is not up to thismoment theoretically explained due to the big number ofindependent parameters which participate.However, it can be explained the mechanism of a abrasivewater jet forming, as it is represented in figure 7 [3] .• β (water compressibility module) is constant withpressure increase ;• β (water compressibility module) is not constant withpressure increase : it decreases. ;The values are obtained during tests performed on aexperimental water jet equipment in ICTCM laboratory.Looking closer to the values from Figure 6 it can beconcluded that, from practical point of view, the water densityincreases linear with pressure increase up to 200 MPa, and notlinear for pressures between 200 ... 400 MPa.Densityvariation[%]20151050ß‐const.ß‐var.100 150 200 250 300 350 400Pressure [MPa]Figure 6. Variation of water density vs. pressureConsidering values of water density and µ quotient fromFigures 3 and 4, the values for velocity of water jet (forpressure between 100 … 450 MPa) calculated with formula 3are:v 0 = 328 ... 786 (m/s) (5)It can be observed that the water jet velocity, for pressures usedfor cutting (200 ... 450 MPa), is grater than the velocity ofsound in air (v s = 347 m/s).2.2. Abrasive water jet techniqueAbrasive water jet is a polyphasic fluid where the constituentsdo not react chemically each other. The phases of this fluid are:• A liquid phase represented by the water (with thelargest mass participation);• A solid phase represented by abrasive particles;Figure 7. Abrasive water jet forming mechanism- 1 – water jet nozzle;- 2 – formed water jet;- 3 – abrasive particle;- 4 – water – abrasive particles focus;- 5 – formed abrasive water jet.The high pressure water jet 2 passes thru a zone called mixingchamber between nozzle 1 exit and entrance in focus 4. In thisarea the pressure is negative due to very high velocity of waterjet.Abrasive particles 3 are sucked in water stream due to thisnegative pressure, in area ranging between points I and II. Inarea between points II and III the abrasive particles are forcedto move in the direction of the water stream, but the velocity ofthem is low. If the abrasive jet should exit from focus in pointIII, the velocity is too low and the efficiency should be lowalso.Between points III and IV the abrasive particles velocity isincreased but this will never reaches the velocity of water jet. Ifthe length of this zone is too big, the velocity of particles willbe decreased due to friction with focus walls.Because there are so many parameters, it is difficult totheoretically determine the length of focus 4. Hence the lengthof focus is experimentally determined by the focusmanufacturers, in order to increase as much as possible thevelocity of abrasive particles.Usually the ratio between focus diameter and its length is (25… 50).In figure 8 is presented an abrasive water jet cutting head froman experimental equipment of ICTCM.Similar to water jet, it was determined the force of an abrasivewater jet (the experimental device is similar).Comparatively (see Figure 9), the forces of a water jet (WJ)and abrasive water jet (AWJ) do not vary too much.42
Page 4 and 5: Keeping identical values for common
Page 6 and 7: Figure 15 shows the influence of di

ABRASIVE WATER JET CUTTING - TECHNIQUE ... - Revtn.ro

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