Waterjet Models and Parameters

Question 1

What is the theoretical pure waterjet velocity according to Bernoulli’s equation?

Answer 1

The pure water velocity is: Vth = √(2p0/ρ)

Question 2

How does water compressibility affect the theoretical jet velocity?

Answer 2

The water compressibility reduces the jet velocity. This is because part of the energy is used to compress the water. Not everything is converted into kinetic energy

Question 3

What is the relationship between pressure and water density in the water compressibility law?

Answer 3

The ration between densities (rho/rho1) = (1 + p/L)^c. Where C is equal to a constant c=0.1368 and L = 300 MPa

Question 4

What are the values of constants (C) and (L) in the water compressibility law?

Answer 4

c=0.1368 and L = 300 MPa

Question 5

How does the density of water change from section 0 to section 1 in the nozzle?

Answer 5

The water density decreases as water expands from section 0 to section 1

Question 6

What is the significance of the compressibility coefficient (\psi)?

Answer 6

The compressibility coefficient relates the actual jet velocity Vth,c with the theoretical jet velocity Vth which accounts for the compressibility of the water

Question 7

How does the compressibility coefficient (\psi) change with increasing pressure?

Answer 7

The compressibility coefficient decreases with increasing pressure indicating that the compressibility effect becomes more significant at higher pressures

Question 8

What is the role of the velocity coefficient (C_v) in calculating the actual jet velocity?

Answer 8

It is used to account for frictional losses in the primary nozzle

Question 9

What is the contraction coefficient (C_c), and how does it affect the water flow rate?

Answer 9

The contraction coefficient is used to estimate the contraciton of the fluid in the nozzle. This contraction is a result of the cross section reduction in the vena contracta causing a detrimental effect on the flow rate

Question 10

How is the overall coefficient of discharge (C_d) defined, and what does it represent?

Answer 10

It is the result of multiplying the compressibily coefficient, the velocity coefficient and the contraction coefficient. It represents the ratio of the actual flow rate in relation with the theoretical one

Question 11

What is the formula for the kinetic power of a pure waterjet?

Answer 11

The formula is Pkin = 1/2rho1(density)Qw (flow rate)*Vj^2(jet velocity)

Question 12

How does the kinetic power of a waterjet depend on the orifice diameter and water pressure?

Answer 12

The flow rate can be related to the orifice diameter and waterpressure through the formula: Qw=CcCvΨπ/4D^2*(2p/ρ)^(1/2)

Question 13

What is the relationship between the power density and the water pressure in a waterjet?

Answer 13

Power density increases with water pressure based in the formula: Qw=CcCvΨπ/4D^2*(2p/ρ)^(1/2)

Question 14

Why does the cross-sectional area of the orifice not significantly affect the power density?

Answer 14

Power density depends mainly on pressure and not on the oriffice diameter based on Bernoulli equation. We have a very high upstream velocity and a very small nozzle diameter

Question 15

What is the role of the velocity coefficient (C_v) in determining the kinetic power of the waterjet?

Answer 15

Cv accounts for the energy losses due to friction reducing the jet velocity and by consequence the kinetic power

Question 16

What is the purpose of the abrasive suction process in the mixing chamber?

Answer 16

To incorporate the water and the abrassive into the water jet for effective mixing and cutting

Question 17

How does the vacuum pressure (p_v) change with increasing jet pressure (p)?

Answer 17

As jet pressure increases, the vacuum pressure decreases creating a stronger suction effect

Question 18

What is the maximum vacuum pressure that can be achieved in the mixing chamber?

Answer 18

In theory 760mmHg if the mixing chamber would be perfectly sealed but this is not possible to achieve in practice

Question 19

How does the air flow rate (Q_{air}) affect the vacuum pressure in the mixing chamber?

Answer 19

As the air flow rate increases, the difference in pressure reduces, reducing the vacuum effect

Question 20

What is the venturi effect, and how does it influence the abrasive suction process?

Answer 20

The venturi effect creates a pressure drop in the mixing chamber, which helps to suck aire and abrasive particles

Question 21

What are the assumptions made in the conservation of momentum for abrasive mixing?

Answer 21

Incoming air and abrasive velocities are negligible, no frictional drag, air mass flow rate is negligible

Question 22

How does the abrasive loading ratio (r_d) affect the abrasive jet velocity?

Answer 22

As the abrasive loading ratio increases, the jet velocity decreases as more energy is transferred to the abrasive particles

Question 23

What is the role of friction in the mixing tube, and how does it affect the abrasive jet velocity?

Answer 23

Friction reduces the actual abrasive jet velocity as it causes energy losses

Question 24

What is the mixing efficiency (\eta_T), and how is it calculated?

Answer 24

eta = Vabr / Vabr,max and it represents the ratio between the actual abrasive jet velocity and the maximum theoretical abrasive velocity

Question 25

How does the mixing tube length affect the mixing efficiency?

Answer 25

Longer mixing tubes improve mixing efficency by allowing more time for momentum transfer between water, air and abrasive

Question 26

What is the typical mixing tube length used in industry, and why is it chosen?

Answer 26

Typical one is 76 mm long, suitable for achieving mixing efficiency of 90% for #80 mesh and above

Question 27

How does the abrasive mass flow rate affect the mixing efficiency and the final jet velocity?

Answer 27

Higher abrasive flow mass rate reduce the final jet velocity but can improve mixing efficiency by transfering momentum quicker

Question 28

What are the two main types of material removal mechanisms in waterjet cutting?

Answer 28

Brittle fracture and ductile fracture

Question 29

How does the angle of attack affect the material removal in brittle and ductile materials?

Answer 29

For brittle materials: Higher angle of attack cause crack propagation For ductile materials: Low angles of attack cause material removal by chip formation

Question 30

What are the three stages of the cutting process in waterjet cutting?

Answer 30

1.- entry cutting process 2.- cyclic cutting process 3.- exit cutting process

Question 31

What is the difference between the cutting wear zone and the deformation wear zone?

Answer 31

Cutting wear zone: material removal at shallow angles of impact (abrasion) Deformation wear zone: material removal at high angles of impact (erosion)

Question 32

How does the feed rate \(v_f\) affect the vertical cutting removal rate \(v_c\)?

Answer 32

As the feed rate increases, the vertical cutting removal rate decreases, as less energy is transferred to the workpiece per unit length

Question 33

What are the key productivity parameters in AWJ cutting?

Answer 33

Material Removal Rate (MRR) and Maximum Cut-through feed rate

Question 34

What are the main geometrical quality parameters in AWJ cutting?

Answer 34

Kerf width, depth of cut, kerf taper, edge rounding, uncut triangle, surface roughness

Question 35

How is the kerf width measured, and what are the most relevant positions for measurement?

Answer 35

Kerf width is measured at the top and at the bottom.

Question 36

What is the significance of the uncut triangle in AWJ cutting?

Answer 36

It is a characteristic of quality in AWJ and represents an area where the waterjet is not able to fully penetrate the material

Question 37

What are the Q levels in AWJ cutting, and how do they relate to cutting quality?

Answer 37

Q levels represent the surface quality in AWJ, each Q level represents a level of quality. Q5 is the one with the best quality, as no marks of striation are visible

Question 38

What are the main process parameters that influence AWJ cutting performance?

Answer 38

The main process parameters are: water pressure, feed rate, stand -off distance, orifice diameter, focuser diameter and abrasive mass flow rate

Question 39

How does the feed rate \(v_f\) affect the cutting depth and surface roughness?

Answer 39

Higher feed rate reduce cutting depth and increase sourface roughness

Question 40

What is the role of the stand-off distance (SOD) in AWJ cutting?

Answer 40

Stand-off distance affects the jet's focus and energy distribution, influencing cutting quality and depth

Question 41

How does the abrasive mass flow rate \(m_a\) influence the cutting performance?

Answer 41

Higher mass flow rate increases material removal rate but can reduce jet velocity and cutting depth if excessive

Question 42

What is the significance of the abrasive particle size (mesh #) in AWJ cutting?

Answer 42

Smaller particles (higher mesh #) improce surface roughness but may reduce cutting efficiency

Question 43

What is the maximum depth model, and how is it derived?

Answer 43

It predicts the maximum cutting depth based on process parameters

Question 44

How does the maximum cutting depth \(h_{max}\) depend on the water pressure \(p\) and feed rate \(v_f\)?

Answer 44

Cutting depth increases with higher water pressure and decreases with higher feed rate

Question 45

What is the empirical model for predicting the maximum cutting depth, and how are its coefficients determined?

Answer 45

The empirical model was derived from testing and the coefficients are determined thrugh linear regresion

Question 46

How does the surface roughness \(R_a\) depend on the feed rate \(v_f\) and abrasive mass flow rate \(m_a\)?

Answer 46

Surface roughness reduces with lower feed rates and higher abrasive mass flow rates

Question 47

What is the relationship between the kerf taper \(T\) and the feed rate \(v_f\)?

Answer 47

Kerf taper decreases with lower feed rates

Question 48

How does the kerf taper \(T\) change with increasing water pressure \(p\)?

Answer 48

Kerf taper decreases with higher water pressure

Question 49

What is the effect of the focuser diameter \(d_m\) on the kerf width and surface roughness?

Answer 49

smaller focuser diameters reduce kerf width and improve surface roughness.