Waterjet Models and Parameters Flashcards
What is the theoretical pure waterjet velocity according to Bernoulli’s equation?
The pure water velocity is: Vth = √(2p0/ρ)
How does water compressibility affect the theoretical jet velocity?
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
What is the relationship between pressure and water density in the water compressibility law?
The ration between densities (rho/rho1) = (1 + p/L)^c. Where C is equal to a constant c=0.1368 and L = 300 MPa
What are the values of constants (C) and (L) in the water compressibility law?
c=0.1368 and L = 300 MPa
How does the density of water change from section 0 to section 1 in the nozzle?
The water density decreases as water expands from section 0 to section 1
What is the significance of the compressibility coefficient (\psi)?
The compressibility coefficient relates the actual jet velocity Vth,c with the theoretical jet velocity Vth which accounts for the compressibility of the water
How does the compressibility coefficient (\psi) change with increasing pressure?
The compressibility coefficient decreases with increasing pressure indicating that the compressibility effect becomes more significant at higher pressures
What is the role of the velocity coefficient (C_v) in calculating the actual jet velocity?
It is used to account for frictional losses in the primary nozzle
What is the contraction coefficient (C_c), and how does it affect the water flow rate?
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
How is the overall coefficient of discharge (C_d) defined, and what does it represent?
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
What is the formula for the kinetic power of a pure waterjet?
The formula is Pkin = 1/2rho1(density)Qw (flow rate)*Vj^2(jet velocity)
How does the kinetic power of a waterjet depend on the orifice diameter and water pressure?
The flow rate can be related to the orifice diameter and waterpressure through the formula: Qw=CcCvΨπ/4D^2*(2p/ρ)^(1/2)
What is the relationship between the power density and the water pressure in a waterjet?
Power density increases with water pressure based in the formula: Qw=CcCvΨπ/4D^2*(2p/ρ)^(1/2)
Why does the cross-sectional area of the orifice not significantly affect the power density?
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
What is the role of the velocity coefficient (C_v) in determining the kinetic power of the waterjet?
Cv accounts for the energy losses due to friction reducing the jet velocity and by consequence the kinetic power
What is the purpose of the abrasive suction process in the mixing chamber?
To incorporate the water and the abrassive into the water jet for effective mixing and cutting
How does the vacuum pressure (p_v) change with increasing jet pressure (p)?
As jet pressure increases, the vacuum pressure decreases creating a stronger suction effect
What is the maximum vacuum pressure that can be achieved in the mixing chamber?
In theory 760mmHg if the mixing chamber would be perfectly sealed but this is not possible to achieve in practice
How does the air flow rate (Q_{air}) affect the vacuum pressure in the mixing chamber?
As the air flow rate increases, the difference in pressure reduces, reducing the vacuum effect
What is the venturi effect, and how does it influence the abrasive suction process?
The venturi effect creates a pressure drop in the mixing chamber, which helps to suck aire and abrasive particles
What are the assumptions made in the conservation of momentum for abrasive mixing?
Incoming air and abrasive velocities are negligible, no frictional drag, air mass flow rate is negligible
How does the abrasive loading ratio (r_d) affect the abrasive jet velocity?
As the abrasive loading ratio increases, the jet velocity decreases as more energy is transferred to the abrasive particles
What is the role of friction in the mixing tube, and how does it affect the abrasive jet velocity?
Friction reduces the actual abrasive jet velocity as it causes energy losses
What is the mixing efficiency (\eta_T), and how is it calculated?
eta = Vabr / Vabr,max and it represents the ratio between the actual abrasive jet velocity and the maximum theoretical abrasive velocity
How does the mixing tube length affect the mixing efficiency?
Longer mixing tubes improve mixing efficency by allowing more time for momentum transfer between water, air and abrasive
What is the typical mixing tube length used in industry, and why is it chosen?
Typical one is 76 mm long, suitable for achieving mixing efficiency of 90% for #80 mesh and above
How does the abrasive mass flow rate affect the mixing efficiency and the final jet velocity?
Higher abrasive flow mass rate reduce the final jet velocity but can improve mixing efficiency by transfering momentum quicker
What are the two main types of material removal mechanisms in waterjet cutting?
Brittle fracture and ductile fracture
How does the angle of attack affect the material removal in brittle and ductile materials?
For brittle materials: Higher angle of attack cause crack propagation
For ductile materials: Low angles of attack cause material removal by chip formation
What are the three stages of the cutting process in waterjet cutting?
1.- entry cutting process
2.- cyclic cutting process
3.- exit cutting process
What is the difference between the cutting wear zone and the deformation wear zone?
Cutting wear zone: material removal at shallow angles of impact (abrasion)
Deformation wear zone: material removal at high angles of impact (erosion)
How does the feed rate (v_f) affect the vertical cutting removal rate (v_c)?
As the feed rate increases, the vertical cutting removal rate decreases, as less energy is transferred to the workpiece per unit length
What are the key productivity parameters in AWJ cutting?
Material Removal Rate (MRR) and Maximum Cut-through feed rate
What are the main geometrical quality parameters in AWJ cutting?
Kerf width, depth of cut, kerf taper, edge rounding, uncut triangle, surface roughness
How is the kerf width measured, and what are the most relevant positions for measurement?
Kerf width is measured at the top and at the bottom.
What is the significance of the uncut triangle in AWJ cutting?
It is a characteristic of quality in AWJ and represents an area where the waterjet is not able to fully penetrate the material
What are the Q levels in AWJ cutting, and how do they relate to cutting quality?
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
What are the main process parameters that influence AWJ cutting performance?
The main process parameters are: water pressure, feed rate, stand -off distance, orifice diameter, focuser diameter and abrasive mass flow rate
How does the feed rate (v_f) affect the cutting depth and surface roughness?
Higher feed rate reduce cutting depth and increase sourface roughness
What is the role of the stand-off distance (SOD) in AWJ cutting?
Stand-off distance affects the jet’s focus and energy distribution, influencing cutting quality and depth
How does the abrasive mass flow rate (m_a) influence the cutting performance?
Higher mass flow rate increases material removal rate but can reduce jet velocity and cutting depth if excessive
What is the significance of the abrasive particle size (mesh #) in AWJ cutting?
Smaller particles (higher mesh #) improce surface roughness but may reduce cutting efficiency
What is the maximum depth model, and how is it derived?
It predicts the maximum cutting depth based on process parameters
How does the maximum cutting depth (h_{max}) depend on the water pressure (p) and feed rate (v_f)?
Cutting depth increases with higher water pressure and decreases with higher feed rate
What is the empirical model for predicting the maximum cutting depth, and how are its coefficients determined?
The empirical model was derived from testing and the coefficients are determined thrugh linear regresion
How does the surface roughness (R_a) depend on the feed rate (v_f) and abrasive mass flow rate (m_a)?
Surface roughness reduces with lower feed rates and higher abrasive mass flow rates
What is the relationship between the kerf taper (T) and the feed rate (v_f)?
Kerf taper decreases with lower feed rates
How does the kerf taper (T) change with increasing water pressure (p)?
Kerf taper decreases with higher water pressure
What is the effect of the focuser diameter (d_m) on the kerf width and surface roughness?
smaller focuser diameters reduce kerf width and improve surface roughness.
