Cavitation effect

North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA

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Cavitation

σ i (vapor pressure) is the performance threshold such that

σ > σ i no cavitation effects

σ < σ i cavitation effects such as performance, degradation, noise and vibration

(non dimensional engrg parameters) po = pressurepv = vapor pressureU = velocity ρ (Greek rho) = atmospheric density σ (Greek sigma) = performance threshold)σi = incipient cavitation


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Physics of Cavitation

• Any device handling liquid is subject to cavitation

• Local pressure that falls below saturated vapor pressure causes cavitation to occur

• Resulting vapor cavities (void bubbles) collapse on surface material with high energy and heat

• Inertial cavitation is commonly destructive


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Collapse on the surface

Spike generation as shown in 1-18. Must have surface nucleation in order to collapse

Surface – note formation of spike 10-18 as vacuum cavity collapses with high energy


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Kinds of cavitation

• Dynamic – produced by pressure variations in a flowing liquid – i.e. die casting

• Acoustic – sound induced• Optic – photons of high energy – i.e. laser

• Particle – elementary particle – proton bubble chamber.

(dynamic and acoustic – liquid tension)

(optic and particle – deposition of energy)


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Dynamic stages

• Traveling – moves with liquid flow• Fixed – steady state fixed (sheet or cloud)• Vortex – in cores of vortices of high sheer

Incipient – term used to describe type and stage where cavitation appears

Desinent – term used to describe when cavitation disappears


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Stages, Cases, Types

Propeller cavitation offers excellent examples of the basic types

cloud cavitation

tip vortex cavitation

hub vortex cavitation

propeller hull vortex cavitation

sheet cavitation

bubble cavitation

Blade root cavitation


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Example Vortex Incipient

Dynamic liquid flow is left to right at a low velocity. Note – the beginning tiny stream of vortex bubbles on the tip of the obstruction

As the velocity increases, actual vortexes can be seen along with sheet and some cloud cavitation – note – how the cavitation occurs on the opposite side of the flow direction.

Similar to how core pins in die casting exhibit deterioration and soldering on the back side of flow


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Video Examples

flow decreasing

flow direction


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Hard to believe factsAs the cavity bubbles collapse on the surface:

• Bubbles collapse to a minute fraction of original size with immense focused force (4500 psi)

• At total collapse a violent mechanism resultstemperature of bubble 5000 Kelvin or 8540 F (4727C)

• By comparison sun’s surface temp is 5778 Kelvin)• Sonoluminescence (light) can be produced at

collapse (Pistol Shrimp)


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Inertial in HPDC dies

Obstruction in a flowing water with flow from left to right - note how incipient cavitationmoves to the back side of the core obstruction in the form of a cloud while at the top right the 150,000 rpm vortex stream is created.This is very representative of core deterioration from molten metal flow during HPDC.

Slow motion photographs of water at high velocity flowing past an obstruction in its path creating pressure drops and cavitation


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Not all cavitation is badNon-inertial

Submarine torpedoes

Ultrasonics for cleaning

Chemistry i.e. water purification

Homogenization of paint or milk

Medical for kidney stone removal or IV drug

Badly soldered H-13 core on left.

After cleaning ultrasonically with NaOH on right

Courtesy of FISA


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OccurrencesInertial & Non inertial

• Aquatic animals – dolphins, tuna, shrimp• Plants – xylem vascular plant system

• Humans – cracking knuckles, bullet wound• Geology – Scablands Mega flood• Dams – Spillways• Industry – Propellers, Pumps, Turbines

High Pressure Die Casting Dies


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Mega flood

When a 2000ft high glacier ice dam broke, the high speed flood of water rushing over the land caused cavitation inception. The water’s massive volume (1/2 Lake Michigan), high speed (over 100ft/sec), and incipient vortex and sheet cavitation from obstacles along the way carved out this area of land between eastern Washington and western Montana about 15,000 years ago.


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The 1964 SDCE Paper

Abstract:Die steel is washed away by the blasting turbulent action of the molten metal entering the cavity. Pitting and breakout is an excessive extension of erosion.

Conclusion:Theory could not be proven since the erosion occurs in the opposite location to where it should. Cavitation is what causes the failure which is evident by observing pressure drop locations and flow separation of the molten metal.

Paper 25 – 01-64-03S


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1981 SDCE Paper Paper G-T81-123

AbstractZinc die casting dies experience cavitation which damage the dies surface due to pressure releases at collapse

RecommendationsControl geometic factors, velocities, design, and material strength all dictate higher process control to reduce the effect and damage from cavitation


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HPDC cavitation components

• Local pressure of the metal falls below the saturated vapor pressure of the liquid

• Occurs at sudden change of flow direction• Occurs at obstructions to flow that

generate pressure variations• High velocities of flow at these locations• Two basic types: sheet and vortex


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Breakout from Cavitation in HPDC

Pitting, breakout, erosion, and washout commonly occur in the opposite locations of where they would be expected - on the low pressure side of the flowing liquid

FLOW


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Chicken and the Egg

Does thermal fatigue cause breakout or does breakout lead to heat checking of the die’s

surface?

Can heat check cracking lead to breakout without incipient inertial cavitation?

YES


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Cavitation & Heat check

Pressure drop location

FLOW

Cavitation Effect Heat check breakout?

Cavitation effect exacerbates itself when more low vapor pressure develops from the disrupted flow patterns


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Cavitation ExacerbatedGate

Sheet and PossibleVortex Cavitation

& Erosion

exhibited sheet or cloud and vortex cavitation effect


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How others buffer cavitation damage

• Dam Spillways - Air slot

• Diesel Engines – Coolant cylinders textured

(patent 7146939 – manganese phosphate)

• Ship Propellers & Rudders – design, pressures, buffer elastomer paint

• Turbines, Pumps – design, operating parameters


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Dam spillways concrete liner – Air slot

Leap frog cavitation in the dam spillway – damages 150ft long – 32ft deepest – 40ft dia.

The air slot installed (4ft wide x 4 ft deep 253 below spillway gate) creates a cushion of non vacuum bubbles that buffer the cavitation destructive force and allow higher flows

Chunk of concrete

Air bubble cushion

Air slot


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Diesel Engine Cavitation

Bigger engines suffer from vibrations of the cylinder walls inducing alternating low and high pressures as the pistons move up and down. The resulting cavitation (like hitting the surface with a microscopic ball peen hammer) causes pit holes to develop in the cylinder wall that let cooling fluid leak into the cylinder wall.


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Diesel Engine - TexturingPatent 7146939 – Dec 2006

It is feasible that this same buffering action takes place in compressive stress textured die surfaces with the cooling molten metal absorbing the cavitation shock along with the induced fatigue resistant substrate.

Drawing of the diesel cylinder liner with textured manganese phosphate coating and resultant textured surface. Coating retains coolant which absorbs the cavitation impact energy of the collapsing cavities that damage the liner.


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Propellers and Rudders

The Navy and Marine industry spend billions on the prevention and cure of cavitation damage to ships’ propellers and rudders. Cavitation also affects power requirements for ships to attain their maximum speed.


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Propeller and Rudder Design and Elastomer Paint

Propeller RedesignUSS ARLEIGH BURKE

Class DDG 51) Destroyer

Twisted Rudder Redesign

Belzona 2141& AER 48E Elastomer

Surface


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Pumps and Turbines

sheet cavitation bubbles acting on a centrifugal pump impeller surface

Impeller drawing showing discharge and cavitation location – low pressure side

cavitation at the blade location of a turbine caused this damage Vortex (tip) cavitation at hub of

turbine


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Design and Operation

Operation• Prevent dead head operation(occurs when the pump's discharge is closed either due to a blockage in the line or an inadvertently closed

valve. )

stationary diffuser vanes

rotating impeller

double-volute wall

Pump and turbine design changes to improve flow and reduce the possibility of cavitation

• Prevent suction recirculation(fluid will start to recirculation or move in a reverse direction at the suction and at the discharge)


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Application to HPDC

Cavitation – controlled but not eliminated

Factors to control or consider• Velocity of metal (dam spillway)

• Extreme pressure variations (all)• Buffering the cavitation effect (diesel)• Fatigue resistance (compressive)• Design & Operation parameters (pumps)


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Considerations

• Simulation studies when designing• Better stronger and tougher die materials• Consideration regarding cavitation inception• Gate designs• Gate velocities• Increase the fatigue resistance• Buffering the harmful cavitation bubbles


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Buffer by compressive texture?

After 30,000 shots, there is no evidence of cavitation damage in the form of breakout, pitting, or erosion. Did the compressive texturing buffer cavitation?

macro photo of casting surface shown belowtypical compressive finish for 47-48Rc


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Conclusions and Summary

1. Anything handling liquid is subject to cavitation2. Found in nature, geology, industry and hpdc3. Inertial cavitation (flow and pressure) is destructive4. Occurs when the local pressure falls below vapor pressure5. Has always been assumed that molten metal is to blame6. Very little investigation to date for hpdc cavitation7. There is a need to study its effects to hpdc dies8. Considerations include parameters from design to operation9. You will never totally eliminate cavitation

10. Are there similar phenomena in HPDC??11. Can the cavitation effect be buffered in HPDC?

Engineering

Cavitation effect