Cavitation

According to Cavitation meaning “It is a process in which the liquid's static pressure falls below its vapour pressure, resulting in the creation of small vapour-filled cavities in the liquid”. 

These cavities, known as "bubbles" or "voids," collapse under higher pressure and can produce shock waves that can harm machinery. These shock waves are powerful when they are close to the imploded bubble, but they weaken rapidly as they travel away from it.

Define Cavitation in Physics

• In Physics there are two cavitations inertial cavitation and non-inertial cavitation. 

• According to Cavitation meaning caused due to inertia “The phenomenon of a void or bubble in a liquid rapidly collapsing and creating a shock wave is known as the inertial cavitation”.

• Inertial cavitation occurs as the bubble diameter expands to at least twice its original diameter over a single acoustic pressure period. The bubble then violently explodes due to the fluid's inertia, potentially fragmenting into several smaller bubbles.

• Local deposition of energy, such as an intense centred laser pulse or an electrical discharge through a spark, is another way to generate inertial cavitation voids. 

• Vapour gases from the surrounding medium evaporate into the cavity, resulting in a low-pressure vapour bubble rather than a vacuum. 

• If the conditions that caused the bubble to form no longer exist, such as when the bubble travels downstream, the surrounding liquid starts to implode due to its higher pressure, accumulating inertia as it moves inward.

• As the bubble eventually bursts, the surrounding liquid's inertia causes the vapour’s pressure and temperature to rise dramatically.

• The bubble gradually collapses to a fraction of its original size, at which point the gas inside dissipates into the surrounding liquid, releasing a large amount of energy in the form of an acoustic shock wave and visible light. 

• The temperature of the vapour inside the bubble can be several thousand kelvins, and the pressure several hundred atmospheres at the point of complete collapse.

• In the presence of an acoustic field, inertial cavitation may also occur. Due to an applied acoustic field, microscopic gas bubbles that are commonly found in liquids would be forced to oscillate.

• If the acoustic strength is high enough, the bubbles can increase in size before quickly collapsing. As a result, even if the rarefaction in the liquid is inadequate for a Rayleigh-like void to form, inertial cavitation will occur.

• For the treatment of surfaces, liquids, and slurries, high-power ultrasonics typically depends on the inertial cavitation of microscopic vacuum bubbles.

• The physical mechanism that causes cavitation to form is close to that of boiling. The thermodynamic paths that precede the creation of the vapour are the most significant difference between the two.

• Boiling occurs when the liquid's local temperature approaches saturation and additional heat is applied to allow the liquid to phase change enough into a gas. 

• Cavitation begins when the local pressure falls far enough below the saturated vapour pressure, which is determined by the liquid's tensile strength at a given temperature.

• Non-inertial cavitation occurs when small bubbles in a liquid are induced to oscillate in the presence of an acoustic field when the amplitude of the acoustic field is inadequate to induce complete bubble collapse. This form of cavitation causes much less erosion than inertial cavitation, and it's often used to clean fragile materials like silicon wafers.

Cavitation in Fluid Mechanics

• The cavitation in fluid mechanics is known as hydrodynamic cavitation.

• The process of vaporisation, bubble formation, and bubble implosion that occurs in a flowing liquid as a result of a decrease and subsequent increase in local pressure is known as hydrodynamic cavitation. 

• Hydrodynamic cavitation occurs only when the local pressure falls below the saturated vapour pressure of the liquid and then rises above it. 

• Flashing is said to have happened when the recovery pressure does not exceed the vapour pressure. 

• Hydrodynamic cavitation usually occurs in pipe systems as a result of an increase in kinetic energy or an increase in pipe elevation.

• Passing a liquid through a constricted channel at a given flow velocity or mechanical rotation of an object through a liquid may also cause hydrodynamic cavitation. 

• The combination of pressure and kinetic energy will create the hydrodynamic cavitation cavern downstream of the local constriction, creating high energy cavitation bubbles, in the case of the constricted channel and based on the particular or special geometry of the system.

• As a hydrodynamic cavitation flow progresses, various flow patterns are detected: inception, developed flow, supercavitation, and choked flow.

• The first time the gas phase occurs in the system is called inception. This is the system's weakest cavitating flow, which corresponds to the highest cavitation amount.

• Established flow is recorded as the cavities in the orifice or venturi structures expand and become larger in size.

• Supercavitation is the most extreme cavitating flow, in which all of an orifice's nozzle region is potentially filled with gas bubbles. The lowest cavitation number in a system corresponds to this flow regime. 

• The system is no longer capable of passing further flow after supercavitation. As a result, velocity does not change as upstream pressure rises. This will result in a higher cavitation number, indicating that a choked flow has occurred.

• For a brief period of time, the process of bubble generation, followed by the growth and collapse of the cavitation bubbles, results in extremely high energy densities, as well as extremely high local temperatures and pressures at the surface of the bubbles. As a result, the total liquid medium environment remains at ambient levels. 

• Cavitation is harmful when it is uncontrolled but by regulating the flow of the cavitation, the power can be harnessed and the damage avoided.

• Since free radicals are formed as vapours trapped in cavitating bubbles dissociate, controlled cavitation can be used to enhance chemical reactions or propagate some unexpected reactions.

• Cavitation is said to be widely produced using orifices and venturi meters. Because of its smooth converging and diverging parts, a venturi meter has an inherent advantage over an orifice in that it can produce a higher flow velocity at the throat for a given pressure drop through it. An orifice, on the other hand, has the advantage of being able to accommodate a greater number of holes in a given pipe cross-sectional area.

• Some industrial processes will benefit from hydrodynamic cavitation. In dry milling facilities, for example, cavitated corn slurry produces higher yields in ethanol production than uncavitated corn slurry.

• Since free radicals are produced in the process due to the dissociation of vapours trapped in the cavitating bubbles, this is often used in the mineralization of bio-refractory compounds that would otherwise require extremely high temperature and pressure conditions. This results in either the intensification of the chemical reaction or the propagation of certain reactions that would otherwise be impossible to propagate.

Advantages or Applications of Cavitation

• Cavitation is often used in manufacturing to homogenise, or blend and break down, suspended particles in a colloidal liquid compound like paint mixtures or milk.

• Cavitating water purification devices have also been developed, allowing contaminants and organic molecules to be broken down by the extreme conditions of cavitation.

• Underwater, cavitation attracts hydrophobic chemicals by forcing them to join together due to the pressure differential between the bubbles and the liquid water. It's possible that this effect may help with protein folding.

• In shock wave lithotripsy, cavitation plays a significant role in the destruction of kidney stones.

• Cavitation is useful for non-thermal, non-invasive tissue fractionation in the treatment of a number of diseases, and it can also be used to open the blood-brain barrier and improve drug absorption in the brain.

• High-intensity focused ultrasound (HIFU), a thermal non-invasive cancer treatment tool, uses cavitation as well.

• Ultrasound is often used to promote bone formation.

• The collapse of cavitation in the synovial fluid within the joint is thought to cause the sound of cracking knuckles.

• Cavitation has enough strength in industrial cleaning applications to withstand particle-to-substrate adhesion forces, loosening pollutants.

• Pasteurization of eggs has been done using cavitation. A rotor with holes creates cavitation bubbles, which heat the liquid from inside. Cavitation strength can be modified, allowing the process to be fine-tuned for minimal protein damage.

Disadvantages of Cavitation Process

• Cavitation is an unpleasant phenomenon in many situations. Cavitation causes a lot of noise, component damage, vibrations, and a loss of efficiency in devices like propellers and pumps.

• Cavitation on the blade surface of tidal stream turbines has been a source of concern in the renewable energy industry.

• When cavitation bubbles burst, they compress energetic liquid into very small sizes, resulting in hot spots and shock waves, all of which are disruptive. 

• Cavitation noise is a particular issue for military submarines because it increases the likelihood of being detected by passive sonar.

• Despite the fact that the collapse of a small cavity is a low-energy occurrence, highly localised collapses can erode metals like steel over time.

• The pitting caused by the collapse of cavities causes a lot of wear on components and can drastically reduce the life of a propeller or pump.

• When a surface is first affected by cavitation, it appears to erode at a faster rate. The cavitation pits increase fluid turbulence and create crevices that serve as nucleation sites for more cavitation bubbles. The pits also increase the surface area of the components and leave residual stresses behind. This increases the surface's susceptibility to stress corrosion.

• When water flows over a dam spillway, the defects on the surface create small areas of flow separation in a high-speed flow, lowering the pressure in these areas. If the flow velocities are high enough, the pressure can drop below the water's local vapour pressure, resulting in the formation of vapour bubbles. When these bubbles are carried downstream into a high-pressure environment, they collapse, resulting in high pressures and the possibility of cavitation damage.

• Due to high compression and undersized cylinder walls, cavitation occurs in some larger diesel engines. Vibrations of the cylinder wall cause the coolant against the cylinder wall to have alternating low and high pressures. Pitting of the cylinder wall occurs as a result of this, allowing cooling fluid to leak into the cylinder and combustion gases to leak into the coolant.

In this article, we studied what is cavitation, the types of cavitation, the advantages of cavitation and the disadvantages of cavitation.

Conclusion

Cavitation is undesirable because it causes substantial corrosion of the spinning blades, as well as additional noise from knocking and vibrations, as well as a major loss of productivity due to flow pattern distortion. As the pressure of the liquid is reduced to its vapour pressure, the cavities form, expand as the pressure is reduced along with the flow, and unexpectedly collapse as they enter higher pressure regions. Extreme pressures are created by the rapid growth and collapse of these vapour cavities, which pit the metal surfaces exposed to the cavitating liquid.