Cavitation Info

What is Cavitation?

Cavitation refers to the process of rapid growth and collapse of vapour pockets about resident nuclei in a liquid due to reduction in the static pressure below the liquid vapour pressure. Hindrance from cavitation is mainly caused by the violent collapses of the flowing cavitation micro-bubbles within very short time scales, which often leads to vibration and damage of mechanical components. Apart from excessive vibrations, cavitation drastically alters the flow field, reducing the hydraulic efficiency of the affected hydraulic components.
Even if vibration and erosion problems are avoided by design or operation, it is likely that the performance of the systems is sub-optimal because countermeasures by design were needed to prevent cavitation problems.

 

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Cavitation Erosion

A serious problem of cavitation is that it often leads to vibration and damage in critical components, for example, in bearings, diesel engine injectors, marine propellers and rudders, pumps and turbines, and even human artificial heart valves. Cavitation erosion when experienced, normally leads to significant additional repair and maintenance costs or, in extreme cases, component replacement. This, in turn, has implications for plant shutdown or, in the case of ships, drydocking costs and loss of trade. As such, the damage caused by cavitation erosion may cost many millions of pounds annually to rectify and in some medical scenarios can also be life threatening.

 

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Cavitation in Automotive Applications

Cavitation is known to occur in various automotive components, where high fluid velocities and rapid accelerations develop; for example fuel may cavitate in high pressure fuel injection systems, or lubricant in piston rings and bearings. As a result cavitation erosion may accumulate, causing damage and affecting engine durability. Cavitation is also known to alter the composition of Diesel fuel properties. In large Diesel engines it might be possible the coolant fluid to cavitate at cylinder walls, due to vibrations produced by the engine operation.
On the other hand, cavitation is believed to enhance atomization, thus improving combustion and reducing emissions in Diesel engines.

Cavitation in Marine Engineering

Marine propellers systems and ducted thrusters, may exhibit cavitation near the tip of the propeller, where velocities are higher. Water jet propulsion systems are also susceptible to cavitation and its related effects. The occurrence of cavitation on propulsion systems is in general unavoidable and therefore accepted; however, it is important to know the extent where cavitation is not harmful in operation, both in terms of noise and erosion. In order to avoid these phenomena as much as possible, many CFD calculations are made during the design process. RANS-CFD methods have been implemented more than a decade ago for performance calculations. Over the years the range of calculations has been extended from open propeller analyses in uniform flow to complete thruster units in oblique inflow In the coming years such methods will be applied to thruster units and to propellers operating in the wake of a ship hull.

 

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Cavitation in Biomedical Engineering

In the field of bioengineering, life-threatening inception of flow-induced cavitation may form during the operation of cardiopulmonary bypass pumps, artificial hearts, and mechanical heart valves (MHV) resulting in device deterioration, blood damage causing haemolysis and air bubbles flowing within the blood circulation, which include high frequency cerebral micro-embolic events that sometimes are regarded as the cause of cognitive disorders.
Despite the aforementioned effects, that greatly contribute to the negative conception existing around the topic of cavitation, there are applications where cavitation is beneficial. Ultrasound and laser-induced cavitation have been tested as cure methods in lithotripsy and kidney stone removal, drug delivery, cancer therapy and even thrombolysis.

Cavitation in Fluid Machinery

In hydraulic machines, such as pumps and turbines, cavitation may occur in areas where the working fluid is forced to accelerate, forming low pressure zones. Typically this happens at the suction side of the impeller blades. The faster the impeller moves, the higher the fluid acceleration will be, resulting to lower pressures. Also, cavitation may occur in abrupt flow direction changes in non-moving hydraulic machinery, such as in sharp elbow turns, partially closed valves, nozzles, etc.
Operation with cavitation leads to excessive vibrations and noise, increased pressure losses, performance degradation and increased maintenance costs.

 

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Harnessing Cavitation

Ultrasound cavitation is used as the standard method for surface cleaning process of precious metals. Similarly, the ongoing downscaling of functional nano-electronic elements in the semiconductor industry makes it increasingly difficult to clean these devices with a physical cleaning method without introducing damage to these structures. High frequency ultrasound cavitation has been successfully utilised as a more effective cleaning processes. Recently, the utilization of high intensity ultrasound has been successfully tested as a facile, versatile synthetic tool for nano-structured materials that are often unavailable by conventional methods.

 

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