There are three dimensionless numbers that may be referred to as the cavitation number in various scenarios: the cavitation number for hydrodynamic cavitation, the Thoma number for cavitation in pumps, and the Garcia-Atance number for ultrasonic cavitation.
The cavitation number (Ca) can be used to predict hydrodynamic cavitation. It has a similar structure as the Euler number, but a different meaning and use:
The cavitation number expresses the relationship between the difference of a local absolute pressure from the vapor pressure and the kinetic energy per volume, and is used to characterize the potential of the flow to cavitate.
It is defined as [1]
where
The cavitation number serves as one of the primary methods for characterizing cavitation within a fluidic system. Low cavitation number indicates a higher probability of cavitation, while high cavitation number indicates no cavitation.
Within a fluid conduit, a pipe or a constraint, as the upstream pressure rises, so does the velocity of the working fluid. However, it's important to note that the increase rate of the square of the velocity greatly surpasses that of the pressure increase. Consequently, the cavitation number exhibits a declining trend with increasing upstream pressure, that is the situation where the system approaches the cavitation.
The inception of cavitation occurs when cavitating bubbles first appear within the system, marking the inception cavitation number. This value represents the highest cavitation number observed within the system in presence of cavitation. Researchers often aim to record cavitation inception at relatively low upstream pressures, particularly when they are pursuing non-destructive applications of this phenomenon.
As the development of cavitating flow progresses, the cavitation number steadily decreases until the system reaches the point of supercavitation, characterized by the highest achievable velocity and flowrate. Lower cavitation numbers are indicative of more intense cavitating flow.
Following supercavitation, the system reaches its fluid-handling limit, even as upstream pressure continues to rise. Consequently, the measured cavitation number embarks on an upward trajectory. This trend is a recurring observation in numerous published articles within the literature. [2]
The Thoma number () is a dimensionless quantity that can be used to predict cavitation in the suction of a pump. It is defined as [3]
Where is the net positive suction head and is the hydraulic head developed by the pump. The fluid will cavitate in the suction of the pump if the Thoma number is smaller than the critical cavitation parameter or the critical Thoma number defined as
Where is the net positive suction head required to prevent cavitation. It is a parameter found experimentally for each pump model.
Cavitation in fluid mechanics and engineering normally refers to the phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, called "bubbles" or "voids", collapse and can generate shock waves that may damage machinery. These shock waves are strong when they are very close to the imploded bubble, but rapidly weaken as they propagate away from the implosion. Cavitation is a significant cause of wear in some engineering contexts. Collapsing voids that implode near to a metal surface cause cyclic stress through repeated implosion. This results in surface fatigue of the metal, causing a type of wear also called "cavitation". The most common examples of this kind of wear are to pump impellers, and bends where a sudden change in the direction of liquid occurs. Cavitation is usually divided into two classes of behavior: inertial cavitation and non-inertial cavitation.
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