Regarding cavitation of liquid film in container mechanical seals?

Regarding cavitation of liquid film in container mechanical seals?

01. Causes of liquid film cavitation

In the middle of the sealing end face of high-speed machines, the microstructure and deformation of the sealing end face cause the flow channel to shrink and expand, which can easily lead to cavitation in the micro gap liquid film.

02. Concept of liquid film cavitation

The process of steam cavitation occurring in the low-pressure region of a liquid flow field. Steam corrosion in seals includes liquid-phase steam corrosion and liquid-phase steam corrosion. There are macroscopic cavitation and microscopic cavitation. When the partial pressure of the protruding surface liquid film drops to the saturation vapor pressure, the vapor nuclei in the liquid are released to produce bubbles, which is known as liquid-phase cavitation; When the liquid itself partially vaporizes and produces bubbles, it is vapor phase cavitation.

03. Application of liquid film cavitation in machine seals

Cavitation has a significant impact on the friction performance (or wetting performance) and service life of machine seals. Domestic and foreign scholars have conducted years of experimental research and theoretical exploration on the cavitation phenomenon of machine seals, and have made favorable progress. However, the theoretical analysis, calculation, and specific product design of machine seals currently mostly ignore cavitation phenomena. This situation has become one of the obstacles to achieving new breakthroughs in high-performance sealing. The generation and cracking of bubbles have a significant impact on the propagation, bearing capacity, and reliability of liquid film pressure.

04. Difference between cavitation and gasification

Cavitation is a physical change mainly affected by partial pressure changes. Unlike liquid film evaporation corrosion, gasification is the situation where the liquid film on the end face of a mechanical seal changes from liquid to liquid phase. This is the result of the combination of boiling and flash evaporation.

05. Establishment process of cavitation in mechanical seals: (macroscopic cavitation)

06. Establishment process of cavitation in confidential seals: (micro cavitation)

Due to the micrometer level flow in the sealed gap, it is no longer usable when the characteristic size of the channel is arbitrarily correlated with the average value of liquid molecules, which changes the relative necessity of damaging various flow forces. In addition, due to its minimal impact on traditional dimensions, the appearance roughness of microchannels is often overlooked, but its influence on microscale flow cannot be ignored. The slight oscillation caused by the roughness of the appearance often affects the flow of the entire mainstream area, which is also an important reason for the cavitation of the sealing micro gap liquid film.

The above figure shows the measurement of irregular ports on the surface of the friction pair stationary ring using a non-contact optical profilometer. The concave and convex protrusions on the surface can be assumed to be quasi cylindrical. When the liquid passes through the sealed end face, a low-pressure area will be formed behind the concave convex surface. When the pressure is less than the saturation pressure, surface cavitation will occur. In addition, microscale flow scale effects, interfacial tension, fluid viscosity, etc. can also affect micro cavitation.

07. Impact of Port Liquid Film Cavitation on on Sealing Performance

The non-contact of the mechanical seal end face depends entirely on the negative pressure effect and dynamic pressure effect of the liquid film. Related studies have shown that due to the resistance of positive and negative pressure, the occurrence of cavitation will not cause the liquid film to lose its load-bearing capacity. Edison, wait. It is believed that cavitation is the main cause of port drilling. In addition, due to the cavitation caused by the liquid film port, it can prevent port leakage, reduce port friction, and achieve cavitation and drag reduction.

But in fast environments, serious cavitation phenomena can occur. When cavitation reaches a certain level, the port loses its opening force, the sealing end face loses its opening force, and even the protruding surface becomes ineffective. As cavitation develops and splits, the protruding surface will experience cavitation, which will then damage the protruding surface.