Brad_Blazer
Veteran Member
You guys are starting to get it right. Cavitation occurs when a low pressure area causes the pressure to drop below the saturation pressure (boiling point) of a liquid forming a pocket of vapor. When the resultant bubble moves out of the low pressure area, it collapses or implodes. Since liquids are incompressible and you have momentum filling the void from all directions, the result is a tiny area of extremely high pressure. When this occurs against the wall of a vessel, it will cause erosion. This is very common in pump impellers and in piping systems where the plumber neglects to ream the pipe ends. In a hot system like a cooling system, the water is already close to the saturation point so cavitation is easy to initiate. In a diesel, there is a lot of knocking vibration due to the high pressure combustion. Those vibrations translate into pressure waves in the coolant and may very well cause cavitation, especially adjacent to the cylinder walls where all of that noise is initiated. I have no direct experience with diesels but I do know that cavitation can be a concern in any fluid system that operates near the saturation point or at low pressure (it only takes -15psig to pull a perfect vacuum.)
One way to prevent cavitation is to remove nucleation /forums/images/graemlins/confused.gif sites. A nucleation site is a small crevice that holds a pocket of air. For a bubble to form, it has to overcome surface tension (intermolecular attraction). When the bubble radius is very small, suface tension is very effective at resisting cavitation. (force = pressure x area and area is small) At a nucleation site, the trapped air provides a big enough radius for cavitation to occur. An additive may work by coating or wetting the surfaces to eliminate nucleation sites. As an example, try this experiment: Partially fill a small pan with cooking oil, then add a few drops of water. The oil should be just deep enough to cover the water and prevent a free water surface on top. Put the pan on a hot burner and watch the drops closely. The temperature of the oil and water will increase way past the saturation point, then the water will evaporate explosively causing you to curse /forums/images/graemlins/shocked.gif when your face gets burned. Actually, I did this experiment many times before I had any clue as to the mechanism at work - maybe you have too. /forums/images/graemlins/grin.gif
Hope that makes sense,
Brad
One way to prevent cavitation is to remove nucleation /forums/images/graemlins/confused.gif sites. A nucleation site is a small crevice that holds a pocket of air. For a bubble to form, it has to overcome surface tension (intermolecular attraction). When the bubble radius is very small, suface tension is very effective at resisting cavitation. (force = pressure x area and area is small) At a nucleation site, the trapped air provides a big enough radius for cavitation to occur. An additive may work by coating or wetting the surfaces to eliminate nucleation sites. As an example, try this experiment: Partially fill a small pan with cooking oil, then add a few drops of water. The oil should be just deep enough to cover the water and prevent a free water surface on top. Put the pan on a hot burner and watch the drops closely. The temperature of the oil and water will increase way past the saturation point, then the water will evaporate explosively causing you to curse /forums/images/graemlins/shocked.gif when your face gets burned. Actually, I did this experiment many times before I had any clue as to the mechanism at work - maybe you have too. /forums/images/graemlins/grin.gif
Hope that makes sense,
Brad