OK, I'll take a whack. I'm reading as I go.

Normally, the classical cavitation that people think of is from areas of low pressure - not high. Like on an impeller on a ship that's run too hard for it's design, areas of low pressure form, and either the pressure drops below the vapor pressure of the liquid (creating liquid vapor bubbles) OR the dissolved air comes out in bubbles. Both collapse at some point and cause damage.

Another form that I'm reading about is cavitation due to literally shearing the liquid apart - making it achieve wicked velocities - which then behave like venturis, which create the vacuum and you again have the same as above.

Correct me if I'm wrong and add what you know here....

Need to think some more. Make sense, Juice, you physics nerd?

 

OK, I'll take a whack. I'm reading as I go.

Normally, the classical cavitation that people think of is from areas of low pressure - not high. Like on an impeller on a ship that's run too hard for it's design, areas of low pressure form, and either the pressure drops below the vapor pressure of the liquid (creating liquid vapor bubbles) OR the dissolved air comes out in bubbles. Both collapse at some point and cause damage.

Another form that I'm reading about is cavitation due to literally shearing the liquid apart - making it achieve wicked velocities - which then behave like venturis, which create the vacuum and you again have the same as above.

Correct me if I'm wrong and add what you know here....

Need to think some more. Make sense, Juice, you physics nerd?

I think you are on the right tack!

I honestly don't know a whole bunch...... but this truck has been a boon to understanding how somethings work.

Yes, I was a Physics Nerd.... Calc sucked .... Chem and O Chem were favorites. EEngineering would have been cool, but I have no patience for all the troubleshooting.

but i digress.

Just a dumb person here looking for a smart answer.

 

Well, Juicey Fruit, we gotta work through this-

The CP3 is a plunger pump, right? So on its intake stroke you could have cavitation if not fed with enough head. Lift pump probably solves that one.

The only other area where you would see rapid expansion and/or velocity is in the injector right where the fuel is let out. I think you could see the effects there, if they exist.

Anyone wanna send me a junk injector? I could cut it open, no problem, and get some photos for shizz and giggles.....

 

I guess it is the worry wart in me.

If cavitation can erode our injectors, at what point will the pump become say... only 75%efficient?

will the erosion even take place on the high pressure side?


I'm not trying to get my post count higher, but i might go get a new pump to compare to a used one.

Steve Cole.....
Paging
Steve Cole.....


Any input on this from what you have been playing with?


Anyone

Anyone

Bueller

Bueller

 

Cavitation comes when bubbles collapse completely. Liquid and gas flowing may cause erosion if the velocities are high enough, but not from cavitation or just from the mere existence of the bubbles.

Most cavitation happens like this:

A fluid flows into a pump, maybe a centrifugal pump. Before the pressure rises, it falls a bit first...then goes back up. The drop in pressure comes when the fluid is accelerated into the impellor. If the pressure drops below the bubble point (where the fluid will vaporize), then you get a few little bubbles. No problem so far.

Now these these bubbles flow to the outside of the impellor, with the fluid, and into the region where the pressure rises back up....and eventually up past where it started. Now at the higher pressure, the bubbles will re-collapse. If this recollapse happens when the bubble is at a metal surface (like the impellor itself, or a wall of the casing), then you have a problem.

If the bubble is at a surface, then as it collapses, the fluid around the bubble is moving inwards, to take up the volume that is disappearing. This fluid has momentum. Some is coming in from the left, some from the right, some from above, etc. At the instant the bubble disappears, all the moving fluid colides with itself at a very small place...at a point. The resultant momentum is downwards toward the surface, and there's very little area to spread it over. It's a bit like a shaped charge explosion...focused. The finite momentum, focused on a point on the surface, creates very high pressures at that point, and that's what does the damage. Of course this all happens really fast, in less than a millisecond, so the fluid velocity (and momentum) is quite high.

It's like hitting a nail with a hammer. The hammer has a finite momentum when it hits, and the force is focused on a very small area (the point of the nail).

It can also happen in valves. Pressurized fluid enters the valve, speeds us as the pressure drops. Then in the outlet of the valve, as the fluid slows back down, the pressure will recover a bit, depending on how it slows back down. In the recovery zone, you can get cavitation if the pressure in the throat is below the bubble point and you got some flashing.

Cavitation in positive displacement pumps is rare. There is very little suction pressure drop in the pump, and the rise is slow (in the reference frame of the fluid). If a PD pump is going to cavitate, it's from acceleration drop in the suction line. if it's too long and/or too narrow. This does happen, but not in good designs, and usually only for piston-type PD pumps...not vane or gear pumps, which do not accelerate the fluid much, and have steady flows.

In the injectors (which are valves followed by nozzles), you could get cavitation in the valves, but the pressure downstream of the valve (but before the nozzle) is high, so I doubt the throat pressure is very low. You want to take the DP at the nozzle not in the valve. If the fluid forms bubbles in the nozzle holes, who cares. After the nozzle, it mixes with air and burns, so it will not recollapse. Besides, if there were cavitation, the injectors would not last very long at all.

If the fuel filter gets very restricted, then you might be able to draw the pressure down enough to vaporize the fuel. It would then recondense before entering the rail. This might be a problem, but then I'd bet the engine wouldn't run well as the rail pressure drops once the backpressure regulator goes closed.

The regulator is probably the most likely place for cavitation. The pressure drops from rail pressure down to 0 on the way back to the tank. If the throat pressure could be low enough anywhere, it's there.

You'd probably hear it though..even over engine noise. The pressure spikes do the damage of a hammer and nail and sound like them too. It sounds like there are rocks flowing in the pipe...lots of high frequencies.

 

I've read a study or two now about micro cavitation wearing out the nozzles on the injectors. Seems the roughness of the boring process on such a tiny hole is significant at these flows and pressures. Micro-honing has all but eliminated the earlier problems.

As mentioned above, the injector valve surface is suseptable to accelerated erosion from cavitation, but I have not seen much on this topic for injectors.