TheSamba.com :: View topic - Fans don't cavitate

Wed Sep 28, 2011 10:11 pm

raygreenwood wrote: jhoefer wrote: wompninja wrote: You're right, they definitely don't "cavitate."

Cavitation: is the formation and then immediate implosion of cavities in a liquid.

It's the local pressure drop behind an object passing through a liquid that causes the liquid to change to gas. You can't cavitate in air because it's already a gas.

And the fan doesn't choke because you'd have to spin the fan to ~20000 RPM to go supersonic.

Yes....you can cause cavitation effects in a gas....but its called turbulence, vortex and/or vacuum...but make no mistake its cavitation nonetheless.
Shear forces can literally strip gas molecules down into components for recombination causing visible instantaneous condensation as well as an energy energy relsease with the change of state. Ray

No, that is not cavitation. The effect is opposite. The pressure and temperature drop simultaneously causing water vapor to condense back into liquid. Cavitation has a very specific definition. As do turbulence or vortexes. Don't conflate them.

It's like saying melting and freezing are the same thing because they happen at the same temperature.

Wed Sep 28, 2011 10:38 pm

RockCrusher wrote: Yes....cavitation is a misnomer for sure. Fans "STALL" not cavitate and the stall occurs when there is enough drive to run into an RPM where the fan simply does not move any more air no matter how fast you run it. In the ACVW it is simply not enough drive traction to spin the fan past a certain point so it slips before it goes into stall. I agree with John that a Serp belt and idler system would improve cooling on stockish engines. You 80mph highway guys could use that.

RC

can you expand a little more on why you guys think this?
Im not calling you out or anything, just want a little more meat as to why it would be better?
I have always been interested in them just never understood the purpose over that of the stock belt system.

Thu Sep 29, 2011 8:40 am

Simply put, cavitation happens in a liquid, think boat propeller and the bubbles it creates. The VW fan pushes air so the word cavitation simply doesn't apply. Sure it's close to what happens but not exactly.

Thu Sep 29, 2011 9:10 am

Quote: I agree with John that a Serp belt and idler system would improve cooling on stockish engines

I disagree that a Serp belt system will improve cooling. It will only eliminate slipping compared to a normal belt drive, so you dont gain more cooling, you only lose less if the belt slips

Thu Sep 29, 2011 9:11 am

so, "losing less cooling" is not improving it?

Thu Sep 29, 2011 9:33 am

the serp belts dont nessarly stop slip, I have sliped plenty of them.if you want one abut want some slip use a spring loaded tensioner with a light spring, or just dont tighten it much(tightening isant real good on the tiny small bearing on the end of the crank either. as for the exploding fans my theroy for what it is worth, is belt slipage, nothing more.then traction&crunch,snatching on the fan,the thinsheetmettel crimped or welded . kinda like side stepping the clutch with big tires, spining the big tires then the big tire grabbing traction= crunch.. it,s a very good article. so do we need the power pully? can we get rid of the whole thing & put a big 16" electric 2100 cfm fan flat on top like a corvaier?? or remove the fan and mount a 12" 1800 cfm fan to the inlet of the oe fan shroud.( I thing a pressure test would tell more than the type of test in the artickle, but I aint no rocket scincetest. somebody got to build&pilot these darn things!!

Thu Sep 29, 2011 3:09 pm

jhoefer wrote: raygreenwood wrote: jhoefer wrote: wompninja wrote: You're right, they definitely don't "cavitate."

Cavitation: is the formation and then immediate implosion of cavities in a liquid.

It's the local pressure drop behind an object passing through a liquid that causes the liquid to change to gas. You can't cavitate in air because it's already a gas.

And the fan doesn't choke because you'd have to spin the fan to ~20000 RPM to go supersonic.

Yes....you can cause cavitation effects in a gas....but its called turbulence, vortex and/or vacuum...but make no mistake its cavitation nonetheless.
Shear forces can literally strip gas molecules down into components for recombination causing visible instantaneous condensation as well as an energy energy relsease with the change of state. Ray

No, that is not cavitation. The effect is opposite. The pressure and temperature drop simultaneously causing water vapor to condense back into liquid. Cavitation has a very specific definition. As do turbulence or vortexes. Don't conflate them.

It's like saying melting and freezing are the same thing because they happen at the same temperature.

No not quite. You are assuming there is a pressure drop because you are assuming a set pressure is involved. The higher the density and pressure on a gas the more it behaves like a liquid. At certain levels....its very hard to tell the two apart due to behavior.

If the fan blade speed is high enough and incoming and outgoing air density is high enough....you get a shearing effect. It actually causes an increase in pressure...at the blade surface. The condensation effect is after the surface and is visible as the water molecules condense in nearby colder air.
The rules are not absolute like the textbook says. It depends on conditions, blade shape, inlet and outlet pressure and speed. Under certain conditions.....fan turbulence in gas acts just like cavitation. It creates the same porblem....slippage. Ray

Thu Sep 29, 2011 3:19 pm

If you want to call an orange an apple because they're both fruit, go ahead.

Thu Sep 29, 2011 4:13 pm

jhoefer wrote: If you want to call an orange an apple because they're both fruit, go ahead.

actually Ray's right -- in conditions where where there ceases to be a distinguishable difference between a liquid and a gas, they are simultaneously both, then really there also ceases to be a difference between what we define as cavitation and what we define as vortices or turbulence. the equations end up being the same.

Thu Sep 29, 2011 4:34 pm

JSMskater wrote: jhoefer wrote: If you want to call an orange an apple because they're both fruit, go ahead.

actually Ray's right -- in conditions where where there ceases to be a distinguishable difference between a liquid and a gas, they are simultaneously both, then really there also ceases to be a difference between what we define as cavitation and what we define as vortices or turbulence. the equations end up being the same. A real smart guy once said "It's all relative" and he was right. Airflow, sound, fluid dyamics, aerodynamics, electronics......all use the same equations in one form or another.

Fri Sep 30, 2011 12:44 am

Bruce wrote: baked beetle wrote: Centrifugal force is a reaction to a centripetal force, and that's why you only need to consider centripetal force.
There's no such thing as centripetal force. It's centripetal acceleration.

:roll:

http://search.yahoo.com/search;_ylt=Aq0hIXILr6fHXZ...=yfp-t-701

Fri Sep 30, 2011 1:39 pm

69 Jim wrote: Bruce wrote: baked beetle wrote: Centrifugal force is a reaction to a centripetal force, and that's why you only need to consider centripetal force.
There's no such thing as centripetal force. It's centripetal acceleration.

:roll:

http://search.yahoo.com/search;_ylt=Aq0hIXILr6fHXZ...=yfp-t-701 so it's a metric type of axcelleration,push the peddel a centameter and go a kilometer faster on your trip???