Jim Sower wrote:

I need a little help here.  I may have got lost back a ways.  Tell me if

this is

true:
1.  Cavitation happens when the pump can't "suck" (read "accelerate") the

coolant

into the impeller with the flow rate (velocity) that is required by output

flow

rate.

2.  The event under discussion took place with the coolant being drawn

>from and

returned to an open tank with coolant level at some distance below the

impeller.

If these things are so, it occurs to me that in a closed system, the coolant
supply to the pump would not be stagnant as in the experiment, but moving

at the

same flow rate as the output side of the pump, and under steady state

conditions

(no acceleration of the coolant supply) no cavitation would or could occur.

I intuit that the coolant supply to the pump being a stagnant pool rather

than a

flowing, closed stream rendered the experiment bogus at the rpm where the

onset of

cavitation occurred.

Where did I go wrong?           Jim S.
PS finally got my date/time straightened out ;-)

Jeff Spitzer wrote:

Your intuition is correct but the magnitudes may be wrong.  Sucking from a
stagnant source will cause the pressure to be lower at the inlet due to
accelerating the fluid.  If the inlet line is of reasonable size this
should be less than 1 psi though.  You are likely to get more pressure drop
from the fittings to close the loop than from the re-acceleration of the
flow from stagnant.

Jeff Spitzer

Now I'm getting tangled up in intuition.  If pump pumps into block, and line out of
block to radiator is 1.5" and line from rad to pump is 1.5", and coolant is
incompressible, and flow rate is whatever it is, pushing coolant out of the pump and
into block is going to result in the very same volume being pushed  into the pump
inlet.  I intuit that cavitation is a bit of volume with no coolant in it (maybe this
is where I'm going wrong) and occurs when you pump coolant out of the pump into the
block, and there is no coolant coming into the inlet side of the pump.  This seems
impossible to me in a closed system.

Could it be that at the temps we are dealing with, just a little 'suction' or reduced
pressure at the inlet causes the coolant to vaporize locally and behave as if there
was a vacuum on the inlet side?  That sounds like a bit of a leap.  But something
doesn't seem to compute.  I can see the impeller spinning and not pumping anything
out OR drawing anything in, but I can't see coolant pumped out and not being
replaced.
And I thought I had a handle on physics ;-)           Jim S.

I think what happens is bubbles are formed in the fluid and it expands.
This expansion is accommodated by the 16 psi pressure cap blow off valve and
some aerated fluid is forced into the overflow tank. Now we have aerated fluid
at the input to the pump allowing room for cavitation. Just a guess on my part.
 
Paul Lamar
 
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