| Here is another picture of a damaged Mazda water pump.
| I initially thought this was caused by cavitation but
| after seeing Perry's destroyed pump I suspect this
| one was also damaged by running it low on water.
|
| Note: Only the upper half is damaged along a horizontal
| line. Just like Perry's pump. This is from an engine I
| bought in the junk yard.
|
| I am beginning to think this is were the myth of Mazda pump
| cavitation got started.
|
| Paul Lamar
Ron Cass wrote:
cavitation depends upon the local pressure being reduced below the water's
vapor pressure. In lay terms it boils locally. The local depression
disappears in short order and the bubble collapses. Lord Raleigh exponded
upon this at some length and produced som high school arithmetic to give
credence to his claims as to the bubbles method of collaps. His theory was
very handsomely supported by some Russian investigators who devised a bubble
maker/collapser machine and photographed the bubbles collapsing. The very
well documented bubble collapse produces a jet of water across the bubble
which (the jet) can reach extraordinary velocities. To get a handle on the
potential velocities try this thought experiment. Imagine a conical cavity
in a body of water. The cavity is defined by a thin layer of water
surrounding the hole thus you have a thin conical hell of water around a
spac. Now start to drive the outer surface of the shell of water down in
size by uniformly moving all surfaces inwards at right angles to the local
surface area as the shell diminishes in size so the wall thickness increases
and if you take it to the limit which is that the hole disappears and if you
move the surface of the shell inwards at a constant speed you should see
that the shell volume is going to get up to a pretty smart clip. raggedy
bubbles collapse in the same way and being raggedy and having already
unstable spots on their surface readily form jets as a way for the shell of
water to get away from the encroaching pressure. (The bubbles do indeed have
a shell as a result of surface tension) The jets that the Russians observed
cheerfully and quite rapidly punched big chunks out of a lead target.
Now Mazda pumps running short of water hmmmmmm! don't think so. I am much
more inclined to think along the lines of cavitation producing
activitiesthat would not of necessity be present in all Mazdas. First
thought is RPM. My lady fiend never ever reved her engine her ex husband
taught her to lug along in the highest gear that would pull the car. the
pump was pristine when I stripped her engine down. Realize that this is all
theory and that there are too many variables for anyone to make a sweeping
conclusion even after looking at multitudes of samples without having the
history in frantic detail. However a theory is a good place to start
testing. Dunno how much you want to spend on checking out a theory but a
test rig aint so tough. a few bits of angle iron and some donor pumps from
the junk yard yeah I know not too many around , but run one at umpteen
thousand RPM and see if it blasts itself. Cant recommend the number of RPM
as I dont know the pulley ratio or the expected op speed of the motor. But
two or three goes should get some answers.
Just as an aside cavitation can be heard so with some electronic gear and a
small bedded mike and some filters for obvious noise sources there will be a
distinct change in noise as cavitation starts.
Different topic same subject.
A young lad named Frank Whittle developed the equations to describe the flow
of gasses and I believe liquids through a rotating compressor. Both axial
flow and centrifugal. Since the Whittle jet engine had teh latter type of
compressor and it looks much like a good water pump design those equatuions
might help some enterprising soul to define(using CAD!!!!) a good water pump
impeller design
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