Bob Darrah wrote:

My point was centrifugal pumps can be stalled by too much back
pressure and cavitate> Paul Lamar

Paul, I need an education here.  I thought  a free-flowing pump was more

apt

to cavitate and cavitation could be stopped by increased back pressure.
Help me out here.

Bob Darrah

My calculus teacher taught me to to always look at the limits :-)
Obviously if you block the outlet of centrifugal pump the the
blades will stall just like a turbo charger compressor stalls
when it encounters too much restriction. The water just goes
around and around in the pump. Technically this may not be called
cavitation but calling it cavitation makes sense to me.

I think cavitation is usually thought of when the centrifugal
pump has too much head to pump the water up from the supply.
In that case then a slight pressure increase on the outlet
side MIGHT help... up to a point. At least Tracy makes this
claim on his artesian well water pump.

If it occurs in a closed system such as a cooling system
I suspect an increase in overall system pressure would accomplish
the same thing without reducing the flow such as would be provided
by an artificial restriction on the output side. In fact any additional
restriction anywhere in the system will worsen the cavitation
as it is closed loop system.

In Bill's case he is operating at atmospheric pressure so he can measure
the flow with a stop watch and a five gallon bucket so the system
pressure increase is not an option.

Paul Lamar

My logic teacher taught me that anything is correct as long as definitions
are arbitrary.
When discussing cavitation, I define it as when the fluid being pumped goes
from liquid to vapor on the low pressure side of the pump impeller blades.
If anyone here can offer a universally accepted definition, it would help
communications a lot.

For my part, I agree with Bob's statement.

Tracy Crook
tcrook@rotaryaviation.com

OK lets agree to call it "stalling" in the future. The end result in
a closed loop systems such as an engine cooling system is the same.
The water stops moving. If the water stops moving from inlet problems
it is "cavitation". If it stops moving because of outlet restrictions
it is "stalling". 

But wait! Inlet and outlet  are connected together
through the block and radiator and water is incompressible.
Is the block and radiator on the inlet side or are they on
the outlet side? 

Will the heat added (Bill mentioned by the water 
re circulating in the pump) and from the block, lower the vapor 
pressure of the water, increase the pressure in the system, cause 
cavitation and pump fluid over board through the pressure cap?

It seems to be well known that the Mazda pump does indeed caviatate
at high RPM.

Perhaps Bill can answer these questions with his setup as it adds
no heat to the water from the block. On the other hand perhaps Bill 
can immerse an evap core in boiling oil and simulate the actual cooling 
system to a degree.

Paul Lamar
 
The AirCraft Rotary Engine NewsLetter.  Powered by Linux.
ACRE NL web site. http://home.earthlink.net/~rotaryeng/            
Copyright 1998-2002 All world wide rights reserved.