Hi Ed, good to see you on the list.
Unincumbered by engineering education when it comes to flows,
restrictions and heat transfer I'm free to ponder the subject.
I keep thinking about a garden hose laying in the sun for a while.
Initially you get hot water, but after a while it gets cold again
when you let it run. So I think a number of factors hide in the
"cp" factor below.
How many of the water/oil molecules get in contact with the walls
(engine and cooler) must be a factor. Thus the turbulators in the
Mazda oil cooler. Time must also be a factor, at least in terms of
letting each molecule move to the wall.
So even if increased speed of flow may not hurt, it seems counter
intuitive to make water/oil flow so fast that heat transfer does
not take place.
There must be some kind of bell curve where you have a fluid speed
for optimum heat transfer. Actually that curve must be a composite
of conditions in the engine and of conditions in the cooler. An
additional factor for the cooler is how well the heat is
transferred from cooler to air. A cold wall should make heat
transfer from fluid to air go that faster?
So what kind of instrumentation (measuring points) would enable us
to optimize speed of fluid flow?
Finn
On 6/24/2018 4:36 PM, Ed Anderson
eanderson@carolina.rr.com wrote:
You said it well, Ernest
What you want is heat removal from the engine. Slowing flow
down through a radiator will indeed show a larger Delta T from
intake to outlet because the coolant(in the radiator) is exposed
to cooling air longer. That has led many to believe erroneously
that slow flow = more heat removal. I once argued an hour with
old man Lou Ross about this issue and when I told him that the
obvious conclusion was that if slowed water cooled better, then
stopped water would cool best – there was silence on the other
end of the phone and then Lou hung up and never spoke to me
again.
Part of the myth also stems the results when attempts were
made to improve flow rate by speeding up the coolant pump
expecting better cooling. When worst cooling occurred, it was
concluded erroneously that the faster flow resulted in worst
cooling. In most if not all of those instances, the poor cooling
resulted from less flow – the faster water pump was actually
cavitating and therefore actually pumping less coolant than at
slower rotating speeds where cavitation did not occur. But, it
all fed into the myth.
Heat duty (Q): Heat duty is defined as the product of mass
flow
rate specific heat capacity and the temperature difference
between inlet and outlet fluid temperatures
Q = m*cp* DT
A rule of thumb regarding heat removal and flow rate is:
The heat
transfer coefficient decreases by ˜10% with a threefold
increase in the mass flow rate
So a 10% decrease in transfer resulting from three times the
mass flow shows that increased mass flow (in of itself) will
result in increased heat removal even though the heat transfer
rate may lessen slightly.
At some point you get pressure losses and other factors - not
to mention the greatly increased power required to get the large
increase in mass flow - makes it impractical to infinitely
increase flow rate. Once you get the flow good enough to cool
your engine under whatever the conditions you are operating
within, it makes little sense to waste power to get more flow
However, we want best heat removal from the engine. Heat is
removed via mass flow of the liquid – no mass flow = no
cooling. So the more mass flow(within reason) - the more heat
is removed from the engine and provided we can get rid of a
certain amount of that heat though the radiator the more cooling
of the engine occurs. So it’s a system, the cooler the oil
returned to the engine the better heat transfer, the more mass
flow from engine to radiator the more cooling, the cooler the
air flowing across the radiator the better the heat rejection,
etc. All factors contribute and you can not focus on just one
factor, the optimum cooling results from the optimization of all
the major variables involved for a particular situation.
Just my 0.02 Back to my cave
Ed