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