Ed wrote:
Ok, Paul
Using the K&W formulas, I
calculate that the 14x10 core area would
handle 120HP at 120MPH TAS, you would need to get to
160 MPH to get the mass
flow to handle 160HP.
Just scaling your
power number on the basis of core area; it comes to about 225hp for my
setup. I designed for 200 hp at 120 mph; so I guess that is reasonable
close correlation. Either we’re both right, or both wrong
J.
Al
Well, for both
your and Paul's sake, I hope we are all correct {:>).
There are enough
uncertainties just in the differences between different types of cooler core
to make that much difference.
Its fairly clear
that air flow and core area are two of the most significant factors as
together they determine the basic Mass flow possible through the cooler.
If that is not sufficient to carry away the heat you are trying to reject then
all else is moot.
In K&W there
are two coefficients they focus on, Kh the heat transfer
coefficient which is dependent on the characteristics of the core,
Kp the pressure drop coefficient - again based on core
characteristics. They end up with a combined coefficient that rates how
well the core transfers heat based on the pressure drop this is Kv =
Kh/Kp. These coefficients are dependent on such things as the
openness ratio, the hydraulic diameter of the individual passage in the core
and the thickenss of the core. Both Kh and Kp
are also weakly dependent on the Reynolds number of the air flow
throught the core. I say weakly because they relate to the inverse
fourth power of the Reynolds number or 1/(Re)^(1/4).
Interestingly enough because it is an inverse relationship the heat transfer
is better to the air with lower velocity through the core. Not quite
certain that I fully understand why, except it appears to do with the shear
force and friction between the air flow in the core passage and the the
passage walls. However, while heat is apparently passed from the core
walls to the air better at the lower Reynolds numbers, the downside is that
low Reynold number also means lower air velocity throught the core which in
turn means Low mass flow which is not good for
cooling.
So from the
equations in K&W that there is an optimum balance between the heat
transfer due to the lower Reynolds effect and the heat removed due to higher
mass flow through the core. Too much of one and not enough of the other and
your cooling is suboptimal.
FWIW
Ed
Anderson