Because at higher speeds, the fan is spitting the flow
out at its tips, creating turbulent disarray in the
air stream and does not evenly distribute the airflow
across the face of the radiator(a good shroud will
REALLY REALLY help this - we made one for Paul
Fitzsimmons at Blackhawk last year out of stuff lying
around and it dropped his temps by 30F). When you
talk about the thermodynamic "size" of a radiator, you
talk about its "UA", the overall heat transfer
coefficient times the effective area. If you don't
use the entire face, the effective area decreases, UA
decreases, stuff gets hotter. Crash referenced
Reynold's number before -it's a dimensionless quantity
- Reynold's Number is density * velocity * effective
diameter divided by the viscosity of the air. At
Reynold's numbers much greater than 3000, flow
transitions from laminar to turbulent. For laminar
flow, the pressure drop is essentially linear, twice
the flow results in twice the pressure drop. Or
conversely, twice the pressure drop results in half
the flow. For turbulent it is a square function -
twice the flow is 4 times the pressure drop, or
conversely twice the pressure drop results in 1/4 of
the flow. That's the biggest benefit of having good
inlet and outlet shrouds with the fan behind the
radiator, which, as we know IS NEARLY IMPOSSIBLE TO DO
IDEALLY!!!!
--- "Robert E. Shlafer" <PilotRob@webtv.net> wrote:
> Then someone, please...
>
> .....explain the racer who finds himself running
> hotter at higher
> forward speeds
> and cooler at lower forward speeds,
> all while racing around the track
=====
Ron Soave
"And another thing.....when I gun the motor, I want the whole world to think
it's coming to an end."
- Homer Simpson
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