----- Original Message -----
Sent: Sunday, September 30, 2007 11:18
AM
Subject: [FlyRotary] Re: Another cooling
question
Mark, if you really had excess air flowing through your radiators
the coolant would drop more than 4 Deg F. In fact, the more air flow
the more coolant Delta T you would drop through the radiator.
<snip>
That's exactly what I HAD thought, until I was told that the air could
pass through too fast and not pick up as much heat. This didn't make
sense to me. Maybe I wasn't listening closely and missed the point
altogether (wouldn't be the first time).
This is one of the oldest myths around -
that air or coolant will flow too fast to pick up the heat. It just IS
NOT factual. The more mass flow you have, the more heat you
will carry away. It appears that some early experimenters noted
that if you slowed the flow of coolant through a radiator that there was a
greater temperature drop of the fluid than if it flowed through
faster. better myth. I once had an debate
Ed, Could you comment on this?:
On race cars I modify the radiators to be double or triple pass. By
inserting baffles in the tanks, to force the coolant through a fewer number of
tubes, and therefore at a higher velocity. The effect is that the radiator has
1/3 or 1/2 the tube count but the tubes are twice or three times as long and
have exactly the same area exposed the airflow. It has never failed to
work for me.
My thinking is that the flow rate remains very energetic and is scrubbing
the inside of the tubes with gusto, dumping more heat than a slower flowing
coolant.
What do you think?
Len, My understanding is that multiple pass
radiators have pros and cons (doesn't every thing?).
I agree with your assessment, the higher velocity,
greater turbulence and multiple passess all contribute to getting more heat
transfered from coolant throught the metal to the air. While you
have the same area (in your case) , you are creating more interaction between
coolant and metal by as well as increasing the velocity of the mass
through the core.
The only down sides that I can think of off the top of
my head is that there more resistance/back pressure results which in turn
takes more engine power to overcome. As the coolant looses heat to
the air along the longer tube length, the deltaT will be
decreasing which means at the end of the longer tube the delta T will be
less - reducing heat transfer at that point. Also, the increased
back pressure might make the pump more prone to cavitation at a lower
rpm.
Here is an extract from the Steward Warner Radiator tech
section:
Double pass radiators require 16x more pressure to flow the same
volume of coolant through them, as compared to a single pass radiator. Triple
pass radiators require 64x more pressure to maintain the same volume.
Automotive water pumps are a centrifugal design, not positive displacement, so
with a double pass radiator, the pressure is doubled and flow is reduced by
approximately 33%. Modern radiator designs, using wide/thin cross sections
tubes, seldom benefit from multiple pass configurations. The decrease in flow
caused by multiple passes offsets any benefits of a high-flow water
pump.
But, if you have the pumping power to force more coolant
through a multipass radiator then there appears to be
benefits. Also since you have reduced the number of tubes, I doubt
the figures cite by Steward Warner apply to the extent indicated - but,
there is undoubted some increase in resistance and at the cost of
more power from the engine. However, if the additional cooling capacity
permits the engine to develop even more power then the net gain is in your
favor.
However, the end result is what tells the story as to which
factors play a dominant role. I am always fascinated about how the
various factors interact and counteract one another. Very
small differences between installations appears to make a major difference in
effectiveness.
Gee, this is fun
Ed