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).=20 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=20 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.=20 What do you think? Len, My understanding is that multiple pass radiators have pros and = cons (doesn't every thing?).=20 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. =20 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. =20 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 -------------------------------------------------------------------------= ----- See what's new at AOL.com and Make AOL Your Homepage. ------=_NextPart_000_0004_01C80360.1F4B6E40 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable

----- Original Message -----
From:=20 Lehanover@aol.com
To: Rotary motors in = aircraft

Sent: Sunday, September 30, = 2007 11:18=20 AM

Subject: [FlyRotary] Re: = Another cooling=20 question

In a message dated 9/30/2007 10:32:24 A.M. Eastern Daylight Time, = eanderson@carolina.rr.com=20 writes:

Mark, if you really had excess air flowing through your = radiators=20 the coolant would drop more than 4 Deg F. In fact, the more = air flow=20 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=20 pass through too fast and not pick up as much heat. This = didn't make=20 sense to me. Maybe I wasn't listening closely and missed the = point=20 altogether (wouldn't be the first time).

This is one of the oldest = myths around -=20 that air or coolant will flow too fast to pick up the heat. It = just IS=20 NOT factual. The more mass flow you have, the more heat = you=20 will carry away. It appears that some early experimenters = noted=20 that if you slowed the flow of coolant through a radiator that there = was a=20 greater temperature drop of the fluid than if it flowed through=20 faster. better myth. I once had an debate=20

Ed, Could you comment on this?:

On race cars I modify the radiators to be double or triple pass. = By=20 inserting baffles in the tanks, to force the coolant through a fewer = number of=20 tubes, and therefore at a higher velocity. The effect is that the = radiator has=20 1/3 or 1/2 the tube count but the tubes are twice or three times as = long and=20 have exactly the same area exposed the airflow. It has never = failed to=20 work for me.

My thinking is that the flow rate remains very energetic and is = scrubbing=20 the inside of the tubes with gusto, dumping more heat than a slower = flowing=20 coolant.

What do you think?

Len, My understanding is that multiple = pass=20 radiators have pros and cons (doesn't every = thing?).

I agree with your assessment, the higher = velocity,=20 greater turbulence and multiple passess all contribute to getting more = heat=20 transfered from coolant throught the metal to the air. = While you=20 have the same area (in your case) , you are creating more interaction = between=20 coolant and metal by as well as increasing the velocity of the = mass=20 through the core.

The only down sides that I can think of off = the top of=20 my head is that there more resistance/back pressure results which = in turn=20 takes more engine power to overcome. As the coolant = looses heat to=20 the air along the longer tube length, the deltaT will be=20 decreasing which means at the end of the longer tube the delta T = will be=20 less - reducing heat transfer at that point. Also, the = increased=20 back pressure might make the pump more prone to cavitation at a lower=20 rpm.

Here is an extract from the Steward Warner = Radiator tech=20 section:

Double pass radiators require 16x more pressure to flow the = same=20 volume of coolant through them, as compared to a single pass radiator. = Triple=20 pass radiators require 64x more pressure to maintain the same volume.=20 Automotive water pumps are a centrifugal design, not positive = displacement, so=20 with a double pass radiator, the pressure is doubled and flow is = reduced by=20 approximately 33%. Modern radiator designs, using wide/thin cross = sections=20 tubes, seldom benefit from multiple pass configurations. The decrease = in flow=20 caused by multiple passes offsets any benefits of a high-flow water=20 pump.

But, if you have the pumping power to force more = coolant=20 through a multipass radiator then there appears to be=20 benefits. Also since you have reduced the number of tubes, = I doubt=20 the figures cite by Steward Warner apply to the extent indicated = - but,=20 there is undoubted some increase in resistance and at the = cost of=20 more power from the engine. However, if the additional cooling = capacity=20 permits the engine to develop even more power then the net gain is in = your=20 favor.

However, the end result is what tells the story as = to which=20 factors play a dominant role. I am always fascinated about how = the=20 various factors interact and counteract one another. = Very=20 small differences between installations appears to make a major = difference in=20 effectiveness.

Gee, this is fun

See what's new at AOL.com=20 and Make AOL Your=20 Homepage.

------=_NextPart_000_0004_01C80360.1F4B6E40--