Paul

 I am about to order a custom dual pass radiator when I read this page
 included from Stewart Components.
 Everything made sense about newer designs having more flow and higher
 turbulence etc. until I got to the bottom of the page where it made the
 claim that dual pass radiators like you suggest  reduce the water pump
 flow about 33% and double the water pressure. These figures seem a
 little high to me and I would like to invite your comments.

 For my 300hp engine  I have room to install one 3" thick radiator,  with
 dimensions 12x26 inches.  Total 936 cubic inches.  The alternative is a
 U-shape dual pass unit with wetted surface 12X24 1.5 thick with 1.5 air
 gap in between.  864 cu. inches.

 According to your book 'How to Cool a Rotary', the dual pass radiator
 would be more efficient in heat transfer both because of: 1. The  higher
 delta Ts experience in each heat exchanger and 2. the first 1/4 inch
 does 90% of the cooling rule.

 You never mentioned the fact about reduction in water flow or increase in
 water pressure with a dual pass, which might lower the heat transfer
 efficiency if turbulence is lost.

 In a single thick 3" radiator if there are (old style) more than one row
 of tubes then of course the ones in the rear are  going to receive
 hotter air and thus have a lower delta T. This is an obvious
 disadvantage.  Below it implies new radiator designs have tubes that are
 very thin but wide.  Three inches wide?  As long as there is adequate
 turbulence within them water will be forced to contact the outer walls.
 That is the theory at least and I don't know what the maximum width a
 tube can be.

 On the other hand a dual pass radiator will have one half the cross
 sectional area of the water passages but double their length.

 Which is more efficient?

 Doug in Japan

        " Tech Tip #5 - Radiators & External Plumbing

 Radiators
 Thicker radiators do have slightly more airflow resistance than thinner
 radiators but the difference is minimal. A 4" radiator has only
 approximately 10% more airflow resistance than a 2" radiator.

 In past years, hot rodders and racers would sometimes install a thicker
 radiator and actually notice decreased cooling. They erroneously came to
 the conclusion that the air could not flow adequately through the thick
 radiator, and therefore became fully heat-saturated before exiting the
 rear of the radiator core. The actual explanation for the decreased
 cooling was not the air flow, but the coolant flow. The older radiators
 used the narrow tube design with larger cross section. Coolant must flow
 through a radiator tube at a velocity adequate to create turbulence.

 The turbulence allows the water in the center of the tube to be forced
 against the outside of the tube, which allows for better thermal
 transfer between the coolant and the tube surface. The coolant velocity
 actually decreases, and subsequently its ability to create the required
 turbulence, in direct relation to the increase in thickness. If the
 thickness of the core is doubled, the coolant velocity is halved. Modern
 radiators, using wide tubes and less cross section area, require less
 velocity to achieve optimum thermal transfer. The older radiators
 benefited from baffling inside the tanks and forcing the coolant through
 a serpentine configuration. This increased velocity and thus the
 required turbulence was restored.

 Radiators with a higher number of fins will cool better than a comparable
 radiator with less fins, assuming it is clean. However, a higher fin
 count is very difficult to keep clean. Determining the best compromise
 depends on the actual conditions of operation.

 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.

 Cross flow radiators are superior to upright radiators because the
 radiator cap is positioned on the low pressure (suction) side of the
 system. This prevents the pressure created by a high-flow water pump
 from forcing coolant past the radiator cap at high RPM. As mentioned in
 the radiator cap section, an upright radiator should be equipped with
 radiator cap with the highest pressure rating recommended by the
 manufacturer. The system will still force coolant past the cap at
 sustained high RPM. "

 --------------------------------------------------------------

 We have to be clear about what we are talking about here.
 Cross counter flow or dual pass.
 Cross counter flow is the first figure. Cross counter flow
 is more efficient than dual pass or single pass due to the first
 half doing 3/4 of the cooling.

 Dual pass is the second and 3rd pictures. I recommend the dual pass
 only because it makes the plumbing simpler for a typical
 aircraft installation. Both inlet and outlet are on
 the back end of the rad. It does however increase the water
 velocity at the expense of consuming more HP from the pump.

 Yes to be sure the cross counter flow is also a dual pass.

 Much of what they say on the Stewart Warner web site is correct.
 However I don't buy the 16X increase in water pressure required. If that
 were true dual pass rads would not work at all. VW would never
 have used one in the Scirocco. I also doubt the 10% more
 airflow resistance than a rad half as thick. Easy to test
 however. Stick both out the side door of a van mounted on a lever
 and measure the forces.

 As far as I know there are no rads available with 3 inch wide tubes.
 Usually the tubes are about one inch wide and they come in rows. One row
 two rows or three rows. Therefore a cross counter flow rad is
 more efficient with one row in front and one row in the rear
 than a non cross counter flow with the same over all thickness.
 Most automotive rads are now one row with large frontal area.

 As Kays and London pointed out however increasing the HP
 on the pump is outweighed by increasing the water flow velocity
 increasing heat transfer and reducing the cooling drag.
 Kays and London is the bible so I would rather believe them.

 Quote:

 "One disadvantage in the use of highly compact surface geometries
 is that the resulting core shapes are characterized by large flow
 frontal area and short flow lengths for the gas-flow path."

 What they are implying here in the use of the term "compact surface
 geometries" Is the most cooling for the smallest core volume.
 In other words the most efficient rad in terms of heat exchanger volume
 is a large frontal area with a thin core.

 In most cars the rad is placed upright between the grill and the engine.
 If you want more cooling your only choice is to use a thicker
 rad. The designers of the RX8 fully understood what Kays
 and London were saying so they mounted the rad at a 45 degree
 angle allowing a thinner rad with a larger frontal area.
 Stewart Warner is mostly concerned with dealing with most cars
 and trucks.

 Kays and London again.

 "The design of a heat exchanger involves a consideration of both
 the heat transfer rates between the fluids and the mechanical
 pumping power expended to overcome fluid friction and move the
 fluids through the heat exchanger. [both air and water] For a heat
 exchanger operating with high-density fluids, [water] the friction-power
 expenditure is generally small relative to the heat transfer
 rate, with the result that the [water] friction-power expenditure is
 seldom of controlling influence."

 "However, for low-density fluids, such as gases, it is very easy
 to expend as much mechanical energy in overcoming friction power
 [aerodynamic drag] as is transferred as heat."

 For an example a 10 pound increase in drag at 300 fps (205 MPH) is
 5.45 net HP out of the prop. With a 80% eff. prop that
 is 6.8 HP more from the engine. While the total HP on the water
 pump is a max of about 3 HP.

 Total drag HP with 2 square foot equivalent
 flat plate area is dynamic pressure 109 pounds per square foot
 at 300 FPS times 2 or 64,400 pound feet per second or 225 HP net
 out of the prop. 281 engine HP. Reducing the cooling drag pays
 off big time compared to 1 or even a 2 HP increase in water pump
 power.

 I highly recommend getting a copy of the Kays and London book and
 reading the whole thing. It is very comprehensive. Hard to understand
 for me but worth it. I had to read it and then re read it several times :)
 It is a complicated subject.

 -------------------------------------------------------------------

 I just had an idea of how you can test a two pass rad for water flow
 by using your municipal water supply. Most municipal water supplies
 have a fixed pressure around 50 psi or lower which is near ideal. I
 have a pressure gage and regulator on my house water supply as if
 I don't the pressure is more like 150 psi and all the toilet valves
 start leaking. The water reservoir is not far away and the top level is
 about 300 or 400 feet above my house in the hills just on the other
 side of a ridge.

 Hook up the water supply to both junk rads 2 pass and not 2 pass
 and time the amount of water coming out. For a quick and dirty test
 you could use two lengths of Home Depot small copper tubing. One
 twice as long as the other.

 Another way of doing it with lower pressure is use a 10 gallon
 container on a stand.

 Water weighs 65 pounds per cubic foot. That means 12 inches of water
 is .45 psi. So to get 4.5 psi the container needs to be 10 feet high.

 BTW don't confuse this dynamic pressure due to the efforts of the
 water pump as being the same as the static pressure in the cooling
 system. The static pressure in the cooling system can be as high
 as 50 psi, depending on the cap pressure setting, due to heat expansion
 while the pump dynamic pressure can be much lower. Continental
 liquid cooled Voyager engines used a 50 psi pressure cap.

 Whether the water pump cavitates or not I suspect is a function
 of the static pressure as well as the pressure drop across the pump.

 BTW a 3 inch wide rad tube would blow up like a balloon with 50 psi.
 That is the fundamental reason rad tubes are only one inch wide.
 We actually had that happen with a new wide tube design oil cooler from
 a well known  company that makes and sells oil coolers. When
 they were shown the blown up oil cooler tubes they then changed the design
 of the wide aluminum tubes by extruding the tubes with walls every .5 inch
 or so
 to keep the tubes from blowing up. The fins in a rad also help
 to prevent the tubes from expanding.

 Here is Bill Schertz test set up and the results of his test.
 The static pressure on the system was atmospheric more or less.


 Paul Lamar


Paul,

Assuming you had room under the cowl, which arrangement do you think would
work best, assuming the same total core area, one 2.5" thick crossflow
radiator, one 2.5" thick dual-pass radiator,
or two 1" cross-counter-flow radiators arranged in a wedge configuration?

Mark S.
Austin

Definitely the one inch thick cores in a wedge for the same core volume.

I am not sure what you mean by:
"Two 1" cross-counter-flow radiators arranged in a wedge configuration?"

Unless you can find a set of .5 inch wide core tubes to build a one inch
thick cross counter flow rad I don't think that is possible.

What is the volume of the rad core you have now? Have you taken
any water manometer pressure data on the front and back.

IMHO I think you should flatten the angle and come in over
the top of the core with an insulated wedge diffuser. That will prevent
any heat from the exhaust affecting the cooling as only cold air will
be going through the rad and out the bottom of the cowl in louvers or
no louvers. Try it with no louvers first. You can get louvers
at Home Depot called wall vents. They come in different sizes.

This company has 18" by 24" aluminum.
http://www.airvent.com/professional/products/staticVents-wall.shtml

The myth is you don't want to do that because the bottom of the cowl
has higher static pressure.  The fact is the dynamic pressure in the scoop
outweighs the low static pressure on the bottom of the cowl.
This can be verified with a few water manometer test on your
current configuration.

BTW Mark I have a set of thin less than an inch cores  from a Dodge Neon
to build a cross counter flow rad.  I will ship them to you if you promise
to build and test them in a cross counter flow configuration.

If you tig weld them up for me and ship them back I can test them.

Paul Lamar