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

Paul,

I would love to do it like your drawing shows.  But what your drawing is
missing is the landing gear.  I used the stock Lancair bed mount which
incorporates the nose gear directly beneath the engine as you can see in
your pictures.  In its present location the radiator is within 1/4" of the
front barry mounts, so I can't raise it any.  Otherwise, it sounds like a
great way to go.  I'm considering relocating the radiator fore/aft down the
right/plugs side.  I have room for a pretty large radiator in that
location.  Using either a dual-pass or a cross-counter-flow radiator would
keep the line lengths to a minimum and save some weight too.  From looking
at a picture of the Dodge Neon radiator, it shows the tanks running along
the long sides, opposite of what I need in order to locate them down the
right side.  I had visualized orienting them like your pictured depicted,
i.e. with the tanks located fore and aft.

Don't get me wrong here, my installation is cooling OK, considering the 100+
degree temps we're seeing in Texas this summer (high today of 105*).
However, it could use a bit of improvement.  Also, it would be nice to
reduce the cooling drag in the process.

The big driver here is in preparation for the higher horsepower
anticipated from the P-ported 20B that I have planned for the near
future.  I need about 25% improvement to stay even with where I am at now.
Otherwise, I'll have to reduce power and/or step-climb to keep the
temps within liimits, or install a water-mister system.

Thanks,
Mark S.
Austin

Since you are going to redo the intake for a p-port with more HP the right
side looks like the right way to do it. That picture does not
look like the cores I have. I'll some pictures today.

Paul Lamar