Paul,

    Thanks for your book it's an interesting read.
    I have put together a quick layout of how I would like to do my belly
    mounted radiator.  The attached pictures I hope explain what I am
  trying to  accomplish.  You mention a separation of about 2" from
  the belly for boundary layer  separation, does this have to extent
  the entire length of the radiator or is  the way I show it ok?  I
  didn't see anything that gave me a frontal opening size
relationship  to  the area of the radiator, is there a percentage
rule of thumb  for this?  Any other suggestions on what I'm showing
to improve  would be appreciated.

    Randy

  Excellent illustrations Randy. Reduces the frontal area over the
  splitter plate idea and is far more cosmetic. A Canard P51.
  Is that Solid Works or Rhino 3D?
  You will still need the Kays & London wedge inlet diffuser page 36.
  Also you will need to take the bottom panel off as it will
  retard the hot air flow out from the front back side of the
  rad page 46. If you just lower the panel it will add frontal
  area. Not good. Just do away with it and let the hot air
  dump into that air stream.
  I would use Venetian blind cowl flaps page 51.
  I found the dims on the Neon rads I have. Here they are.
  The rule of thumb is 20% of the frontal area of the rad.
  With the P51 style scoop you are committed :) I dislike
  commitments all my old girlfriends used to tell me :)
  Paul Lamar

  I see you have shown two radiators.  Are you going to use
  both of them somehow?
  Doug in Japan.

  Yes. I was going to build a cross counter flow  rad (book  page 25
  )  for testing but that is now far down on the priority list as
  Perry Mick  has already tested it and it works well. If you are
  pressed for  space it is worth it.
  Paul Lamar
  --

  Paul

  For years I was excited about that cross counter flow design you
  introduced and actually did drawing to incorporate it under my
  engine. That is until I read a comment last year on Stewart
  Component websites claiming the such setup increase the back
  pressure  and therefore flow significantly.

  http://www.stewartcomponents.com/tech_tips/Tech_Tips_5.htm

  I don't believe the longer effective water passages it will
increase  pressure 16 times but it will be significantly more than double.
   This got me thinking about some to the water flow challenges I
have  had in my line of work.  In essence, when the  water channels
lengthen or  restrictions are increased,  the effect of back
 pressure on flow will rise in a  non linear fashion.

  In a water heat exchanger, high velocity enhances turbulence, thus
  wall contact time.  Velocity is upper limited by restriction and
  internal pump fluid slippage.Engine pumps are centrifugal
   devices hence produce highest flow when the system has the  least
  restriction.  Said another way, maintain as high as water flow as
  possible through the engine.  To get  300hp out of the 13B engine
  I'll need,  according to Jim at Racing Beat, the man who's been
  there, at least  40 gals a minute water flow. Choke the flow
  anywhere in the system and pumping rate falls off as water pressure
  goes up. Knowingly increasing the water pressure may not be a good
  thing for thin wall items tending to corrode over time.

  For comparison purposes oil heat exchangers have positive
  displacement pumps, have to squirt  a viscous liquid through
  inherently small openings  and thus have a slower flowing, robust
  heat exchanger in need of built-in turbulators and must be able to
  withstand high pressures..  Many oil heat exchangers are designed
to  be cross flow because the characteristics of wall contact time
are  enhanced by the slower flow. Oil  by nature as you know is less
 thermal conductive than water.

  This discussion basically boils down to whether it is more prudent
  to have a single thick core thick radiator or two thinner ones in a
  reverse crossed mode RCM like you have shown.Actually I have
  found that when the air gap between the two radiators in the
reverse  cross flow is taken into account, the total installed
package is  thicker than a single thicker unit.

  Paul your thoughts that  proper air flow through heat exchanger is
   paramount and on that I wholeheartedly agree.  I'm not so sure the
  old rule of the first 1/4 of the radiator does 50% of the cooling
is  correct anymore however.  That would depend on fin count air
 resistance, tube design, number of  rows and how well the unit is
 maintained.  A clogged heat exchanger is as good as the dirt it
 catches. Dave Garber showed me a very compact  design unit he had
 made that used two rows of  the newer  overside elongated tubes.
 These are about 1.5 in" in length but  essentially the same
 standard width. The Dave Garber design installed package thickness
came out to about 2.5 inches.  compared to close to 4 inch thick for
 a similar RCM design.  That is assuming a one inch air gap between
the heat exchanges  helps isolate infrared heat transmission.

  In Dave's design, air flow had to traverse about three inches to
get  through it though.  The tubes were angled as was the heat exchanger
  so air flow was actually parallel to the fins and that might be a
  deciding factor in the high performance he experienced.So high
  that he had to cover up the first few inches of it as you can see
by  the photo. His  heat exchanger wetted area is about 12 inches
tall  by 28 inches long.  .  Thats my 8cm x 11cm (3.1/8" x  4 3/8") wallet
  in the foreground for scale purposes.

  I will try an identical system but lay it on its side under the
  engine and make it 16" wide (max 20" with the side tanks included)
  by 19" or 20"  in depth.  I 'm not sure how to angle or curved the
  side tanks to help insure even flow distribution though.  Logic
  dictates at center inlet and outlets with the tanks sides curving
  out toward the ends. Understanding how fluids  flow is hard to
  predict other than it takes the path of least resistance.

  Doug in Japan

  Well apparently Stewart did not read Kays & London's bible Compact
  Heat Exchangers  :)

  ==========================================================================
  "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. For a heat exchanger operating
  with high-density fluids, the friction-power expenditure is
  generally small relative to the heat transfer rate, with the result
  that the friction-power expenditure is seldom of controlling influence."

  [What they are saying here is the HP consumed by the water pump is
  of minor importance. In fact the Mazda pump only consumes about 3 HP.]

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

  [What is being said here is HP is consumed by cooling drag. Since
  cooling drag can be as high as 30% of over all drag the HP could be
  as much as 30% of 200 HP or 60 HP. As you can see cooling drag
  becomes a very important issue.]

  "And it should be remembered that in most thermal power systems
  mechanical energy is worth 4 to 10 times as much as its equivalent
  in heat."

  [Here they are referring to the heat balance of most internal
  combustion engines that only derive about 25% to 10% of the heat
  energy in a gallon of fuel as mechanical HP.]

  ==========================================================================

  Builders have done some really dumb things like use electric water
pumps  with 1/10 the HP and got away with it. The early Mazda pumps,
many people  still use, have a restricter in them. Mazda woke up
with the RX8 pump.

  You need my book or Kays & London's book Doug :)

  Paul Lamar

  Doug,
  As you stated, water will flow the path of least resistance. If
you put both  inlet and outlet in the middle of the respective tanks,
 the flow will want to  pass through the center few tubes. The
correction would be to put a difuser in  one (inlet) tank to force
some flow to the edges. More weight and restriction.

  It would be better to put connections on opposite sides and the
flow will even  out on it's own. Geothermal installs do this by
hooking up the loops so the  closest to the supply is connected the
farthest from the return on the  respective headers.

  Years ago, Caterpillar radiators had the tubes angled slightly so
that the air  passing between two tubes in one row was directed at
the side of a tube in  the row behind. The tubes were only angled
something like 10 degrees. The  problem they had is with trapping
dirt and debris. In that application it is  hard to design to shed
the dirt. AC applications should not have a big problem  with that.

  Dale Davies

Dale

Thanks.  So if I understand you correctly the design is like  what
is found on lots of air heat exchangers.

For instance two wedged shaped tanks that are reversed in
orientation on either side of the heat exchanger.  The  wide end on
each would contain the inlet or exit for the fluid. If  I understand
correctly, the hottest (less dense) water should be brought to the
farthest end of the heat exchanger.That works  for me as the coolest
denser liquid would end up right next to the diffusion tank/ and
water pump inlet.

Doug in Japan



Hi Doug,

First, I must let you know I am not an engineer. I am an auto, heavy duty and
instrument mechanic. That said, I do look at and file stuff in my department
of useless info for future reclassification.

That is what I was getting at. Which end gets the hot less dense fluid
probably has a minimal effect on the system. The hot fluid should be less
viscous and have less resistance to flow. The longer the piping the more
friction with the pipe wall. If the flow is laminar, the less the wall effects
the resistance to flow. I doubt we can rely on laminar flow.

Automotive radiators have straight tanks with the connections on opposite
sides. They seem to work well. They are not concerned with weight to the
extent aircraft builders are. For AC the tapered tanks save a little material
which is weight. Down side is the slight extra cost of construction. Paul may
have some useful input. I will have to study his book when I get it. I do hope
this is good food for thought.

Dale Davies