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Pressure Differential in heat Exchangers



 
 
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  #1  
Old July 2nd 03, 06:09 AM
Bruce A. Frank
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Default Pressure Differential in heat Exchangers



Morgans wrote:
Why not use a little electric fan to draw air past the radiator for extended
ground operations, like cars do?


It has been done. Bayard DuPont did it on the rear engine in his Defiant
and it worked well. Others who tried it found problems with ram air in
flight spinning the fans and wearing out the bearings.

I'll get into this a bit more. A primary consideration to get enough air
flow is to locate the radiator exhaust air opening in a low pressure
area. It has proven virtually impossible to push air through a radiator.
When there is no low pressure area into which the radiator exhaust air
can flow, regardless of the size of the intake opening, the engine
doesn't cool.

An advantage to the radiator in the cowling is that prop wash will
provide enough air movement to cool the engine during extended holds.
Bayard found that he never had cooling problems with the Defiant's front
engine once he had created a lip on the bottom firewall edge of the
cowl. The lip on the trailing edge of the cowl creating the opening
through which the radiator air exhausted dropped the pressure enough to
"suck" the air through the radiator. Bayard has sat for as long as 30
minutes with no over heating.

A little bit about drag through the radiator. The air going through the
radiator has to slow down a lot in the duct from the cowl opening to the
radiator. This requires some consideration of the divergence of the duct
from the intake opening to the radiator. In a perfect world proper
divergence of the walls of the duct should be about 7 degrees. In
practical application up to 15 degrees works well. The air expands and
slows and the air pressure differential from the front side to the back
side of the radiator is what gets the air through. The duct should then
converge to re-accelerate the air before it dumps into the slipstream
again.

To create a mental image, too large an intake opening sort of tries to
gulp too big a chunk of air. The plenum created by the divergent duct
can't flow that much and the radiator acts like a closed window because
it cannot pass that high speed air. The air spills back out of the
intake opening creating swirls and burbles like a cup under a wide open
faucet...or like trying to drink out of a garden hose shot straight from
the nozzle into your mouth (you know, you try to pucker you lips to slow
the flow to keep it from squirting out of your nose). This is where the
high drag is created.

The route through the radiator is a relatively easy one, even making a
right angle turn through the fins, because it is a pressure thing rather
than an air speed thing easily navigated by the air that has traded its
velocity for pressure through the correctly divergent plenum/duct.

Hanging a radiator out in the slip stream works poorly except on the
slowest planes. The air just piles up in front of it and spills around
it. Even a short straight wall duct leading into and out of such an air
dam will improve the efficiency of the radiator.

Bruce A. Frank, Editor "Ford 3.8/4.2L Engine and V-6 STOL
Homebuilt Aircraft Newsletter"
| Publishing interesting material|
| on all aspects of alternative |
| engines and homebuilt aircraft.|
*------------------------------**----*
\(-o-)/ AIRCRAFT PROJECTS CO.
\___/ Manufacturing parts & pieces
/ \ for homebuilt aircraft,
0 0 TIG welding

While trying to find the time to finish mine.

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  #2  
Old July 2nd 03, 05:51 PM
Corky Scott
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Default

On Wed, 02 Jul 2003 05:09:00 GMT, "Bruce A. Frank"
wrote:


I'll get into this a bit more.

Bruce A. Frank, Editor "Ford 3.8/4.2L Engine and V-6 STOL
Homebuilt Aircraft Newsletter"
| Publishing interesting material|
| on all aspects of alternative |
| engines and homebuilt aircraft.|
*------------------------------**----*
\(-o-)/ AIRCRAFT PROJECTS CO.
\___/ Manufacturing parts & pieces
/ \ for homebuilt aircraft,
0 0 TIG welding

While trying to find the time to finish mine.


Me too. I've got a video at home that was produced by Jerry
Schweitzer. He used to build Ford V-6's for people to use for auto
conversions, don't know if he does that anymore. He had two airplanes
that were Ford powered, a Pacer and an RV-4.

The video I have is about the Pacer installation. In it, the radiator
is mounted flat underneath the engine. It's a pretty big sized
radiator. The air is drawn through the stock openings in the chin
cowl, passes by the PSRU, which is good because it needs to have
cooling air going over it. Then the air has to get around the engine
and the hot exhaust pipes and THEN pass through the radiator and
remove heat from it's fins.

But we aren't done yet. After that convoluted journey, the air then
has to bend 90 degrees to head out the exit, which is just a hole in
the bottom of the cowling underneath the firewall. The hot air exit
had a lip in the front of the opening to promote a low pressure area
right around the opening.

Despite all the problems with this layout, it not only worked, it
worked well.

Corky Scott
  #3  
Old July 3rd 03, 12:24 AM
Bruce A. Frank
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Default

And several more who have just sealed the radiator to the bottom of the
cowl. Cut a hole in the cowling for the radiator exhaust air to exit;
placed a lip at the front edge or several rear facing louvers over the
opening and it cools....even bringing with it all that hot air radiating
from the engine exhaust pipes (though temps are higher than setups using
a dedicated duct system).

Many different styles of radiator arrangement have worked...radiators
behind the engine, under the engine, above the engine, even in front of
the engine...the common element is exhausting that hot radiator air into
a low pressure area. I have talked to builders who were absolutely vexed
and ready to abandon the project. In most cases simple installation of a
2" lip on the leading edge of the exit opening completely solved the
problems.

Let me add that the dedicated duct offers a better opportunity to
control temps and drag, but many arrangements have worked exceedingly
well...even radiators mounted behind the baggage compartment (Blanton's
original design with the V-6 STOL)

Corky Scott wrote:
Me too. I've got a video at home that was produced by Jerry
Schweitzer. He used to build Ford V-6's for people to use for auto
conversions, don't know if he does that anymore. He had two airplanes
that were Ford powered, a Pacer and an RV-4.

The video I have is about the Pacer installation. In it, the radiator
is mounted flat underneath the engine. It's a pretty big sized
radiator. The air is drawn through the stock openings in the chin
cowl, passes by the PSRU, which is good because it needs to have
cooling air going over it. Then the air has to get around the engine
and the hot exhaust pipes and THEN pass through the radiator and
remove heat from it's fins.

But we aren't done yet. After that convoluted journey, the air then
has to bend 90 degrees to head out the exit, which is just a hole in
the bottom of the cowling underneath the firewall. The hot air exit
had a lip in the front of the opening to promote a low pressure area
right around the opening.

Despite all the problems with this layout, it not only worked, it
worked well.

Corky Scott



--
Bruce A. Frank, Editor "Ford 3.8/4.2L Engine and V-6 STOL
Homebuilt Aircraft Newsletter"
| Publishing interesting material|
| on all aspects of alternative |
| engines and homebuilt aircraft.|
*------------------------------**----*
\(-o-)/ AIRCRAFT PROJECTS CO.
\___/ Manufacturing parts & pieces
/ \ for homebuilt aircraft,
0 0 TIG welding

While trying to find the time to finish mine.

  #4  
Old July 3rd 03, 04:50 AM
Bruce A. Frank
external usenet poster
 
Posts: n/a
Default

The fans that wear out seem not able to take the thrust loading of being
spun by the air load, but as I said, the one in Bayard's Defiant had no
problems. You may be correct that those other systems had too high air
velocity.

Morgans wrote:
"Bruce A. Frank" wrote in message
...


Morgans wrote:

Why not use a little electric fan to draw air past the radiator for


extended

ground operations, like cars do?


It has been done. Bayard DuPont did it on the rear engine in his Defiant
and it worked well. Others who tried it found problems with ram air in
flight spinning the fans and wearing out the bearings.



Seems to me that if the fan was spinning so fast that it is wearing out
bearings, the air speed past the radiator is waaaaaay to high. Design the
inlet (read small and divergent ducts) so that it keeps the air speed past
the radiator low at cruise speed, and then plan on using the fan for ground
operations, and climb, if need be.

As far as pushing air, I know that it needs to be drawn through to an area
of lower pressure; that's what the fan on the back side of the radiator is
all about.

See any flaws in my reasoning? BOb, I know what you have to say; that the
idea of using anything other than a lycosarus is a flaw! ;-)



--
Bruce A. Frank, Editor "Ford 3.8/4.2L Engine and V-6 STOL
Homebuilt Aircraft Newsletter"
| Publishing interesting material|
| on all aspects of alternative |
| engines and homebuilt aircraft.|

  #5  
Old July 3rd 03, 05:18 AM
Bruce A. Frank
external usenet poster
 
Posts: n/a
Default

The sides of the duct angle away, diverge from the intake opening, at an
angle of 7 degrees. A round duct would assume a section conical shape
with the walls tilted 7 degrees from the centerline. The problem is that
the usual distance needed for the duct to expand from the intake
opening to the size of the radiator takes more length than is available
in the typical prop to firewall distance.

With a square or rectangular cross section duct the sides would diverge
at 7 degrees relative to the centerline. What works is left and right
sides diverging, or top and bottom diverging or both top, bottom and
sides diverging. Most installations expand the duct more rapidly because
one is moving from an in intake opening of about 55 sq. inches to a
radiator of 400 to 500 sq. inches in a distance of only 18 to 24 inches
(this if aprox for a 200 hp installation).

There is more to this. The opening of the intake, if nearly flush to the
nose bowl surface, should have rounded edges to split the air stream as
an airfoil would with smooth flow both down the throat and along the
cowling. An intake that sticks forward of the surface of the nose bowl
can also have a rounded leading edge, but relatively sharp sheet metal
edges seem to work well also...spliting the air stream before the
interference of the surface of the nose bowl.

There have been articles over the years, I believe, in both Sport
Aviation and Kitplanes on air duct divergence and leading edge shape.
but I cannot point you to a specific article.

Ernest Christley wrote:
Bruce A. Frank wrote:

from the intake opening to the radiator. In a perfect world proper
divergence of the walls of the duct should be about 7 degrees. In
practical application up to 15 degrees works well. The air expands and



Very informative post, Bruce. Thank you. But could you explain this?
I've heard this 7 degree figure knocked around a couple of times, but
have no idea what it refers to. Does it mean that if I have a square
opening in the XY plane and a line extending through the middle of it in
the Z plane, then an 'extrusion' of the square will form a box that
moves away from the line at a rate of 7 degrees?

Then the biggest question is, how does this correspond to the rounded
intake plenums that I keep seeing?




--
Bruce A. Frank, Editor "Ford 3.8/4.2L Engine and V-6 STOL
Homebuilt Aircraft Newsletter"
| Publishing interesting material|
| on all aspects of alternative |
| engines and homebuilt aircraft.|
*------------------------------**----*
\(-o-)/ AIRCRAFT PROJECTS CO.
\___/ Manufacturing parts & pieces
/ \ for homebuilt aircraft,
0 0 TIG welding

While trying to find the time to finish mine.

 




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