Pressure Differential in heat Exchangers

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"
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