Sent: Wednesday, December 24, 2003 7:38
PM
Subject: [FlyRotary] seven degree
divergence?
I keep seeing reference to 7 degree duct
divergence, and the carpenter in me can't figure out how you place a typically
straight-sided protractor against a curve to measure it. anyone
know? seems to me something is missing, such as "per inch" so that the
curve can be broken down into a series of small triangles.
Kevin,
In some early studies on ducts,
experimentation showed that if the straight walls (in this case not curved)
of a duct did not diverge more than 7 Deg from the intake to the cooler
core that losses would be at a minimum. Apparently 7 Deg was the magic
angle (I have seen references that indicated you might get away with as
much as 10 Deg) which if you did not exceed would provide flow with
no boundary layer separation.
It is not 7 Deg per inch ( that would make
it much more useable for aircraft), but 7 deg total. So as you might
imagine with only 7Deg if you core were any size at all the inlet would have
to be many inches away. Probably OK for wind tunnels
{:>)
That is one reason why attempts were made to
find low loss diffusion configurations that provided for shorter ducts.
I personally have never seen the original study, but I have seen
it referenced in many NACA studies on radiators, diffusers and
ducts.
Ed Anderson