Only for those who (Dave? Bill?, Rusty?.....) wish to
punish themselves with more minutia on air flow in
ducts/diffusers.
A month or two ago, I posted some slides extracted from
a university study/course on the effects of airflow separation inside a
duct. This involved the boundary layer which appears to act somewhat
different inside the constraining walls of a duct as compared to its free
flow across an airfoil. The cause of flow separation in the duct
being the pressure build up by the expansion in duct area which led
to two counteracting forces.
The pressure build up actually helps the boundary layer
stay attached to curving duct wall - for a time. But,
this same pressure that helps "push" the boundary layer against the duct wall
also slows down the boundary layer which ultimately leads to flow
separation and reversal.
This "understanding" led me to my "pinched duct"
design to accelerate the boundary layer and cause it to penetrate further into
the higher pressure area before separation. I also inferred that
this effect was what made the Streamline duct of K&W so
effective. No claim was made that the pinched ducts were anywhere nearly
as effective as the Streamline duct, but were an attempt to meet a space
constraint.
Most attempts to use the streamline duct in a space too small involves
truncating the duct from the inlet end. However, while this does tend to
preserve some of its effectiveness, if the distance is very short (like my 3
-6 inches) the large expose core area likely increases cooling drag
considerably. So I decided to keep the inlet small (unlike what
truncating the streamline duct would have resulted in) but to pinch it down to
keep the boundary layer velocity high resulting (hopefully) in further
penetration down the duct before flow separation occurred.
Some questioned my interpretation (always a smart thing
to do, as I only had one short course in aerodynamics as an Electrical
Engineering student - so my attention was probably not as keenly
focused as it should have been {:>)). In any case, I went
looking for the source document so that any interested could read it and draw
their own conclusion.
The original source for this material was http://www.me.dal.ca/site2/courses/mech3300/5_Separation.ppt. However, they have (as Universities
frequently do) apparently rotated material presented and removed this
briefing from their website. This leaves the .html portion I saved
when first reading the presentation which is attached.
I was only partially successful in providing the source
- in that I found the original script that went with the slide
presentation - but unfortunately the slides are not present with it. I
do have a number of the slides I had previously extracted (used in my
presentation) but since they could be "tainted" by my "explanation" of the
slides, I will not present them.
I reviewed the script again and still believe my interpretation is
correct, but others should have the opportunity to decide for
themselves. We do that sort of thing on this list.
But, if you do decide differently, please don't tell my pinched
ducts {:>)
Ed