Okay,
searched my email storage (power to gmail!!) and found everything you sent ( and I cared to archive - just about all)
IF all the pics are still current:
a) I would split lengthwise to separate the oil-cooler from the radiator, rather then across.
b) Make sure there is a big radius duct from the upper side of the intake opening to the upper end of the radiator.
c) Make sure there is no stray eddies coming from the spinner opening (don't see wether you have a sealed duct or whether you use the cowling as a duct - for all I can see your spinner might syphone air out the spinner opening...)
d) round off the lower fire-wall edge. You are not using any exit ducting, but dump the air into the engine"room"
e) Have a look at using your exhaust for some augmenting. It seems to be fairly close to the duct exit.
Extending the exit according to some augemntation rules might be the ticket in connection with d)
d) would be the first thing I'd go after: bend some alu, cut out any openings needed and clamp in place temporary.
You might even get away with a 4" PVC-tube cut in half or 3/4 - for a trial. Depending on the width of your exit opening, the round part has only to be over the exit, but I would try that also coming from the sides, if the exit is not across the whole width.
See pics
TJ
PS: Be warned, all my "wisdom" is harvested from this site and the other one....
All theory, nothing flying yet ( at least nothing I built!:))
On 8/7/07, Thomas Jakits <rotary.thjakits@gmail.com
> wrote:
Going with gut-feelings:
Dennis, you mentioned your inlet duct represents a wedge.
I believe (other expression for gut-feeling :)), a streamline duct redirects the airflow to about 90º towards the radiator surface (never mind the fin/cooling tube orientation).
So you might want to contact that PL guy on that other list and have him make you a custom 3D drawing for your situation.
It should not be too much work to make a duct according to streamline philosophy - cheaper than than a new custom radiator anytime!
(Keep your intake opening and distance and have Mr. 3D-Rhino make a coordinate list from which you can cut foam for a duct .....)
I did not study deep enough into the cooling theory, but I understand that the incoming air has to be slowed substantially to make the cooling system efficient (less drag). At some point it will break flow and become turbulent/stagnant - the trick seems to be (or so my gut tells me) to get this point as close or right at the face of the radiator - here it can do its magic (take all the heat out of the radiator) and return to orderly flow right after the cooler.
Hoe does your exit (duct) look, maybe we can squeeze some more delta-P from there.....
IF you go the PL route you might as well ask for the "perfect exit" solution, too.
Did you post pics in the past?
TJ
On 8/7/07, Ed Anderson < eanderson@carolina.rr.com
> wrote:
I agree, Tracy.
Our flow is without doubt never laminar, a boundary layer of laminar flow has only(mostly?) the molecules next to the metal absorbing significant heat. Those molecules in the middle of the stream have no(little) opportunity to pick up heat. A boundary layer with turbulence on the other hand has molecules shuffling all over the place and every one (most?) get an opportunity to contact the hot metal and take away some of the heat. So turbulent flow is better for conducting away heat.
Chaotic macro flow (boundary layer folding over itself, eddies, etc) on the other hand impedes pressure recovery, increases drag and overall adversely effects cooling.
My research and experiments leads me to believe that many factors are relative minor compared to the large scale adverse effect of poor duct design which leads to early boundary layer separation. However, significant macro turbulence at the entrance to the core channels might have a large effect - just speculation on my part.
Ed
----- Original Message -----
Sent: Tuesday, August 07, 2007 11:03 AM
Subject: [FlyRotary] Re: RV -7A Cooling Update 8/6/07
No scientific analysis here, just my sum total of gut feel after reading & experimenting.
In my understanding, the airflow through the common rads we use is fully turbulent. the little louvers in the fins are there to guarantee this. So what difference does it make whether the air goes turbulent at the leading edge of the fins of a dozen or so thousandts later. Again according to my very fallible gut feel, the whole story is whether or not you converted the air velocity to air pressure. Either you did or you didn't. I can't remember if you have measured pressure at the face of the rad or not but that will tell the whole story. Angle of tubes, fins, face of rad, etc is all relatively insignificant.
The motorcycle rad stuff someone mentioned is not a good indicator. They do not depend on high pressure recovery the way we do so the design and operation of their rads is not necessarily applicapable.
As always, YMMV and I will gladly amend my gut feel to match reality if you find it is wrong.
Tracy
On 8/6/07, Dennis Haverlah <clouduster@austin.rr.com
> wrote:
I've been busy with Family vacation, dealing with the exceptional wet weather in
central Texas and my tennis playing but finally I have some more
thoughts on radiators and cooling. My cooling is marginal for Texas in
the summer. I want to climb at 120 kts and 26 + inches MP on a 100 deg
F day without exceeding 215 on water and oil.
I have the Griffin radiator (core 19 X 13 X 2.5 inches) and stock RX-7
'89 oil cooler as shown on pictures I have previously posted. The
radiators are mounted under the engine at about a 30 deg. angle. My
latest test flight with OAT of 92 deg F on the ground was encouraging.
I had temp. probes on the outlet side of the oil and water radiators
to measure the temp. of the heated air. The temp. probes had an upper
limit of 160 deg. F. The air exiting the water radiator exceeded the
160 Deg. limit soon after take-off. I estimate the air temperature
rise through the water radiator was at least 80-90 deg. Cooling water
temp. never exceeded 210 deg. F.
The air exiting the oil radiator was at 135 - 140 deg. F. (A delta T of
about 40 - 45 deg F.) Oil temperature rose to 213 deg. F. max and at
cruse 24 in. MP, 160 mph at 5500 feet the oil temp. decreased to 210 deg. F.
I'm close to ideal cooling but I've been surprised how little effect my air
flow modifications have have had on overall oil and water cooling. After
studying K&W Chapter 12 some more I've decided I mounted my cooling radiators
incorrectly!! As mentioned above, the radiators are below the engine at about
a 30 Deg angle (alpha = 60 deg.) to the incoming air stream. The tanks are
orientated fore and aft. This positions the fins across the air stream.
Ch. 12.2 of K & W Fig. 12.6 shows a radiator block at an oblique angle (alpha)
to the incoming air. The tubes are at the angle alpha to the flow. In the
K & W analysis the tubes are slightly aerodynamic in shape they turn the flow
as it enters the radiator fins. In the radiators I am using the tubes are
separated about 1/2 inch. My fins are separated by about 0.080 inch. Because
I mounted my radiator with the tanks fore and aft, the fins are at the angle
alpha to the flow and the fins turn the air. The fins are very sharp thin metal
and I believe air flow separation and turbulence is occurring at the leading
edge of each fin. Because the fins are very close together the flow is restricted
through the entire radiator surface. I believe the separated, turbulent flow at
the leading edge of the fins limits the amount of air flowing through the
radiator regardless of how "good" the diffusers are ahead of the radiators.
If I have to do it over, I will defiantly mount my radiators with the tanks on the left
and right side of the incoming air so that the tubes turn the air through alpha - not
the fins!!
Any comments - Am I out to lunch on this one?
PS. The end of the first paragraph in Ch. 12.2. states "We shall consider first the
simple case of parallel inflow at an angle alpha to the tubes, as shown in Fig. 12.6"
I have not found a consideration in Chapter 12 of the case of the fins being at
an angle alpha.
Dennis Haverlah
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