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  #71  
Old 05-13-2019, 07:59 PM
M McGraw's Avatar
M McGraw M McGraw is online now
 
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Default NACA Vent

Thank you for the information.

My CHTs run in the very low 300s but my OIL temps run around 185-195. I want to throttle the mass flow in exchange for higher CHTs, but I need to do so without increasing the oil temps.

Im considering a NACA vent on the flat side of my lower cowl with a 4 SCAT to the oil cooler. That SCAT would be a straight shot of only a few inches length as opposed to the current SCAT which is 18 length with a large flow direction change.

Below the cooler I will need a nozzle to exit the flow outside the lower cowl. That would separate the oil cooling air from the cylinder heads.

That is my plan when I have time to take the plane down for a few weeks. In the meantime, I like your diffuser and I will try one soon.
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  #72  
Old 05-13-2019, 09:13 PM
BillL BillL is offline
 
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Marvin, Check these photos out. They may help your thoughts on separating the cooler circuit.

https://photos.app.goo.gl/jEy8cvbiqohdprd98

Let me know if the link does not work. OK - Now, try again.

Bill

EDIT #2 - Marvin, search VAF for Alan Judy's posts and he talks about the test results of his externally blown oil cooler.
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Last edited by BillL : 05-14-2019 at 02:56 PM.
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  #73  
Old 05-13-2019, 09:58 PM
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Toobuilder Toobuilder is offline
 
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NACA ducts are great low drag vents but they really need negative pressure on the back side to move significant air. Trying to use them to ram air into an aerodynamic mess like an oil cooler is a challenge.
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  #74  
Old 05-13-2019, 10:53 PM
scsmith scsmith is offline
 
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Quote:
Originally Posted by Mike S View Post
That must chafe at your engineer soul......
Sometimes.

But sometimes, using understanding of the physics to move a design in the direction of improvement is satisfying and sufficient, even if I can't measure how much.

And sometimes engineering intuition can help make a leap in design space more efficiently than a methodical approach. In those cases, you just need to test enough to insure your intuition was right.
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  #75  
Old 05-15-2019, 08:03 AM
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Quote:
Originally Posted by M McGraw View Post
My CHTs run in the very low 300s but my OIL temps run around 185-195. I want to throttle the mass flow in exchange for higher CHTs, but I need to do so without increasing the oil temps.
That was the situation here, early on. I've run several changes over the years. The most effective change was removing an SW/Meggit 10599 and installing a 10611.

Quote:
I’m considering a NACA vent on the flat side of my lower cowl with a 4” SCAT to the oil cooler. That SCAT would be a straight shot of only a few inches length as opposed to the current SCAT which is 18” length with a large flow direction change.
I ran a fully ducted system for a few years, one which took its oil cooler inlet air from just inside the low Vi/Vo cooling inlets. The goal was delivering air to the oil cooler without pre-heating it in the upper plenum. Before, taking air from the rear plenum wall, the typical temp rise (OAT to cooler face) was around 16F. Despite duct losses, the result was slightly lower oil temp, similar to what might be expected from lower OAT conditions. I eventually removed the upper plenum delivery duct and returned to simply tapping the rear baffle wall, although I did reshape the scat tube entry with better lip radius and install the larger cooler.

You're talking about far, far less duct length. I kinda doubt you would get higher pressure at the duct entry, but the short duct is probably a big deal.

Quote:
Below the cooler I will need a nozzle to exit the flow outside the lower cowl. That would separate the oil cooling air from the cylinder heads.
As you know, I've had the cooler outlet flow ducted all the way to the exit since birth. Originally it was a scat duct and some glass transitions, then a full glass duct to cut losses:

See post 95:

http://www.vansairforce.com/communit...=68241&page=10

Post 197

http://www.vansairforce.com/communit...=68241&page=20

It dumps into the exit bell as close as possible to its smallest area section. Pressure measurements at stations progressively closer and closer to the bell's exit showed steadily decreasing static pressure. Best would be an oil cooler exit into a low pressure freestream, as you outline, but I wanted to keep frontal area low (no bluff body exit). Everything is inside the cowl.

It might be interesting to see what sort of performance you might get with an oil cooler exit which was simply a nozzle leading to a flush opening in the bottom of the cowl. Local pressure on the belly may be higher than freestream static (it usually is, judging from the CFD plots of fuselages), but less than inside the lower cowl with a throttled exit, and maybe even less than where I dump into the exit bell.

It would be easy to place a few pressure taps on the bottom, for pressure data prior to cutting holes in your beautiful cowl.
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Last edited by DanH : 05-15-2019 at 02:26 PM.
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  #76  
Old 05-15-2019, 09:23 AM
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Quote:
Originally Posted by scsmith View Post
And sometimes engineering intuition can help make a leap in design space more efficiently than a methodical approach.
If I understand correctly, the idea is to make the cooler flow like its exit is connected to a lower static pressure. Is there a calculation which would estimate the reduction in pressure? Is pressure reduction inversely proportional to area increase? For example, if a cooler was 4" x 6" (24 sq in) and local static was 5" H2O lower plenum pressure, would a 6" x 8" diffuser exit (48 sq in) make the cooler flow like local static was 2.5" H2O?

I ask because it would help with Marvin's design choice, i.e between the internal diffuser (with X expected pressure drop) vs a quickly measured pressure at an external location.
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Last edited by DanH : 05-15-2019 at 02:28 PM.
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  #77  
Old 05-15-2019, 10:11 AM
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I read through the thread (ok Carl, ha ha). I love all the innovation. There was mass thread drift but to the OP's #1 post. You have air coming out of cooler you want to promote with low pressure. Would it work better if you put a exit shroud with big smooth radius angling outlet down 90 degrees, because ALL AIR must go out the cowl exit Why not aim the coolers exit air that way..

READING POST #69 from OP, it seems he has made more mods to inlet and will be getting more data. I am not a fan of the wedge plenum, sharp 90 degree turn, and anything less than 4" SCAT. He states a few degrees cooler just from initial impressions.... Cool! (pun intended). Keep us posted. Thanks for the thread/

In this thread I see folks doing things I like a lot
  • NOT mounting direct on back of baffling (crack city)
  • Using larger coolers.
  • Large 4" Dia ducts or larger.
  • The felt prop to cowl seal nice (did it help?)

Quote:
Originally Posted by scsmith View Post

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Last edited by gmcjetpilot : 05-15-2019 at 10:27 AM.
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  #78  
Old 05-15-2019, 11:50 AM
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Quote:
Originally Posted by gmcjetpilot View Post
You have air coming out of cooler you want to promote with low pressure. Would it work better if you put a exit shroud with big smooth radius angling outlet down 90 degrees, because ALL AIR must go out the cowl exit Why not aim the coolers exit air that way..
A duct to simply aim flow toward the exit? Because (1) it doesn't change the cooler outlet pressure, and (2) the duct walls add parasitic drag.
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  #79  
Old 05-15-2019, 12:35 PM
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M McGraw M McGraw is online now
 
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Quote:
Originally Posted by DanH View Post
...You're talking about far, far less duct length. I kinda doubt you would get higher pressure at the duct entry, but the short duct is probably a big deal...
I read the old NACA source document a few days ago. That data would suggest the NACA vent is a poor choice for an oil cooler. For effectiveness the vent needs lower pressure on the back side and an oil cooler would make that very difficult. Because I want a nozzle on the backside of the cooler due to space constraints the NACA vent would be very unlikely to work. Two bad situations for a low drag vent. TooBuilder would call it “messy”. This is why you normally see a scoop for coolers.

I’m am working toward all the mods you suggested above. I plan to radius the inlet on the baffle. Try the inner tube duct I saw from RVBuilder2002 and add a few piccolo tubes to my cowl exit at various edge distances to look for low pressure. Space constraints will very likely require a nozzle rather than a diffuser.
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Last edited by M McGraw : 05-15-2019 at 01:02 PM.
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  #80  
Old 05-15-2019, 12:41 PM
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Quote:
Originally Posted by DanH View Post
A duct to simply aim flow toward the exit? Because (1) it doesn't change the cooler outlet pressure, and (2) the duct walls add parasitic drag.
Good points. The short length, diverging duct would not add much parasitic friction. However my theory is interference (drag) from turbulent airflow swirling around cowl, acting at acute angles across the cooler outlet or even opposing it, blocks free flow, aka interference drag... This is all wild guessing on my part.. OP claims a few degrees lower temps (not clear if combined with other mods other than outlet duct/shroud) is good if his detailed flight test prove that out.

With the advent of rugged high resolution sport cameras, in flight video of area with tuff's to show what air is doing in the cowl might be interesting? BTW your test set up is great.
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Last edited by gmcjetpilot : 05-15-2019 at 12:46 PM.
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