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Exit diffuser on oil cooler to increase flow through oil cooler

in post #47 you can see the amount of flex the felt will do, so it shouldn't have a problem sealing. remember there is a lot of engine movement in turbulence or aerobatics. yes, i cut the small flange off the inner edge of my cowl so the cowl was flat to put the felt on. i used rtv between the first aluminum ring and the cowl to seal that area. i suspect if you didn't what to cut the cowl you could do some some foam and/or micro fill and just make it flat. if you wanted to move it toward the flywheel a bit just make the foam ring a bit thicker which depending on how your cowl goes on at the spinner it wouldn't interfere with the normal placement as the front edge of the cowl goes on, if it a rotate into positon fit. just a thought after 2000 hrs of service.
 
Just finishing some upgrades/modifications I have done to my RV-6A while replacing all of the baffling (which includes some of the other mods being tried).

Some parallel some of the mods being discussed here.

Here is my version of the fwd cowl seal.
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New remote mounted oil cooler installation with low flow resistance ducting (15/600-6 RV-10 main wheel inner-tube)

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I have since added a cockpit controllable throttle plate to the home made oil cooler duct to use for oil temp regulation in cold temps.

I plan to experiment with an exit defuser for the oil cooler similar to what Steve has described, once I have done some testing.

None have been flight tested yet.
 
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All looks really good Scott. Very clever using an inner tube. What is the diameter of the tube?

In thinking about mounting the cooler on the firewall with two angles like that, I've been concerned about doing that for the big 200006A cooler. I think I want some kind of diagonal bracing and a bigger contact footprint onto the firewall. I've been kind of thinking of an integrated mounting bracket+exit diffuser.

I can see how much easier it would be to put the nose seal on with the prop off. I really don't want to do that, so I will struggle thru.
 
All looks really good Scott. Very clever using an inner tube. What is the diameter of the tube?

In thinking about mounting the cooler on the firewall with two angles like that, I've been concerned about doing that for the big 200006A cooler. I think I want some kind of diagonal bracing and a bigger contact footprint onto the firewall. I've been kind of thinking of an integrated mounting bracket+exit diffuser.

I can see how much easier it would be to put the nose seal on with the prop off. I really don't want to do that, so I will struggle thru.

The tube is just over 4" in diam. I wanted to use a size that could be replace with standard SCAT hose size if it turned out to be too stiff, so all of my flange diameters are sized for 4" SCAT and the very slightly over sized rubber inner tube compresses down to fit fine. It is stiffer than SCAT but because of it's long length I think it will be ok. The interior profile of SCAT is not the greatest for clean airflow. particularly if it is making sharp bends. That was the reason for trying the inner tube.
If you look close, you should see that my cooler is supported at the engine mount. Since it is the smaller 7 row, it probably would have been ok just cantilevered off the firewall, but I agree, the larger coolers should probably have additional support.
 
. I wanted to use a size that could be replace with standard SCAT hose size if it turned out to be too stiff, so all of my flange diameters are sized for 4" SCAT and the very slightly over sized rubber inner tube compresses down to fit fine. It is stiffer than SCAT but because of it's long length I think it will be ok. The interior profile of SCAT is not the greatest for clean airflow. particularly if it is making sharp bends. That was the reason for trying the inner tube..

Interesting. I only grudgingly resorted to scat when I did my new oil cooler installation last year because of the interior air flow friction. Inner tube seems like a better solution. I don’t understand why you describe it as stiffer than scat though. Seems like inner tube is way more flexible to me.

Erich
 
Interesting. I only grudgingly resorted to scat when I did my new oil cooler installation last year because of the interior air flow friction. Inner tube seems like a better solution. I don?t understand why you describe it as stiffer than scat though. Seems like inner tube is way more flexible to me.

Erich

It depends on what inner tube you are talking about I guess.
Bicycle inner tube is very thin and flexible. Aircraft inner tube is .090 to .100 thick. When opened into a tubular shape its column stiffness is quite high.
There may be other tubes in the same size range (maybe for ATV's or something similar) that aren't as thick, but I used what I had readily available.
Putting some hrs on it is the only thing that will prove whether it is flexible enough, but I think it will be ok for my installation since the overall tube run is rather long and the flow direction changes 90 degrees. I don't think the inner tube would work well for say an engine mount mounted cooler with just a short run of inner tube.
 
Semi-Related Advice Wanted

I hope this isn't too big a diversion.

On my RV-3B project, while I have a way to go, I've been thinking of mounting my small non-standard oil cooler directly to the top engine baffle.

Would it be best to plan build a rounded entry or just leave it bare, flush with the baffle?

Dave
 
I hope this isn't too big a diversion.

On my RV-3B project, while I have a way to go, I've been thinking of mounting my small non-standard oil cooler directly to the top engine baffle.

Would it be best to plan build a rounded entry or just leave it bare, flush with the baffle?

Dave

Dave, the upper cowl is essentially stopped flow, just mixing around with fairly low velocity. Where the flow rushes into the cooler, it has to turn around a corner along the edges, and a separation bubble forms, called a "vena contracta". This has the effect of making the opening smaller. If you can put a small radius circular lip around the perimeter of the opening into the cooler, it will help get the flow turned and into the cooler without the restriction formed by the small separation bubble. It doesn't take much of a lip, maybe 1/4" diameter tube cut in half to make semicircles.

The same thing happens when people put their air filter in the front left side cooling intake ramp. It really should have a radius lip around the opening.
 
that was a really great idea scott. i love simple ideas. nice prop/cowl seal.
some people have asked about the paper i had put together for the west coast flyin of the testing i did over 10+ years on my plane. i have a copy of the original and recreated it as a word.docx. if anyone is interested you can email me and i can send it.
 
DaveB asked what felt i used for the prop seal. i tried to respond privately but got mailer-daemon back. so:
it's a really simple way to get a good seal. McMaster-Carr
Soft Felt Sheets and Strips

The higher the wool fiber content, the better the vibration absorption and abrasion resistance.

Use F10 and F13 felt for grease and oil retention. F10 felt absorbs grease and oil even when compressed.

F26 felt is the softest wool felt we offer and is often used for packing and padding applications where the felt will be held between other materials.
1/2" 95% 4.20 Durometer 20A -80? to 200? White 225 2842T29 $23.20
this # is a 12"X 12" piece. they have cheaper but not as high a wool content.
i felt that since it was unsupported for about 1" or so that this was the best type to use. i cut the hole about 1/4"-1/2" smaller then the hub and then ground a taper on the end flat edge (a die grinder with a course disc does pretty good grinding) then as you could see in the pics i put the taper toward the inside so increasing pressure would produce a tighter seal. also soak the edge contacting the hub with silicone spray. i found that an 1" or so clearance around the hub to the supporting alum rings was enough clearance for me. when the felt is shaped like that it didn't require much support. i hope DaveB gets this.
 
Thanks

Got it Dave--thank you for getting the info to me--my order goes in tomorrow.

Cheers,

db
 
Update?

Any update on your progress? I?m in the process of building a cowl flap. However, I will need to lower my oil temps after raising the CHTs. I?m interested in how your oil cooler diffuser worked out.
 
Update

In shrinking my exit, a new prop seal was in order. Using the felt recommended by F1Boss (ordered before Dave posted his) my nose internals were modified. Rather than leaving the lip shape to chance, a form was made with PVC to 3/8" less diameter than my spacer. Stabilizing rings were made, water applied and the felt pushed over the form. Heat and resting overnight to dry and set did the trick.

An unseen element are spacers to allow controlled crush of the felt, and ability to fully hold the cap and backplate with the screws. A late addition to the design.

The glass cap was made to provide some radial backing to the compliant felt.

Now I can move to testing the shrunken exits, #1 and #2 (#X??).

Huge thanks to Dave Anders and F1Boss for the information posted. I liked the rubber one like DanH, but this seems to fit better in my space.

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Bill, nice work on the spinner seal. I know that you have a lot to do but I have a suggestion for you to consider. Your new bottom shape gradually expands as you go aft. This expansion tends to slow the exiting air which is not what we want to accomplish here.
Since it looks like you might be going to screw this shape to the bottom of the cowling you could make another straight sided duct that could be screwed to the same location. This would allow you to swap the outlet bottoms and compare speed and cooling changes.
 
Any update on your progress? I?m in the process of building a cowl flap. However, I will need to lower my oil temps after raising the CHTs. I?m interested in how your oil cooler diffuser worked out.

Marvin,
Just curious what sort of oil temp you are getting, now that we are in the warmer weather.
 
Any update on your progress? I?m in the process of building a cowl flap. However, I will need to lower my oil temps after raising the CHTs. I?m interested in how your oil cooler diffuser worked out.

Marvin,
Just curious what sort of oil temp you are getting, now that we are in the warmer weather.
 
Oil temps

Marvin,
Just curious what sort of oil temp you are getting, now that we are in the warmer weather.

My normal summer time oil temps are 185 to 195F depending on how hard I run. I have a butterfly valve to reduce winter time oil cooler flow.
 
Any update on your progress? I?m in the process of building a cowl flap. However, I will need to lower my oil temps after raising the CHTs. I?m interested in how your oil cooler diffuser worked out.

Hi Marvin,

It works. My oil is cooler. It is hard to put quantitative value on that. I don't have hard data before and after under similar conditions. As spring is here and temps are warming, I can confidently say the oil is 3--5? cooler at top of climb, and cools down to steady state cruising temp faster. I don't think the cruising temp is much different because it is still cool enough that the vernatherm is interacting. Maybe slightly cooler. Later into summer the cruise temps will be more meaningful.

I have now instrumented the airplane and will get some measurements of oil cooler inlet and outlet temp before and after I switch to 4" scat and a well-designed transition to the cooler inlet plenum, going away from my 3.5" scat and abrupt connection to oil cooler inlet plenum. I will report back on that change.

I probably won't test the before/after oil cooler diffuser temps. I don't want to bother to take it off and re-install it, with accompanying orange RTV seals. It helps. It can't hurt. I just can't really quantify with valid data.
 
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.

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.

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.
 
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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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.
 
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.
 
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.

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.

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/community/showthread.php?t=68241&page=10

Post 197

http://www.vansairforce.com/community/showthread.php?t=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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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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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?)

 
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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.
 
...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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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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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?

Some of us have already done this years ago and learned lots. As has been discussed in this thread previously, CFD and theory is great but only flight testing with accurate instrumentation will truly validate those ideas.
 
Update?

I want to do some cowl work to reduce my cooling exit flow area and at the same time reduce overall frontal area.

But before I can do that, I have to do two other things. One is make a seal around the crankshaft/spinner gap. As you close off the cooling exit, raising the pressure in the lower cowl plenum, you want the higher pressure to result in higher exit velocities out of the cooling exit, not just spill out around the spinner. So I have to do that.

The other thing I need to do is increase the flow through the oil cooler. The angle-valve engines put a lot more of the heat load into the oil, and efficient oil cooling is crucial. I've been very happy with my firewall-mounted cooler arrangement with the large 20006A cooler, I have good oil temperatures even during a long climb at Vy on a hot day. But as I close off the cowl exit and increase the pressure in the lower plenum, the oil cooler flow will decrease because of the reduced pressure differential across the oil cooler. So..how to get that flow back? put a diffuser on the cooler exit. The trick is to do this smoothly enough get good pressure recovery without a lot of length that wouldn't fit in the cowl area. Short, wide-angle diffusers can work pretty well. The pressure in the oil cooler flow at the exit of the diffuser will match the local static pressure in the area around it. Ideally, if the diffuser is flowing well, this results in lower pressure at the exit face of the cooler itself. Its like immersing the cooler in the throat of a venturi.

I made a mold out of wood, painted and waxed it, and laid up a fiberglass diffuser. The diffuser geometry has an area ratio of 1.45. This will also be a good trial of the high-temperature hardener system (3136R) I got for the Jeffco 3107 (Rhino) epoxy we use. The diffuser was room temperature cured, then removed from the mold and post-cured, 1/2 hr at 150F, 1/2 hr at 175F, then 2 hr at 200F. Hopefully this will give a high enough glass-transition temperature (T_g) for the epoxy to survive the hot environment in the lower cowl. Here are a couple of pictures of the installation:

img_1056.jpg

img_1057.jpg


You can see, especially in the second picture, that I made the diffuser a little bit too small. I had measured the mounting flange length and assumed that the cooling channels were the same length, when they actually extend about a 1/4" beyond. So I will add a 1/2" plank on the mold and make another one. In the mean time, I can test this one.

Another issue is that I have an engine mount tube crossing the exit face of the oil cooler. Without the diffuser, this likely reduced the flow through the cooler a little because of the "blockage". With the diffuser, there is some suction pulling on the exit of the cooler that will help pull flow around that tube. Hard to say if the unsteady flow around the tube will help or harm the flow in the diffuser itself. It may cause an oscillating flow separation on the diffuser walls, but hopefully the average pressure recovery will be OK. When/if I put a slightly wider diffuser on, I will also put a spiral wrap of wire on the tube to break up the coherence of the oscillating separated flow downstream of the tube. I'll report back when I get a chance to fly with this.

Any news or data on this project?
 
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