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Trapped air in oil cooler

Bavafa

Well Known Member
I was wondering*with the oil cooler that is mounted with its port in horizontal*direction and on the sides, would it be possible to get air trapped in it and not fill completely*due to trapped air.**
 
I just went through this; I've heard for years that you should mount an oil cooler with with both ports up or with one port higher than they other because of potential air entrapment.

In my build it worked better to mount it upside down so I asked the question on the Aero-Classics tech email. Here's the response I got back when I asked specifically about mounting the Aero-Classic P/N 8000215 upside down, i.e. level, but with both ports on they bottom.

Terry,
Your question is very common and equally as common as the myth that aviation oil coolers trap air.

Aviation oil coolers do not trap air because like there is an air fin on the outside of the oil cooler there in an oil fin on the inside.
The purpose of the fin structure is multiple but to include increasing surface area, breaking up flows for increased heat transfer and ensuring even pressure drop across both sides of the oil cooler.
With that, everything flows in the direction of least resistance. So, air will not get trapped because oil will push air rather than oil because it is less resistant to being pushed out of the oil cooler over oil.

I edited the above a couple times while trying not to write a book.
Hopefully it is at least a little bit clearer than mud!
Please advise if you have any questions.
 
I just went through this; I've heard for years that you should mount an oil cooler with with both ports up or with one port higher than they other because of potential air entrapment.

In my build it worked better to mount it upside down so I asked the question on the Aero-Classics tech email. Here's the response I got back when I asked specifically about mounting the Aero-Classic P/N 8000215 upside down, i.e. level, but with both ports on they bottom.

Terry,
Your question is very common and equally as common as the myth that aviation oil coolers trap air.

Aviation oil coolers do not trap air because like there is an air fin on the outside of the oil cooler there in an oil fin on the inside.
The purpose of the fin structure is multiple but to include increasing surface area, breaking up flows for increased heat transfer and ensuring even pressure drop across both sides of the oil cooler.
With that, everything flows in the direction of least resistance. So, air will not get trapped because oil will push air rather than oil because it is less resistant to being pushed out of the oil cooler over oil.

I edited the above a couple times while trying not to write a book.
Hopefully it is at least a little bit clearer than mud!
Please advise if you have any questions.

Interesting. I've designed a lot of Heat Exchangers. What is being described above are commonly called turbulators. It keeps the fluid velocities near the HEx walls at locally high velocities, prevents stratification of the fluid thus reduces reliance on thermal conductivity of the fluid both of these increase heat xfer. They also add pressure losses to help ensure equal flows in all of the passages, etc. They never mentioned the headers, only the fin-pack.

While any cooler that doesn't have a high point vent can get air-bound, I'm guessing the header velocities are relatively high (>6ft/sec) which would mean high "tube" losses to compensate. This would help keep the headers filled.

A surprise but trust the OEM here.
 
This is the Aero Classic 8000215 oil cooler.
- If it´s mounted with in/outlet facing up, no air is trapped.
- If it´s mounted with in/outlet on the side AND inlet bottom, no air is trapped.
- If it´s mounted with in/outlet on the side AND inlet on top, air is trapped.
- If it´s mounted upside down, air is trapped.


Good luck
 

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This is the Aero Classic 8000215 oil cooler.
- If it´s mounted with in/outlet on the side AND inlet bottom, no air is trapped.
- If it´s mounted with in/outlet on the side AND inlet on top, air is trapped.

Good luck
I am not sure if I understand this. Aren't the in/outlet on the same plane/level?
My airflow cooler has both ports on the same level and this is what I have seen on other brands.
 
The picture backs my assumption. It appears the designer is relying on high header fluid velocities. These are typically kept low for manifolding effect but most likely means they're probably compensating with higher fin pack passage velocities. Since related pressure losses only affect power consumption versus flow by nature of the positive displacement pump, it allows the HEx designer to remove a typical boundary condition. No local low fluid velocities -> move any collected air along. Smart.

Again, I see no reason not to trust the HEx OEM here.
 
Again, I see no reason not to trust the HEx OEM here.

I agree. I have "known" for years about the potential for entrapped air. At this point, I don't even know if I learned it 30+ years ago in A&P school, or if it was just tribal knowledge picked up along way somewhere.

Due to the positioning of the big engine on my RV7 I went with the RV10 firewall mounted plenum but it ended up being a tad higher than ideal. I was going to end up with a cluttered line routing if I went with traditional port up orientation.

I started noodling around the inter web to get ideas and realized that the Showplanes remote cooler kit most definitely mounts the cooler upside down so I started asking the question.

I'm a few months away from engine runs, so this may blow up in my face, but when the manufacturer says with no waffling whatsoever that this is the way it is, it's hard not to take that at face value.
 
Enclosed is a clip of Davies Craig instruction.
It points out how to mount an oil cooler.
 

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I don't know anything about Davies Craig, but a google search reveals the they appear to make oil and transmission coolers for cars or construction equipment for something? On the picture you posted it looks like the end caps are completely different that they are on the Aero-Classic. Perhaps that's where the difference lies?

Look, all I know is this:

The RV-14 lays it horizontally with the ports fwd in an orientation that leaves a good bit of the cooler above the highest port.

The Showplanes installation mounts it upside down.

Aero-Pacific told me that I could mount it upside down with a detailed explanation of why, which I posted here.

I really don't know what else you're hoping to accomplish here. It obviously works. If you have a problem with statements made by Aero-Pacific about how their own product works then you should take it up with them.
 
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The OEM is responsible for their installation instructions, application recommendations, etc for their product. There are a ton of variables to consider when designing heat exchangers, surface area, material conductivity, fluid specific heats, boundary layer coefficients (internal and external), turbulation, fin efficiency, more. There’s still a hand full of variables for application. Trying to apply one OEM’s requirements to another isn’t necessarily applicable. When the applications are different, it makes even less sense. Sure, if you could conveniently apply an approach from one application to another that lands on the conservative side why wouldn’t you? Per the relevant OEM here, the subject approach isn’t necessary.

So I’ll state again for the aforementioned technical and practical reasons, I see no reason not to trust the OEM here. If they’re wrong, I’ll bet Desert Rat will tell us. Have a good weekend.
 
Easy Fix

Every time you change the oil you will trap some air. Just do a loop and pull some negative G's. Should remove any trapped air.
 
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What is the function or operational issue being solved here?

Every time you change the oil you will trap some air. Just do a loop and pull some negative G's. Should remove any trapped air.

This is funny and fun - - and the best answer of all.

Fundamentally, air won't stay in the cooler very long. At 80psi air will dissolve in the oil, AND, air viscosity is lower than oil - - lower resistance in flow, so will be gone.
 
Every time you change the oil you will trap some air. Just do a loop and pull some negative G's. Should remove any trapped air.

You know, this actually crossed my mind as well :)

Does anybody but me remember how you could tell if an airplane had been upside down by the air bubble in the ball in the turn and bank?
 
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