Exit diffuser on oil cooler to increase flow through oil cooler

[rv6ejguy]

Originally Posted by Toobuilder

I intend to duct the outlet back to the wing root near the point of max thickness to harvest that fat slice of low pressure that always shows up so well in CFD plots. A very simple flanged 3 sided duct could be bent up and attached with speed tape to the fuselage to validate the theory in no time.

I have a box full of wedge, sheet metal outlet ducts from various experiments over the years... Tape 'em on and fly, observe pressures.

Yup, this is a good thread. Interested to see what Steve finds out here.

[F1Boss]

[quote=Toobuilder;1322413]Interesting and timely thread since I'm working on my cowl right now. Looks like I'm not as clever as I thought I was considering the same ideas I'm implementing are being discussed here.

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Also looking at the prop seal thing. Though Dans flywheel seal works, the Rockets have an extra long hub which provides plenty of real estate to seal. Not only is the overall sealing surface smaller on the Rocket, but the surface speeds are much lower on the small diameter hub. Both are good things. I plan on teflon tape around the hub and a dry felt seal media contained in a floating seal retainer.

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I bought 1/2" thick felt on Amazon - I get the F7 1/2" x 12" x 12". I think I got these part numbers from Dave Anders?

I closed off the cowl front bulkhead area, and cut a hole 1.5" larger than the shaft dia in the fiberglass. I cut a hole in the felt about 1/2" smaller than the shaft, then cut the felt in two (rounded to match the cowl bulkhead). I made an alum panel the retains the felt. I did put some liquid Teflon into the edge of the felt. I see that the felt polishes the shaft like Dave said it would; the shaft needs no protection (tape etc). I doubt this setup will need any maintenance, as it has not changed since I put it in.

I see the felt is pushed fwd around the shaft, so it must be seeing increased pressure too...

I recall the felt costing about $20...

My change to the round inlets was the most dramatic change I have ever made. CHTs dropped to low 300s and oil went to ~160F - so I had to tape off an area of the cooler. Previous temps were 400 CHTs and 240 on the oil - not the best numbers...

The strange and confusing part was that the aircraft speed did not change. So, it seems that I should remove some outlet area, and add a cowl flap..and have my own Incredible Shrinking Outlet.

[Toobuilder]

Great minds Mark...

I have the big inlets now but after looking at the benefits of a sealed lower cowl and the dynamic extraction of an exhaust augmentor, there is compelling evidence that adequate Pd can be maintained between the upper and lower deck with significantly reduced inlet size. Look at the inlets Andrew F is using on his Lancair - he is cooling 700 HP with less than 40 inches of inlet area! I'm set up for 8 inch round inlets now (100 inches total) but plan to do significant flight test to choke those down to optimum. As a contrast, my outlet is only 30 inches with the cowl flap closed.

The inlets can be modified with easily fabricated restrictor plugs and the exhaust can be opened up with a reduced size splitter duct. It was designed this way to be modular and facilitate flight test. Once I get it correct, the new "final" cowl gets built in carbon fiber and I'll be done. In the meantime my Rocket will be sporting rough primer and filler.

[Ralph Inkster]

Mark, on Bill's Rocket I did a conical rubber seal (Vans baffle rubber seal material) similar to Dan's scheme that runs on the extended prop hub. It resulted in a slight smudge on the surface, no surface wear on the hub. I planned for a lot of engine movement so the seal had a lot of flex room.

[rv6ejguy]

Originally Posted by F1Boss

My change to the round inlets was the most dramatic change I have ever made. CHTs dropped to low 300s and oil went to ~160F - so I had to tape off an area of the cooler. Previous temps were 400 CHTs and 240 on the oil - not the best numbers...

The strange and confusing part was that the aircraft speed did not change. So, it seems that I should remove some outlet area, and add a cowl flap..and have my own Incredible Shrinking Outlet.

Impressive temperature drops. Mass flow must have increased to drop the temps but without constricting the outlet to regain lost momentum, I wouldn't expect to see much in the way of speed gains. Cowl flap allows you to optimize exit area for climb and cruise. That should gain you something.

[DanH]

Originally Posted by F1Boss

The strange and confusing part was that the aircraft speed did not change.

Just imagine some softly rounded bazoombas out in front of those inlets. The fact they are made of increasing static pressure need not cool your ardor

Originally Posted by Toobuilder

Look at the inlets Andrew F is using on his Lancair - he is cooling 700 HP with less than 40 inches of inlet area!

Careful with that line of thought. Andrew has the magic of velocity squared working for him. Standard day dynamic pressure at 350 knots and 5000 ft is 68" H2O. Here in the sluggo world, we need to cool RV's at 120 knots at full power for climb...where available dynamic pressure is 8".

68 is better than 8

[scsmith]

Originally Posted by DanH

Just imagine some softly rounded bazoombas out in front of those inlets. The fact they are made of increasing static pressure need not cool your ardor

Sometimes I wish this forum had a "like" button! Made me smile..... a lot!

[scsmith]

Originally Posted by rv7charlie

OK, I'm going to demonstrate my untrained ignorance. If Bernoulli says that speed and area have an inverse relationship, and speed and pressure have an inverse relationship (redneck engineering expressions), then I'm struggling to see how putting an expanding cone on the exit of a heat exchanger (when it's located in a volume of basically static air) will reduce pressure at the heat exchanger exit.

When I struggle through books like Kuchemann & Weber, and Kays & London, 'diffuser' is the term used to describe an expanding duct that *slows the air and increases pressure*.

Please educate me.

Charlie

HI Charlie, lets see if this helps:
The oil cooler is dumping its flow into the lower cowl plenum, and as always with subsonic flow exits, the pressure in the oil cooler flow at the exit must match the pressure in the local environment around the exit - which is the lower cowl plenum pressure. It doesn't matter what the relative flow velocities are, the pressures must match. With no exit diffuser on the cooler, this results in a bunch of little jets of flow coming out of all the cooler passages, at whatever velocity is needed to match the pressure to the local environment at the exit. If instead, you put a diffuser on the cooler, and lets just say it flows ideally with no separated flow, all those little jets of flow out of the cooler coalesce into a flow that can now slow down and increase pressure in the diffuser, until once again that flow must match pressure with the local environment around the exit of the diffuser. In both cases (with and without a diffuser on the cooler) the lower cowl pressure is about the same. So if the pressure at the diffuser exit matches the lower cowl plenum pressure, then the pressure must be lower at the entrance to the diffuser, at the cooler exit. So what the diffuser has done is lower the pressure at the exit of the oil cooler, and now with more pressure differential across the cooler from inlet to exit, there must be more airflow through the cooler.

Hope that helps. Keep banging on us - -there are no doubt others that want to understand, just as you do. Happy to try to help.

[Toobuilder]

Originally Posted by DanH

...Careful with that line of thought. Andrew has the magic of velocity squared working for him. Standard day dynamic pressure at 350 knots and 5000 ft is 68" H2O. Here in the sluggo world, we need to cool RV's at 120 knots at full power for climb...where available dynamic pressure is 8"....

That's true but he also has a bit of help from the aurgmented exhaust too. As evidence I have had some extensive conversations with a well known high HP RV driver with tiny inlets that can demonstrate a climb from sea level to near the flight levels at a constant 110 indicated and never even get close to 400 CHT. He attributes this performance significanly to the augmented outlet.

If you can cool 230 HP continuously with upper deck dynamic pressure in the low single digits then there is some magic at the outlet to create the delta P through the fins.

[rv6ejguy]

Augmentors are really magic when done right. Look at the size of the inlets on Andy Findlay's Lancair. He's cooling a lot more than 700hp, though some of that is happening with spray bar water.

The big deal with augmentors is you can regain all or most of the cooling air momentum lost passing through coolers and fins. Of course there is a lot of energy available at these power levels from the exhaust.

If the aim is to boost flow through coolers, that not so hard with some attention to detail, if we want to reduce drag at the same time though, we need to increase the velocity at the exit and direct it aft. That is much harder to do on an RV with the limited space available under the cowling and layout of the parts.

Ideally, if we had room, we'd want to do something like this with the oil cooler ducting: