Fluid Dynamics question
 

I originally posted the following question in the "Glow Engines" forum. I got some good suggestions, nobody really answered the question. It's more of a fluid dynamics question than it is an engine question. I don't know where else to ask, so I'll try here...

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Apparently I don't have enough outlet area in my .46-size World Models P-51 with an RCV-60SP. One of the "cons" of the RCV-SP engines is that they are more difficult to keep cool, as I am experiencing.

[link=http://image.rcuniverse.com/forum/upfiles/13495/Bz79072.jpg]Picture #1[/link] is how I cool the engine now. This is the side cross-section of the cowl. Engine is grey. Cowl is black. Cool air is blue. Hot air is red. The outlet:inlet ratio is about 2:1. But apparently the hot air doesn't want to circulate down like I want it to.

[link=http://image.rcuniverse.com/forum/upfiles/13495/Up49024.jpg]Picture #2[/link] is how I want to add additional outlet area at the top of the cowl.

The question is how big should I cut the lip at the top of the cowl? And what dimension should I use to calculate the additional outlet area?

In [link=http://image.rcuniverse.com/forum/upfiles/13495/Pu50610.jpg]Picture #3[/link], should I use dimension A or B to calculate the exit area?

Thanks in advance for your help.

Juice
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RE: Fluid Dynamics question
 

Because the air has to flow through, area A you should use this area for calculations.

/Red B.

RE: Fluid Dynamics question
 

The real problem is not outlet /inlet size.
calculate all you want --
You have to have a perfectly sealed - single path from inlet to outlet--plus- directed flow THROUGH the fins.
In order to make the air siphon (that's what is really happening) -- the outlet must be in a low pressure area -or-a low pressure outlet must be created along the skin of the fuselage -using a lip/dam whatever you want to call it .
Honestly, the fin orientation is awful on that engine setup for good flow.
On full scale craft --where airspeed in much higher ( and air is operating in higher RN setup) you can use relatively tiny inlets and outlets -- not so with this model stuff at small sizes
basically the air is reluctant to flow along the walls of the cooling path and TOTAL flow is greatly impeded.
Think of this
If the air in a pipe -sticks to the wall in a 1/8" thick layer -- a small pipe -say 1/2" ID --has a hard time flowing any air
A bigger pipe has the same 1/8" layer of air sticking -- but the larger ID lets RELATIVELY more air flow thru
In your case you have the worst of all situations
small size , and a convoluted path.

RE: Fluid Dynamics question
 

Hi Dick!

I agree completely with you. I have previously indicated this in my [link=http://www.rcuniverse.com/forum/fb.asp?m=4414261&key=]posting[/link] in the original thread.

/Red B.

RE: Fluid Dynamics question
 

You are on the right track with your cooling setup. While it winds around within the airplane it should work. You have the airflow directed over the head to maximize the cooling to the head of the motor, then back down and out of the engine compartment. That area should be large enough to allow for the expansion of the air due to heat.

Now as far as how you size the exit vent to the intake, the exit needs only to be around 1.4 times larger than the intake.
Now if drawing 3 is the vent shape you have in mind, in spite of the restriction of area A, the vent area will be B.

A few observations….

You have the area under the motor just after the intake baffled upward to direct the air upward. In the drawing you have it as a squared angles, that would cause your first restriction to fluid flow, that should be rounded gently. The same would follow at the top of the forward edge of the cowl over the motor, and the same at the top of the cowl next to the firewall, the square corners will cause a high-pressure area to develop.
These tweaks should improve the situation without any external mods. to your bird.
However, the one thing I noted beyond that, is that you show the forward edge of your vent to be higher than the trailing edge that is probably one of your biggest problems. That is going to cause an area of high-pressure right where you need to be developing an area of low pressure. The leading edge of your vent must be lower then the trailing edge, not much but at least 1/8 inch.
Like Dick said, you need to you need to make an air siphon. You exit right now is you major problem you are creating an area of high pressure, the down flow off the nose cannot stop the air from causing a high pressure area at your vent.

I think that the hole on top of the cowl probably won't buy you that much more cooling, and won't look all that great.

Good luck

RE: Fluid Dynamics question
 

In my opinion, picture 2 without the lower outlet would be a better configuration. Air does not like to make a ~180 degree turn. It would much rather continue on the same path that it is currently on. Also, you are wanting the air to pass over the colling fins. If it doesn't need to pass over the whole engine, then don't let it.

If you do end up going with an outlet at the top, use a NACA duct. These are a lot more efficient than having just a hole.

RE: Fluid Dynamics question
 

As far as I know NACA ducts were developed as inlets.
Do they work equally well as outlets?

/Red B.

RE: Fluid Dynamics question
 

Yes, they are used as inlets, but we have also used them as outlets and have not had any problems with them so far. I don't have any tests to back this up. However, we did not use an exact NACA duct. We modified ours and made it a bit more rounded, but the same idea is still there. I would love to know if anyone has any tests on this. I could set up an experiment, but if someone else has data, it would save me a lot of time.

Now, keep in mind that I could be wrong, so I don't want to hear anyone bash me.

RE: Fluid Dynamics question
 

Dick is right on. I don't think you need another exit hole at the top. Just add a fence or dam at the forward edge of the exit hole on the bottom of the cowl to encourage low pressure behind it. That will cause air from inside the cowl to "fill" that low pressure, and more air will flow through the cowl. I have done this several times to improve cooling, without altering hole size at all. I've seen it on some full-scale airplanes too. A small fence (even only 1/4" or 1/2" tall) should make a significant difference in flow rate. No calculations necessary.

RE: Fluid Dynamics question
 

OUTLET NEEDS TO BE BIGGER THAN INLET.. USE DAMS TO DIRECT AIR TO COOLING FINS. IF AIR DOES NOT EXHAUST, IT JUST KEEPS REHEATING,,.. RD

RE: Fluid Dynamics question
 

The discussion here has been very helpful and I even got to learn a few new things.

The green in this new picture is what I plan on doing next... before I try opening up the top of the cowl. The side walls of the baffle used to go up to the middle of the engine. They now go all the way up almost to the top of the cowl. The top/forward edge of this new duct is rounded. This is already done. Now, I am going to round off the top back of the cowl. Last, I am going to use 1/64 ply with epoxy and microballoons to create a lip on the forward edge of the outlet hole.

This should help reduce some of the unwanted high pressure pockets inside the cowl.

Honestly, this is great information in this thread and I appreciate all of you sharing your knowledge.

RE: Fluid Dynamics question
 

This is an upper shroud--on my ZDZ160 -- the forward section slides under the returned edge on top of cowl.
th lower section is integrated with the cowl- for ease of removal