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Louver Sizing Question

Louver Sizing Question

Louver Sizing Question

(OP)
Gotta crazy question here.

We have need to vent out cooling air outta of the airframe from within. Unfortunately, we are extremely constrained by lack of space for a proper exit ducting. A crazy suggestion of using louvers and positioning them on the skin in a region of low external
air pressure may be enough to do the job. Suggestion was provided by an external consultant and apparently his team have used similar solution on a couple of aircraft programs.

I am being tasked to do some basic research on feasibility of the same. I think I need to come up with some basic numbers on louver blade dimensions (how high from surface), blade sizing etc. All my searching so far have led to me louver materials for HVAC applications. I suppose, I could use it as a starting point.

Just posting here to request if any folks have idea about louver design, selection or experience, please do provide pointers on how to proceed.

Thanks

RE: Louver Sizing Question

how would you know the outside aero-pressure ? I suspect that almost everywhere will give you suction for some flight points, and +ve pressure for others.

What is the advantage of louvers over conventional ducting ? Either would take advantage (or disadvantage) of external suction. Or is it that you have no ducting, just an "opening", so you'll exhaust your hot air into a compartment and hope it finds it's way out ? What sort of temperature ?

Maybe mount a fan over the opening to suck air out of the compartment ?

another day in paradise, or is paradise one day closer ?

RE: Louver Sizing Question

(OP)
Hi RB1957,
I think we have the surface pressure distribution contour from CFD analysis on both the upper & lower skins of the aircraft. I think the idea is to just position the opening with louvers at a place which has lowest pressure and as you have mentioned, hope the air will find its way out. Of course, I am told the "hope" will be verified using a CFD analysis. So there will be some sort of simulation activity to verify the design intent.

RE: Louver Sizing Question

Can you indicate roughly the flow rate that needs to be handled? 10 CFM or 10,000 CFM?
A heated drain mast is suitable for many small exhausts. Cabin pressurization is enough to drive it out. If your aircraft if pressurized.
You can force the discharge of a slightly larger flow with an ejector pump, fed from the bleed air (if your engines have that).
It will exhaust at a fairly high pressure, but enough to overcome external pressure depends on design.
If you don't want pressure but want lots of flow, then consider an exhaust fan and ducting the exhaust to some innocuous place. Like near the outflow valve.

STF

RE: Louver Sizing Question

(OP)
SparWeb,
The aircraft is not a commercial one. It's an UAV. So, no pressurized chamber inside the airframe (for now we are assuming an operating condition of atmospheric pressure).

We have a mass flow rate of around 1.2 Kgs/sec, which roughly translates to around 2100 CFM.

Also, we don't have space to accommodate a ducting and hence looking at alternate means of ejection. Our engine has bleed air feature but I was told the flow rates are very low. Probably low enough not to act as an useful tool. I have to talk to our CFD/Systems Engineer again to get some more clarity.

I tried to apply some existing HVAC louver design guidelines to come up with blade height & blade spacing, but our operating region is beyond supplied HVAC/Building guidelines. The outcomes from such guidelines don't make any sense. (For ex: the free air velocity I am getting is 42,200 m/s) :)

Still searching on usage of louvers in aircraft application.

RE: Louver Sizing Question

Is this problem related to avionics cooling... or some other [mechanical, propulsion, etc] systems cooling??

Random Fud-4-Thot to consider...

Air cooling for most systems demands ambient-air-in-flow, momentary pressure-stagnation [slow-down air to allow heat transfer to occur] and then exhaust flow of the heated air.

Temperature extremes can be particularly difficult to design for with fixed geometry... especially when the vehicle could also operate in extreme heat and extreme cold climates. What works for one general climate might be disastrous in other climate extremes.

Air inlets, ducts and louvers are subject to environmental effects such as air-flow induced buzz/flutter and environmental intrusion or blockage due to moisture, rain, ice, snow, sand/dust, FOD, etc.

Also, must be careful to protect against [or plan-for?] intrusion' due to 'critter' and airborne-debris when not operating etc [bugs/birds/mice/etc]

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Louver Sizing Question

2100 CFM seems like quite a lot for passive airflow, where is all the inlet air coming from? You will need to have an inlet duct somewhere since the body of air inside your UAV would need to be cycled many times per minute to meet 2100 CFM.

Can you attach what ever it is you are trying to cool to the frame/skin with some heat pipes and use the external surface of the UAV for heat transfer?

Can you stick a couple of cooling fins outside of the body in the airflow?

RE: Louver Sizing Question

It seems would at least need something feeding air into the area to be cooled for this to work. Just have an exit near a low pressure area would reach semi-stable equilibrium without a way to flow air into the area from elsewhere (maybe you have that already, but it wasn't mentioned).

As WKTaylor noted, a simple louver system would probably require a screen of some kind to reduce/eliminate FOD as well.

If you have a stagnation point on the airframe somewhere, you could possibly run an inlet duct, or at least just an opening, in that region since you say you don't really have room for proper ducting. Not sure what else you can do based on the limited info here.

RE: Louver Sizing Question

(OP)
Folks,
Thanks for the replies.

We do have a dedicated inlet duct. The intent is to cool the oil sump surrounding (in an annular chamber) the gas turbine. Looking for means to eject the air after it has done its job. As of now, the idea is to position the exit louvers on the top skin (at the bay near the oil sump and position it with-in the bay where external pressure point is low enough in hope to create suction). Being told that environmental effects, moisture ingress, FOD are not a concern at this point. The UAV will be like a tech demonstrator and hence is not expected to fly in inclement weather.

I know this is not good engineering, but the thought process here to put out fires as they come by i.e. the environmental aspect issues will be handled as they arise later I guess. This is the directive I've given and asked to proceed.

Kwan, thanks for the NACA website. I guess, I will try to search for some thing there as a starting point.

RE: Louver Sizing Question

Quote:

The intent is to cool the oil sump surrounding (in an annular chamber) the gas turbine.

That might explain why the sump needs so much cooling; you're trying to remove heat it gained from the gas turbine.

I assume it's too late to use a sump remote from all the heat...

Mike Halloran
Pembroke Pines, FL, USA

RE: Louver Sizing Question

(OP)
Yes, the oil sump is part of the engine supplied by a vendor. The engine systems mod is a NO GO.

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