Off the wall cooling idea

airguy · #1 10-04-2006, 03:17 PM

I hope to buy my 9A kit soon and get started, I'm still in the process of researching all the particular parts and pieces associated with building my first aircraft before writing the check - including engine choices. I was talking the other day with a friend about the Thielert turbodiesel, and possibly using it in my aircraft. We started talking about the liquid cooling loop, and he had an idea about using a liquid-liquid heat exchanger to dump the heat from the coolant into the diesel fuel in the tanks, instead of using a draggy radiator. Then the fuel can dissipate the heat through the wet wing.

Initially, I see several issues with this, all of which could be dealt with by using some good analysis and careful engineering, and a couple real nice benefits. Coolant leaks, increased vapor pressure of the fuel, CG changes, failure modes and heat exchanger design jump up real big in my mind, but the more I think about it the more I can see design changes that could handle these issues. The loss of the radiator would mean a nice decrease in drag with a redesigned cowl, and it would look pretty cool to boot. I'm not too sure about this being thermodynamically feasible, though, so I'm asking for some additional input from some guys with more engineering knowledge than me before I waste any more dreamtime on this.

The coolant loop and oil cooler on this engine will need to dump roughly 160,000 btu/hr at full power. The fuel (pump diesel or Jet A) will be stable to about 300 F, so no issue there. The coolant is thermostatically controlled to 210F nominal, 250 max,and pressurized to 25 psig. A minimum quantity of fuel would obviously be required for adequate heat removal. Assuming a high flow, low pressure fuel pump drawing from the inboard end of the tank through a heat exchanger and returning to the far outboard point of a properly baffled tank, you can assume fairly decent thermal mixing and complete bottom tank skin exposure to the slipstream. The issue then becomes twofold. Can the tank skin dump that much heat in a worst-case-scenario of a hot day with a full-power climb (think extended touch and go session here, in summertime West Texas heat) and what kind of heat exchanger/pump setup would be needed to handle that heat flow?

For takeoff/climbout on a hot day, you can safely assume (especially with cool tanks) that you'll be effectively "storing" heat for the first 10 minutes or so in the fuel, to be dumped once you climb into cool air. What would be the equilibrium temp of the fuel at common cruise conditions, say 75% power(120,000 btu/hr heat rejection), 140ish kias, and 40F OAT?

At first blush, I thought this was a pretty cool idea. Then I started seeing the problems associated with it, and thought it was a disaster looking for place to happen. Then, the more I thought about it, I starting seeing solutions to the problems posed, and now I'm back to the point of thinking it just might be feasible from a design/engineering standpoint, if the heat flows work out. Any suggestions?

Kyle Boatright · #2 10-04-2006, 03:23 PM

Think about what happens to the mass of your heat sink and to the effective radiator area of the fuel tanks as you burn off fuel.

airguy · #3 10-04-2006, 03:27 PM

Considered that - hence the need to establish a minimum quantity of fuel in the tanks for cooling. As long as your pumping it to the far end of the baffled tank, it should cover all the surface area of the tank on the way back to the pump, and you are still dumping the same amount of heat through the same number of square feet of wing, just with a smaller quantity of fuel over a shorter period of time. The fuel will not care about the rate of it's thermal cycle. The key will be ensuring that you're still covering enough square inches of tank surface to dump that same quantity of heat.

PJSeipel · #4 10-04-2006, 03:44 PM

It's a neat idea, but there's definitely a lot of challenges. You would need to design the system to make it impossible to run the fuel tanks below that minimum quantity required to keep the engine cool on a worst case hot day. That means you're probably going to carry around more weight than a traditional radiator setup would have added.

Also, wouldn't your operating limitations have to require that you not start the plane unless it's been stored in the shade and the fuel temperature is below a certain point to ensure you have enough cooling mass for your engine to survive a hot summer day taxi when the tower decides you have to go 2 miles to the far runway and wait in line for 20 minutes?

PJ
RV-10 #40032

videobobk · #5 10-04-2006, 03:52 PM

I recall that the amount of heat disapated by a surface levels out at about 40 mph, so your "radiator" would always be working at full capacity. Area would be a factor, and as noted, it would diminish as fuel level dropped. As a precaution, you might consider a cowled and vented aux radiator that would open (or be opened by the pilot) at higher temps. This would limit added drag to these times. Considering the power wasted in cooling an engine, I would think this would be worth investigating, although I have no idea of how well it could be made to work. It sure could make one "hot" plane, though!

Bob Kelly, Taxi testing

Mike S · #6 10-04-2006, 03:52 PM

Remember a commerical jet blowing up over New England a few years ago????

If you do this, just DONT have anything in the way of an ignition source in the tank--------capactance fuel guage comes to mind.

Mike

RV_7A · #7 10-04-2006, 04:08 PM

One issue I can see that would largely affect this wonderful concept is once the aircraft has landed, if the fuel was cooler than ambient air, condensation would be a certain factor.

hevansrv7a · #8 10-04-2006, 04:24 PM

What about cooling vanes on the wing, aligned with the airflow? They might have the additional advantage of acting like fences to inhibit spanwise motion of the air. Maybe get a winglet effect? There might be a slight weight penalty which might be offset by the reduced cooling drag. What kind of lightweight fluid coolant could be used?

captainron · #9 10-04-2006, 05:03 PM

Wouldn't it be great if someone would just build a simple, lightweight, direct drive, air-cooled engine for these little planes? Hopefully, one designed for, and capable of running at high, or full power for a couple thousand hours? Lemme see, at 150 MPH, that would mean you could travel and explore new places for, golly, 300,000 miles! No complex systems or cooling leaks to worry about? An engine you wouldn't have to think twice about when your wife, or son or daughter wanted to go for a ride with you? I sure hope someone invents one soon!

L.Adamson · #10 10-04-2006, 05:16 PM

Quote:

Originally Posted by captainron

Wouldn't it be great if someone would just build a simple, lightweight, direct drive, air-cooled engine for these little planes? Hopefully, one designed for, and capable of running at high, or full power for a couple thousand hours? Lemme see, at 150 MPH, that would mean you could travel and explore new places for, golly, 300,000 miles! No complex systems or cooling leaks to worry about? An engine you wouldn't have to think twice about when your wife, or son or daughter wanted to go for a ride with you? I sure hope someone invents one soon!

I know what you mean! I'm in the heating and cooling business, and have seen home grown heat extraction systems that seemed to take half the basement, as well as a wall full of pumps, valves, relays, etc.

And then I just throw in these little 90+ efficient heating systems, that are an example of compactness & symplicity, & most of all, reliability.

And that's exactly what we're comparing here. A plane weighed down by plumbing, pumps, and probable headaches; when something as simple as a four cylinder air cooled engine would solve all problems.

L.Adamson RV6A 180 Lyc