Heat Dissipation with a Belly Pan

[Thenorm]

Originally Posted by drrbc

@Thenorm- so as airflow/convective loss decreases, radiant loss will become the dominant mechanism. My thoughts then would be to either avoid interfering with airflow, or maximize radiation.

I'm saying that radiation is not an effective mechanism at our temperatures. Convection (forced air) is really the only real way to shed heat in vehicles.

[chefdave]

Hi
with a full or nearly full belly pan could you fit nasa? Style ducts to help air flow without drag penalty. as fitted to bonnets on nasa cars. this is just a random thought but could work.

[aerohead]

Originally Posted by chefdave

Hi
with a full or nearly full belly pan could you fit nasa? Style ducts to help air flow without drag penalty. as fitted to bonnets on nasa cars. this is just a random thought but could work.

I found a forward bellypan section of some kind of Audi laying in an intersection and brought it home to look at.
It has 3-NASA submerged inlets leading upwards towards some under-engine components,so they were thinking exactly as you are.

[aardvarcus]

I am the one with the failed belly pan in question in the first post. My gas tank overheated with a partial aluminum belly pan, complete 4’ wide down the middle but with the sides fully open. The air from the engine bay was free to enter and flow through the pan and out the back. The sides were open by several inches as it wasn’t yet completed, yet it still didn’t allow the heat to dissipate. It melted plastic parts up under my car. It heated the bolts holding my seat down up so much you couldn’t hold your finger on them for more than a second or two. And it boiled over a gallon of gas out of my tank. I went back and cut holes in the pan under the exhaust to let more heat escape, but things kept heating up and I ended up removing the back portion of the pan halfway home.

I thought the exact same thing about heat dissipation with my belly pan, painting the top and bottom, cutting holes, insulating the exhaust pipe, etcetera. I have some even crazier/better ideas than those for dealing with the issue. Note the pan overheated while driving down the interstate and highway, 65-75 MPH the entire time. I don’t want to think about what would have happened if I had suddenly been stuck in stop and go traffic for an hour.

What I ended up deciding for myself is that a 90% belly pan is good enough for me. As I rebuild by belly pan V2 I am going to leave the 10% or so of the exhaust exposed. I am going to radius the edges of the pan as I near the exhaust to limit the aerodynamic consequences of having a small section of the pan missing. If you look at other “full” pans, many of them have screen wire under where the exhaust is, so basically the exhaust is kind of “exposed” on those pans anyway.

Just conceptually, my vehicle is less than ideal in a number of areas. There are still a lot of aerodynamic areas for improvement with my car. It isn’t impossible to reduce the heat transfer from the exhaust to the car as it travels from the engine bay out the back. Honestly it isn’t rocket science or really that hard. All it takes is thinking, time, money, and effort. But since my car isn’t perfect in so many other areas, I decided that I will start with my 90% pan, and then focus on fixing all the other areas on my car that need work. As it sits now, I don’t even know if you would be able to measure the difference in effectiveness between a 90% and 100% pan. Maybe if I get all the other areas taken care of I will revisit the 10% of the pan under the exhaust.

I tell you nothing makes you want to stop eco-modding more than spraying a boiling gas tank with a garden hose or trying to take a belly pan off a low car on the side of the road with no lift. Frankly it is very discouraging. I would try to stick to the easier no risk almost guaranteed reward projects first.

Maybe your vehicle will work with a 100% full pan with no special work around the exhaust. Several people on here state it will work fine. Why do you think I tried it in the first place? All I know is mine won’t.

[aerohead]

aardvarcus,that's a lot of work to go through to end up 'punished' after your good deed.
Thanks for attempting it and sharing all the warts and moles of the project.Countless individuals will no doubt benefit from the info.
If we all lived closer we could laugh,scream,cuss,kick,and throw things together.
Collective brainstorming would be easier too.Tool and materials sharing also.
Your project is still a success as we've access to invaluable information thanks to your contribution.Thanks again.

[aerohead]

Here is a link to illustrate how Audi is harvesting air under their belly pans
Sorry,tools don't show above.
The image is at :GOOGLE IMAGES @ Audi smooth belly pans

[drrbc]

aardvarcus,

Firstly, please understand my mentioning your experience was academic and not for fun.

I value what the folks here can teach me, but in general we seldom mention our failures- and these are priceless. Personally, when I find someone with the humility and honesty to willing write up an (again, please forgive) "Epic Fail", I am forced to acknowledge they are probably a better person than I.

I tip my hat to you sir.

That said, I assume the gas tank doesn't warm itself and most tanks are not attached to parts that generate large amounts of heat. And you mention yours was only partially covered. We can infer (since you didn't mention it) that there was no "forced convection" to cool the tank.

From this I would suggest the mechanism of heat transfer to your tank was via radiation, and that this radiation would have normally been transferred to another site (by conduction) where convective loss could occur, or (with respects to Thenorm) the primary mechanism of heat loss from the heat producing agent is via radiation.

I also have noticed some modders using screen coverings, I'm assuming steel or Aluminum. As I write this I'm looking at a 12'X12" Al screen with 1/8" holes and can easily imagine it absorbing radiant energy from a 1,000º catalytic converter or a 700º muffler. This screen would begin radiating the same energy downhill. First to a cool vehicle itself (that warms up quickly because it dissipates heat poorly), and then into the planet itself- which wouldn't even notice it.

As my screen is mostly just "holes", there is nothing there to prevent radiation going into the ground to start with. But I also imagine there IS enough there to promote laminar flow of air over it as well as significant pressure drops though it.

In other words, again with respects to Thenorm, I'm not buying that radiation is "not effective" and that "Convection (forced air) is really the only real way to shed heat in vehicles.". Indeed, it seems the opposite may be true.

Convection IS important, but it is also linear and I do not see how it can account for even half of the heat flux, especially (as Thenorm mentioned) radiant loss increases to the 4th power of the dT.

I may be wrong though, and will continue reading on this.

Once more, my respects to you and your work. Thank You.

drrbc

[mcrews]

Originally Posted by aerohead

Here is a link to illustrate how Audi is harvesting air under their belly pans
Sorry,tools don't show above.
The image is at :GOOGLE IMAGES @ Audi smooth belly pans

audi



mine:

[aardvarcus]

Thanks for the kind words, I mainly posted my story for fear of others safety.

I agree that most of the problem was radiant heat transfer, aggravated by the reflective aluminum panel right under the exhaust reflecting the additional rays back up. Without the belly pan though, the convective heat transfer of the cool air blowing up under the car would have removed all that built up heat. With the belly pan though there was only a limited amount of air that could not remove the heat. Hence problem.

The more I think about the issue with the exhaust heating, the solution I like is instead of trying to only deal with the heat after it has left the exhaust, but first to significantly reduce the heat transfer out of the exhaust pipes. Several ways would be ceramic coating the exhaust system, using exhaust wraps, putting the exhaust pipe within another pipe, putting reflective barriers or other insulation above the exhaust pipe. Additionally, going a step further in complexity the exhaust or exhaust cover could be made with a flat bottom where the bottom of the exhaust was actually part of the belly pan, but allowing direct conductive cooling of the exhaust or outer exhaust cover.

As I said before, it is just thinking, time, money, and effort.

[drrbc]

Originally Posted by aardvarcus

...putting reflective barriers or other insulation above the exhaust pipe. Additionally, going a step further in complexity the exhaust or exhaust cover could be made with a flat bottom where the bottom of the exhaust was actually part of the belly pan, but allowing direct conductive cooling of the exhaust or outer exhaust cover.

As I said before, it is just thinking, time, money, and effort.

My thoughts as well, and I imagine that may be the future. On the down side, no matter who does this would make serviceability of the system a nightmare. In the mean time however I think my wife may have a nice aluminum cookie sheet that I could small scale.