Radiant Heat Transfer 101

[Geico266]

Some -12 builders / flyers are having some discussions about physics, heat transfer, and radiant heat transfer.

Can a knowledgeable engineer type help us understand radiant heat and what factors increase it or decrease it. Does humidity have an effect on radiant heat transfer? We have discovered a common theme among -12 flyers with "higher than normal" oil temps in that the level of humidity seems to be a factor. What role does humidity play on radiant heat transfer?

The muffler on the -12 is about 1 1/4" from the oil cooler and my heat shield / baffle seems to be working well to reduce oil temps about 15F.

Anyone have any experience in radiant heat transfer? PM me if you would rather talk on the phone.

Thanks in advance.

[mcencula]

Although I'm not an expert in heat transfer by any means, it would help if you consider that radiant heat is actually radiation (light). The amount of humidity would have essentially no effect on how much radiation is emitted from a surface.

IIRC, the amount of energy emitted by radiation really ramps up at the higher temperatures since the rate of radiated energy transfer is proportional to the fourth power of absolute temperature. At lower temperatures, the radiant heat is minimal.

The other factor which increases the rate of energy transfer is surface area. For a given temperature, if you double the area of the radiating body, you double the amount of energy radiated.

To prevent radiated heat from getting to another component, a heat shield is perfect. For a mental picture, imagine the hot item glowing brightly...any barrier you put between that and the thing you're trying to protect is going to put that item in the shade. That's pretty much what happens even if the radiation wavelengths aren't visible.

Hope that helps.

[AlexPeterson]

Mike's points are correct. The only clarification is that the heat transfer from radiation is a function of the difference of the absolute temperatures to the fourth power, among other variables. One of those variables is a parameter called a shape factor, which basically says, "how much of the sphere around me is at the hot temperature?" The skin of a muffler is probably in the 5 to 700 F range (guessing), but it is not a huge percent of the oil cooler's "sphere". I.e., most of what the oil cooler sees is not hot. Another is the surfaces' respective emissivities. A totally black body is considered to have an emissivity of 1 (interestingly, water is also nearly 1, as is white paint). I've learned that it is not easy to guess at something's emissivity - light colors are not necessarily lower. The higher the emissivity (1 is the maximum), the higher the rate of transfer. In other words, don't paint the oil cooler - polished is better!

Regarding humidity's affect on convection (air through the oil cooler), I'd expect also very little. At an extreme dewpoint of 80F (which rarely occurs), the air is only about 3% water vapor. While water molecules are lighter than the predominate nitrogen molecule, water's specific heat capacity is a bit higher than nitrogen. I didn't do any calculations, but I'd say dry vs humid air will cool the same going through the cooler. Density altitude will matter though (fewer molecules going through the cooler).