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Thermodynamics: Heat absorption / rejection vs colour

by SouthSide Sean
Tags: absorption, colour, heat, rejection, thermodynamics
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SouthSide Sean
Apr7-13, 03:42 PM
P: 3
I am working on an automotive project and am looking to pick up any gains that I can. In doing so, I've been studying the effect that colour has on thermodynamic characteristics.

To be specific, I am interested in finding:

A) What colour(s) will help retain the most heat i.e. minimize thermal radiation & losses via reduced emissivity. My desire is to retain as much heat within the exhaust system as is possible.

B) What colour(s) will minimize heat transfer via reduced absorption. My desire is to keep the incoming air as cool & dense as possible via minimizing thermal transfer of heat generated from underneath the hood of the vehicle into the air intake system.

While the parts / materials located under the hood in the intake system will not be exposed to direct light, the exhaust system will see both reflected light & heat from the black driving surface.

Any & all help appreciated. Sean
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Apr7-13, 05:49 PM
P: 11,589
Colors describe how the material reacts to (and emits) visible light, but heat is in the infrared range. There is no fixed relation between those regions.
SouthSide Sean
Apr7-13, 09:50 PM
P: 3
I keep reading & seeing stuff like this, but I have a hard time believing it.

If this is true, how can two identical items of two different colours measure two different temperatures in the same exact environment? I am NOT being a smart-*** ( maybe a dumb-*** ), as I am asking a legit question. Sean

Apr8-13, 04:25 AM
P: 11,589
Thermodynamics: Heat absorption / rejection vs colour

Different materials can have different emission/absorption coefficients in the infrared as well.
If you have additional visible light, the temperature is determined both by the emission/absorption coefficient in visible light (larger: hotter) and infrared (larger: colder).
As a result, black cars are hotter in sunlight, for example: They absorb more visible light, but their infrared emission (at the same temperature) is similar to cars of other colors.
SouthSide Sean
Apr8-13, 10:50 PM
P: 3
Thank you for taking the time to respond and for the clarification. I still don't fully understand but at least have a better idea of both what you speak & the direction that I need to head in. Best wishes and THANKS again!
Apr9-13, 10:51 PM
P: 17
As has been stated, color does not really play an important role in absorption/reflection of "heat." In wavelengths most associated with thermal transfer, the material used and surface condition play a far greater role. For instance, fresh brushed aluminum has an emissivity of ~.06, while the as received plate may have an emissivity of ~.09. Aluminum foil has very low emissivity, usually around .03 depending on the thickness and how it was rolled. This is probably the best material you can use strictly for low-e applications. Wrapping the exhaust in fiberglass header wrap seems like the ideal solution, the thermal conductivity between the wrap and the exhaust is relatively low, so the header will be allowed to heat up to a temperature above that of the wrap.

As far as absorption, Kirchoffs law says that emissivity = absorption, so reflectivity + transmission + absorption = 1. In other words, if an object has an emissivity of .03, it will reflect 97% of radiation hitting it, assuming no transmission. It does not seem like one should be worried about reflected or emitted radiation from a black driving surface in the case of the exhaust. Asphalt typically has an emissivity of ~.9+, so it will reflect almost no energy. It will emit some radiation depending on its temperature, nearly perfectly, but in my mind this is not a significant factor in performance.

Regarding the intake, minimizing absorption of radiation may not be very effective. Heat transfer between intake air and intake plumbing is not that efficient anyway, so lowering the temperature of intake plumbing (the goal of applying reflective coatings) may not lower intake air temperatures much. I would imagine that given the amount of time the flow of air remains in the plumbing, the temperature would not increase much at all from when it was pulled in. This is unless you are pressurizing your intake air with a turbo or something.

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