Radiator coatings

[daveismissing]

A topic with lots of opinions


Watercooler discussions below Earl's comments:



Earl:
If it matters. The color/paint used to increase radiation doesn't do anything. Now there is a paint that is designed to absorb longwave radiation and convert it to heat.....http://www.builditsolar.com/Experimental/PaintAbsorbancy/GlossvsFlat.htmIt performs almost as well as plain ole flat black 
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Bruce the science guy:

Actually, there is a lot of reason not to paint it blackit should be running at around 350K. That makes its thermal emissionpeak at about 8.4 microns.this is mid infrared... so to enhance heat loss, it's fine to paint itwith somethIng that has high emmisivity in the mid infra-red. You can be fairly sure that black paint is not such a material. Neither is anodising but sandblasting may work well.
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Wayne the Mechanical Engineer:

There is a theoretical reason, although in practice I don't know if it
really matters. Most of the heat exchanged to the air from the
radiator is through conduction with direct contact with the air flow
(through boundary layer effects). The type of surface on the radiator
will have a drastic effect on the boundary layer, and thus the cooling
ability of the radiator. Therefore, one would think the surface
texture of the paint is far more important that the color. However,
some small amount will be radiated via black body radiation to the
surrounding air by the Stefan-Boltzmann law, H=A*sigma*epsilon*(Th^4
- Tl^4). Th is the high temp, Tl is the lower temp, H is heat, A is
area, sigma is Stefan-Boltzmann constant (5.67×10-8 W m-2 K-4) and
epsilon is the emissivity of the surface. The important thing in this
conversation is the emissivity. Believe it or not, black paint has
higher emissivity than other colors so it will radiate more heat. Here
is a table of emissivities:

http://snap.fnal.gov/crshield/crs-mech/emissivity-eoi.html

Now, aren't you sorry you asked. Thermodynamics 101.
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SP the electrical engineer:

The emissivity won't help for the side pointed at the (I think)
hotter engine, it will actually hurt. This comes up in heat sinks
used in hybrid tube-semiconductor equipment where the heatsink is
looking at a rather hot vacuum tube. I think ideally you would
want the front and sides painted black and the back mirror-like
(very low emissivity).

Also, you must consider the emissivity in the correct wavelengths..
which are rather long due to the relatively low temperature.

http://www.thermalcities.com/images/planck_curves.gif

We know the peak wavelength is b/T where b is Wein's displacement
constant of 2.89777E+6 K-nm, so for a car radiator operating at
80°C = 353.15K the peak wavelength is ~8.2 microns.

An object may appear black to visible light
but be shiny to deep IR. I had a customer complaining that the
optical pickup was not working properly and it turned out that the
black ink used in his inkjet printer was transparent to near-UV
so that it was looking at white-on-white markings. A photocopy of
the pickup pattern cured it.

Anyway, in this temperature range and range of temperature differences,
radiation cooling is going to be UTTERLY negligible compared to the forced
convection from the fan and forward motion of the car. So, anodize it
chartreuse or teal if it turns your crank.

[Steve Wood]

In the real world, what is important....a ny coating applied cannot be thick enuf to impede air flow or act as an insulator....c onvection is where it's at

[earlbrown]

It's been a long long time since I read into emissivity of radiators but I recall something about it not being physically possible. The small gain during perfect conditions is more than taken away by the insulating value of the paint (at all conditions)


I also just realized typing that first line that a radiator is actually a convector. Unless you submerge the front of the car into a lake of course.

[Steve Wood]

In just a few years, coatings have gone from something many racersconsider ed to be little better than snake oil, into a tool that's usedjust about everywhere on a race car. Coatings are widely thought of as away to reduce friction, but other blends are great at controlling heat.The trick, according to Tech Line Coatings' Leonard Warren, is tounderstand what you want to do with the heat your race car produces.Coati ngs, known as thermal dispersants, can be used to help hot parts onthe car radiate heat more efficiently. Tech Line sells coatings designedto be applied to radiators, oil pans, brake calipers, and just aboutanything else you can think of to help them stay cooler. "We have seenthermal dispersants improve the thermal transfer efficiency of aradiator by 20 percent," he says. "I've applied it to an alternator andwatched it drop 30 degrees of operating temperature the next time it wasinstalled on the same engine."

Read more: http://www.circletrack.com/techarticles/ctrp_0507_ways_to_keep_your_engine_cool/viewall.html#ixzz24Ql4LoBx

[Scott87]

Only reason I asked what he coated the radiator with was for balance between thermal dissipation and protection.

With any paint there is a certain amount of insulating properties unless it is high temperature aluminum used in engine rooms...

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[daveismissing]

IMHO: Surface area and volume (air and water). When you've maxed those then coatings and surface finish. We also fail to use the chassis as a heat sink.

[earlbrown]

I like coatings in exhaust ports, piston tops, combustions chambers etc....


I just can't wrap my head around coating a radiator and expecting a return. There's only three ways to move heat energy: conduction, convection and radiation.

Lets say that a radiator rejects 99% of it's heat energy with convection, .75% through radiation, and .25% through conduction (that's being nice as a hot transmission line is conducting into the radiator shell)

Now let's say that Wiz-Bang LLC invents a coating that "Doubles your emissivity or your money back!!!!   but wait there's more, you get these free wind chimes just for trying it!"

Lets say your cooling system is maxed out at 400HP. Now your emissivity has doubled you have a cooling system that can now handle 401HP!!!!  (That's also assUming the coating has 0 insulating effect)     

It's be like standing next to a pumper truck with a squirt gun...     Then training a gorilla to pull the trigger harder.

On the other side, the coating will be great for bench racing. It'd be like being the only guy in the parking lot with a powerband.

[Steve Wood]

the TLTD from Techline seems to produce some results.  Most uses I have seen has been on blocks and heads but there are some out there with it on intercoolers and such.

[earlbrown]

Isn't TLTD designed to help hot oil fall off of parts so it doesn't act as an insulator (and to give cold oil a chance to do it's thing)?

[Steve Wood]

That is one of the uses...bottoms of pistons and such...keeps the temps down by promoting the oil to drain more quickly but it also transfers heat to the oil as it picks up more heat than bare metal alone. Should keep combustion chamber temps more in control.  These days, it is used on blocks and heads  plus radiators and intercoolers.  Claimed to be about 20% more efficient in heat transfer than bare metal.  I doubt one could tell the difference to any substantial degree if the radiator is of proper size.