Felix E Posted September 29, 2004 Share Posted September 29, 2004 I seem to remember from Physics lessons that black bodies radiate heat far more efficiently than any other colour, so would it improve engine cooling to spray the radiator with a high temperature black paint? Just a thought for a wet Wednesday afternoon. Best Regards, Felix. Link to comment Share on other sites More sharing options...
Sheds Moderator Posted September 29, 2004 Share Posted September 29, 2004 Yes it would help a bit. Don't forget though that rads work by passing air through (convection/conduction), not radiation so the difference is minimal. As for high temp, it's only going to hit tops 110°C. Where's it raining? Sunny here 😬 Link to comment Share on other sites More sharing options...
Red SLR Posted September 29, 2004 Share Posted September 29, 2004 Mine is black? LeMans 2004 pics here Seven related chat here Link to comment Share on other sites More sharing options...
Pelico Posted September 29, 2004 Share Posted September 29, 2004 Mine is black too, except for the addition of a layer of Arthropodal conduction enhancing material 😬 My Caterham owner's site Link to comment Share on other sites More sharing options...
Peter T Posted September 29, 2004 Share Posted September 29, 2004 Most brass ones are black, most alloy ones are silver being the colour of the aloy.A brass rad that has not been sprayed looks very messy with all the solder on it, hence it is coloured black which shows a lot less dirt. Support the 7 Society...... Ehhh? Link to comment Share on other sites More sharing options...
Red SLR Posted September 29, 2004 Share Posted September 29, 2004 Mine is a tripple core ali rad. I think it was painted though. (I posted ages ago on the best way to remove the paint...) LeMans 2004 pics here Seven related chat here Link to comment Share on other sites More sharing options...
Ferrino Posted September 29, 2004 Share Posted September 29, 2004 Is it OK to spray paint a rad? I am picking up a used rad which is flaking paint a wee bit, but was worried about paint setting between the small spaces between the fins. Guess I could relax my 'liberal coating' technique! Link to comment Share on other sites More sharing options...
Ken Sailor Posted September 29, 2004 Share Posted September 29, 2004 I repainted mine with a radiator paint I got from Eastwood here in the US. Same kind of company as Frost so they probably have it also. Not 'gloppy' at all dispite my liberal spray technique too. Ken Sailor Link to comment Share on other sites More sharing options...
Graham Perry Posted September 29, 2004 Share Posted September 29, 2004 I am no expert but if you are going to paint it black shouldn't you paint it that colour on the inside ? 😬 😳 Link to comment Share on other sites More sharing options...
puremalt Posted September 30, 2004 Share Posted September 30, 2004 I sprayed my alloy one black without any problems as it enhances the se7en grille in front of it! Clamshells...You Know It Makes Sense Link to comment Share on other sites More sharing options...
Peter Carmichael Posted September 30, 2004 Share Posted September 30, 2004 I hadn't realised there was "radiator paint" on the market. Any paint will be an insulating layer and will make the radiator less effective. Sounds like "radiator paint" might be a specialist product - I would hope such a specialist product would produce a thin film and have reasonable conductivity. The key thing here is that "radiators" disperse heat using forced convection predominantly. Radiation is very much a secondary effect. The Stefan-Boltzmann law of radiation can tell us how much energy is dissipated by radiation, if we make a bunch of assumptions. H = esAT^4 e = emissivity (0-1) s = Stefan-Boltzmann constant = 5.67 x 10-8 J/(s.m^2.K^4) A = surface area of object T = Kelvin temperature Presume black body emissivity of 1. Presume all radiation goes to front and rear of the radiator Ignore all the surface area of the intricate bits of the core, because in radiation terms they cancel out Presume a surface area of 400mm x 500mm x 2 Presume ambient temperature of 17 degC to the front of the radiator (290 K) Presume an engine bay temperature of ~40degC average for most of the radiating items (bit of a fudge, but it will do) Presume something about the temperature distribution across the radiator core - only at standstill will it all be hot which will give us a high bracketing figure. 90degC all the way across the radiator. Hout= 1 x 5.67 x 10^-8 x .4 x .5 x 2 x 363^4 = 394 J/s The radiation received from surroundings to the front of the radiator given my assumptions is: Hinf= 1 x 5.67 x 10^-8 x .4 x .5 x 290^4 = 80J/s The radiation received from the engine bay to the rear of the radiator given my assumptions is: Hinf= 1 x 5.67 x 10^-8 x .4 x .5 x 313^4 = 109J/s Therefore the net heat lost by radiation is... wait for it...394-80-109 = 205 J/s unless I've got my sums wrong. That is a miniscule quarter horsepower being dissipated. Think about making a vacuum flask out of a radiator and you'll see how the numbers might stack up - remember that this is an upper bracketing figure. Now the actual heat dissipated can be worked out from the power output of the engine and typical thermal efficiency. At a compression ratio of 10.5:1 the Otto cycle (4 stroke spark ignition) is about 61% thermally efficient, so for every 61bhp you get 39 horsepower rejected as heat - a lot of it goes out of the exhaust. The majority of the rest of it is handled by the radiator. The radiator has to dissipate tens of horespower; the maximum contribution that radiation makes is to dissipate 0.27 horespower. Therefore all the bulk of the work done by a radiator is done by forced convection - don't paint a radiator if you can help it. Link to comment Share on other sites More sharing options...
Peter Carmichael Posted September 30, 2004 Share Posted September 30, 2004 And the most important point is that Ludwig Boltzmann looks a bit like Felix Klauser with a wild beard... Link to comment Share on other sites More sharing options...
Peter Carmichael Posted September 30, 2004 Share Posted September 30, 2004 The emissivity of aluminium depends very much on its surface condition. Polished aluminium has a very low emissivity - typically 0.05. Roughened or anodized aluminium has an emissivity of ~0.2-0.3. Using my previous calcs, this means that the actual radiated heat from my Seven radiator is one sixteenth of a horsepower. And just to make life complicated... ...matt black paint has a typical emissivity of 0.5-0.8 So you are only looking at getting rid of an extra sixteenth of a horsepower or so with matt black paint. And to make it really confusing, white paint has an IR emissivity of ~0.97 - i.e. it is almost BLACK in radiation terms!!! Link to comment Share on other sites More sharing options...
Peter T Posted September 30, 2004 Share Posted September 30, 2004 Peter, you need to get out more. 😬 Support the 7 Society...... Ehhh? Link to comment Share on other sites More sharing options...
MikeSA Posted September 30, 2004 Share Posted September 30, 2004 Now there's how to kill a perfectly good thread 😬 Hey, I just drive em - fast! Link to comment Share on other sites More sharing options...
Tony Martyr Posted September 30, 2004 Share Posted September 30, 2004 The conclusion Peter correctly draws has produced a significant sift in radiator design over the years. How did they become to be called radiators? Heat exchangers is what they are. The old honeycomb or cellular construction looked good but suffered from the important heat transfer problem of a metal/air : metal/water ratio of about unity. Modern radiators are covered by grills and are designed to have turbuent airflow and as efficient as possible heat transfer. Aluminium is not easy to work in thin section and high temperature because it tends to anneal so ali 'radiators' tend to be an interesting compromise of surface area and mass Link to comment Share on other sites More sharing options...
Ken Sailor Posted September 30, 2004 Share Posted September 30, 2004 Peter C: The radiator paint I mentioned was in response to Ferrino's problem of painting a radiator that had flaking paint and his concern about clogging up the spaces between the fins. The radiator paint here that I used was thinner than regular paint and was heat resistant. Don't know anything about heat transfer. Ken Sailor Link to comment Share on other sites More sharing options...
Chelspeed Posted September 30, 2004 Share Posted September 30, 2004 They still look nicer black. Link to comment Share on other sites More sharing options...
Anthony Posted September 30, 2004 Share Posted September 30, 2004 Heat exchanger iss bound to put the price up 😬 Anthony Link to comment Share on other sites More sharing options...
Roger King Posted October 1, 2004 Share Posted October 1, 2004 Nice one Anthony. Link to comment Share on other sites More sharing options...
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