With an uncoated copper radiator you are loosing a lot of the radiation part.
The radiator has two components how he gets rid of heat:
1. Convection with the air
At a car radiator the convection part is the most important one but the
radiation is important too.
A metallic copper surface has a radiation transfer of 0,15.
A black coated copper surface can transfer 0,98.
That's the reason why all the engines are black coated.
In this connection I use this type of transfer with radiation heating
plants for energy efficient houses. This type of heating reduces the
enegy consumption a lot and avoid the build up of mould fungus. If
the shiny copper tubes are directly coated with roughcast the heat
transfer through radiation is about 0,92-0,95.
Hope this helps
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