Radiation, usually associated with the sun, is visible light (short wave) containing heat (long wave, or infrared) radiated to the Earth through a vacuum.
Black surfaces absorb heat (high absorptivity: 0.9), while white and bright surfaces reflect short waves (low absorptivity: 0.25).
Upon touching the Earth, these waves are immediately reflected back into the surrounding environment, dispersing heat to cooler surfaces by means of long-wave radiation (the infrared wave or radiation).
“Radiators” for indoor heating are the most common long-wave radiative heaters, in which heat is transmitted not only by convection, but mainly by radiation. This is easier to understand in the outdoor infrared heaters used on some terraces during the colder months.
Moreover, according to Boltzman law, when the temperature is low, radiation occurs between objects with a temperature difference. According to the law, the thermal radiation is proportional to the fourth power value of the temperature.
For long-wave radiation, there is no difference in the emissivity ε between white surface and black surface (value range: 0.9 to 0.95). Metal basically does not emit any heat (aluminum, copper: ε in the range of 0.04). Therefore, when using an infrared thermometer camera, the temperature of a bare conductor cannot be measured properly because the camera cannot capture the infrared radiation emitted by the object.
When the temperature rise of a naked conductor is measured by natural convection, the conductor is painted black. Theoretically, black has the highest emissivity (0.95).
When the test is carried out with bare copper (tape only used at the measuring point), an increase in temperature to 70 ° C represents an additional temperature increase of 26 ° C or 60%. Painting a bare conductor has the same effect as adding an insulating layer to the conductor (insulating material has a higher emissivity ε). This means that moving from a multi-layer or “simple” bus bar using uninsulated copper to a laminated “high performance” bus bar can have a significant impact on conductor temperature.
It is intuitively assumed that adding an additional insulation layer will increase the temperature of the bus bar (observe the conductivity of the 0.44mm insulation layer at a temperature increase of 0.66 ° C), but in contrast, the radiation effect will decrease the temperature by 40% (26 ° C).
When fans are used for forced cooling, the effect of radiation on the bus exhaust performance is reduced because convection is dominant. Therefore, natural cooling fins are usually painted black (shock radiation), while forced fan cooling fins are usually made of uncoated aluminum.
Post time: Mar-29-2022