Black painted stainless steel radiates heat better than the same plain clean stainless steel metal.
"Physicists calculate how much light a perfectly emitting object would give off at a given temperature, and then they multiply that by a number between 0 and 1. objects that are black at room temperature are close to 1, objects that are shiny or white are close to 0.
That’s because to conserve energy, the best absorbers (those looking black) have to be the best emitters."
At what temp will stainless steel glow red?
"Doesn’t really matter what the emitter is...stainless steel, cast iron, tungsten in your light bulb, the temps are about the same for a given color. Generally accepted colors/temps are:
C F Color
400 752 Red heat, visible in the dark
474 885 Red heat, visible in the twilight
525 975 Red heat, visible in the daylight
581 1077 Red heat, visible in the sunlight
700 1292 Dark red
800 1472 Dull cherry-red
900 1652 Cherry-red
1000 1832 Bright cherry-red
1100 2012 Orange-red "{*}
RADIANT HEAT TRANSFER
Radiant heat transfer is thermal energy transferred by means of electromagnetic waves or particles.
Thermal Radiation
Radiant heat transfer involves the transfer of heat by electromagnetic radiation that arises due to the temperature of a body. Most energy of this type is in the infra-red region of the electromagnetic spectrum although some of it is in the visible region. The term thermal radiation is frequently used to distinguish this form of electromagnetic radiation from other forms, such as radio waves, x-rays, or gamma rays. The transfer of heat from a fireplace across a room in the line of sight is an example of radiant heat transfer. Radiant heat transfer does not need a medium, such as air or metal, to take place. Any material that has a temperature above absolute zero gives off some radiant energy. When a cloud covers the sun, both its heat and light diminish. This is one of the most familiar examples of heat transfer by thermal radiation.
Black Body Radiation
A body that emits the maximum amount of heat for its absolute temperature is called a black body. Radiant heat transfer rate from a black body to its surroundings can be expressed by the following equation.
Q = б AT^4
where:
Q = heat transfer rate (Btu/hr)
б = Stefan-Boltzman constant (0.174 Btu/hr-ft^2-°R^4)
A = surface area (ft^2)
T = temperature (°R) (unit is in Kelvin)
Two black bodies that radiate toward each other have a net heat flux between them. The net flow rate of heat between them is given by an adaptation of Equation.
Q = б A (T1^4 – T2^4 )
where:
A = surface area of the first body (ft^2)
T1 = temperature of the first body (°R)
T2 = temperature of the second body (°R)
All bodies above absolute zero temperature radiate some heat. The sun and earth both radiate heat toward each other. This seems to violate the Second Law of Thermodynamics, which states that heat cannot flow from a cold body to a hot body. The paradox is resolved by the fact that each body must be in direct line of sight of the other to receive radiation from it. Therefore, whenever the cool body is radiating heat to the hot body, the hot body must also be radiating heat to the cool body. Since the hot body radiates more heat (due to its higher temperature) than the cold body, the net flow of heat is from hot to cold, and the second law is still satisfied.
Emissivity
Real objects do not radiate as much heat as a perfect black body. They radiate less heat than a black body and are called gray bodies. To take into account the fact that real objects are gray bodies, Equation is modified to be of the following form.
Q = Є б A T^4
where:
Є = emissivity of the gray body (dimensionless)
Emissivity is simply a factor by which we multiply the black body heat transfer to take into account that the black body is the ideal case. Emissivity is a dimensionless number and has a maximum value of 1.0.
Radiant Heat Transfer Summary
Black body radiation is the maximum amount of heat that can be transferred from an ideal object.
Emissivity is a measure of the departure of a body from the ideal black body.
Radiation configuration factor takes into account the emittance and relative geometry of two objects.
http://www.infrared-thermography.com/material.htm
Material condition Emissivity
Stainless steel: type 18-8,buffed 0.16
Stainless steel: type 18-8, oxidized at 800°C 0.85
Steel: sheet, nickel plated 0.11
Steel: oxidized 0.79 "{*}
Backup reference (with a neat diagram)
http://en.wikipedia.org/wiki/Thermal_radiation
