Absorptivity and Net Radiation Heat Transfer

Ch 4, Lesson B, Page 23 - Absorptivity and Net Radiation Heat Transfer

• Radiation heat transfer is a two-way street We know that the sun radiates energy and some of it hits the Earth.
• A portion of the energy that hits the Earth is reflected and a portion is absorbed.
• A very small portion may be transmitted right through the Earth.
• We are interested in the fraction of the incident radiation that is absorbed.
• This fraction is called the absorptivity and is represented by the symbol a.
• Alpha must lie between zero and one, but for this course, we will boldly assume that alpha is always 1
• This means that we will assume that ALL incident radiation is absorbed.
• We are often interested in the NET rate of heat transfer between two objects or systems.
• For example the NET rate of radiation heat transfer from the sun to the Earth is equal to the rate at which heat is transferred from the sun to the Earth minus the rate at which energy is transferred from the Earth BACK to the sun !
• As interesting and important as energy exchange between the Earth and the sun can be, it isn’t the kind of problem that will typically come up in this course.
• The most common radiation heat transfer problem involves a relatively small object sitting within its surroundings.
• We assume that the surface of the object is at a uniform temperature, Ts.
• We also assume that the surroundings are at uniform temperature, Tsurr.
• As I just said, we assume that the absorptivity is one for both the object and the surroundings.
• In the absence of better information, we further assume that the emissivities of the object and of the surroundings are the SAME. This is a big assumption, so if you have better information about the emissivities, don’t be afraid to use it.
• When we use the Stephan-Boltzmann Equation and put all of the equations together, we get a fairly simple equation for the net rate of radiation heat transfer FROM the object. Be careful here with our sign convention !
• You probably noticed that epsilon doesn’t have any subscript in the last equation.
• That is because we assumed that the emissivity is the same for both the object and the surroundings.
• Since there is only one e in the problem, we no longer need a subscript to say where it applies !
• Well, concludes our discussion of heat transfer.
• Take a look at a couple of example problems and then give the quiz a try.
• In the next lesson, will finally put everything we have learned about energy together and formulate the 1st Law of Thermodynamics !