Heat Transfer (Unit-8) Thermal Radiation IIT lecture videos

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Heat and Mass Transfer

Prof. S.P. Sukhatme

Department of Mechanical Engineering

Indian Institute of Technology, Bombay

Lecture No. 10

Thermal Radiation-1

You will recall from the introductory lectures that we had 3 modes of heat transfer – conduction, convection and radiation. We have studied 1 mode namely heat conduction in solids. Today, we will now start talking about the second mode that is thermal radiation. If you recall from our introductory lectures, I had said thermal radiation, there are certain laws which we will have to study in order to be able to calculate what is the heat exchange by radiation between surfaces. So let us now state certain basic concepts about thermal radiation. The first thing which we would like to state is that all solid and liquid surfaces at all temperatures emit thermal radiation.

Lecture No. 11

THERMAL RADIATION 2

We were discussing the laws of back body radiation and last time when we stopped, we had stated 2 laws - Planck’s law for giving us the monochromatic emissive power of a black surface and Wien’s law which gives us the value of the wavelength at which the maximum value of the monochromatic emissive power occurs. Today we will take up the next law, the third law that is the Stephen Boltzmann law; the Stephen Boltzmann law is, gives us the emissive power of a black surface.

Lecture No. 12

Thermal Radiation-3

We now turn our attention to the directional nature of thermal radiation and in that context we are going to define 2 terms - solid angle and radiation intensity.

Lecture No. 13

Thermal Radiation – 4

Today we are going to first look at radiant heat exchange between 2 finite sized black surfaces.

Lecture No. 14

Thermal Radiation-5

So far we have calculated the radiant heat exchange between black surfaces - 2 black surfaces at 2 different temperatures - and in the process of deriving expressions for the radiant heat exchange between 2 black surfaces, we have defined a term called as the shape factor. Now today we would like to extend these ideas to first calculate radiant heat exchange rates in an enclosure consisting of a number of black surfaces. Consider the following enclosure as shown here.

Lecture No. 15

Thermal Radiation-6

In the previous lecture, we calculated or we rather we derived an expression for the heat exchange by radiation between 2 infinite parallel plates which are maintained at different temperatures T1 and T2. We got the expression for the heat exchange rate and we today we want to do a similar derivation but we would like to calculate the heat exchanged by radiation now in the annular space between 2 infinitely long concentric tubes. So once again, it is a two surface enclosure - the outer surface of the inner tube is one surface the inner surface of the outer tube is the other surface. These two surface make up a two surface enclosure.

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