Radiation heat transfer

Question 1

What happens to the thermal radiation incident on a surface?

Answer 1

Some of it is reflected, some of it is absorbed and if the surface is glass some of it is transmitted through the glass.

Question 2

Explain emissivity values

Answer 2

A surfaces ability to absorb thermal radiation is the same as its ability to emit it. A hypothetical object that absorbs all radiation is called a black body and is used as a refference value to compare the emissivity of real surfaces.

Question 3

Explain emissive power (E), Irradiation (G), Radiosity (J), Net radiative flux

Answer 3

Emissive power (E) is the rate at which radiation is emmitted from a surface per unit area. (w/M^2)

Irradiation (G) is the rate at which radiation is incident on a surface per unit area (w/M^2).

Radiosity (J) Is the rate at which radiation leaves a surface per unit area (including radiation that is emitted, reflected and transmitted) (w/M^2)

Net radiative flux (J-G) is the balance of radation from a surface (w/M^2).

Question 4

What is the formula for emitted radiation per unit area?

Answer 4

Emitted radiation=SB constant*emissivity*temperature^4

Note temperature is measured in kelvin. Because it is a fourth power then emitted radiation raises sharply in proportion to temperature.

Question 5

Explain view-factor in radiation heat exchange

Answer 5

View factor means that not all of one surface can see another and hence cannot exchange radiation. View factor is the fraction of radiation emitted from one surface that is incident upon another. The notation is F_1-2 the fraction of radiation emitted by surface one that can be seen by surface 2. F_2-1 is the fraction of radiation emitted by surface 2 that can be seen by 1.

Question 6

Explain the Hottel interchange factor and its notation

Answer 6

This is only applicable to two surfaces that form an enclosure such as the different internal sides of a cube. There are a number of factors that affect the exchange of radiation between the two surfaces including: temperature, emissivity, size, distance and geometry. The Hottel interchange factor incorporates all of these into a single variable. The notation for total radiation from one surface 1 incident on surface 2 is F_{H 1-2.}

Question 7

How do you calculate the maximum wavelength emitted by a black body at a given temperature? (Weins law)

Answer 7

Maximum wavelength = temperature (K) * 2897.8 (micrometers)