Questions relating to Ahmed radiation notes

Diffuse surface

A surface whose radiative properties are independent of direction

Gray surface

A surface whose radiative properties are independent of wavelength

Is a gray surface necessarily diffuse?

No, gray refers to wavelength independence while diffuse refers to directional independence

Emissivity (ε)

Ratio of radiation emitted by a surface to that emitted by a blackbody at the same temperature

Irradiation (G)

Radiation incident on a surface per unit area from all directions

Radiosity (J)

Total radiation leaving a surface = emitted + reflected radiation

Relation between α, ρ, τ

α + ρ + τ = 1

Opaque surface property

Transmissivity τ = 0, so α + ρ = 1

Does emissivity always equal absorptivity?

No, only at thermal equilibrium

Kirchhoff's Law

At thermal equilibrium, emissivity equals absorptivity (ε = α)

Blackbody definition

A perfect emitter and absorber of radiation (ε = 1, ρ = 0)

Stefan-Boltzmann Law

E = σT^4

Real surface emission

E = εσT^4

Wien's displacement law

The wavelength of peak emission from a black body is inversely proportional to the absolute (kelvin) temperature of the body.

Wien's Displacement Law Equation

λ_max T = 2898 μm·K

Effect of increasing temperature on λ_max

Peak wavelength decreases (shifts to shorter wavelengths)

Planck's law of radiation

relates black body radiation energy density and inverse wavelength to 5th power( and temperature)

Diffuse surface emission characteristic

Emits equally in all directions

Is a blackbody diffuse?

Yes, a blackbody is also diffuse

Gray surface property

Emissivity is independent of wavelength

What does radiosity include?

Emission and reflection

Does radiosity include absorption?

No

Ideal solar collector surface

High absorptivity at short wavelengths and low emissivity at long wavelengths

Why low emissivity at long wavelengths (for ideal solar collector)?

To minimize radiative heat loss

Greenhouse effect explanation

Glass transmits shortwave solar radiation but blocks longwave infrared radiation

View factor definition

Fraction of radiation leaving one surface that reaches another

Reciprocity relation

A1F12 = A2F21

Summation rule of view factors

Sum of all view factors from a surface equals 1

Can a flat surface see itself?

No, Fii = 0 for flat or convex surfaces

Which of the two cannot see itself: concave or convex

convex

Net radiative heat flux

q'' = εσT^4 − αG

What happens if both surfaces are black?

Heat transfer is maximized

Radiation wavelength range

Approximately 0.1 to 100 microns

Blackbody emissive power

Eb = σT^4

Spectral property definition

A property that depends on wavelength

Directional (specular) property definition

A property that depends on direction

Diffuse vs specular

Diffuse is direction-independent, specular depends on direction

Intensity definition

Energy leaving a surface per unit area, per unit solid angle, per wavelength

Hemispherical emission relation

E = πI

Wien's law interpretation

Higher temperature objects emit radiation at shorter wavelengths

Band emission fraction

Fraction of total radiation within a wavelength range

View factor physical meaning

Geometric relationship describing how surfaces exchange radiation

Radiation heat transfer between plates

Depends on emissivities and temperature difference to the fourth power