Radiation and Heat Transfer Concepts

Radiation

Emission of energy in wave form.

Irradiance

Power per unit area received from radiation.

G = αG +ρG + τG

Reflectivity

Fraction of irradiance (irradiation) reflected by a surface.

ρ

Transmissivity

Fraction of irradiance (incident radiation or irradiation) transmitted through a surface.

τ

Absorptivity

Fraction of irradiation absorbed by a surface.

α

Sum of Reflectivity, Transmissivity, and Absorptivity

α +ρ + τ = 1

0 ≤ α, ρ, τ ≤ 1 (for all of them)

Heat Flow for radiation (heat loss) Equation

*Q = εσA(Ts^4 - Tsurr^4) (temp in K)

Stefan-Boltzmann Constant (σ)

5.67E-8 W/(m^2 K^4)

E _emission

*Qrad/A = εσ (Ts^4 - Tsurr^4) (temp in K)

if no Tsurr, then E = εσ (Ts^4)

0 < ε < 1

Ts (source) eq

Ts = (E/εσ)^1/4

Eq of Enet and J

Enet = J- G

J = Enet + αG +ρG + τG

Duality Nature of light

Concept that radiation behaves as both wave and photons.

Emissivity

Ratio of radiation emitted by a body to black body.

0 < ε < 1

E_Net Radiation

Total radiation energy received minus emitted.

(absorbed - emmited)

Black Body

Ideal body (ex- the Sun)

Object that is (perfect) emitter and absorber of radiation

α = ε = 1

Gray Body

αλ = ελ < 1

Opaque Body

Anything you can't see through (non-transparent)

Material that does not transmit light

α≠0 ρ≠0 τ=0

G_op = αG + ρG

Semi-Transparent Body

Absorbs nothing

Material that partially transmits light.

α = 0 ρ ≠ 0 τ ≠ 0

G_st = ρG + τG

Grey-opaque

0 < α_λ < 1

ρ + α_λ = 1

ρ + ε_λ = 1

ελ = αλ < 1

τ = 0

G = ρG + αG = ρG + εG

White Body

Irregular surface that reflects everything (ex - mirror)

α = τ = 0

ρ = 1

Spectral Irradiance

Gλ = ∫∫∫ Iλ cosθ sinθ dθ dφ dλ

Graph of Wavelength λ vs Intensity I_λ (looks like even mountain graph like in statistics) (could not get pic on here)

Wavelength

Distance between successive peaks of a wave.

Radiosity (symbol J)

Total radiation leaving a surface per unit area.

Energy that leaves the object

Surface Temperature

Temperature of a surface in thermal equilibrium.

Solar Radiation

Radiation emitted by the sun, primarily visible light.

Thermal Conductivity

Ability of a material to conduct heat.