Heat & Mass Transfer

Notes on Thermal Radiation-1

Introduction

  • Radiation is the energy emitted by matter in the form of electromagnetic waves.
  • Here we will look into thermal radiation (10-7 m to 10-4 m) since we are dealing with heat transfer.
  • In radiation the internal energy of an object decreases.

Rate of emission of radiation by a body depends on-

  • Surface temperature
  • Surface Nature
  • Wavelength or frequency of radiation

Important Definitions

Total Emissive Power (E)

  • Total amount of radiation (all wavelength range) emitted by a body per unit area and time.
  • A/C to Stefan-Boltzman, for a black body emissive power is proportional to absolute temperature to the fourth power.

Eb = σAT4 W/m2

σ = Stefan-Boltzman Constant (5.67*10-8 W/m2K4)

Monochromatic Emissive Power (Eλ)

  • Rate of energy radiated per unit area of the surface per unit wavelength.
  • At any given temperature the amount of radiation emitted per unit wavelength varies at different wavelengths. So, Monochromatic Emissive Power (Eλ) of the surface is used.

Emissivity (ε)

  • Ability of the body surface to radiate heat.
  • It is also defined as the ratio of emissive power of any body to emissive power of a black body of equal temperature.
  • Its value varies for different substances ranging from 0 (white body) to 1 (black body).
  • Emissivity may vary with temperature and wavelength.

Irradiation (G)

  • Total incident radiation on a surface from all directions per unit time per unit area of the surface.

Radiosity (J)

  • It refers to all of the radiant energy leaving a surface per unit area of the surface.

J = ε + ρG

ρ = reflectivity

Reflectivity (ρ)

  • Fraction of incident radiation reflected.
  • For white body :- ρ = 1, α = 0, τ = 0

Absorptivity (α)

  • Fraction of incident radiation absorbed.
  • For Black Body α = 1, ρ = 1, τ = 0

Transmittivity (τ)

  • τ = 0 for black, white and opaque body (α + ρ = 1).
  • Fraction of incident radiation transmitted.

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