Summary of Heat Transfer 20 | Radiation Heat Transfer [1] | Mechanical Engineering | GATE Crash Course

Summary of "Heat Transfer 20 | Radiation Heat Transfer [1] | Mechanical Engineering | GATE Crash Course"

This lecture primarily introduces the concept of Radiation Heat Transfer, the third mode of heat transfer after conduction and convection. The instructor explains fundamental theories, characteristics, and laws governing Radiation Heat Transfer, focusing on thermal radiation relevant to engineering applications such as GATE exams.

Main Ideas and Concepts

1. Introduction to Radiation Heat Transfer

Radiation is the third mode of heat transfer, distinct from conduction and convection.
It involves transfer of heat via electromagnetic waves and does not require a medium; it can occur in a vacuum.
Radiation Heat Transfer is most effective in a vacuum, where the speed of radiation equals the speed of light (~3 × 10⁸ m/s).

2. Theories Explaining Radiation

Two main theories explain Radiation Heat Transfer:
- Maxwell’s Electromagnetic Wave Theory: Radiation is electromagnetic waves.
- Photon Theory (Planck’s Photo Theory): Radiation consists of photons (energy bundles).
For engineering heat transfer problems, Maxwell’s Electromagnetic Wave Theory is primarily used.

3. Electromagnetic Spectrum and Thermal Radiation

Radiation spans a wide electromagnetic spectrum from gamma rays (short wavelength) to radio waves (long wavelength).
Thermal radiation important in heat transfer lies between 0.1 micrometer to 100 micrometers wavelength.
Thermal radiation includes:
- Ultraviolet (UV) rays
- Visible light
- Infrared (IR) rays
Energy and wavelength are inversely related: shorter wavelengths have higher energy (e.g., X-rays), longer wavelengths have lower energy (e.g., radio waves).

4. Optical Laws and Radiation Behavior

Radiation follows optical laws similar to light:
- Reflection
- Absorption
- Transmission
- Scattering
Radiation can be reflected, absorbed, or transmitted by surfaces.

5. Medium in Radiation Heat Transfer

Unlike conduction and convection, radiation does not need a medium.
Medium between two bodies can be:
- Non-participating: Does not absorb, reflect, or scatter radiation.
- Participating: Absorbs, reflects, or scatters radiation.
In many engineering problems, the medium is assumed non-participating.

6. Radiation Heat Exchange Between Bodies

Any body above absolute zero (0 K) emits radiation.
When two bodies at different temperatures exchange radiation, net Radiation Heat Transfer occurs.
The medium between bodies can influence this exchange if it is participating.

7. Surface Phenomena in Radiation

Radiation is a surface phenomenon.
Incident radiation on a surface can be:
- Reflected
- Absorbed
- Transmitted
Energy balance on a surface:
Incident radiation = Reflected + Absorbed + Transmitted
(Reflectivity + Absorptivity + Transmissivity = 1)

8. Types of Bodies in Radiation

Opaque Body: Transmissivity = 0; only reflection and absorption occur.
Reflectivity + Absorptivity = 1.
Black Body: Hypothetical perfect absorber that absorbs all incident radiation (Absorptivity = 1).
Also a perfect emitter of radiation.
Perfect Reflector: Hypothetical body that reflects all incident radiation (Reflectivity = 1).

9. Black Body Concept

A Black Body is an idealized body that absorbs all radiation incident on it.
It is a perfect emitter as well.
Black Body radiation is a standard for comparing real bodies.
Black bodies are hypothetical but essential for theoretical and comparative purposes.
Real bodies approximate Black Body behavior to varying degrees.
Examples include hollow cavities with small openings (cavity radiators).

10. Diffuse Body

A diffuse body emits radiation uniformly in all directions, independent of angle.
Black bodies are diffuse emitters.

11. Planck’s Distribution Law

Describes the spectral distribution of radiation emitted by a Black Body at a given temperature.
Monochromatic emissive power: Radiation power emitted at a single wavelength.
The monochromatic emissive power varies strongly with wavelength and temperature.
The curve of emissive power vs. wavelength:
- Rises to a maximum at a certain wavelength (λ_max)
- Then decreases for longer wavelengths
As temperature increases, λ_max shifts to shorter wavelengths (Wien’s Displacement Law).
The total emissive power is the area under the monochromatic emissive power curve.

12. Stefan-Boltzmann Law

Total emissive power of a Black Body is proportional to the fourth power of its absolute temperature (T⁴).
This law is