Conduction, convection, radiation

1. Heat transfer is the movement of thermal energy from a region of higher temperature to a region of lower temperature.
2. There are three primary modes of heat transfer: conduction, convection, and radiation.

Conduction

1. Conduction is the transfer of heat through a material without the movement of the material itself.
2. It occurs mainly in solids, where particles are tightly packed.
3. The rate of conduction is described by Fourier’s Law: Q = -kA(dT/dx), where:
   • Q is the heat transfer rate.
   • k is the thermal conductivity of the material.
   • A is the cross-sectional area.
   • dT/dx is the temperature gradient.
4. Good conductors, such as metals, transfer heat efficiently due to free electrons.
5. Poor conductors or insulators, like wood and plastic, transfer heat slowly.
6. Conduction is significant in systems like heat exchangers and thermal insulation materials.
7. Thermal resistance determines how easily heat can flow through a material.

Convection

1. Convection is the transfer of heat by the movement of a fluid (liquid or gas).
2. It occurs due to density differences created by temperature variations.
3. There are two types of convection:
   • Natural convection: Driven by buoyancy forces caused by density differences.
   • Forced convection: Involves an external force, such as a fan or pump, to move the fluid.
4. Convection plays a vital role in weather patterns, ocean currents, and heating systems.
5. The rate of convective heat transfer is described by Newton’s Law of Cooling: Q = hA(Ts − Tf), where:
   • h is the convective heat transfer coefficient.
   • A is the surface area.
   • Ts and Tf are the surface and fluid temperatures, respectively.
6. Convection is enhanced in designs such as radiators and air conditioning systems.
7. Thermal boundary layers form near surfaces due to fluid flow and temperature gradients.

Radiation

1. Radiation is the transfer of heat through electromagnetic waves, without requiring a medium.
2. All bodies emit thermal radiation based on their temperature.
3. The energy emitted by a surface is described by the Stefan-Boltzmann Law: Q = εσAT⁴, where:
   • ε is the emissivity of the surface.
   • σ is the Stefan-Boltzmann constant (5.67 × 10⁻⁸ W/m²·K⁴).
   • A is the area of the surface.
   • T is the absolute temperature in kelvin.
4. Black bodies are ideal emitters and absorbers of radiation.
5. Radiative heat transfer depends on the surface's emissivity and temperature difference.
6. Unlike conduction and convection, radiation can occur in a vacuum.
7. Applications of radiation include solar panels, thermal imaging, and infrared heaters.
8. Radiative heat transfer can be minimized using reflective coatings or thermal insulation.

Combined Heat Transfer

1. In real-world systems, heat transfer often involves a combination of conduction, convection, and radiation.
2. For example, in a boiling pot of water:
   • Conduction transfers heat through the pot's metal base.
   • Convection circulates heat in the water.
   • Radiation transfers heat from the pot to the surroundings.
3. Understanding these modes is critical for designing energy-efficient systems and insulation.
4. Proper materials and geometries optimize heat transfer processes in engineering applications.