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Introduction to Heat Transfer

• Heat transfers from a higher temperature body to a lower temperature body spontaneously without any external work.
• Heat transfer in solids happens through three mechanisms: conduction, convection, and radiation.

Conduction

• Heat transfer through a material by the exchange of thermal energy between the molecules.
• Molecules vibrate and transfer energy through collisions.
• Good conductors transfer heat quickly, while poor conductors transfer heat slowly.
• Free electrons in solids also contribute to heat transfer.

Convection

• Heat transfer through the movement of molecules.
• Warmer molecules move, carrying thermal energy to cooler regions.
• Takes place only in liquids and gases.

Radiation

• Heat transfer through electromagnetic waves.
• No medium is needed for radiation-based heat transfer.

Thermal Conductivity

• Different materials have different thermal conductivities.
• Metals are excellent conductors.
• Nonmetals, liquids, and gases are poor conductors.
• To measure thermal conductivity correctly for poor conductors, precautions should be taken to prevent heat transfer by convection or radiation.

Coefficient of Thermal Conductivity

• Heat transfer via conduction through a thin metallic sheet depends on several factors:
  • Cross-sectional area
  • Temperature difference
  • Thickness of the sheet.
• Rate of heat transfer formula is:

dQ/dt = -KA(dT/dx)

Where: - dQ/dt is the rate of heat transfer - K is the thermal conductivity - A is the cross-sectional area - dT/dx is the temperature gradient

• Thermal conductivity (K) is a material constant that reflects a material's ability to conduct heat.
• The negative sign indicates that heat flows from higher temperature to lower temperature

Other Important Concepts

• Temp-gradient: Rate of temperature change with respect to distance

• Boundary condition: Specific values of temperature, flux, or materials, etc, at boundaries of a system.

• Isolated bar: A bar isolated from its surroundings.