Heat transfer is transfer of energy due to temperature difference in a medium or between two or more media.

  • Heat transfer rate and heat flux: \[Q = qA\] where

    \(Q\) = heat transfer rate (W)
    \(q\) = heat flux (W/m\(^2\))
    \(A\) = heat transfer area (m\(^2\))

  • Heat is always transferred in the direction of decreasing temperature.

  • Temperature is a scalar, but heat flux is a vector quantity.

  • Difference between thermodynamics and heat transfer:

    Thermodynamics tells us:

    • how much heat is transferred (\(Q\)).

    • how much work is done (\(W\)).

    • final state of the system.

    Heat transfer tells us:

    • how (with what modes) \(Q\) is transferred.

    • at what rate \(Q\) is transferred.

    • temperature distribution inside the body.

  • Different types / modes of heat transfer processes:

    • Conduction—heat transfer is due to a temperature gradient in a stationary medium or media. Conduction needs matter and does not require any bulk motion of matter.

    • Convection—heat transfer occurs due to temperature difference between a surface and a moving fluid.

    • Radiation—heat transfer occurs due to emission of energy in the form of electromagnetic waves by all bodies above absolute zero temperature. Net radiation heat transfer occurs when there exists a temperature difference between two or more surfaces emitting radiation energy. Radiation does not need matter.