1.2 Convection

Convection refers to transfer of thermal energy by motion of a medium. The rate equation or the convective heat transfer (regardless of particular nature) between a surface and an adjacent fluid is prescribed by Newton’s law of cooling. \[Q=hA(T_s-T_\infty) \tag*{(2)}\] where

\(A\) = area exposed to heat transfer (m\(^2\))
\(T_s\) = surface temperature of solid (\(^\circ\)C)
\(T_\infty\) = fluid temperature (\(^\circ\)C), and
\(h\) = feat transfer coefficient (W/m\(^2\).\(^\circ\)C)

\[h = f (\text{geometry, fluid motion, fluid properties, $\Delta T$})\]

  • There are two types of convection:

    • Forced convection—refers to the case when the fluid is made to flow by some external agent, using a pump, fan, stirrer, etc.

    • Free or natural convection—refers to fluid motion which naturally occurs from the heat transfer itself, due to buoyancy differences (“hot air rises and cold air sinks...”).

  • Typical values of heat transfer coefficient: (W/m\(^2\).K):

    Free convection gases: 2–25
      liquids: 50–100
    Forced convection gases: 25–250
      liquids: 50–20,000
    Boiling/Condensation 2500–100,000