Convection
Convection is heat transfer by the bulk motion of a fluid (liquid or gas) combined with conduction at boundaries. In natural (free) convection, flow is driven by buoyancy, as warmer, less dense fluid rises; in forced convection, flow is driven externally by fans, pumps or wind. The heat flow from a surface is usually written q = hAΔT, where h is a heat-transfer coefficient that depends on the fluid, geometry and flow.
Curious
Heat a pan of water from below and the warmed water at the bottom expands, becomes less dense and rises, while cooler water sinks to replace it, setting up circulating currents. That is natural convection. Blow on hot soup or switch on a fan and you force the fluid to move faster, carrying heat away much more quickly: forced convection.
Convection shapes the world on every scale: sea breezes, thunderstorms, ocean currents, the slow churning of Earth's mantle that drives plate tectonics, and the boiling granules visible on the Sun's surface (see Stellar Structure).
Exploring
| Situation | Typical h (W m−2 K−1) |
|---|---|
| Natural convection, gases | 2–25 |
| Natural convection, liquids | 50–1,000 |
| Forced convection, gases | 25–250 |
| Forced convection, liquids | 100–20,000 |
| Boiling and condensation | 2,500–100,000 |
Deep Dive
Onset of convection. Henri Bénard (1900) observed hexagonal convection cells in thin heated fluid layers; Lord Rayleigh (1916) showed that convection begins when buoyancy overcomes viscous damping and thermal diffusion, measured by the Rayleigh number. The critical value 1,708 applies for rigid boundaries; with free surfaces it is about 658 (and surface-tension effects, the Marangoni effect, actually drove Bénard's original cells). At very high Ra convection becomes turbulent, and scaling of heat transport with Ra is still an active research topic.
Dimensionless groups. Engineering correlations express h through the Nusselt number Nu = hL/k as a function of the Reynolds number (forced flow) or Rayleigh and Prandtl numbers (natural flow). Boundary layers, thin regions near surfaces where velocity and temperature change rapidly, control the transfer (see Heat Transfer).
Geophysics and astrophysics. Earth's mantle convects on timescales of tens of millions of years despite being solid rock, because over such times it flows like an extremely viscous fluid; this was the flaw in Kelvin's nineteenth-century estimate of Earth's age, which assumed pure conduction (see Lord Kelvin). In stars, convection transports energy wherever the temperature gradient exceeds the adiabatic gradient (the Schwarzschild criterion; see Adiabatic Process).
Concept Relationship Map
Requires: conduction, thermal expansion (buoyancy)
Enables: weather and ocean circulation, mantle dynamics, stellar energy transport
Commonly confused with: h as a material constant
Sources
- Rayleigh, Lord (1916). Philosophical Magazine, 32, 529–546.
- Bénard, H. (1900). “Les tourbillons cellulaires dans une nappe liquide.” Revue Générale des Sciences Pures et Appliquées, 11, 1261–1271, 1309–1328.
- Incropera, F.P., DeWitt, D.P., Bergman, T.L., Lavine, A.S. (2007). Fundamentals of Heat and Mass Transfer (6th ed.). Wiley.