Heat Transfer: Convection

Heat Transfer: Convection

Convection is the second mode of heat transfer, and it is possible only within a fluid -- a liquid or

a gas -- because, unlike conduction, it works by the actual bulk (large-scale) movement of the heated

fluid itself, physically carrying its thermal energy from one place to another as it flows.

Why convection happens: buoyancy-driven flow

When a fluid is heated, most fluids expand and so become locally less dense than the surrounding cooler

fluid. This heated, lighter fluid then experiences a net upward buoyant force (exactly the same buoyancy

that makes a less-dense object float in a denser fluid) and rises, while cooler, denser fluid from

elsewhere sinks down to take its place, where it in turn is heated and rises. This continual cycle of

rising warm fluid and sinking cool fluid sets up a circulating flow pattern called a convection current, which keeps carrying heat energy away from the source of heating and distributing it through

the bulk of the fluid, far more effectively (in most situations) than conduction alone could manage

through a fluid at rest.

Natural (free) convection

When the circulating flow is driven purely by the density differences that heating itself produces, with

no external device pushing the fluid along, the process is called natural or free convection. Many

familiar phenomena are examples of natural convection: water in an open pot on a stove convects visibly as

it approaches boiling; a room heated by a radiator placed near the floor warms throughout its height as

warmed air rises from the radiator and cooler air sinks to replace it; and the land-and-sea breeze cycle

experienced at a coastline (onshore breeze by day, offshore breeze by night) arises from convective

circulation set up because land and sea heat up and cool down at different rates under the same sunlight.

Forced convection

When a fluid is instead driven past a hot (or cold) surface by some external agency -- a fan, a pump, or

simply the wind -- rather than relying on buoyancy alone, the process is called forced convection. …