# Convection

## 03 Mar 2021 Convection

Convection describes the heat transfer (https://www.fea-solutions.co.uk/heat-transfer-analysis/) through the movement of fluids (liquids and/or gases) or between fluids and solids.

There are two types:
– Free convection (natural convection)
– Forced convection

When a gas heats up, it will expand in volume. This will cause its density to drop, compared to the surrounding cooler gas. Consequently, the warm gas bubble will rise, displacing an area of cold fluid which is denser. The latter will sink into areas which cause it to heat up, starting a cycle of heat transfer in a process called a convection current. This is called free convection or natural convection, as the current is independent of an additional. A household radiator is a good example of this. The radiator heats the air surrounding it, this air will expand and rise to the top of the room, forcing the cold air down, which in turn will heat up.

In forced convection, the fluids are forced to move in order to increase heat transfer. This ‘forcing’ can be done with a fan or pump. Forced convection allows significant amounts of heat energy to be transported very efficiently, more so than with free convection. An example of forced convection is a fan pushing cool air over a hot component in order to cool it down.

When a heat transfer analysis is performed on an assembly of solid bodies, without modelling the fluid, convection is implemented as a boundary condition, defined by a convection coefficient and an ambient temperature. The larger the coefficient, the faster heat can be transferred.

The convection coefficient, also called heat transfer coefficient, is measured in the following units:  W/(m2*K)

The convection coefficient can be obtained from data tables, or from CFD analyses, where it is referred to as film coefficient.

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