Convection Mechanisms
Convection may happen in fluids at all scales larger than a few atoms. There are a variety of circumstances in which the forces required for natural and forced convection arise, leading to different types of convection, described below. In broad terms, convection arises because of body forces acting within the fluid, such as gravity (buoyancy), or surface forces acting at a boundary of the fluid.
The causes of convection are generally described as one of either "natural" ("free") or "forced", although other mechanisms also exist (discussed below). However the distinction between natural and forced convection is particularly important for convective heat transfer.
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