Everything about Free Convection totally explained
Convective heat transfer is a mechanism of
heat transfer occurring because of bulk motion (observable movement) of fluids. This can be contrasted with
conductive heat transfer, which is the transfer of energy by vibrations at a molecular level through a solid or fluid, and
radiative heat transfer, the transfer of energy through
electromagnetic waves.
As convection is dependent on the bulk movement of a fluid it can only occur in liquids, gases and multiphase mixtures.
Convective heat transfer is split into two categories: natural (or free) convection and forced (or advective) convection, also known as heat
advection.
Natural convective heat transfer
Natural convection is a mechanism, or type of heat transport in which the fluid motion isn't generated by any external source (like a pump, fan, suction device, etc.) but only by density differences in the fluid occurring due to temperature s. In natural convection, fluid surrounding a heat source receives heat, becomes less dense and rises. The surrounding, cooler fluid then moves to replace it. This cooler fluid is then heated and the process continues, forming a convection current. The driving force for natural convection is buoyancy, a result of differences in fluid density. Because of this, the presence of gravity or an equivalent force (arising from the
equivalence principle, such as
acceleration,
centrifugal force or
Coriolis force) is essential for natural convection. For example, natural convection doesn't operate as it does on Earth in the micro gravity environment of the orbiting International Space Station, where other heat transfer mechanisms are required to prevent electronic components from overheating.
Natural convection has attracted a great deal of attention from researchers because of its presence both in nature, seen in the rising plume of hot air from
fire, oceanic currents, and sea-wind formation, and in engineering applications such as formation of microstructures during the cooling of molten metals and in shrouded fins and solar ponds. A very common industrial application of natural convection is air cooling: this can happen on small scales (computer chips) to large scale process equipment.
Mathematically, the tendency of a particular system towards natural convection relies on the
Grashof number (Gr), which is a ratio of buoyancy force and
viscous force.
»
When the Peclet number is much greater than
unity (1), advection dominates diffusion. Similarly, much smaller ratios indicate a higher rate of diffusion relative to advection.
Further Information
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