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Sabtu, 13 Maret 2010

Convection
Main articles: convection and convective heat transfer
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Convection is the transfer of thermal energy by the movement of molecules from one part of material to another. As fluid motion goes more quickly the convective heat transfer increases. The presence of bulk motion of fluid enhances the heat transfer between the solid surface and the fluid.[2]

There are two types of Convective Heat Transfer:

* Natural Convection: is when the fluid motion is caused by buoyancy forces that result from the density variations due to variations of temperature in the fluid. For example in the absence of an external source, when the mass of the fluid is in contact with a hot surface its molecules separate and scatter causing the mass of fluid to become less dense. When this happens, the fluid is displaced vertically or horizontally while the cooler fluid gets denser and the fluid sinks. Thus the hotter volume transfers heat towards the cooler volume of that fluid.[3]

* Forced Convection: is when the fluid is forced to flow over the surface by external source such as fans and pumps. It creates an artificially induced convection current.[4]

Internal and external flow can also classify convection. Internal flow occurs when the fluid is enclosed by a solid boundary such as a flow through a pipe. An external flow occurs when the fluid extends indefinitely without encountering a solid surface. Both these convections, either natural or forced, can be internal or external as they are independent of each other.[3]

The formula for Rate of Convective Heat Transfer:[5]

q = hA(Ts − Tb)

A is the surface area of heat transfer. Ts is the surface temperature and Tb is the temperature of the fluid at bulk temperature. However Tb varies with each situation and is the temperature of the fluid “far” away from the surface. The h is the constant heat transfer coefficient which depends upon physical properties of the fluid such as temperature and the physical situation in which convection occurs. Therefore, the heat transfer coefficient must be derived or found experimentally for every system analyzed. Formulas and correlations are available in many references to calculate heat transfer coefficients for typical configurations and fluids. For laminar flows the heat transfer coefficient is rather low compared to the turbulent flows, this is due to turbulent flows having a thinner stagnant fluid film layer on heat transfer surface.[6]

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