Convection heat transfer is defined as the transfer of heat through the agency of particles of fluid which receive heat from a high temperature source and move to the locality of a lower temperature sink to reject heat. The sink may be some body exposed to the fluid, or it may be cooler particles of fluid. It should be noted that convection heat transfer occurs by means of fluid motion, in contrast to conduction heat transfer, which takes place entirely by means of intermolecular energy transfers.
Convection heat transfer may be further classified according to the mode of motivating the flow. If the convection currents are caused by density differences brought about by temperature gradients within the fluid, the convection is said to be natural. If the flow is aided by some mechanical device such as a pump (fan) the resulting convection currents are forced.
Newton’s Law of Cooling:
The rate of heat transfer by convection between a surface and a fluid may be computed by the relation
qc = h̅CA.ΔT
Where qc = rate of heat transfer by convection (kJ/hr)
A = heat transfer area (m2)
ΔT = difference between the surface temperature Ts and the temperature of the fluid T∞ at some specified location (°C).
h̅c = average unit thermal convective conductance often called the surface coefficient of heat transfer or the convective heat transfer coefficient kJ/hr.m2.°C.
Thermal conductance K for convective heat transfer Kc = h̅c. A and thermal resistance
The relation expressed by this equation was originally proposed by the British scientist, Issac Newton in 1701. This is also known as Newton’s law of cooling. The evaluation of the convective heat transfer coefficient is difficult because convection is a very complex phenomenon.
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