QnA#2 Heat Transfer Mechanisms or modes of heat transfer

QnA#2 Heat Transfer Mechanisms or modes of heat transfer

Answer
Conduction involves the transfer of heat by the interactions of atoms or molecules of a material through which the heat is being transferred.

Convection involves the transfer of heat by the mixing and motion of macroscopic portions of a fluid.

Radiation or radiant heat transfer, involves the transfer of heat by electromagnetic radiation that arises due to the temperature of a body.


1- Conduction
Conduction Is The Heat Transfer Between Two Substances By Direct Transferring Of Molecular Kinetic Energy . Conduction Occurs When A hot Substance Comes Into Contact With A cold Substance : Heat Is transferred From Hotter Substance Has A higher Temperature Than The Colder Substance , The Substance With A higher Temperature Has Molecules With Higher Average Kinetic Energy Than The Substance With The Lower Temperature .
heat transfer rate by conduction speed related to four factors :-
∙ Thermal conductivity of the substance
∙ The surface area over which the heat is applied
∙ The thickness of the material subject to the heat transfer
∙ The temperature difference across the materials

Fourier's Law of Conduction Q = k A ΔT/L
˙Q = rate of heat transfer (Btu/hr)
A = cross-sectional area of heat transfer (ft2)
Δx = thickness of slab (ft)
Δr = thickness of cylindrical wall (ft)
ΔT = temperature difference (°F)
k = thermal conductivity of slab (Btu/ft-hr-°F)

2- Convection
∙ Convection heat transfer involves fluids, either liquids or gases in motion
∙ convection heat transfer occurs in two parts :
o Heat transfer into or out of a fluid by the process of conduction
o The movement of the fluid transports the heated fluid, as well as its thermal energy, to another location
∙ Convection either natural or forced (using fans, pumps or other devices).
∙ Also the increased turbulent of the forced flow will increase convective heat transfer rate as a result of increased heat transfer coefficient.
Newton's Law Of Cooling ˙Q= h A ΔT
˙Q = rate of heat transfer (Btu/hr)
h = convective heat transfer coefficient (Btu/hr-ft2-°F)
A = surface area for heat transfer (ft2)
ΔT = temperature difference (°F)

3- Radiation
Is the energy emitted by matter in the form of electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules.
∙ Unlike conduction and convection, the transfer of energy by radiation does not require the presence of an intervening medium .
∙ All bodies at a temperature above absolute zero emit thermal radiation.