Modes of Heat Transfer

As already mentioned, Thermodynamics alone cannot tell how or how rapidly (rates) heat transfer will take place.

RATES -> how + driving force + resistence

Three ``hows'' (modes) of heat transfer: conduction, convection, and radiation.

Conduction

DEFINITION:

Conduction is the transfer of energy due to either random molecular motion or due to the motion of ``free'' electrons

In different phases of matter, the modes of conduction are slightly different:

• gases: conduction is due to collisions of randomly moving molecules.
  
• liquids: similar to gases, but with a much smaller ``mean free path''.
  
• solids: conduction is due to lattice vibrations and/or motion of ``free'' electrons.
  

Convection

DEFINITION:

Convection refers to any transfer of thermal energy by motion of a medium.

In this sense, convection can refer equally well to a fluid (gas or liquid) flowing along or to a chuck of a solid begin transported (perhaps thrown!).

NOTE:

In typical engineering application, convection is more broadly defined, so that it may also refer to transfer of thermal energy from a solid mass to a fluid flowing past that mass (where clearly conduction is also going on!).

A distinction is made between ``forced'' and ``natural'' convection.

Forced vs. Natural

• forced convection refers to the case when the fluid is made to flow by some external agent, like a pump for example.
  
• natural convection refers to fluid motion which naturally occurs from the heat transfer itself, due to buoyancy differences (``hot air rises...'').
  

Radiation

DEFINITION:

In radiative heat transfer, objects emit and absorb electromagnetic waves/particles (photons).

NOTE:

There need not be any medium (mass) through which this form of heat is transported!

The amount of energy (photons) which is radiated depends on the temperature (thermal energy) of the radiator. (Radiator in this sense does not mean an apartment heater, it means a source of radiation. An apartment heater in fact acts more as a ``convector'' than a ``radiator''.)

Obviously, the way that heat is transfered in this mode is by an object emitting and absorbing different amounts! If a photon is absorbed the thermal energy of the mass increases, if a photon is emitted the thermal energy of the mass decreases. (Obviously, if something emits more than it absorbs it cools down!)

OBJECTIVE 1.3:

Explain and give examples of the three primary modes of heat transfer

TEST YOURSELF

What modes of heat transfer dominate inside a oven? on a stove-top? in a "fry-heater" at McDonald's?