Heat Transfer

There are three possible modes of heat transfer

• Conduction
• Convection
• Radiation

Conduction	Transfer of energy via the vibration of atoms in some medium. The best conductors are silver and copper.
Convection	Transfer of energy via the bulk motion of a medium (e.g. air, water)
Thermal Radiation	Cooling of a material to thermal equilibrium via radiation losses. Stored energy re-emitted (Power per unit Area) goes as T4

Thermal Conduction:

• heat energy flows from regions of high temperature to low temperature
• wide variation among different materials depends on the density of free electrons in the material

  Thermal Conductivity of some Materials in relative Units

  • Silver 1.0
  • Copper 0.93
  • Gold 0.70
  • Aluminum 0.48
  • Steel 0.12
  • Concrete 0.004
  • Brick 0.002
  • Wood 0.0002
  • Air 0.00005

So Clearly metals are required for efficient heat transfer.

Heat flow through a wall:

Convection: liquids and gases transfer heat this way motion of the medium between regions of different temperatures.

Fireplaces produce natural convection warm air rises and is replaced by cold air

Most space heating systems operate via convective heat transfer (forced air)

Thermal Radiation:

Material absorbs sunlight and heats up and then re-radiates that as long wavelength infrared radiation (heat radiation).

Was effectively used centuries ago in American Southwest ( Mesa Verde).

Sandstone dwellings and walls would absorb much sunlight during the day and then re-radiate that as heat at night.

In general, recovering incident solar radiation via subsequent thermal radiation of materials is not practical and large amounts of thermal mass are required. This can be a) expensive and b) space intensive.

Thermal Mass:

• efficiency depends upon specific heat of material and its thermal conductivity
  • Specific Heat measure of how much energy a substance can store. This is measured on a scale of 0 to 1. Water has a specific heat of 1.

  • Thermal conductivity measure of efficiency of heat transfer (i.e. getting it back when you want it)

Values for typical materials:

Material	Specific Heat	Thermal Conductivity
Water	1.0	4.2
Iron	0.1	320
Glass	0.2	4.0
Stone	0.2	3.0
Wood (Oak)	0.6	1.4
Brick	0.2	4.6
Concrete	0.15	12
Sand	0.2	2.3

Water is the clear winner followed by concrete. So thermal mass is most effectively used in the form of large tanks of water or several tons of concrete in an insulated container.

Phase Change

Matter exists as solids, liquids and gases (or vapours).  Transitions from one phase to another require or produce heat, and are usually accompanied by a change in volume.

The latent heat of fusion, L_f is the quantity of heat which must be supplied to unit mass of a solid at its melting point to convert it completely to liquid at the same temperature.

The latent heat of vaporisation, L_v is the quantity of heat which must be supplied to unit mass of a liquid at its boiling point to convert it completely to gas at the same temperature.

When a vapour condenses (or liquid solidifies) it gives up its latent heat of vaporisation (or fusion).

The units of latent heat are J kg^-1.  Latent heats may be added by the method of mixtures - e.g. adding a known mass of ice (or steam) to a known mass of water and measuring the temperature change. 

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