""HomeHeat TransferSolidificationMechanica lPropertiesCatalysisCircular MotionMicrogravity
Heat Transfer MenuIntroductionConductionConvectionRadiationHeat PipesConductionGlossary
SideNavArea

The three main methods of heat exchange are conduction, convection and radiation.

Conduction:

Conduction in solids and liquids operates by part of the kinetic energy of one particle being passed to its immediate neighbour. In simple terms the bonds between neighbouring atoms and molecules in a solid can be thought of as elastic links; as one particle vibrates it causes the next in the line to vibrate also. This process repeats, particle after particle allowing thermal energy to be passed from the hot face of a body to the cold face. In the case of liquids however their ability to flow means that in most cases convection is a more significant method of heat transfer.

Conduction in metals operates very differently. In a metal ‘valence’ electrons are free to move through the body of the metal. These are the same electrons that are responsible for electrical conduction. At the hot face of the metal the valence electrons gain kinetic energy and rapidly spread through the whole of the metal. There is a simple link between electrical and thermal conductivity in metals, this is the Wiedemann-Franz Law. It states that the thermal conductivity k of a metal should be proportional to its electrical conductivity σ(the inverse of its resistivity ρ), any difference is due to the contribution made by the metal ions acting like an ordinary, non-metallic, solid. Due to the effects of their impurities the values for alloys, e.g. steel and bronze, are a little more complicated.

 

 

Conductivity (at 273.2 K)

Metal

Electrical

Thermal

Ratio σ /k

 

σ(10 8 S m -1)

k (W m -1 K -1)

(S K W -1)

Aluminium

0.400

236

0.00169

Copper

0.645

403

0.00160

Gold

0.488

319

0.00153

Lead

0.052

36

0.00144

Platinum

0.102

72

0.00142

Potassium

0.164

104

0.00158

Silver

0.680

428

0.00159

Zinc

0.182

117

0.00156

Conduction in gases can take place but it is very slow indeed. It relies on collisions between rapidly moving ‘hot’ gas particles and their slower moving ‘cold’ counterparts. This is very similar to the diffusion process and so is extremely inefficient. Unless all other means of heat transfer can be suppressed its contribution is vanishingly small. It does however become important if the temperature difference between hot and cold is quiet small and the gas can be held stationary. This is the case in double glazing.

For most practical applications therefore conduction relies on the presence of a solid as with liquids, gases and plasmas other heat transfer mechanisms are generally more important.

Radiation:

Radiation operates by the hot object emitting electromagnetic radiation. The precise characteristics of the radiation depend on the temperature of the hot object. Cooler objects (e.g. people) radiate in the near infrared and so can be detected with IR cameras. Hot objects (e.g. incandescent lamps (ordinary light bulbs)) radiate strongly in the visible spectrum and so we can see by their light. Very hot objects (e.g. electrical sparks and arcs) radiate strongly in the near ultraviolet, as a result people using arc welding equipment need to use special masks in order to avoid sun burn to the eyeballs (also called “welder’s flash” or “arc eye”). In addition as the temperature of an object increases the amount of energy it emits at any given wavelength increases. This increases as the fourth power of the temperature. As a result if the (absolute) temperature of an object is doubled (say from 400 K to 800 K) then the amount of energy it radiates will increase by a factor of sixteen and a tenfold increase in the temperature will increase the radiation level by a factor of ten thousand!

Radiation can be affected by the surface of the objects. Matt black objects are typically excellent emitters (and absorbers) of radiation.

Since all other methods of heat transfer require the presence of matter in one form or another, radiation is the only means of an object losing heat if it is held in a vacuum.

Heat loss by radiation can occur in any situation but becomes more important as the temperature increases.

Convection:

Convection operates when a fluid (a liquid or gas) is heated resulting in a change in density. Usually the fluid will expand on heating and so become less dense. The difference in density with the surrounding fluid leads causes the fluid to flow, carrying thermal energy with it.

Convection is by far the hardest form of heat transfer to produce theoretical models for. The rate of heat flow is affected not only by the temperature differences involved but also the viscosities and rates of thermal expansion of the fluid and size, shape and surface texture of any objects in contact with the fluid (such as the heater).

Heat loss by convection requires that there be a fluid that changes its density when heated. Furthermore there must be a gravitational field (or an equivalent acceleration) so that the density difference can produce movement. Finally the fluid must be free to move in the direction that the density difference is trying to drive it.

One example where heat loss by convection breaks down is when a lake freezes over. As the water at the surface of the lake cools it contracts and falls to the bottom of the lake. However this only works down to about 4 °C, below this point the water begins to expand again and so does not sink. This, now buoyant, water continues to float at the surface until it freezes (expanding still further). The result is that a lake freezes from the top down. All the water below the ice is at a temperature between 0 °C (at the top) and 4 °C (at the bottom). Heat transfer through this water can only continue through conduction.

Top of PageClose Window

ESA Logo