Conduction is a type of heat transfer that deals with contact between to objects. Each object has an initial temperature that is different than the other, but when the two objects come in contact over a period of time they reach an equilibrium temperature. In everyday life this experience is observed if a metal spoon is left in the hot pan, then eventually the handle will be hot even though it is not touching the hot pan.
Conduction is analyzed by looking at a transfer of heat per unit time. Conduction specifically deals with the transfer of energy from more energetic particles to less energetic particles of a particular substance at the atomic level. This can be thought of as a transfer of heat from hot to cold until an equilibrium temperature is reached.
Fourier’s law expresses heat transfer per unit area in the direction perpendicular to the direction of transfer. This expression is known as heat flux and it is proportional to the temperature gradient of the object. The heat flux is also dependent on the type of material that is conducting the heat. This property is known as the thermal conductivity of a material.
Gases such as carbon dioxide, and air have the poorest thermal conductivity along with foam, fiberglass and other materials used for insulation. In contrast, the best materials for thermal conductivity are pure metals such as silver, copper, and aluminum.
There are many mechanisms that control the transfer of heat energy by the method of conduction. Thermodynamic analysis of heat transfer pertains to the equilibrium state of a system. This type of analysis is dependant on a variety of material properties such as density, and volumetric heat capacity, which is the materials ability to store thermal energy. The ratio of the thermal conductivity to the heat capacity is an important property known as the thermal diffusivity. It measures the ability of the material to conduct thermal energy relative to its ability to store thermal energy. Naturally, materials with a high thermal diffusivity will respond quickly to changes in their thermal environment while materials with low thermal diffusivity will take longer to reach an equilibrium temperature.
There are very detailed equations for all of the different scenarios in conduction analysis. Contact resistance, thermal resistance, and temperature distribution all need to be considered in heat transfer analysis. These mechanisms of heat transfer allow for engineers to come up with very predictable designs in a variety of thermodynamic applications.
