Methods based on experiment are available for evaluating energy transfer by heat. These methods recognize two basic transfer mechanisms: conduction and thermal radiation. In addition, empirical relationships are available for evaluating energy transfer involving certain combined modes. A brief description of each of these is given next.
Energy transfer by conduction can take place in solids, liquids, and gases. Conduction can be thought of as the transfer of energy from the more energetic particles of a substance to adjacent particles that are less energetic due to interactions between particles. The time rate of energy transfer by conduction is quantified macroscopically by Fourier’s law.
Thermal radiation is emitted by matter as a result of changes in the electronic configurations of the atoms or molecules within it. The energy is transported by electromagnetic waves (or photons).
Unlike conduction, thermal radiation requires no intervening medium to propagate and can even take place in a vacuum. Solid surfaces, gases, and liquids all emit, absorb, and transmit thermal radiation to varying degrees.
The rate at which energy is emitted, from a surface of area A is quantified macroscopically by a modified form of the Stefan–Boltzmann law which shows that thermal radiation is associated with the fourth power of the absolute temperature of the surface.
In general, the net rate of energy transfer by thermal radiation between two surfaces involves relationships among the properties of the surfaces, their orientations with respect to each other, the extent to which the intervening medium scatters, emits, and absorbs thermal radiation and other factors.
Energy transfer between a solid surface at a temperature and an adjacent moving gas or liquid at another temperature plays a prominent role in the performance of many devices of practical interest. This is commonly referred to as convection.
The heat transfer coefficient is not a thermodynamic property. It is an empirical parameter that incorporates into the heat transfer relationship the nature of the flow pattern near the surface, the fluid properties, and the geometry.
When fans or pumps cause the fluid to move, the value of the heat transfer coefficient is generally greater than when relatively slow buoyancy-induced motions occur. These two general categories are called forced and free (or natural) convection, respectively.