There are three types of heat transfer. These are:
Thermal analysis calculates the temperature distribution in a body due to some or all of these mechanisms. In all three mechanisms, heat energy flows from the medium with higher temperature to the medium with lower temperature. Heat transfer by conduction and convection requires the presence of an intervening medium while heat transfer by radiation does not.
Types of heat transfer 1: Conduction
Conduction is the one of three types of heat transfer in which thermal energy transfers from one point to another through the interaction between the atoms or molecules of the matter. Conduction occurs in solids, liquids, and gasses.
Conduction does not involve any bulk motion of matter. Gases transfer heat by direct collisions between energetic molecules, and their thermal conductivity is low compared to solids since they are dilute media. The conduction of energy in liquids is the same as in gases except that the situation is considerably more complex since the molecules are more closely spaced and molecular force fields exert a strong influence on the energy exchange in the collision process. Nonmetallic solids transfer heat by lattice vibrations so there is no motion of the media as heat propagates through. Metals are better conductors than nonmetals at normal temperatures because they have free electrons that carry thermal energy.
The heat transfer by conduction obeys Fourier’s law which states that the rate of heat conduction Qconduction is proportional to the heat transfer area (A) and the temperature gradient (dT/dx), or:
Qconduction = – K A (dT/dx)
where K, the thermal conductivity, measures the ability of a material to conduct heat. The units of K are W/m.oC or (Btu/s)/in.oF. For the planar layer shown below, the rate of heat conduction is given by,
Q conduction = – K A ( T H – T C )/L
This figure shows range values for the thermal conductivity for liquids, nonmetallic solids, and pure metals at normal temperature and pressure.
Temperature Dependence of the Thermal Conductivity (K)
For most materials, K varies with temperature. It rises with temperature in gases at low pressures, but it may rise or fall in metals or liquids.
The following table lists thermal conductivities (in W/m.oK) versus temperature (in oK) for selected materials:
Types of heat transfer 2: Convection
Convection is the second Types of heat transfer mode in which heat transfers between a solid face and an adjacent moving fluid (or gas). Convection has two elements:
- Energy transfer due to random molecular motion (diffusion), and
- Energy transfer by bulk or macroscopic motion of the fluid (advection).
The mechanism of convection can be explained as follows: as the layer of the fluid adjacent to the hot surface becomes warmer, its density decreases (at constant pressure, density is inversely proportional to the temperature) and becomes buoyant. A cooler (heavier) fluid near the surface replaces the warmer fluid and a pattern of circulation forms.
The rate of heat exchange between a fluid of temperature Tf and a face of a solid of area A at temperature Ts obeys the Newton’s law of cooling which can be written as:
Qconvection = h A (Ts – Tf)
where h is the convection heat transfer coefficient . The units of h are W/m 2 . K or Btu/s.in 2 .F. The convection heat transfer coefficient (h) depends on fluid motion, geometry, and thermodynamic and physical properties.
Generally, there are two modes convection heat transfer:
Natural (Free) Convection
The motion of the fluid adjacent to a solid face is caused by buoyancy forces induced by changes in the density of the fluid due to differences in temperature between the solid and the fluid. When a hot plate is left to cool down in the air the particles of air adjacent to the face of the plate get warmer, their density decreases, and hence they move upward.
An external means such as a fan or a pump is used to accelerate the flow of the fluid over the face of the solid. The rapid motion of the fluid particles over the face of the solid maximizes the temperature gradient and increases the rate of heat exchange. In the following image, air is forced over a hot plate.
Types of heat transfer 3: Radiation
Thermal radiation is the thermal energy emitted by bodies in the form of electromagnetic waves because of their temperature. All bodies with temperatures above the absolute zero emit thermal energy. Because electromagnetic waves travel in vacuum, no medium is necessary for radiation to take place. The following figure shows the range, in wavelength, of thermal radiation compared to radiation emitted by other means (X-rays, g-rays, cosmic rays, etc).
The thermal energy of the sun reaches the earth by radiation. Because electromagnetic waves travel at the speed of light, radiation is the fastest heat transfer mechanism.
The following Types of heat transfer topics are discussed in the next subsections:
- Basic Radiation Definitions
- Stefan-Boltzmann Law
- Radiation Emission from Real Surfaces
- Radiation Exchange Between Surfaces
- Radiation View Factors