Skip to main content

​
Common Terms

Heat is the form of energy transferred between systems or objects with different temperatures, flowing from the high-temperature system to the low-temperature system.SI unit: Joule (J)
Heat flux is the flow of energy per unit of area per unit of time. It has both a direction and a magnitude, so it is a vector quantity.SI unit: Watts per square meter (W/m2)
Heat generation, or “internal heat generation,” is the heat generated by the bulk matter (volume) of a body. It is typically used to model heat sources or sinks in an analysis.SI unit: Watts per cubic meter (W/m3)
Temperature is a physical quantity that expresses the degree of hotness or coldness.SI unit: Kelvin (K)

​
Heat Transfer Modes

There are three fundamental modes of heat transfer
Thermal conduction is the transfer of heat energy by microscopic collisions of particles and movement of electrons within a body. Conduction takes place in most phases: solid, liquid, and plasma. Thermal conduction can be represented by Fourier’s Law:q= -k∇ Twhere:
  •  q is the local heat flux density, W/m__2
  •  k is the material’s thermal conductivity, W/(m·K)
  •  ∇ T is the temperature gradient, K/m
Convection is the transfer of heat energy from one place to another by the movement of fluid. The basic relationship for heat transfer by convection is:Q̇ = hA(T-Tf)where:
  • Q̇ is the heat transferred per unit time, W
  • h is the heat transfer coefficient, W/(m2K)
  • A is the area of the object, m__2
  • T is the object’s surface temperature, K
  • Tf is the fluid temperature, K
The convective heat transfer coefficient relates the heat flux to the temperature difference between the boundary and the fluid medium. It is dependent on the physical properties of the fluid and the physical situation.
Thermal radiation is the transfer of heat energy via electromagnetic waves from an emitting body. Thermal radiation can be represented by the Stefan-Boltzmann’s Law: q =  εσT4where:
  • q is the radiation heat flux density, W/m2
  • ε is the emissivity (a measure of the effectiveness of the body to emit energy) and ranges between 0 and 1
  • σ is the Stefan-Boltzmann’s constant, equal to 5.670374419×10-8 W/m2K4
  • T is the absolute temperature, K