Heat flux refers to the flow of heat energy. Sometimes called thermal flux, it is the calculated flow rate of heat energy across a specified surface as it flows from place to place. Like other forms of energy, heat flows by conduction and convection in the direction of potential energy drop — temperature in the case of heat energy.
When scientists measure the flow of heat, they first sample the temperature in two locations between which the heat energy is transferred. Heat, like all forms of energy, flows from high to low potential — from high to low temperature. The faster the atoms or molecules of a medium move and vibrate, the higher the temperature of that medium and the greater the pressure forcing flow toward areas of less movement and less pressure. Heat flux, in response to temperature differences over time, balances out temperature and approaches a state in which temperatures cease to vary, and heat ceases to flow — a state known as thermal equilibrium.
Given a large enough volume of fluid, such as a liquid or gas, heat flux occurs mostly through convection, by the diffusion of atoms and molecules. Heat flows from volumes where particles are moving rapidly to volumes where particles are moving slowly. Fluid particles move out into cold regions where there is more space between slower moving particles. Since atoms cannot move in solids, heat is conducted by the vibrations of atoms or molecules in their constrained lattices, with further energy distribution due to the movement of free electrons. Since the movement of free electrons is also a characteristic of electrical conduction, good conductors of electricity are also good conductors of heat.
Heat flux due to temperature differences is called sensible heat flux as opposed to latent heat flux, which refers to the flow of heat due to phase changes in the medium. Critical heat flux is the amount of heat that can be pumped through a given surface in a given amount of time, in which the flow characteristics of the medium change — a change in state for example. When a material becomes solid, it incorporates energy that constrains its lattice structure. At critical heat flux, as a certain amount of heat is pumped into the material, it reaches its melting point or gas transition temperature, and this bonding energy is released as latent heat.
