Heat flux is the flow of thermal energy from a high to low potential. It occurs through conduction and convection and is measured by temperature differences. Heat flows by diffusion in fluids and by vibrations in solids. Sensible heat flux is due to temperature differences, while latent heat flux is due to phase changes. Critical heat flux is the maximum amount of heat that can be pumped through a surface area before flow characteristics change.
Heat flux refers to the flow of thermal energy. Sometimes called heat flux, it is the calculated flow rate of heat energy across a specified surface area as it flows from one place to another. Like other forms of energy, heat flows by conduction and convection in the direction of potential energy fall: the temperature in the case of thermal energy.
When scientists measure heat flux, they first measure the temperature at two points between which heat energy is transferred. Heat, like all forms of energy, flows from high to low potential, 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 higher the pressure forcing the flow to areas of less motion and lower pressure. The heat flow, in response to temperature differences over time, balances the temperature and approaches a state where temperatures cease to vary and heat ceases to flow, a state known as thermal equilibrium.
Given a sufficiently large volume of fluid, such as a liquid or a gas, the heat flow occurs mainly by convection, by diffusion of atoms and molecules. Heat flows from volumes where particles move rapidly to volumes where particles move slowly. Fluid particles move to cold regions where there is more space between the slower moving particles. Since atoms cannot move in solids, heat is conducted by the vibrations of the atoms or molecules in their bonded lattices, with an additional distribution of energy due to the movement of free electrons. Since the movement of free electrons is also a feature of electrical conduction, good conductors of electricity are also good conductors of heat.
Heat flux due to temperature differences is called sensible heat flux in contrast to latent heat flux, which refers to heat flux due to phase changes in the medium. The critical heat flux is the amount of heat that can be pumped through a given surface area in a given amount of time, where the flow characteristics of the fluid change, such as a change of state. When a material becomes solid, it incorporates energy which constrains its lattice structure. At critical heat flux, when a certain amount of heat is pumped into the material, it reaches its melting point or gas transition temperature, and this binding energy is released as latent heat.
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