Glossary term
Heat Flux
The heat-transfer rate per unit area through a surface, boundary, wall, interface, or control surface.
Definition
quantityHeat flux is heat-transfer rate divided by the area through which heat crosses, usually expressed in watts per square metre.
Heat flux describes the intensity of thermal energy transfer through a surface. It can result from conduction, convection, radiation, boiling, condensation, or combined modes. It is central to thermal design because temperature limits, material stress, insulation sizing, cooling capacity, burn risk, and heat-exchanger performance often depend on heat rate per unit area rather than total heat rate alone.
Heat flux is the rate at which heat crosses a surface per unit area:
where q'' is heat flux, \dot{Q} is heat-transfer rate, and A is the area normal to the heat flow. A large heat rate spread over a large area may be manageable, while the same heat rate concentrated over a small area can overheat a component, damage insulation, boil a fluid, or create thermal stress.
Conduction, convection, and radiation
For one-dimensional conduction through a material, Fourier’s law relates heat flux to temperature gradient:
where k is thermal conductivity. The negative sign indicates that heat flows from higher temperature toward lower temperature.
For convection, heat flux is often modelled as:
where h is convective heat-transfer coefficient, T_s is surface temperature, and T_\infty is bulk fluid temperature. The value of h depends on fluid properties, flow regime, geometry, surface condition, and whether the flow is forced or natural convection.
For radiation between a surface and its surroundings, heat flux depends strongly on absolute temperature and emissivity. At high temperatures, radiative heat flux can dominate conduction and convection.
Engineering use
Heat flux is used to size heat sinks, insulation, heat exchangers, furnace walls, reactor cooling jackets, building envelopes, brake systems, batteries, electronics packages, and spacecraft thermal protection. It is also a safety quantity: burn hazard, fire exposure, and material degradation often depend on incident heat flux.
Critical heat flux is especially important in boiling systems. Above a certain heat flux, stable liquid contact can be lost and a vapor film may insulate the surface. Surface temperature can then rise rapidly, causing burnout or severe damage. This makes heat flux a limiting variable in boilers, nuclear systems, evaporators, and high-power electronics cooling.
Common mistakes
A common mistake is confusing total heat rate with heat flux. A component may meet total heat removal requirements while still failing locally because a hot spot has excessive flux density. Another mistake is using the wrong area: projected area, wetted area, fin area, contact area, and effective radiating area can differ substantially. Good thermal documentation states the area basis, heat-transfer mode, boundary conditions, temperature reference, and whether the reported value is average, peak, incident, absorbed, or net heat flux.