Heat flux or thermal flux is the rate of heat energy

Energy

In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 transfer through a given surface. The SI derived unit of heat rate is joule per second, or watt. Heat flux is the heat rate per unit area. In SI

Si

Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

 units, heat flux is measured in [W

Watt

The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

/m²]. Heat rate is a scalar quantity, while heat flux

Flux

In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...

 is a vectorial quantity. To define the heat flux at a certain point in space, one takes the limiting case where the size of the surface becomes infinitesimally small.

Heat flux is often denoted , the subscript q specifying heat rate, as opposed to mass or momentum rate. Fourier's law is an imporant application of these concepts.

Measuring heat flux

The measurement of heat flux is most often done by measuring a temperature difference over a piece of material with known thermal conductivity

Thermal conductivity

In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

. This method is analogous to a standard way to measure an electric current, where one measures the voltage

Voltage

Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

 drop over a known resistor

Resistor

A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...

.

Relevance to science and engineering

One of the tools in a scientist's or engineer's toolbox is the energy balance

Energy balance

Energy balance may refer to:* First law of thermodynamics, according to which energy cannot be created or destroyed, only modified in form* Energy balance , a measurement of the biological homeostasis of energy in living systems...

. Such a balance can be set up for any physical system, from chemical reactors to living organisms, and generally takes the following form

where the three terms stand for the time rate of change of respectively the total amount of incoming energy, the total amount of outgoing energy and the total amount of accumulated energy.

Now, if the only way the system exchanges energy with its surroundings is through heat transfer, the heat rate can be used to calculate the energy balance, since

where we have integrated the heat flux density over the surface of the system.

In real-world applications one cannot know the exact heat flux at every point on the surface, but approximation schemes can be used to calculate the integral, for example Monte Carlo integration.

See also

• Eddy covariance
  Eddy covariance
  The eddy covariance technique is a key atmospheric flux measurement technique to measure and calculate vertical turbulent fluxes within atmospheric boundary layers...
  
   flux (eddy correlation, eddy flux)
• Heat Notation
• Heat conduction
  Heat conduction
  In heat transfer, conduction is a mode of transfer of energy within and between bodies of matter, due to a temperature gradient. Conduction means collisional and diffusive transfer of kinetic energy of particles of ponderable matter . Conduction takes place in all forms of ponderable matter, viz....
  
  
• Heat transfer
  Heat transfer
  Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...
  
  
• Critical heat flux
  Critical heat flux
  Critical heat flux describes the thermal limit of a phenomenon where a phase change occurs during heating , which suddenly decreases the efficiency of heat transfer, thus causing localised overheating of the heating surface.The Critical heat flux for ignition is the lowest thermal load per unit...
  
  
• Latent heat flux
• Rate of heat flow
  Rate of heat flow
  The rate of heat flow between two systems is measured in watts .The formula for rate of heat flow is ∆Q/∆t = -K×A×∆T/x ,...
  
  
• Insolation
  Insolation
  Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter or kilowatt-hours per square meter per day...
  
  
• Heat flux sensor
  Heat flux sensor
  A heat flux sensor is a transducer that generates an electrical signal proportional to the total heat rate applied to the surface of the sensor. The measured heat rate is divided by the surface area of the sensor to determine the heat flux....
  
  
• Relativistic heat conduction
  Relativistic heat conduction
  The theory of relativistic heat conduction claims to be the only model for heat conduction that is compatible with the theory of special relativity, the second law of thermodynamics, electrodynamics, and quantum mechanics, simultaneously...