### Heat Flux Thermo Flux Equation Thermodynamic Properties

**Thermodynamics Directory**** | Heat Transfer Directory**

*Heat Flux Thermo Flux Equation*

**Heat flux**or

**thermal flux**is the rate of heat energy transfer through a given surface, per unit surface. The SI derived unit of

*heat rate*is joule per second, or watt.

**Heat flux density**is the heat rate per unit area. In SI units, heat flux density is measured in [ W /m 2 ]. Heat rate is a scalar quantity, while heat flux 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.

The rate at which heat is transferred is represented by the symbol Q. Common units for heat transfer rate is Btu/hr. Sometimes it is important to determine the heat transfer rate per unit area, or heat flux, which has the symbol Q" . Units for heat flux are Btu/hr-ft^{2}. The heat flux can be determined by dividing the heat transfer rate by the area through which the heat is being transferred.˙

Q ^{"} = Q/A

where:

Q" = heat flux (Btu/hr-ft^{2})

Q = heat transfer rate (Btu/hr)

A = area (ft^{2})