Heat flux

In physics and engineering, heat flux or thermal flux, sometimes also referred to as heat flux density, heat-flow density or heat flow rate intensity, is a flow of energy per unit area per unit time. Its SI units are watts per square metre (W/m2). It has both a direction and a magnitude, and so it is a vector 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 flux, as opposed to mass or momentum flux. Fourier's law is an important application of these concepts. Thermal conduction#Fourier's law For most solids in usual conditions, heat is transported mainly by conduction and the heat flux is adequately described by Fourier's law. where is the thermal conductivity. The negative sign shows that heat flux moves from higher temperature regions to lower temperature regions. The multi-dimensional case is similar, the heat flux goes "down" and hence the temperature gradient has the negative sign: where is the gradient operator. Heat flux sensor The measurement of heat flux can be performed in a few different manners. A commonly known, but often impractical, method is performed by measuring a temperature difference over a piece of material with known thermal conductivity. This method is analogous to a standard way to measure an electric current, where one measures the voltage drop over a known resistor. Usually this method is difficult to perform since the thermal resistance of the material being tested is often not known. Accurate values for the material's thickness and thermal conductivity would be required in order to determine thermal resistance. Using the thermal resistance, along with temperature measurements on either side of the material, heat flux can then be indirectly calculated. A second method of measuring heat flux is by using a heat flux sensor, or heat flux transducer, to directly measure the amount of heat being transferred to/from the surface that the heat flux sensor is mounted to.