Heat Flux Measurement
The measurement of heat flux can be performed in a few different manners.
- Measurement based on the temperature difference. 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 assumes that the material’s thermal conductivity is well known. This method is analogous to a standard way to measure an electric current, where one measures the voltage drop over a known resistor.
- Measurement based on the use of the heat flux sensor. Heat flux can be directly measured via heat flux sensors or heat flux transducers. The most common type of heat flux sensor is a differential temperature thermopile which operates on essentially the same principal as the first measurement method. A heat flux sensor should measure the local heat flux density in one direction. The result is expressed in watts per square meter. This measurement has the advantage in that the thermal conductivity does not need to be a known parameter.
Heat Flux Density – Thermal Flux
The rate of heat transfer per unit area normal to the direction of heat transfer is called heat flux. Sometimes it is also referred to as heat flux density. In SI its units are watts per square metre (W.m−2). It has both a direction and a magnitude, and so it is a vector quantity. The average heat flux is expressed as:
where A is the heat transfer area. The unit of heat flux in English units is Btu/h·ft2. Note that heat flux may vary with time as well as position on a surface.
In nuclear reactors, limitations of the local heat flux is of the highest importance for reactor safety. Since nuclear fuel consist of fuel rods, the heat flux is there defined in units of W/cm (local linear heat flux) or kW/rod (power per fuel rod).