Example – Heat flux through a window
A major source of heat loss from a house is through the windows. Calculate the rate of heat flux through a glass window 1.5 m x 1.0 m in area and 3.0 mm thick, if the temperatures at the inner and outer surfaces are 14.0°C and 13.0°C, respectively. Calculate the heat flux through this window.
At this point, we know the temperatures at the surfaces of material. These temperatures are given also by conditions inside the house and outside the house. In this case, heat flows by conduction through the glass from the higher inside temperature to the lower outside temperature. We use the heat conduction equation:
We assume that the thermal conductivity of a common glass is k = 0.96 W/m.K.
The heat flux will then be:
q = 0.96 [W/m.K] x 1 [K] / 3.0 x 10-3 [m] = 320 W/m2
The total heat loss through this window will be:
qloss = q . A = 320 x 1.5 x 1.0 = 480W
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).