In both compressible and incompressible fluid flow, the stagnation temperature equals the total temperature at all points on the streamline leading to the stagnation point.
Applying the steady flow energy equation[1]: Eq (5.50) and ignoring the work, heat and gravitational potential energy terms, we have: where: Substituting for enthalpy by assuming a constant specific heat capacity at constant pressure (
However, invoking the common assumption of a calorically perfect gas, enthalpy can be converted directly into temperature as given above, which enables one to define a stagnation temperature in terms of the more fundamental property, stagnation enthalpy.
A bimetallic thermocouple is frequently used to measure stagnation temperature, but allowances for thermal radiation must be made.
The aforementioned figures are 'worst case scenario values' that allow collector designers to plan for potential overheat scenarios in the event of collector system malfunctions.