The fluid temperature (Tb), referred to as the
bulk temperature, varies according to the details of the situation. For flow adjacent to a hot or cold surface, Tb is the temperature of the fluid that is "far" from the surface, for instance, the center of the flow channel. For boiling or condensation, Tb is equal to the saturation temperature.
The concept of the bulk temperature is that adiabatic mixing of the fluid from a given cross section of the duct will result in some equilibrium temperature that accurately reflects the average temperature of the moving fluid, more so than a simple average like the film temperature .