Ans: In laminar flows, the flow is said to be fully developed when the normalized temperature profile remains unchanged along with the velocity profile after the entrance region.

- The hydrodynamically fully developed flow is equivalent to fully developed flow if the temperature of the fluid remains the same that is there is no heating or cooling of the fluid in the pipe.

- Thermal entry length for laminar flow is typically longer than the hydrodynamic entry length(for Pr>1). Therefore if the fluid is thermally fully developed, it must be hydrodynamically fully developed.

- For turbulent flows in the most practical applications the thermal and hydrodynamic entry lengths are **very short **(compared to laminar flow) and approximately the same.