The entrance length is the length in a tube or duct after an obstruction - until the flow velocity profile is fully developed.
A fluid flow need some length to fully develop the velocity profile after passing through components like bends, valves, pumps, turbines or similar.
The entrance length can be expressed with the dimensionless Entrance Length Number as
El = le / d (1)
El = Entrance Length number
le = length to fully developed velocity profile (m, ft)
d = tube or duct diameter (m, ft)
Entrance Length Number for Laminar Flow
Ellaminar = 0.06 Re (2)
Re = Reynolds Number
Entrance Length Number for Turbulent Flow
Elturbulent = 4.4 Re1/6 (3)