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Fluid Flow - Entrance Length and Developed Flow

The entrance length is the length in a tube or duct after an obstruction - until the flow velocity profile is fully developed.

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A fluid flow need some length to fully develop the velocity profile after passing through components like bends, valves, pumps, turbines or similar.

Entrance Length

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

The Entrance length number correlates with the Reynolds Number  and for laminar flow the relation can be expressed as:

Ellaminar = 0.06 Re                       (2)


Re = Reynolds Number

Entrance Length Number for Turbulent Flow

The Entrance length number correlation with the Reynolds Number for turbulent flow can be expressed as:

Elturbulent = 4.4 Re1/6                         (3)

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  • Engineering ToolBox, (2003). Fluid Flow - Entrance Length and Developed Flow. [online] Available at: [Accessed Day Mo. Year].

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