Entry Length

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There is an entrance region where a nearly inviscid upstream flow converges and enters the tube. Viscous boundary layers grow downstream, retarding the axial flow v(x, r) at the wall and thereby accelerating the center-core flow to maintaintain the incompressible continuity requirement

Q = òv dA = constant

At a finite distance from the entrance, the boundary layers merge and the inviscid core disappears. The flow is then entirely viscous, and the axial velocity adjusts slightly further until at x = Le it no longer changes with x and is said to be fully developed, v = v(r) only. Downstream of x = Le the veocity profile is constant, the wall shear is constant, and the pressure drops linearly with x, for either laminar or turbulent flow.

Le/D = 0.06 ReD for laminar

Le/D = 4.4 ReD1/6 Where Le is the entry length; and
ReD is the Reynolds number based on Diameter.

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Last Modified on: 14-Sep-2014

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