Abstract

DU PLESSIS, J. P.  and  COLLINS, M. R.. A new definition for laminar flow entrance lengths of straight ducts. R&D j. (Matieland, Online) [online]. 1992, vol.8, pp.11-16. ISSN 2309-8988.

A new definition for hydrodynamic entrance lengths of developing laminar flow in straight ducts of arbitrary cross-section is proposed. The suggested length of flow development is theoretically based on hydrodynamic characteristics of the flow development and can be retrieved analytically whenever the fully developed velocity profile is known. A property is obtained which is both unique and physically more meaningful than the customary hydrodynamic entrance length. The latter corresponds to the maximum velocity attaining an arbitrary percentage of the fully developed limit, this value being obtainable only indirectly by numerical means. The proposed new definition forsakes detail of the velocity profile for the more fundamental equalisation of the two basic conditions of developing and developedflow. The definition is applied to some commonly encountered duct shapes and corresponding hydrodynamic entrance lengths are provided.

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