Developing Turbulent Flow in Pipes and Analysis of Entrance Region

Abstract

Turbulent flows have complex structures due to its nature and its’ analyses are hard either by numerical or experimental means. Hydrodynamic development of turbulent flow is also complex. In this study, velocity and turbulence distributions in hydrodynamic entrance length of pipes are investigated numerically depending on axial and radial locations. Implications of these distributions are qualitatively evaluated in terms of heat transfer. Literature was surveyed for a single empirical expression that provides velocity profile directly according to Reynolds number, radial and axial locations. Requisite for computational fluid dynamics in hydrodynamic entry length of pipes is stressed by assessing turbulence magnitudes in radial and axial directions. Definition of the development length and effects of the definition in respect of heat transfer are discussed. An axisymmetric pipe entrance region was analyzed by means of a commercial CFD code with nondimensional parameters. Therefore, dimensional parameters reduce into one dimensionless independent parameter, i.e. Reynolds number. Four different Reynolds numbers that are 5x103 , 1x104 , 5x104 , 1x105 were used in calculations. k-? turbulence model and standard wall functions were used for turbulence modeling. Hydrodynamic entry length, velocity and turbulence values are presented by means of axial and radial profiles. According to the obtained results, two different directions of radial velocity component values exist in the hydrodynamic entry length that would lead to different radial thermal convection effects. It is found that simultaneously developing velocity profiles and turbulence quantities leads to a characteristic centerline velocity profile. Also, it is seen that a good resolution in hydrodynamic entrance length can be easily achieved by computational fluid dynamics. A detailed composition of hydrodynamic turbulent entrance length analysis, its physical explanations due to simultaneously developing hydrodynamic boundary layers and turbulence production, definition aspects of the entrance length in terms of heat transfer and literature survey for analytical solution of the region are provided.