Experimental study of the influence of the rows of vortex generators on turbulence structure in a tube

摘要

An experimental study was carried out on the effects of a cascade of longitudinal vortices on the turbulence structure of the flow inside a tube equipped with seven rows of vortex generators. This mixing process is tested in order to be used as a chemical reactor. Laser-induced fluorescence technique (LIF) visualizations of the flow structures show their role in the radial transport mechanism. Measurements of the mean velocity and turbulence quantities were carried out by laser Doppler velocimetry (LDV). The mechanism of radial transfer induces a deformation "in double U" of the axial velocity profile.The study of the axial velocity fluctuation rate showed that the rows increase notably turbulence in the flow. There is a turbulent flow establishment length, equivalent to the first three rows lines. Besides the axial velocity fluctuation rate does not seem to be sensitive to the Reynolds number, in the studied range. In order to solve the difficulties related to the measurement of the space-time correlations velocity, the fluctuations macro-scale length has been evaluated starting from a relation similar to that obtained by Taylor. A temporal macro-scale, and a suitable convection velocity scale, which take into account the turbulence heterogeneity generated in the static mixer, have been defined. The turbulence dissipation rate was defined based on the Batchelor non-dimensional assumption and the local isotropy of the high frequency fluctuations assumption. It is heterogeneous through the flow cross-section. The hydrodynamic structures generated by the rows increase considerably the turbulence level but they do not diffuse turbulence in the radial flow direction. The Reynolds number does not have a qualitative influence on the turbulence dissipation rate distribution in the studied range. The knowledge of this turbulence characteristic is of a great utility for the prediction of the selectivity of fast chemical reactions.