Experimental characterization of flow regimes in various porous media - II: Transition to turbulent regime

This work deals with hydrodynamics in porous media beyond the end of the stable laminar regime, at higher Reynolds numbers. Local measurements of current fluctuations were carried out with electrochemical probes located at different positions in the porous media. Owing to the use of the electrochemical transfer function, the spectrum of velocity gradient fluctuations at the micro-electrodes and the velocity gradient fluctuating rate were determined. In packed beds of particles, a stabilization of this fluctuation rate was observed at most electrodes in the Reynolds number range covered. It is shown that this stabilization corresponds to a locally turbulent How. In reticulated media, no such stabilization was observed. The characteristic length scales of the flow, i.e. the order of magnitude of the Bow eddies dimensions, were evaluated from the autocorrelation function of the velocity gradient fluctuations calculated from the spectrum. They were compared to the pore diameter calculated from the capillary model proposed by Comiti and Renaud [(1989), Chem Engng Sci. 44, 1539-1545]. The stable values of the length scales obtained for high Reynolds numbers confirm the turbulent nature of the How regime. The flow regime transition is gradual from laminar to turbulent in the entire bed, it is characterized with the pore Reynolds number, Re-p, based on the employed capillary model: in packed beds presenting an isotropy in the plane perpendicular to the main direction of the fluid flow, the laminar regime ends at Re-p = 160 whereas a value of Re-p = 900 corresponds to the stabilization of the velocity gradient fluctuating rate at 90% of the electrodes. Calculations, based on a pressure drop model related to the capillary representation, show that the percentage of inertial effect on the pressure drop is then about 90% at this Reynolds number in these packed beds. (C) 1998 Elsevier Science Ltd. All rights reserved.