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 t he stable laminar regime, at higher Reynolds numbers. Local measuremen ts of current fluctuations were carried out with electrochemical probe s located at different positions in the porous media. Owing to the use of the electrochemical transfer function, the spectrum of velocity gr adient fluctuations at the micro-electrodes and the velocity gradient fluctuating rate were determined. In packed beds of particles, a stabi lization of this fluctuation rate was observed at most electrodes in t he 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 fl ow, i.e. the order of magnitude of the Bow eddies dimensions, were eva luated from the autocorrelation function of the velocity gradient fluc tuations calculated from the spectrum. They were compared to the pore diameter calculated from the capillary model proposed by Comiti and Re naud [(1989), Chem Engng Sci. 44, 1539-1545]. The stable values of the length scales obtained for high Reynolds numbers confirm the turbulen t 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 m ain 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. Calculat ions, based on a pressure drop model related to the capillary represen tation, show that the percentage of inertial effect on the pressure dr op is then about 90% at this Reynolds number in these packed beds. (C) 1998 Elsevier Science Ltd. All rights reserved.