HYDRODYNAMICS OF LIQUID FLOW IN PACKED-BEDS - AN EXPERIMENTAL-STUDY USING ELECTROCHEMICAL SHEAR RATE SENSORS

A literature survey on the electrochemical sensor technique shows that the simple relation between the average measured diffusional current and the local shear rate of the liquid flow holds even for extremely n on-homogeneous flow conditions with high-amplitude fluctuations. Even though the frequency response of electrochemical probes is unsatisfact ory, their transfer function has been recently determined and low-ampl itude high-frequency fluctuations of the instantaneous local shear rat e can be investigated. Small circular probes have been successfully us ed to study hydrodynamics of liquid flow in packed-bed reactors. The l ocal flow is extremely non-homogeneous in space and, for high Reynolds numbers, quite fluctuating in time. The onset of chaotic, time-depend ent flow occurs at superficial Reynolds numbers of about 110-150. The analysis of the characteristic times of the velocity gradient fluctuat ions in the chaotic flow regime indicates the existence of liquid aggr egates having the characteristic dimensions of the porous media.