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 non-homogeneous flow conditions with high-amplitude fluctuations. Even though the frequency response of electrochemical probes is unsatisfactory, their transfer function has been recently determined and low-amplitude high-frequency fluctuations of the instantaneous local shear rate can be investigated. Small circular probes have been successfully used to study hydrodynamics of liquid flow in packed-bed reactors. The local flow is extremely non-homogeneous in space and, for high Reynolds numbers, quite fluctuating in time. The onset of chaotic, time-dependent flow occurs at superficial Reynolds numbers of about 110-150. The analysis of the characteristic times of the velocity gradient fluctuations in the chaotic flow regime indicates the existence of liquid aggregates having the characteristic dimensions of the porous media.