Flow regime and liquid–solid mass transfer investigation in a designed porous structure using electrochemical micro-probes

A revisit of the electro-diffusional theory for the wall shear stress measurement

“…If the simple geometry as a straight channel is used, the strip probe can be uniquely positioned on the wall so that its leading edge is perpendicular to the flow direction. In many applications [36,37], the situation is less clear, and the flow direction is also dependent on the input conditions. In such cases, the probe cannot be placed universally in the space so that the liquid flow is always perpendicular to the leading edge, and the existence of transverses flow directions must be taken into account in addition to the main axial component.…”

Comments on the paper “Flow regime and liquid-solid mass transfer investigation in a designed porous structure using electrochemical micro-probes” by Altheimer et al. (2016)

“…A comparison of the porous structure studied in Altheimer et al (2016) with the geometry studied in Huchet et al (2007) appears to be unsuitable due to the aforementioned comments, while the rise in the liquid-solid mass transfer underscored in their conclusion is likely to have been overestimated.…”

“…The diffusion-convection equation is solved in the vicinity of the microelectrode, thus leading to an analytical relationship between the local wall mass transfer ( , which itself is correlated with the measured limiting diffusion current (Eq. 5 in the paper by Altheimer et al (2016)), and the wall shear rate (Eq. 6).…”

“…None of the underlying assumptions have been mentioned in the reviewed paper, even though one of them is crucial, namely: the size of the microelectrode must be of the same order of magnitude as the turbulent eddies. In Altheimer et al (2016), the size of the microelectrode has been set equal to 0.4 mm. Flow uniformity above the electrochemical sensor is another important issue; the circular probe flush-mounted to the wall is capable of producing diffusional edge effects.…”

“…Generally speaking, the authors implement a working electrode with a large active surface area in order to measure the liquid/solid mass transfer (see, for example, Montillet et al (1994) or Huchet et al (2007)). Altheimer et al (2016) have thus compared their local mass transfer results with the average liquid-solid mass transfer measured from three macro-electrodes mounted in a well-designed geometry (Huchet et al (2007)), composed of a network of crossing minichannels. This millifluidic device was designed to mimic porous media and the results (Huchet et al (2007)) have shown that the liquid-solid mass transfer is similar to the values established in the packed beds of spheres (Wakao and Funazkri, 1978), whereby:…”