Investigation on particle size and packing tortuosity by coupling image analysis and permeability tests

This experimental study aims to enhance the understanding of the correlation among equivalent particle diameters measured using two analytical techniques: optical analysis (assisted by computer aided image analysis) and permeability tests. The presence or absence of a specific analytical method or instrument can lead to the use of an incorrect equivalent diameter. Therefore, it can be beneficial to establish conversion rules between different equivalent particle diameters obtained through various methods and instruments. The optical analysis returns an equivalent diameter value inherently independent of particle arrangement since it deals with isolated particles. In contrast, the permeability test offers an equivalent mean diameter dependent not only on the size of the particles but also on their packed arrangement. A suitable correlation between the two diameters has been proposed, shown to be a decreasing function of porosity following a power law. An unexpected outcome of the comparison between the optical method and permeametry is the possibility to isolate and characterize the effect that the packing arrangement has on pressure losses and to characterize it in terms of the tortuosity of the path that the fluid must travel through the packed bed. Our findings confirm a strong alignment between our tortuosity model, which contains the ratio between the two equivalent diameters considered here, and an empirical correlation from literature often utilized for predicting packed bed tortuosity.