Controlling flow-induced segregation in a granular mixture is highly relevant to many industrial settings. To enhance mixing or promote segregation, the continuous gravity flow of a bidisperse granular mixture through a series of narrow vertical channels with exit slots is investigated. The bidisperse mixture is composed of two different sizes of particles, but of the same density. In dense flow, segregation occurs, leading to formation of bands. The bands of large particles appear at a distance away from the walls. This finding is in contrast to that in shear-driven segregation in a dense flow where large particles segregate towards the walls. Using a phenomenological model, it has been shown that rolling and bouncing induced segregation is the dominant mechanism. When cylindrical inserts are placed to modify flow patterns, that significantly influences segregation patterns. The symmetrical placement of a cylindrical insert close to the exit slot vanishes the bands and enhances mixing. However, with two inserts placed symmetrically and close to the exit slot, the degree of segregation in the reverse direction is greatly enhanced compared to that without insert. In the former, small particles accumulate in thin regions adjacent to the walls, and large particles comprise the bulk of the domain and the flowing stream. The heap formation above the insert in a narrow channel, when the insert is close to the exit, enhances mixing in one configuration, whereas it amplifies reverse segregation in the other.
Granular mixing and segregation are two important unit operations frequently encountered in the powder and grain industries (Ottino & Khakhar, 2001;Rao & Nott, 2008;Bridgwater, 2012). It has long been known that granular mixtures constituted by particles of different sizes, densities, or surface properties segregate under conditions, such as external vibration or shear (Ahmad & Smalley, 1973;Gray & Hutter, 1997;Shinbrot & Muzzio, 1998;Ottino & Khakhar, 2000;Johnson et al., 2012). Extensive research has been conducted on segregation in various flow configurations, including free surface flow inside a rotating tumbler (Das Gupta et al., 1991;McCarthy et al., 1996;Gray, 2001;Jain et al., 2005), flow over an inclined chute (Khakhar et al., 1999b;Wiederseiner et al., 2011;Tripathi & Khakhar, 2013), bounded heap flow (Fan et al., 2012(Fan et al., , 2017)), flow inside cylindrical Couette geometries (Hill & Fan, 2008;Golick & Daniels, 2009), and flow through a silo (Artega & Tüzün, 1990;Samadani et al., 1999;Ketterhagen et al., 2008). In flow-induced segregation, the pressure gradient induced by the gravity and the gradient of the shear rate can be dominant to influence segregation (Khakhar et al., 1999a;Gray & Thornton, 2005;Fan & Hill, 2011b;Umbanhowar et al., 2019;Jing et al., 2022;Sahu et al., 2023;Duan et al., 2024). It has been shown that the interplay among an advective flow field, diffusion due to grain temperature and percolation determines mixing and segregation (Khakhar et al., 1997;Fan et al., 2014). Despite a deep understanding, less is explored how segregation can be controlled in a flow configuration under the same operating conditions, such as the effect of overburden and the mass flow rate. To address this former issue, one possible mechanism can be the use of flow-modifying inserts (Cliff et al., 2021;Irvine et al., 2023).
Since many decades, flow of grains past inserts or obstacles was investigated (Wieghardt, 1975;Tüzün & Nedderman, 1985a,b;Tardos et al., 1998;Chehata et al., 2003;Gravish et al., 2014;Agarwal et al., 2021). An important aspect in such flows is to design inserts based on the forces (drag and lift) they encounter, and how the flow dynamics influence these forces (Ding et al., 2011;Guillard et al., 2014;Debnath et al., 2017;Dhiman et al., 2020). However, there is a little progress in this area of granular flow due to their complex rheology and the lack of a suitable constitutive law (Debnath et al., 2022a). Numerous studies showed complex flow patterns and regimes in simple flow configurations that existing theories do not capture (Krishnaraj & Nott, 2016;Bharathraj & Kumaran, 2019;Dsouza & Nott, 2021;Debnath et al., 2022bDebnath et al., , 2023)).
Furthermore, mixing and segregation are complex processes. The coupling between the flow dynamics and segregation is not straightforward even in simple flow configurations (Guillard et al., 2016;Jing et al., 2022;Yennemadi & Khakhar, 2023;Liu et al., 2023). Because of these reasons, segregation in flow past an insert is less investigated.
In flow past an insert, a dense stagnant zone and shock wave arise in the upstream and a grain-free wake zone appears in the downstream. The sizes of the stagnant zone and the wake zone depend on the upstream velocity, the shape of the insert, and distances of the free surface and the exit slot from the insert (Wassgren et al., 2003;Cui & Gray, 2013;Mathews et al., 2022;Tregaskis et al., 2022). If the insert is cylindrical, the size of the wake zone reduces, adopting a triangular shape as the upstream velocity decreases (Chehata et al., 2003;Cui & Gray, 2013;Chen et al., 2022;Tregaskis et al., 2022). There is an interesting observation related to the stagnant zone. A recent work (Mathews et al., 2022) showed that the stagnant zone takes the shape of a stable heap due to a continuous flow. It dissolves in absence of the flow. The height of the heap is a function of the diameter of the cylinder and strongly depends on the solids fraction.
However, it shows weak dependence on the grain diameter and the upstream velocity.
In a dense flow, the maximum height of the heap is approximately the diameter of the cylinder.
It can be speculated that if the cylinder’s diameter is comparable to the flow dimensions in a bounded dense flow, such heap formation can significantly alter the surrounding flow patterns. Therefore, it raises a question: can the extent of segregation be controlled by modifying the flow patterns in a bounded dense flow using cylindrical inserts?
Here we investigate a continuous flow through a series of narrow vertical channels with exit slots. The cylindrical inserts of diameter comparable to the narrow dimension of the channel are placed inside the bed at different locations to modify the flow patterns. We adopt a simulation based approach using the discrete element method (DEM) to simulate the flow. We observe that without insert, large particles segregate away from the wall regions, in
This content is AI-processed based on open access ArXiv data.