Research on clogging mechanisms of bulk materials flowing trough a bottleneck

Highlights

• Clogging mechanisms are examined experimentally and numerically.

• There is a indeed region with larger fluctuation characteristics near the outlet.

• The dynamic characteristics are not always related to the formation of clogging.

• The different influencing factors and mechanisms of clogging are clarified.

Abstract

Dense granular systems flowing through bottleneck is prone to spontaneously form clogging. Clogging is common in various industries, which makes the process requirements of continuous production might not be accomplished. Therefore, it is very important to clarify the dynamic characteristics and clogging mechanism for the optimal design of anti-clogging system. In this work, clogging mechanisms are examined experimentally and numerically. The results indicate that there is indeed a region with larger fluctuation of kinematic characteristics near the outlet, which we called “dynamic arch”. But, the fluctuation characteristics are not always related to the formation of clogging which can help to distinguish two clogging mechanisms, including “collision clogging” and “jamming”. Further, we also illustrated microscopic mechanisms of clogging and discussed fully the different clogging mechanisms. The definite clogging mechanisms lay foundations for a better designs of devices for preventing formation of the clogging.

Introduction

In a large number the industry of chemical production, such as food and pharmaceutical industry, in order to make multiple processing equipment form a continuous operation unit, the granular materials usually inevitably flow through a bottleneck to feed into the next processing equipment [1,2]. However, convergence of dense particles towards the bottleneck is prone to developing blockage, which will cause the interruption of the entire production and seriously affect the production efficiency [3]. In recent years, although anti-clogging devices emerge in various forms, there is no yet a satisfactory way to avoid clogging completely. Up to now, the clogging problems in various systems still urgently require to be solved, such as microfluidic devices [4,5], extrusion process used for forming pastes into designed shapes [6] and the suspension system with fluid as the driving force [7]. Therefore, it is very important to clarify the clogging mechanism to guide the optimal design of anti clogging equipment for the processing process with continuous production requirements.

In fact, clogging is a rather complex problem. First, clogging are influenced by a large number of factors [8], such as silo geometry [[9], [10], [11]], outlet types [[12], [13], [14]], outlet size [15,16] and characteristics of granular material including particle size [17], shape of particles as well as particle surface roughness [[18], [19], [20], [21]], and even packing of particles [22]. These variables affect mechanisms of clogging, clogging probability and prevention of clogging leading to the thorny problem of construction of the unified clogging theory. Secondly, clogging is considered to be a rapid and self-organized process. Bianconi and Marsili [23] detected clogging is related to the self-organized critical state indicating the clogging is a critical state of natural evolution without intrinsic time or length scale, therefore clogging is seen as a unpredictable process. For the above reasons, at present, the underlying mechanism of clogging is still unclear. But recently, some studies have shown that the clogging arch is similar to the expansion zone [19,24] which is called “dynamic arch”, near the outlet in the discharging process. Therefore, there should be some relationship between “dynamic arch” and clogging. Specifically, the “dynamic arch” may be the initial form of the clogging arch. And the dynamic effect caused by the formation and collapse of “dynamic arch” may determine the possibility of clogging. These possibility provide a promising research direction for understanding the mechanism of clogging, which could help to solve the clogging troubles.

However, the dynamic effect is difficult to be observed in the experiment. Over the years, different experimental methods have emerged to discern dynamic properties of granular materials during discharging process. Observing a two-dimensional silo with image processing techniques exhibits many advantages over the other methods, and has been adopted very widely to understand discharging process. To [25] experimentally studied jamming transition of monodisperse metal disks flowing through two-dimensional silo. Using the method of visualization, they minutely recorded the number of particles passing through the hopper before jamming and construct the function of the jamming probability according the distribution function for the number of particles through the silo before the jam occurs. Further, Zuriguel et al. [26] tracked velocity fluctuations of particles in a two-dimensional silo during discharging process using the method of visualization and they confirmed that the conclusions obtained from the 2D discharging experiments are also applicable to 3D silo. However, for the research of clogging process, this method face a thorny problem that the transient time of clogging is unable to determine in the 2D discharging experiments. In this regard, Discrete Element Method (DEM) is an effective tool for solving this problem, since the numerical method can provide microscopic stress information [27].

Therefore, in this work, we use a combination of experiment and simulation to study the relationship between the clogging arch and the “dynamic arch” (fluctuation effect) during the discharging. It is worth noting that we constructed two completely different discharge conditions to observe different clogging phenomenons. The results clarify the influencing factors and mechanisms of clogging, and lay a foundation for the prediction and prevention of clogging.