3. Results and Discussion
3.1. Experiment #1
Particle paths of four particles obtained by the DEM study were presented in Figure 1. These particles were picked randomly, each taken from one of the four quadrants of the circular section. In addition, a Poincare plot is utilized to present the path of the particles within the sections in the vertical static mixer. The Poincare plot locates a colored dot for each particle at the position at which the particle passes through a cut plane (termed as a Poincare section). In Figure 4, the positions of particles at six different Poincare segments in the vertical static mixer were presented at heights: 0·H, 0.5·H, 1·H, 1.5·H, 2·H and 3·H. The color parameter is a logical expression employed to mark the initial color of particles at positions x < 0 (yellow) and x > 0 (blue). As the particles start to follow the flow of particles, they start to blend. By the end of the mixer, the particles have not blend totally—there are critical pockets of just yellow and blue particles to be mixed more appropriately.

 

Figure 4. Three-element vertical static mixer scheme. (a) Poincare maps of the particle trajectories from different Poincare sections in the three-element vertical static mixer (H—mixing element height) (b).

The relative standard deviation values decayed in all three sets of trials and accompanied numerical studies, for all three experimental groups. The quality of mixing was augmented with the increase in the count of blending elements within a vertical static mixer. The contrast between the experimental and the DEM model results (presented as RSD values) is illustrated in Figure 5 (for Experiment #1), which outlined the obtained conduct of the performed experiment.

 

Figure 5. The RSD estimation for Experiment #1.

The DEM approach was applied to investigate particle motions throughout the mixing procedure, involving particle mechanics (tracking, location and movement) in the three Cartesian dimensions. The position of the specific particle is fundamentally varied through blending, with the expressive changes in mechanics parameters (position, velocity, and acceleration), which provide the potential to enhance the mixing quality.

The uniformity result acquired following after the passage of the limestone aggregate mixture particles through only one element of the vertical static mixer exhibited the RSD values of 90.68; 94.50; 95.78; 91.23; 91.65 and 90.28%, for binary mixture: (1 ÷ 2 mm)—(2 ÷ 3 mm), (1 ÷ 2 mm)—(3 ÷ 4 mm), (1 ÷ 2 mm)—(4 ÷ 5 mm), (2 ÷ 3 mm)—(3 ÷ 4 mm), (2 ÷ 3 mm)—(4 ÷ mm) and (3 ÷ 4 mm)—(4 ÷ 5 mm), respectively, which went far beyond the recommended value of 10% [41]. The acquired result of the DEM investigation showed almost the same results, indicating the RSD value of 90.26; 98.79; 99.57; 87.24, 89.07, and 93.87%, respectively for the above-mentioned limestone aggregate mixture.

The uniformity of a limestone aggregate mixture falling through a vertical static mixer equipped with 1, 3 or 5 elements illustrated a declining trend of RSD value specifying the better mixing action, but the goal of RSD < 10% was not nearly fulfilled. Besides, Figure 5 expressed the dependence of RSD value and the count of passages through the vertical static mixer, for the experimental and the numerical study. The RSD could be lowered, by boosting the mixing elements count, but not enough to reach the level below 10%. In addition, by increasing the blending parts count, the structure of the vertical static mixer could induce the augment in the installing investments. Figure 5 also demonstrated that numerical results agreed with trial conduct, cross-referenced the acquired RSD values (r2 was 0.968).

3.2. Experiment #2
The investigation of the possible opportunities to diminish the relative statistic deviation in the conveyor mixer was accomplished in Experiment #2. The mixing duration throughout the conveyor mixer was set to: 4, 12 or 20 min, and the outcomes of the uniformity tests were discovered by the RSD criteria evaluation, Figure 6.

 

Figure 6. The RSD results in Experiment #2.

After mixing in the conveyor mixer, the uniformity results estimation of the limestone aggregate mixture (which did not fall beforehand through the vertical static mixer) explained that RSD were between 45.10 and 54.76%, after 20 min, for experimental and between 45.25 and 55.60% for the DEM simulation, respectively, vertical static mixer. The uniformity plateau was not accomplished. In Figure 6, the similarity of the conduct acquired in the trial study and the conduct using the DEM calculation was presented, while the acquired r2 value between experimental and DEM results was equal to 0.983.

3.3. Particle Flow Field throughout the Conveyor Mixer
The effect of the feed height on the flow field for the conveyor mixer was investigated in Experiment #2, using the DEM calculation. The height of the particle feed was set to 400, 450 and 500 mm. During the mixing, the material was taken from the bottom of the cone part using the central screw conveyor and continuously transferred to the top of the mixer, where it fell gravitationally (Figure 3b). Figure 7 explained the velocity field allocation for distinct heights of feed. The downward stream and a clockwise deviation in velocity magnitude were monitored in three cases of feed height (400, 450 and 500 mm), Figure 7. Velocity magnitudes and velocity vectors of particles positioned at higher levels showed reduced values. With the augment of feed height, a substantial growth in the downward velocities of particles in the whole hopper could be observed, which collides with the results of Han, et al..

 

Figure 7. Distributions of the particle stream with distinct heights in the hopper of the conveyor mixer, in case of feed height: (a) 400 mm; (b) 450 mm and (c) 500 mm.

3.4. Experiment #3
In Experiment #3, the limestone aggregate mixture, was previously mixed using the Komax-type vertical static mixer equipped with five elements (in Experiment #1), was blended in the conveyor mixer. An attempt was made to improve the result and shorten the blending duration to 3/4 of that time applied for Experiment #2. The blending duration was set to 3, 9 or 15 min, in Experiment #3.

The conduct acquired in this experiment was given in Figure 8, and the results show an increase in the blending outcome, with the simultaneous reduction in the mixing duration (RSD value reached the level between 3.34 to 13.79 after 15 min for the experimental investigations and 3.50—13.95%, after 15 min, for DEM study. Thus, all three blending periods in Experiment #3 were shortened to 3/4 of the periods obtained in Experiment #2, with the RSD values reduced. However, the required uniformity plateau was not accomplished for all mixtures, after 15 min, and the mixing period should be increased to 20 for 1 ÷ 3 mm limestone aggregate mixture, or even 25—30 min for 1 ÷ 4 mm or 2 ÷ 4 mm limestone aggregate mixture. These outcomes provoked the decision that less engagement and the minor effort of the final blender in the line could be required, if a vertical static mixer was applied prior to the principal mixer.

 

Figure 8. The RSD results obtained in Experiment #3.

Figure 8 shows the RSD value for the mixing duration of the experiment and DEM simulation results. It was obvious that as the blending duration increased, the RSD value diminished. The similarity of the results of the experimental investigation and the DEM study, regarding the blending quality calculation is evident, as was the case in the previous sets of experiments (obtained r2 was 0.995).

4. Conclusions
Within this manuscript, an experimental study and the discrete element method (DEM) were performed to investigate the homogenization process of the fine limestone aggregate used for self-compacting concrete mixtures in the vertical static mixer (type Komax), the conveyor mixer and the coupled action of these mixers. The crucial point of this examination was to study the potential outcomes to construct an affordable, easy to manufacture, uncomplicated device, which could be inserted in the blending line, with a goal to expand the final quality of the mixing process and lower the mixing time. This task was investigated through experimental and numerical study, using the DEM approach.

Three experiments were considered. The blending was performed using the vertical static mixer, with one, three or five elements, in the initial experiment. The optimal outcome was accomplished with five mixing parts, according to the RSD criteria. Notwithstanding, the mixing conduct was unsuitable, which was concluded by the RSD value greater than 10%. In Experiment #2 the mixing process in the conveyor blender was examined, and the outcome of the process was estimated according to the blending duration. The inadequate RSD value between 45.10 and 54.76%, for experimental and between 45.25 and 55.60% for the DEM model, respectively for trials study and the DEM simulation, accordingly was managed after 20 min.

The limestone aggregate mixture was previously homogenized in the vertical static mixer, encompassing five elements, in the final experiment. The mixing process was executed in the conveyor mixer, and the mixing duration in Experiment #3 was shortened to only 3/4 of the duration in the preceding trials. The results of Experiment #3 exhibited that the RSD plateau was reached after only 15 min of mixing, for the mixtures with similar particle diameters. However, the mixtures composed of higher differences in particle diameter (such as 1 ÷ 4, 2 ÷ 4 or 1 ÷ 3 mm mixtures) were not thoroughly mixed (RSD was not underneath 10%), after 15 min, even if the vertical static mixer was used for pre-mixing. However, the additional decrease of the RSD value could be accomplished with a longer duration of the blending process.

 

© 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

Pezo, L. L., Pezo, M., Terzić, A., Jovanović, A. P., Lončar, B., Govedarica, D., & Kojić, P. (2021). Experimental and Discrete Element Model Investigation of Limestone Aggregate Blending Process in Vertical Static and/or Conveyor Mixer for Application in the Concrete Mixture. Processes, 9(11), 1991. https://doi.org/10.3390/pr9111991