Abstract:
In this work an experiment study has been conducted to investigate the effects of the physical properties of two granular materials and the operating condition of the process of mixing of the binary mixture in a rotating drum on the quality of mixing. Four types of agricultural granular materials of different shapes were used in these experiments. These are rice, lentils, mung beans, and chickpea. The physical properties of the granular materials, such as particle size, shape, bulk density, static angle of repose, and dynamic angle of repose are measured. At least one dimension of the particles is about 4 - 5 mm. The shapes are characterized by a term called sphericity and its values for rice, mung beans, lentils and chickpeas are 0.43, 0.64, 0.78 and 0.92, respectively. The bulk densities are almost close to each other ranging from 1.565 gm/cc for lentils to 1.666 gm/cc for rice.
The static angles of repose of rice, mung beans, lentils and chickpeas are 35.9, 32.8, 27.9 and 30.8, respectively. The angle of repose is found to decrease as the sphericity increases. However, in case of chickpeas with a high value of sphericity, the angle of repose is found to be higher than that expected, most likely due to the presence of a spike on the surface of chickpeas. The dynamic angle of repose of the granular particles is determined using a rotary drum apparatus of 14-inch diameter and 6 inch width. It has been found that the sphericity influences the dynamic angles of repose in the same fashion it influences the static angles of repose. In general, the dynamic angles of repose increase with the increase of speed for spherical and almost spherical particles. For particles with a low value of sphericity, the trend is opposite.
In the second part of experiments, tests are conducted by mixing two granular materials placed in horizontal layers in the rotary drum. The experiments are performed in three different drum fill levels: 25%, 50%, and 75% and three rotational speeds: 1, 2 and 5 rpm and mixing time and mixing index are determined in each run. Among the operational parameters, fill level has the most dominant effect on the mixing time. At 25% fill level, the mixing time is the shortest for all four binary systems at all rpms. At a 50% fill level, the mixing time is slightly longer. However, mixing time is significantly longer at the 75 percent fill level in all experiments. The values of mixing index follow the same trend in reverse order.
The particles of the granular materials have enough free space for avalanching and mixing at 25% fill level. During avalanching, granular particles attain enough kinetic energy, which helps them mix quickly. At 75% fill level, the granular materials have less free space for avalanching and have low kinetic energy during avalanching. In addition, a stagnant zone is developed around the center of the rotating drum. The mixing is slowed down by these two factors. As a result, mixing time at 75% fill level is significantly longer than that at 25% and 50% fill levels. For the very same reason mixing index is highest at 25% fill level and lowest at 75% fill level. With higher RPM, the avalanching frequency of the granular materials inside the rotating drum increases. This causes faster and good mixing and hence, less mixing time and higher mixing index.
In case of the mixing of materials of dissimilar shapes, the mixing index is lower. Binary systems with smaller difference between the values of the sphericity as well as the static angle of repose have higher mixing index. In case of mixing of same particles, there is zero difference between the values of sphericity as well as the values of the angle of repose and very high values of mixing index are achieved.