- 上一篇：空调和空调系统英文文献和中文翻译
- 下一篇：混凝土构件中裂纹建模英文文献和中文翻译
For the Canica vertical shaft impact crusher, the DEM prediction of product size is in good agreement with the experimental data at a size range coarser than 10 mm, but over-predicts for fine product. This may be due to the fact that more than 8000 particles smaller than 2.36 mm were removed from the DEM simulation. Although this fraction of materials only account for 4% by weight in the feed, the cushion effect of these particles on the collision energy may be significant. Thus the total cumulative specific energy should be smaller than the one presented in this work. Similarly, the DEM prediction for the BJD horizontal shaft crusher is good at coarser size fractions (>2 mm), but over-predicts fines. This may be attributed to the over-simplified one-shaft configuration of hammers used in the DEM, while the BJD mill operates with double-shaft swing hammers.
Nevertheless, the DEM predictions seem to follow the general trends of the experimental data very well. This verifies that the DEM simulations can provide reasonable insight of the impact crusher performance. A number of simulations regarding the effects of machine design and operational conditions on the impact crusher were then conducted.
3. Descriptions of particle kinematics by DEM simulations
The impact crushers usually operate at very high rotational speed (1000 rpm for the vertical shaft crusher and 3000 rpm for the horizontal shaft hammer mill in this study). Dynamic motion of particles inside the crushing chamber and the interaction between the crushing element and the particles exerts a dominant influence on product size distribution. However, such knowledge is rare in the literature. Although high-speed cine camera study on the motion of coal particles inside the BJD hammer mill was attempted by researchers (e.g. Callcott, 1960), it was difficult to determine the impact energy quantitatively. DEM simulations provide a suitable tool to establish the particle kinematics and the energy distribution patterns.
3.1. Specific energy in relation to particle size
Investigation was conducted of the effect of particle size on the intensity of the introduced energy and the nature of the energy split for various rotational speeds of the impellers in the vertical shaft crusher. The particle size studied was in the range of 10–70 mm. Single spherical particles were dropped from a height of 1 m, at a position that corresponds to half of the impeller length. The results show that kinetic energy is a dominant form of energy and the amount of specific kinetic energy (kW h/t) is a function of the particle size and the rotational speed of impellers (Fig. 4a and b).
3.2. Specific energy in relation to feed position
The effect of particles feeding position to the vertical shaft impact crusher was investigated numerically for the case of 40 mm-diameter particles in the crusher with impellers rotating at a speed of 650 rpm. The energy introduced to the particles is a function of the position along the impeller where contact with the particle occurs. Radial velocity of the specific point along the impeller is determined by the angular velocity of the impeller and the distance from the rotation centre. In order to minimise the rolling effect of the particles, the friction coefficient between particles and crusher surfaces were increased to a high value. This may be reflected to the effect of irregular particle shape of natural rock material, which often has a relatively high effective coefficient of friction. Results show that both specific kinetic and strain energy increase as the impact point becomes closer to the tip of the impeller (Fig. 5).
It is interesting to observe that both large and small fragments tend to migrate along the surface of the impeller towards its tip. When they reach the tip, the fragments take-off with a maximum possible velocity towards the surrounding anvils of the crusher. The dominant phase of fragmentation occurs when the particles hit the anvils.
4. Analytical modelling of rock fragmentation in impact crushers