摘要：
环模制粒成型技术以其高效率、高成型率、强适应性等优点广泛应用于生物质能源和饲料产业等领域.该文以苜蓿草粉为原料,应用POLYFLOW软件对环模制粒机挤压区流场进行数值模拟,研究了喂料量、环模转速和物料含水率的变化对流场压力、速度、剪切速率和黏度分布特性的影响规律,并以试验验证,旨在为生产工艺的有效控制提供参考依据.结果表明:在结构参数等条件一定的情况下,增大喂料量,流场压力升高,挤压成型区域扩大,且物料以较快的速度作层流运动,流场剪切速率降低,黏度增大,出模压力和成型密度较大;减小环模转速,流场压力和挤压成型区增大,流动速度减小,剪切速率降低,黏度较大,但出模压力和成型密度降低;物料含水率降低使流场压力、黏度、出模压力和成型密度增加.通过比较得出:当喂料量为6 t/h、环模线速度为6.5 m/s和物料含水率为15%时所形成的流场有利于苜蓿草粉的制粒成型.%Rotary roll extrusion pelleting technology has been widely used in the bioenergy industry, feed industry, chemical industry, pharmacy industry, and so on, because of the advantages such as high productivity, high forming rate, high adaptability and low pollution. By working principle analysis, numerical simulation, and experimental research, this paper revealed the distribution laws of internal flow field at different process parameters of ring die pellet mill in order to provide reference for the effective control of actual production process. First, the numerical simulation of two-dimensional isothermal flow field in the extruding domain of the ring die pellet mill MUZL420 with granulated alfalfa as the pelletizing material was performed by the POLYFLOW based on the reasonable simplification. The structural parameters of the pellet mill MUZL420 were as follows: The ring die diameter was 420 mm, the ring die width was 180 mm, the number of rollers was 2, the roller diameter was 203 mm, the percentage of the die opening area was 44.2%, the die hole diameter was 8 mm, and the die hole length was 54 mm. Effects of the changes in the feeding rate, ringdie speed and material moisture content on the distribution laws of pressure, velocity, shear rate and viscosity in the fluid field were analyzed on basis of the contour plots and cloud charts generated by CFD-POST. Then the extruding force in the exit of each die hole was captured by the Probe function in the CFD-POST. Next, according to the different process parameters set by the above numerical simulation, the experiments were carried out to measure the density of forming alfalfa pellet using the ring die pellet mill MUZL420. Finally, it was concluded by comparison that the forming density of alfalfa pellet increased with the increase of extruding force, which verified the validity of numerical simulation to some extent. The results showed that: 1) In the case of the same structural parameters and other conditions, increasing the feeding rate made the pressure in flow field gradually increase from higher negative pressure to positive pressure along the material flow direction, and the extruding area enlarged and the material here approximated to laminar flow, which improved the granulation yield and the product compactness. At the same time the shear rate was smaller and the viscosity and extruding force and forming density were larger. 2) Decreasing the ringdie speed led to the positive increase in fluid pressure along the material flow direction and the decrease in flow velocity and shear rate, in addition to obtaining larger extruding area and viscosity. However, the extruding pressure and forming density were reduced with the decrease in ringdie speed. 3) Decreasing material moisture content made the increase in pressure and viscosity and extruding pressure and forming density. 4) Only considering the pelletizing density, the optimal parameters combination in various combinations of the factor levels in this paper was as follows: The feeding rate was 6 t/h, the ringdie speed was 6.5 m/s, and the material moisture content was 15%.
