双螺杆挤出机流场数值模拟中流道进出口边界条件的探讨

Investigation of the inlet and outlet boundary conditions in numerical simulations of twin-screw extruders

在对双螺杆挤出机流场的数值模拟中,流道进出口边界条件的设置一直是一个颇具争议的问题。由于事先无法获得计算域进出口平面上的真实边界条件,研究人员在进行双螺杆挤出机的流场分析时,大都采用放松边界条件。为了考察放松边界条件对双螺杆挤出机流场数值模拟结果的影响,本文采用聚合物流动分析软件POLYFLOW,在流量恒定的前提下对双螺杆挤出机流道进出口给定三种不同分布形式的速度边界条件,对其流场进行了数值模拟。数值计算结果表明,在体积流量恒定的条件下,流道进出口不同分布形式的速度边界条件对流场的影响主要集中在进出口附近区域,但对离进出口边界较远的流场影响很小。一般而言,当计算域所对应的螺杆较长时,可以忽略流道进出口的放松边界条件所引起的误差;当计算域较短时,不宜直接采用放松边界条件,而应根据螺杆的实际构型,在计算域的进出口增加适当长度的发展段。

In the numerical simulations of flow in twin screw extruders, imposition of boundary conditions on the inlet and outlet of the flow domain has long been a controversial problem. Because detailed profiles of velocity or pressure on the inlet and outlet of the flow domain are previously unknown, generally relaxed boundary conditions are imposed to override this problem. In order to investigate the influence of relaxed boundary conditions on the simulation results, three different velocity profiles with the same flowrate are imposed on the inlet and outlet of the solving domain and the resulted flow fields were simulated using POLYFLOW package. The numerical results indicate that, under constant flowrate, different velocity profiles imposed on the inlet and outlet has a very significant impact on the nearby flow field of the twin-screw extruder; for the flow field far away from the entrance and exit planes the impact of relaxed boundary conditions is rather small. Therefore it can be further concluded that, when the solving domain is long, errors caused by relaxed boundary conditions on the inlet and outlet could be ignored; whereas when the solving domain is short, it is strongly recommended that development screw sections based on the practical screw configurations be added to both ends of the original screw sections.