Secondary flow (also termed as stagnation flow, dead space, recirculation zone, and vortex) is rheological phenomenon occurring during flow of polymer melts through abrupt contraction channels as result of flow separation from solid boundary leading to accelerating flow regime with recirculating material in corners. Polymer melt captured in secondary flow slowly rotates in direction opposite to main flow direction and simultaneously moves in third direction through helical motion. This may first reduce flow stability and second increase residence time initiating highly undesirable thermal degradation of polymer melt. Since the first visual experimental observation performed by Tordella and preliminary theoretical prediction made by Langlois and Rivlin at the end of the 1950s, this phenomenon represents one of the most fundamental rheological problems ever with many practical and theoretical impacts discussed here. This comprehensive review written in historical perspective summarizes key factors (Newtonian viscosity, shear thinning, viscoelasticity, flow geometry, and extensional viscosity) influencing secondary entry flows for polymer melts and provides deep and critical discussion of the most important experimental and theoretical works on this topic (such as branched low-density polyethylene, LDPE, linear low-density polyethylene, LLDPE, high-density polyethylene, HDPE, polystyrene, PS, isotactic polypropylene, PP, polymethyl methacrylate, PMMA, polyamide, Nylon PA 66, or polybutadiene, BR).

二次流（也称为停滞流，死区，再循环区和涡流）是流变学现象，其发生在聚合物熔体通过突然收缩通道流动的过程中，这是由于流与固体边界分离而导致的流动状态加速，其中再循环材料位于拐角处。在二次流中捕获的聚合物熔体在与主流相反的方向上缓慢旋转，并同时通过螺旋运动在第三方向上移动。这可能首先降低流动稳定性，其次增加停留时间，引发高度不希望的聚合物熔体热降解。自从1950年代末Tordella进行了首次视觉实验观察以及Langlois和Rivlin进行了初步的理论预测以来，这种现象代表了最基本的流变问题之一，在这里讨论了许多实际和理论上的影响。这份以历史观点撰写的全面综述总结了影响聚合物熔体二次入口流动的关键因素（牛顿粘度，剪切稀化，粘弹性，流动几何形状和拉伸粘度），并对有关该主题的最重要的实验和理论工作进行了深入和批判性的讨论。 （例如支化低密度聚乙烯，LDPE，线性低密度聚乙烯，LLDPE，高密度聚乙烯，HDPE，聚苯乙烯，PS，全同立构聚丙烯，PP，聚甲基丙烯酸甲酯，PMMA，聚酰胺，尼龙PA 66或聚丁二烯，BR ）。