The Baron fiber classifier is an instrument used to separate fibers by length. The flow combination section (FCS) of this instrument is an upstream annular region, where an aerosol of uncharged fibers is introduced along with two sheath flows; length separation occurs by dielectrophoresis downstream in the flow classification section. In its current implementation at NIOSH, the instrument is capable of processing only very small quantities of fibers. In order to prepare large quantities of length-separated fibers for toxicological studies, the throughput of the instrument needs to be increased, and hence, higher aerosol flow rates need to be considered. However, higher aerosol flow rates may give rise to flow separation or vortex formation in the FCS, arising from the sudden expansion of the aerosol at the inlet nozzle. The goal of the present investigation is to understand the interaction of the sheath and aerosol flows inside the FCS, using computational fluid dynamics (CFD), and to identify possible limits to increasing aerosol flow rates. Numerical solutions are obtained using an axisymmetric model of the FCS, and solving the Navier–Stokes equations governing these flows; in this study, the aerosol flow is treated purely aerodynamically. Results of computations are presented for four different flow rates. The geometry of the converging outer cylinder, along with the two sheath flows, is effective in preventing vortex formation in the FCS for aerosol-to-sheath flow inlet velocity ratios below ∼50. For higher aerosol flow rates, recirculation is observed in both inner and outer sheaths. Results for velocity, streamlines, and shear stress are presented. Copyright 2014 Prahit Dubey, Urmila Ghia, and Leonid A. Turkevich

中文翻译：

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Baron纤维分级机流组合段中鞘流和气溶胶流的数值研究

Baron 纤维分级机是一种用于按长度分离纤维的仪器。该仪器的流动组合部分 (FCS) 是一个上游环形区域，在该区域，不带电纤维的气溶胶与两个鞘流一起被引入；长度分离通过流分类部分下游的介电泳发生。在 NIOSH 目前的实施中，该仪器只能处理非常少量的纤维。为了制备大量用于毒理学研究的长度分离纤维，需要增加仪器的吞吐量，因此需要考虑更高的气溶胶流速。然而，较高的气溶胶流速可能会导致 FCS 中的流动分离或涡流形成，这是由入口喷嘴处的气溶胶突然膨胀引起的。本研究的目标是使用计算流体动力学 (CFD) 了解 FCS 内鞘流和气溶胶流的相互作用，并确定增加气溶胶流速的可能限制。数值解是使用 FCS 的轴对称模型获得的，并求解控制这些流动的 Navier-Stokes 方程；在这项研究中，气溶胶流是纯粹的空气动力学处理。给出了四种不同流速的计算结果。当气溶胶与鞘流入口速度比低于 50 时，会聚外圆柱体的几何形状以及两个鞘流可有效防止 FCS 中的涡流形成。对于更高的气溶胶流速，在内鞘和外鞘中都观察到再循环。显示了速度、流线和剪切应力的结果。