A study of the flow structures generated by oscillating flows in a helical baffled tube
A study of the flow structures generated by oscillating flows in a helical baffled tube
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DOI：
10.1016/j.ces.2017.05.032
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发表时间：
2017-11
影响因子：
4.7
通讯作者：
J. Mcdonough;Safaa M. R. Ahmed;A. Phan;A. Harvey
中科院分区：
文献类型：
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作者：
J. Mcdonough;Safaa M. R. Ahmed;A. Phan;A. Harvey
如果选择适当的振荡条件，振荡折流板反应器 (OBR) 能够在层流净流条件下产生活塞流。与其他挡板设计相比，包含螺旋挡板的中尺度 OBR 具有更宽的活塞流“操作窗口”（即更宽的振荡强度范围）。据推测，流动中的额外涡流提供了另一种限制轴向扩散的机制。这些旋流先前已被定性识别，但在本研究中，首次使用 CFD 和 PIV 对这些旋流进行数值和实验研究。通过模拟（层流求解器）获得的流动结构使用 Q 准则和 3D 流线的等值面进行可视化。流动的特征是在挡板后面形成螺旋形涡流。流线既可以径向移动（环绕涡流结构），也可以切向移动。使用旋流数和类似的“径向”数，观察到涡旋主导和旋流主导混合之间的转变，这提供了该假设有效的证据。研究发现，当振荡强度增加时，流动的切向运动增加的速度快于径向流动的增加，因为涡流尺寸受到柱直径的限制。
Oscillatory baffled reactors (OBRs) are able to generate plug flow at laminar net flow conditions, providing appropriate oscillation conditions are selected. Mesoscale OBRs containing helical baffles exhibit wider “operating windows” (i.e. a broader oscillation intensity range) for plug flow than other baffle designs. It has been hypothesised that additional swirling in the flow provides another mechanism to limit axial dispersion. These swirling flows have previously been qualitatively identified, but in this study these flows were investigated both numerically and experimentally using CFD and PIV for the first time. The flow structures obtained via simulation (laminar solver) were visualised using isosurfaces of the Q-criterion and 3D streamlines. The characteristic feature of the flow is a helically shaped vortex that forms behind the baffle. Streamlines move both radially (wrapping around the vortex structure) and tangentially. Using the swirl number and analogous ‘radial’ number, a transition between vortex-dominated and swirl-dominated mixing was observed providing evidence that the hypothesis is valid. It was found that when the oscillation intensity is increased, the tangential motion of the flow increases faster than the increase in radial flow because the vortex sizes are bounded by the column diameter.