极窄流化床中的结晶与重新流化
Crystallization and refluidization in very-narrow fluidized beds
AI总结：
本文研究了极窄固液流化床中的结晶与重新流化现象，发现单分散床中两者交替发生并提出了量化扰动的新参数，同时证明双分散床底层扰动可阻碍顶层结晶，为缓解脱流化提供了新方法。
摘要：
由于能实现高质量和热量传递速率，固体颗粒被上升流体流化在工业中很常见。然而在某些情况下会发生堵塞，阻碍流化床的正常运行。本文研究了在稳态流动条件下，极窄固液流化床中发生的结晶（脱流化）与重新流化。为此，我们进行了实验，将单分散或双分散床浸没在速度高于流化所需速度的水流中，且管径与粒径之比小于6。对于由规则球体组成的单分散床，我们观察到结晶与重新流化随时间交替连续发生，并通过宏观结构和单个颗粒的扰动对其进行了量化。我们得出了结晶的特征时间，并提出了一种量化床层扰动程度的新宏观参数。双分散床由放置在规则球体层（后者与所测试的单分散床相同）底部的不太规则的球体组成。我们测量了宏观结构和颗粒扰动所经历的变化，并表明底层较高的扰动阻碍了顶层的结晶。我们的结果为极窄床的动力学带来了新见解，并提出了一种缓解脱流化的方法。
英文摘要：
Fluidization of solid particles by an ascending fluid is frequent in industry because of the high rates of mass and heat transfers achieved. However, in some cases blockages occur and hinder the correct functioning of the fluidized bed. In this paper, we investigate the crystallization (defluidization) and refluidization that take place in very-narrow solid-liquid fluidized beds under steady flow conditions. For that, we carried out experiments where either monodisperse or bidisperse beds were immersed in water flows whose velocities were above those necessary for fluidization, and the ratio between the tube and grain diameters was smaller than 6. For monodisperse beds consisting of regular spheres, we observed that crystallization and refluidization alternate successively along time, which we quantify in terms of macroscopic structures and agitation of individual grains. We found the characteristic times for crystallization, and propose a new macroscopic parameter quantifying the degree of bed agitation. The bidisperse beds consisted of less-regular spheres placed on the bottom of a layer of regular spheres (the latter was identical to the monodisperse beds tested). We measured the changes that macroscopic structures and agitation of grains undergo, and show that the higher agitation in the bottom layer hinders crystallization of the top layer. Our results bring new insights into the dynamics of very-narrow beds, in addition to proposing a way of mitigating defluidization.
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