Computational fluid dynamics simulations of unsteady mixing in spacer-filled direct contact membrane distillation channels
Computational fluid dynamics simulations of unsteady mixing in spacer-filled direct contact membrane distillation channels
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DOI：
10.1016/j.memsci.2020.118931
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发表时间：
2021-01-13
影响因子：
9.5
通讯作者：
Tilton, Nils
中科院分区：
文献类型：
--
作者：
Lou, Jincheng;Johnston, Jacob;Tilton, Nils
直接接触膜蒸馏(DCMD)是一种将卤水浓缩到饱和极限的很有前途的方法。在这一过程中，膜间隔物在温度极化、浓差极化和矿物结垢中起着关键作用。这些相互作用还没有被很好地理解，因为它们很难进行实验和数值研究，而且流型也没有完全被绘制出来。因此，我们开发了一个定制的内部CFD程序，模拟了具有圆柱隔板的板框DCMD系统中的非定常二维传热和传质。该程序结合了空间有限体积法、时间投影法和间隔物表面浸没边界方法的最新进展。利用该程序，我们研究了向非定常层流旋涡脱落的转变如何影响DCMD系统的极化和渗透产生。我们发现，间隔物的影响可以通过研究在膜体内和膜附近产生的各种定常和非定常涡流结构来解释。总体而言，我们表明，尽管非定常涡旋结构倾向于将温度偏振层与主体混合，但它们不能类似地混合浓度层。相反，涡旋结构往往会形成有利的盐分聚集区。在旋涡脱落状态下，最终结果是间隔物往往以增加矿物结垢的风险为代价增加水蒸气的产量。
Direct contact membrane distillation (DCMD) is a promising means of concentrating brines to their saturation limit. During that process, membrane spacers play a key role in temperature polarization, concentration polarization, and mineral scaling. These interactions are not well understood, because they are difficult to study experimentally and numerically, and the flow regimes are not fully charted. We consequently develop a tailored in-house CFD code that simulates unsteady two-dimensional heat and mass transport in plate-and-frame DCMD systems with cylindrical spacers. The code uses a combination of finite-volume methods in space, projection methods in time, and recent advances in immersed boundary methods for the spacer surfaces. Using the code, we explore how the transition to unsteady laminar vortex shedding affects polarization and permeate production of DCMD systems. We show that the impact of spacers can be explained by examining the various steady and unsteady vortical flow structures generated in the bulk and near the membranes. Overall, we show that though unsteady vortex structures tend to mix temperature polarization layers with the bulk, they are not similarly able to mix the concentration layers. Rather, vortical structures tend to create regions of preferential salt accumulation. In the vortex shedding regime, the net result is that spacers often increase vapor production at the expense of increasing the risk of mineral scaling.