Modeling transpiration in synthetic trees
Modeling transpiration in synthetic trees
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DOI：
10.1016/j.ijheatmasstransfer.2021.122121
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发表时间：
2022-02
影响因子：
5.2
通讯作者：
Ndidi L. Eyegheleme;K. Peng;J. Boreyko
中科院分区：
文献类型：
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作者：
Ndidi L. Eyegheleme;K. Peng;J. Boreyko
人造树木通过将水库和导管连接到湿润的纳米孔来模拟天然树木的蒸腾周期，纳米孔在蒸发过程中表现出负的拉普拉斯压力。在这里，我们开发了合成树蒸腾的综合理论模型，该模型由顶部连接到纳米多孔合成叶的垂直定向管阵组成。我们的模型表明，当树叶暴露在对流气流中时，扩散边界层可以忽略不计，环境湿度直接决定了树叶中水的负拉普拉斯压力。由此产生的毛细管驱动的蒸腾速率是拉普拉斯压力和树木几何形状的函数，拉普拉斯压力设定水力负荷，树木几何形状设定水力阻力。相反，当树叶暴露在大气环境中时，正是蒸发率和树木的几何形状规定了保存质量所需的拉普拉斯压力。在半月板上匹配拉普拉斯和开尔文压力会导致局部湿度与环境不同，从而必然会形成扩散边界层。我们的模型还解释了半月板的动态演化，特别是它们调整接触角的能力，以及在必要时部分退回到纳米孔以自我稳定的能力。在各种各样的树木几何形状和环境条件下，我们识别蒸腾速率何时是蒸发受限的，而不是压力受限的(瓶颈是树叶的最大拉普拉斯压力)。这些发现应该会为下一代合成树的设计和开发提供参考，这些树将应用于水提取和太阳能蒸汽发电。
Synthetic trees mimic the transpiration cycle of natural trees by connecting a reservoir and conduits to wetted nanopores that exhibit a negative Laplace pressure during evaporation. Here, we develop a comprehensive theoretical model of transpiration for synthetic trees comprised of a vertically-oriented tube array connected at the top to a nanoporous synthetic leaf. Our model illustrates that when the leaf is exposed to a convective gas flow, the diffusive boundary layer is negligible and the ambient humidity directly prescribes the negative Laplace pressure of water in the leaf. The resulting capillary-driven transpiration rate up the tree is then a function of both the Laplace pressure, which sets the hydraulic load, and the tree geometry, which sets the hydraulic resistance. Conversely, when the leaf is exposed to an atmospheric environment, it is the evaporation rate and tree geometry that prescribe the necessary Laplace pressure to conserve mass. Matching the Laplace and Kelvin pressures at the menisci results in a local humidity that differs from the ambient, such that a diffusive boundary layer necessarily forms. Our model also accounts for the dynamic evolution of the menisci, in particular their ability to tune their contact angle and, when necessary, partially retreat into the nanopores to self-stabilize. Over a wide variety of tree geometries and ambient conditions, we identify when the transpiration rate is evaporation-limited versus pressure-limited (bottlenecked by the leaf’s maximum Laplace pressure). These findings should inform the design and development of next-generation synthetic trees with applications in water extraction and solar steam generation.