Abstract Distinct from monoporous structures with only one characteristic pore size, biporous structures are characterized by two distinguished pore sizes. In this paper, novel double-layered biporous coatings consisting of a biporous top layer and a monoporous base layer are fabricated by freeze casting from aqueous nickel slurry, where the particle size (D50) is 400 nm. The biporous layer of the freeze-casted coating is characterized by lamellar vertical large pores formed by templating ice crystals and small pores formed by particle accumulation. The sizes of the large pores are in the order of 10~100 μm depending on coating thicknesses and freezing temperatures, while the sizes of small pores are in the order of ~1 μm. Pool boiling heat transfer performances of freeze-casted double-layered biporous coatings are investigated. For comparison purposes, saturated pool boiling experiments under the same conditions are also carried out on a slurry-evaporation-deposited surface as well as on a smooth surface. Experimental results indicate that freeze-casted coatings enhance boiling heat transfer performance significantly by increasing nucleate site density, decreasing bubble departure diameter, increasing departure frequency, and enhancing surface wickability. Compared to a smooth surface, the inception temperature on a freeze-casted surface is significantly reduced from about 10 K to less than 5 K while the heat transfer coefficient (HTC) and the critical heat flux (CHF) are improved by 180% and 67%, respectively. It is found that there is an optimum coating thickness to maximum HTC due to a tradeoff between nucleation site density and vapor removal efficiency. In addition, liquid slurry frozen at low supercoolings has better boiling heat transfer performance since the pore size is bigger as a result of larger ice crystal size.

中文翻译：

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增强冷冻铸造表面上的池沸腾传热

摘要 与只有一种特征孔径的单孔结构不同，双孔结构具有两种不同的孔径特征。在本文中，新型双层双孔涂层由双孔顶层和单孔基层组成，通过冷冻浇铸镍水浆制成，粒径 (D50) 为 400 nm。冻铸涂层双孔层的特点是模板化冰晶形成的层状垂直大孔和颗粒堆积形成的小孔。根据涂层厚度和冷冻温度，大孔的尺寸在 10~100 μm 的数量级，而小孔的尺寸在~1 μm 的数量级。研究了冻铸双层双孔涂层的池沸腾传热性能。为了比较，在相同条件下的饱和池沸腾实验也在浆液蒸发沉积表面以及光滑表面上进行。实验结果表明，冷冻铸造涂层通过增加成核点密度、减小气泡脱离直径、增加脱离频率和增强表面芯吸性来显着提高沸腾传热性能。与光滑表面相比，冷冻铸造表面的起始温度从大约 10 K 显着降低到小于 5 K，同时传热系数 (HTC) 和临界热通量 (CHF) 提高了 180% 和 67 ％， 分别。发现由于成核位点密度和蒸汽去除效率之间的权衡，存在最大 HTC 的最佳涂层厚度。此外，