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对于饱和蒸气壁面凝结过程, 蒸气分子在体相与过冷壁面间过渡区的微观演化机制尚不清晰, 分子团聚模型认为分子到达壁面凝结前首先在体相中形成一定团簇分布, 但由于观测近壁边界层微小空间中微观粒子的动态演化较为困难, 对该模型的实验验证并不充分. 基于团簇内部的氢键网络, 利用衰减全反射傅里叶红外光谱技术, 实时检测了近壁薄层内蒸气分子凝结过程中的动态行为, 直接验证了近壁区的团簇分布, 表明团簇是凝结和液滴生长的主要单元, 且平均团簇尺寸沿着靠近壁面方向逐渐增大. 利用团簇体的氢键特征, 又观测了乙醇蒸气的近壁面团聚行为, 进一步验证了壁面凝结过程团簇演化的合理性. 此外, 实验发现乙醇蒸气冷凝的团簇分布空间范围要小于同样条件下的水团簇分布范围, 这可能间接表明乙醇蒸气凝结的传热边界层范围小于水蒸气凝结的传热边界层范围, 而导致其传热性能较弱. 利用壁面结构调节近壁区团簇分布, 将为含有不凝气的蒸气冷凝传热或气相水汽捕获等过程的强化提供新方向.For the saturated vapor condensation on the cooled surface, the evolution mechanism of vapor molecular in the transition zone between the bulk phase and the cooled surface is not clear yet. The molecular clustering model considers that the vapor molecules first form clusters in the gas phase before condensing on the cooled surface. However, it is difficult to observe the dynamic evolution of nanoparticles in the near-wall boundary layer, hence, the experimental verification about this model is not sufficient now. Based on the hydrogen bonded network formed inside the cluster, in this paper, the attenuated total reflection Fourier transform infrared spectroscopy is introduced to follow and detect the dynamic behavior of vapor molecules in the near-wall thin layer during the condensation process. The infrared spectra of the gas phase at different positions from the cooled surface during the condensation process are obtained. The experimental results directly verify the distribution of clusters in the near-wall region, indicating that clusters are the main units of vapor condensation and droplet growth. Moreover, the average cluster size n increases gradually along the direction near the cooled surface. Based on the hydrogen bond characteristics of clusters, the ethanol molecular clustering near the surface is also observed, which further verifies the rationality of this model. In addition, it’s found that the distribution region along the cooled surface of ethanol clusters during the process of condensation is smaller than that of water clusters under the same condition. This may indirectly indicate that the heat transfer boundary layer of ethanol vapor condensation is thinner than that of water vapor condensation, resulting in its weaker performance of heat transfer. This method, where we use the microstructures manufactured on the surface to regulate the distribution of clusters in the near-wall region, will provide a new insight into enhancing the process of steam condensation with non-condensable gas or efficient water capture from air.
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Keywords:
- molecular clustering /
- attenuated total reflection infrared spectroscopy /
- condensation on cooled surface
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图 1 ATR技术检测团簇的原理 (a) 单点衰减全反射; (b) 在ATR附件上的冷凝平台
Fig. 1. Condensation experiment platform based on ATR: (a) The single attenuated total; (b) a schematic of the ATR accessory integrated with cooled surface.
图 2 实验装置示意图(1, 控温炉; 2, 热管; 3, 三维位移平台; 4, 冷凝腔室; 5, ATR附件; 6, 电脑; 7, 洗气瓶; 8, 流量计; 9, 氮气瓶)
Fig. 2. Schematic diagram of experimental setup. 1, temperature control furnace; 2, copper heat pipe; 3, three-dimensional displacement platform; 4, condensing chamber; 5, ATR accessory; 6, computer; 7, scrubber; 8, flow meter; 9, nitrogen cylinder.
图 3 饱和水蒸气在近壁面冷凝的红外光谱 (a) 各位置处的吸收光谱, 各谱线由下而上依次为d = 1700−0 μm; (b) 各吸收光谱的频率, peak 1红移14 cm–1, peak 2红移5.8 cm–1; (c) 各位置处吸收光谱的峰高及两者的相对强度随距离的变化
Fig. 3. Infrared spectra of saturated vapor of water condensing near the cooling surface: (a) The absorption spectra at each distance, d = 1700−0 μm from bottom to top; (b) the frequency of maximum intensity in OH bonded stretching region of each spectrum, peak 1 is red-shifted by about 14 cm–1, peak 2 is red-shifted by about 5.8 cm–1; (c) the peak high and the relative intensity of the peak 1 and peak 2 as a function of distance.
图 4 饱和乙醇蒸气在近壁面冷凝的红外光谱 (a) 距壁面不同距离的气相的红外光谱, 各谱线自下而上依次为d = 2000−0 μm; (b) 各距离处的ATR-FTIR光谱的频率
Fig. 4. Infrared spectra of saturated vapor of ethanol condensing near the cooled surface: (a) The absorption spectra at each distance, d = 2000−0 μm from bottom to top; (b) the frequency of maximum intensity in bonded OH stretching region of each ATR-FTIR spectrum of Fig. 4(a).
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[1] Jakob M 1936 Mech. Eng. 58 729
[2] Tammann G, Boehme W 1935 Ann. Phys. 414 77
Google Scholar
[3] Hashimoto H, Kotake S 1995 Therm. Sci. Eng. 3 37
[4] Song T Y, Lan Z, Ma X H, Bai T 2009 Int. J. Therm. Sci. 48 2228
Google Scholar
[5] Ma X H, Song T Y, Lan Z, Bai T 2010 Int. J. Therm. Sci. 49 1517
Google Scholar
[6] 兰忠, 王爱丽, 马学虎, 彭本利, 宋天一 2010 物理学报 59 6014
Google Scholar
Lan Z, Wang A L, Ma X H, Peng B L, Song T Y 2010 Acta Phys. Sin. 59 6014
Google Scholar
[7] 徐威, 兰忠, 彭本利, 温荣福, 马学虎 2014 工程热物理学报 35 774
Xu W, Lan Z, Peng B L, Wen R F, Ma X H 2014 J. Eng. Thermophys. 35 774
[8] Xu W, Lan Z, Peng B L, Wen R F, Ma X H 2015 J. Chem. Phys. 142 054701
[9] 兰忠, 徐威, 朱霞, 马学虎 2011 物理学报 60 120508
Google Scholar
Lan Z, Xu W, Zhu X, Ma X H 2011 Acta Phys. Sin. 60 120508
Google Scholar
[10] Lan Z, Wang D, Cao K J, Xue Q, Ma X H 2017 Sci. Rep. 7 987
Google Scholar
[11] Vernon M F, Krajnovich D J, Kwok H S, Lisy J M, Shen Y R, Lee Y T 1982 J. Chem. Phys. 77 47
Google Scholar
[12] Buck U, Huisken F 2001 Chem. Rev. 101 205
Google Scholar
[13] Buck U 1994 J. Phys. Chem. 98 5190
Google Scholar
[14] Buck U, Ettischer I, Melzer M, Buch V, Sadlej J 1998 Phys. Rev. Lett. 80 2578
Google Scholar
[15] Mizuse K, Hamashima T, Fujii A 2009 J. Phys. Chem. A 113 12134
Google Scholar
[16] Hamashima T, Mizuse K, Fujii A 2011 J. Phys. Chem. A 115 620
Google Scholar
[17] Zurheide F, Dierking C W, Pradzynski C C, Forck R M, Flüggen F, Buck U, Zeuch T 2015 J. Phys. Chem. A 119 2709
[18] Fujii A, Mizuse K 2013 Int. Rev. Phys. Chem. 32 266
Google Scholar
[19] Pradzynski C C, Forck R M, Zeuch T, Slavíček P, Buck U 2012 Science 337 1529
Google Scholar
[20] Buck U, Pradzynski C C, Zeuch T, Dieterich J M, Hartke B 2014 Phys. Chem. Chem. Phys. 16 6859
Google Scholar
[21] Buch V, Sigurd B, Paul Devlin J, Buck U, Kazimirski J K 2004 Int. Rev. Phys. Chem. 23 375
Google Scholar
[22] Forck R M, Pradzynski C C, Wolff S, Ončák M, Slavíček P, Zeuch T 2012 Phys. Chem. Chem. Phys. 14 3004
Google Scholar
[23] Hu Y J, Fu H B, Bernstein E R 2006 J. Chem. Phys. 125 154305
Google Scholar
[24] Shi Y J, Consta S, Das A K, Mallik B, Lacey D, Lipson R H 2002 J. Chem. Phys. 116 6990
Google Scholar
[25] Fahrenfort J 1961 Spectrochim. Acta 17 698
Google Scholar
[26] Harrick N J 1967 Internal Reflection Spectroscopy (New York: John Wiley & Sons) pp301−320
[27] Lenz A, Ojamäe L 2006 J. Phys. Chem. A 110 13388
Google Scholar
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