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张建文江裕孙艳云姜爱国王若鹏 《低温与超导》2017,(9):1-6
壁面结霜致使低温换热设备传热性能下降,甚而产生设备安全和生产安全问题。通过实验研究了涂覆疏水涂料壁面结构的结霜特性,不同倾斜角度壁面的水滴凝结与滑落特性,以及添置了太阳能吸热膜的表面传热特性。结果表明:涂有疏水材料的壁面较普通壁面初始结霜时间能延迟约10分钟;一定的壁面倾斜角度通过影响水滴的滚动角,促进凝结的水滴滑落,可阻碍霜层生长;当换热壁面利用太阳能吸热膜对低温壁面进行补热时,吸热量为5.27J/cm2,为普通壁面的1.44倍。这些改进措施可以减少冷壁面结霜量,有助于提升低温设备传热效果。 相似文献
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本文以LNG冷能发电装置中的中间介质气化器中冷凝器丙烷管外凝结过程为研究对象,通过建立相应数值模型,采用VOF模型追踪气液相界面,利用LEE模型作为相变传热传质模型,对管外气态丙烷凝结成液膜流动及凝结传热问题进行了瞬态二维CFD模拟。分析了冷凝器丙烷端管外液膜流动和凝结换热特性,将管外换热系数的模拟值与Nusselt公式理论解进行对比,偏差在10%以内,证明了该模型对模拟冷凝器中丙烷管外凝结过程的准确性。此外,还将该IFV中丙烷端凝结换热特性与常规IFV进行了对比,发现在较小壁面过冷度时,该IFV管外换热系数要明显小于常规IFV。 相似文献
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针对流体在纳米通道的小尺度效应,采用分子动力学方法模拟了传热效应以及流体流动行为,研究在壁面温度影响下,不同润湿性壁面上方气层生成状态以及流体流动时气层的稳定特性和相应的减阻性能.结果表明:当壁面为纯疏水壁面时,不能形成气层;疏水基底+亲水组合壁面形成不规则气层;纯亲水壁面和亲水基底+疏水组合壁面能形成规则气层.当流体流动时,疏水基底+亲水组合壁面气层消失,而纯亲水壁面和亲水基底+疏水组合壁面气层较为稳定.纯疏水壁面主流区域速度较大,而纯亲水壁面主流区域最低.对于壁面滑移速度,存在气层的壁面滑移速度与纯疏水表面相对接近,甚至稍优于纯属疏水表面,而疏水基底+亲水组合壁面滑移速度最小. 相似文献
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基于空气源热泵在低温寒冷地区运行中遇到的结霜问题,对不同风速工况下,结霜过程中设备性能的变化进行分析,以换热量、换热系数为指标对不同翅型换热器的换热特性进行研究。实验结果显示:换热器结霜过程中,换热过程主要分为初始增加段、换热平稳段、缓慢衰减段、后期平稳等四段,结晶体在增加空气湍流度强化换热的同时,也增加了换热热阻使换热效果变差,因此换热效果本质而言是两种换热效果的综合体现;空气阻力随风速的增大、结霜量的增加而增大,而蒸发压力随着风速的增加而升高、随结霜量的增大而减小;百叶窗翅片表面结霜量大于平翅片,因此平翅片翅型当量换热系数更大,翅片结霜量、当量换热系数随风速的增加而增大,风速由1 m/s增至4 m/s时,结霜量、当量换热系数增加约三倍。 相似文献
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A new method of determination of the lateral structure of crystal surfaces is presented. The method is based on earlier work showing the existence of resonances in the elastic scattering of low-energy electrons at crystal surfaces. The method consists of: (a) Measurement of the surface resonance band structure Ek∥E, k∥ respectively denote the electron energy and surface-parallel momentum for which resonances occur) and (b) Interpretation of E(k∥) to determine the lateral variation of the effective potential acting on electrons at the surface.The surface resonance band structure is measured by a net-current electron reflection method. The measurement method is basically the same as used previously but here its precision is greatly enhanced by the use of digital methods of data handling including a digital filter to remove background due to inelastic and non-resonance elastic scattering. The surface resonance band structure is interpreted by a two-dimensional nearly-free electron scheme. In this scheme the interaction elements are Fourier coefficients of an effective potential which is an average of a pseudopotential with respect to the depth distribution of electron density in a surface resonance — the surface-weighted pseudopotential. Experimental surface resonance band structure for Ni(001), Ni(001) p(2 × 2)O and two different Ni(001) c(2 × 2)O surfaces (one of them with an oxygen-saturated Ni substrate) are presented for E = 1–30 eV and k∥ running halfway from gG towards H? in the surface Brillouin zone for Ni(001). The experimental results are fitted, using the nearly-free electron scheme, to determine the Fourier coefficients of the surface-weighted pseudopotential. Surface potential variations synthesized from the above data are discussed in comparison with the atomic arrangements known from LEED. It is demonstrated that the new method can give a correct picture of the lateral structure of surfaces. It is emphasized that these results are obtained without costly equipment or computations called for by other methods. 相似文献
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In this paper, we demonstrated a simple method to create either a hydrophilic or hydrophobic surface. With femtosecond laser irradiation at different laser parameters, the water contact angle (WCA) on polystyrene’s surface can be modified to either 12.7° or 156.2° from its original WCA of 88.2°. With properly spaced micro-pits created, the surface became hydrophilic probably due to the spread of the water droplets into the micro-pits. While with properly spaced micro-grooves created, the surface became rough and more hydrophobic. We investigated the effect of laser parameters on WCAs and analyzed the laser-treated surface roughness, profiles and chemical bonds by surface profilometer, scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). For the laser-treated surface with low roughness, the polar (such as C—O, C=O, and O—C=O bonds) and non-polar (such as C—C or C—H bonds) groups were found to be responsible for the wettability changes. While for a rough surface, the surface roughness or the surface topography structure played a more significant role in the changes of the surface WCA. The mechanisms involved in the laser surface wettability modification process were discussed. 相似文献
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The surface photoelectric effect and the surface plasmon resonances appear when a p/transverse magnetic polarized laser hits a gas-solid interface. We model this effect in the long wave length (LWL) domain (λvac > 10 nm, ?ω < 124 eV) by combining the Ampère-Maxwell equation, written in classical approximation, with the material equation for the susceptibility. The resulting model, called the vector potential from the electron density (VPED), calculates the susceptibility as a product of the bulk susceptibility and the electron density of the actual system. The bulk susceptibility is a sum of the bound electron scalar susceptibility taken from the experiment and of the conduction electron non-local isotropic susceptibility tensor in a jellium metal (Lindhard, 1954 [1]). The electron density is the square of the wave function solution of the Schrödinger equation. The analysis of observables, the reflectance R and the photoelectron yield Y as well as the induced charge density permits to identify and characterize the multipole surface plasmon resonance of Al(111) appearing at ωm ∼ 0.8ωp or 11-12 eV. 相似文献
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A novel formalism (the effective surface potential method) is developed for calculating surface states. Like the Green function method of Kalkstein and Soven and the transfer matrix method of Falicov and Yndurain, the technique is exact for simple tight binding Hamiltonians. As well as offering an alternative viewpoint, the present method provides a simple analytic expression describing the surface states. At each point ks in the surface Brillouin zone the semi-infinite solid is viewed as an effective linear chain where each element of the chain is a planar layer. The solution to the linear chain problem can be expressed in terms of an effective potential h(ks,E) at each energy E. A number of examples are presented in detail; “spd” Hamiltonians for a linear chain (d = 1), the honeycomb lattice (d = 2), the 111 surface of silicon (d = 3), and a dissected Bethe lattice. Various exact results are given, e.g. the extremities of surface state bands and the surface density of states of p-like (delta function) bands. The results of Kalkstein and Soven for the 100 surface of a simple cubic solid with a perturbation on the surface layer are rederived. 相似文献
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Recent progress in the spectroscopy of empty electronic states at metal surfaces allows for measuring the energy vs. momentum dispersion of both crystal-induced and image-potential surface states with high precision. This allows for deriving the effective barrier potential for an electron near a metal surface with considerable accuracy by comparing the experimental data with corresponding calculations based on the one-step model of inverse photoemission. The method is demonstrated for Cu(100) where four empty surface states are known experimentally. 相似文献
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《Solid State Communications》1987,64(3):375-378
Surface enhanced Raman scattering from copper phthalocyanine thin films deposited onto Ag films roughened by underlayers of gas-evaporated Si particles was investigated. The surface roughness was systematically varied by varying the average size of Si particles. Results of quantitative intensity measurements indicate that there exists an optimum surface roughness depth for SERS. The maximum enhancement factor obtained is ≈1.5 x 104 and a crude estimate of the optimum roughness depth is of the order of 100 Å. The origin of presently observed enhancement is thought to be purely electromagnetic, involving the excitation of the surface plasmon modes via the surface roughness. 相似文献
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Rubber surface is subjected to ultraviolet radiation (UV) in the presence of allylamine and radiation sensitizer benzophenone (BP). Fourier transform infrared spectral studies reveal the presence of allylamine on the surface. The presence of irregular needle shapes on the surface as observed in scanning electron micrographs also confirms the polymerized allylamine on the surface. Allylamine coatings have been further confirmed from atomic force microscopy (AFM) analysis. Thermogravimetric analysis (TGA) reveals that allylamine coating on the rubber surface lowers the thermal degradation rate. The contact angle between the water and rubber surface decreases for the modified rubber surface confirming the surface modification due to UV surface grafting. 相似文献
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Damascelli A Lu DH Shen KM Armitage NP Ronning F Feng DL Kim C Shen ZX Kimura T Tokura Y Mao ZQ Maeno Y 《Physical review letters》2000,85(24):5194-5197
The electronic structure of Sr2RuO4 is investigated by high angular resolution ARPES at several incident photon energies. We address the controversial issues of the Fermi surface (FS) topology and the van Hove singularity at the M point, showing that a surface state and the replica of the primary FS due to sqrt[2]xsqrt[2] surface reconstruction are responsible for previous conflicting interpretations. The FS thus determined by ARPES is consistent with the de Haas-van Alphen results, and it provides additional information on the detailed shape of the alpha, beta, and gamma sheets. 相似文献
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Surface photovoltage transients were measured at clean cleaved silicon (111) faces in ultrahigh vacuum. The temperature and doping of the samples, the intensity of the stimulating light pulses (energy less than band gap), and the surface coverage (clean and adsorbed water vapor) were varied systematically. The results yield information on the charge transfer at the surface and on surface recombination. The calculation of the surface photovoltage (using only the generation rates into and out of the surface states and data of thermal equilibrium) shows, that only one bulk band (conduction band for n-doped samples and valence band for p-doped samples) controls completely signal height and its relaxation via charge transfer to the surface states. The determined surface state parameters are: relaxation time constants, capture cross-sections for photons and transition probabilities. On the basis of the model all decay curves can be reproduced quantitatively. 相似文献