Dropwise condensation heat transfer of steam on a polytethefluoroethylene film

bouctionThe higher heat tranSfer phae due todroPwise condensation as cOmPared with filIncondensation had bo inveshgated extensively by manyresearchers durin the past sixty years["n. Grea PrOgI'esshad been Inade in understanding the IneCanisms fordroPwise condensation pessP'q, However unscondensation mode had not been widely aPPlied toPIaedcal heat exchanger devices due tO the crucialProblem of han an effeChve method formanhaurin the bopwise condensation sbos.ReCetly, the successful aP…     

Evaluation of organic coatings for the promotion of dropwise condensation of steam

Several new polymer coatings, as well as gold and silver, were evaluated for their ability to promote and sustain dropwise condensation of steam. Long-term behavior on 25-mm-square specimens was qualitatively observed in an endurance apparatus and heat transfer coefficients on single horizontal tubes were determined in a separate heat transfer apparatus. The organic coatings were successful in promoting good quality dropwise condensation for prolonged periods of times (> 12,000 h). Dropwise heat transfer coefficients as large as six times the film condensation value were obtained with these coatings and results were not dependent upon the thermal conductivity of the wall.     

Theoretical investigation of stable dropwise condensation heat transfer on a horizontal tube

The heat transfer model of stable dropwise condensation for saturated vapor on a horizontal tube is developed based on previous theoretical models. Through a comprehensive analysis of all the contributing thermal resistances, the convection effect inside the droplet itself is taken into consideration in the model. For the stable dropwise condensation process in dynamic conservation, a method of double integration of heat flux through numerous inclined plates with different inclination angles is introduced to obtain the overall heat flux through the horizontal tube surface. The model can predict the variation of heat transfer of stable dropwise condensation with different contact angles outside a horizontal tube. The influences of contact angle, temperature difference, and other typical parameters on both a single droplet and the whole condensation process are discussed. The results indicate that a high contact angle can cause a size reduction of falling droplets from condensing surface and thus taking more heat away. The adsorbed condensate film adds an extra thermal resistance and its thickness plays a significant role on the dropwise condensation heat transfer.     

Lifetime test of dropwise condensation on polymer-coated surfaces

In this paper, a special lifetime testing apparatus is installed for dropwise condensation heat transfer on polymer-coated surfaces. Film and dropwise condensations are simultaneously carried out in the same condensing chamber to get rid of any containments. The discontinuous experimental results demonstrate that one surface has sustained dropwise condensation for about 1000 hours. The condensation heat transfer characteristics tend toward stability. In addition, the experiments prove that a polymer film prepared by the dynamic ion-beam mixed implantation (DIMI) method has good adhesion with the metal substrate. © 1999 Scripta Technica, Heat Trans Asian Res, 28(7): 551–558, 1999     

新型复合防腐表面烟气对流凝结换热实验研究

烟气对流凝结换热强化和换热表面防腐是天然气热能动力设备烟气余热回收利用关键技术。不同防腐表面耐腐蚀性能不同,且换热性能也不同。采用CCD高速摄像仪,对烟气在新型复合防腐表面上的凝结形态和凝结过程进行了可视化观测和换热实验研究,采用对图像边缘提取法,获得凝结液的边缘曲线。研究表明,烟气在新型复合防腐表面上的凝结为珠状凝结,凝结液珠最大粒径为0.2～0.28 mm,与其他表面形成的膜状凝结相比,在实验范围内,珠状凝结换热可提高约7倍。为增强烟气对流凝结换热和开发烟气冷凝余热回收利用技术提供了参考和依据。     

Effects of surface roughness and tube materials on the filmwise condensation heat transfer coefficient at low heat transfer rates

The laminar filmwise condensation heat transfer coefficient on the horizontal tubes of copper and stainless steel was investigated. The outside diameter of the tubes was 15.88 mm, and the tube thickness ranged from 1.07 to 1.6 mm. The polished stainless steel tube had an RMS surface roughness of 0.37 μm, and commercial stainless steel tubes had maximum surface roughness of 15 μm. The tests were conducted at saturation temperatures of 20 and 30 °C, and liquid wall subcoolings from 0.4 to 2.1 °C. The measured condensation heat transfer coefficients were significantly lower than the predicted data by the Nusselt analysis when the ratio of the condensate liquid film thickness to the surface roughness, δ / R_p–v, was relatively low. When the condensate liquid film was very thin, tube material affected the condensation heat transfer coefficient in the filmwise condensation.     

Condensation heat transfer on tubes in actual flue gas

In order to improve boiler efficiency, latent heat recovery from flue gas is a very important concept. Condensation heat transfer on horizontal stainless‐steel tubes was investigated experimentally by using an actual flue gas from a natural gas boiler. The experiment was conducted at different air ratios of the flue gas and a wide range of tube wall temperatures. The condensation pattern was similar to a dropwise condensation near the dew point. By decreasing the wall temperature, the wall region covered with a thin liquid film increased. The heat and mass transfer behavior was well predicted with the analogy correlation at the high‐wall‐temperature region. At the low‐wall‐temperature region, the total heat transfer was higher than that predicted by the analogy correlation. © 2001 Scripta Technica, Heat Trans Asian Res, 30(2): 139–151, 2001     

Dropwise Condensation Heat Transfer on Plasma-Ion-Implanted Small Horizontal Tube Bundles

Stable dropwise condensation of saturated steam was achieved on stainless-steel tube bundles implanted with nitrogen ions by plasma ion implantation. For the investigation of the condensation heat transfer enhancement by plasma ion implantation, a condenser was constructed in order to measure the heat flow and the overall heat transfer coefficient for the condensation of steam on the outside surface of tube bundles. For a horizontal tube bundle of nine tubes implanted with a nitrogen ion dose of 10¹⁶ cm^{? 2}, the enhancement ratio, which represents the ratio of the overall heat transfer coefficient of the implanted tube bundle to that of the unimplanted one, was found to be 1.12 for a cooling-water Reynolds number of about 21,000. The heat flow and the overall heat transfer coefficient were increased by increasing the steam pressure. The maximum overall heat transfer coefficient of 2.22 kW · m^?2· K^?1 was measured at a steam pressure of 2 bar and a cooling-water Reynolds number of about 2,000. At these conditions, more dropwise condensation was formed on the upper tube rows, while the lowest row received more condensate, which converted the condensation form to filmwise condensation.     

铜陵电厂凝汽器黄铜管换不锈钢管

经过多方面的调查、试验和研究,铜陵电厂将1号机凝汽器HSn70-1黄铜管全部换成了304不锈钢管,与黄铜管相比,不锈钢管具有较好的耐蚀性能、机械性能、经济性能,,总的传热性能也不差,取得了较好的效果。     

Study on transport phenomena for flow film condensation in vertical mini-tube with interfacial waves

The Kelvin-Helmholtz instability of phase-change interface during flow film condensation in vertical mini-diameter tube was studied here by means of energy analysis. According to the interfacial boundary conditions, the film thinning effect and the phase-change area enlarging effect by interfacial waves on heat transfer enhancement were analyzed for flow condensation in tubes with different diameter. It is indicated that, in mini-diameter tube, more obvious heat transfer enhancement due to interfacial waves can be expected than that in normal-sized tube, and the interfacial waves enhance the heat transfer mainly by film thinning effect.     

Effect of surface free energy difference on steam-ethanol mixture condensation heat transfer

The condensation heat transfer of steam-ethanol mixture with different weight fractions was investigated experimentally at atmospheric pressure. The results indicate that the heat transfer coefficients (HTC) and condensation modes, i.e. filmwise condensation (FWC), transition state and dropwise condensation (DWC), varied with the mixture compositions and the vapor-surface temperature differences. The interface effect, in terms of equivalent surface free energy difference between condensate and ultra thin liquid film, was introduced to analyze the variation of condensation modes and heat transfer coefficient. The equivalent surface free energy differences under various vapor conditions and vapor-surface temperature differences are calculated quantitatively. The experimental results show that as equivalent surface free energy difference was gradually increased, the condensation mode alternates from filmwise to transition state and finally to dropwise condensation, with heat transfer coefficient simultaneously increasing. The effect of surface free energy difference was also introduced to analyze the data in literature, and the effect of subcooling on heat transfer coefficient was discussed from the perspective of interface effect. The results show that as the vapor-surface temperature difference was gradually increased, the surface free energy difference increase accordingly and then reaches its peak value. The heat transfer coefficient exhibits the same tendency as equivalent surface free energy.     

功能表面降膜蒸发传热特性的实验研究

研究了处理表面镀铬铝管、PTFE铜管和纯铝氧化管水平管降膜蒸发传热，研究了喷淋密度、热流密度、管内蒸汽速度和管表面处理对降膜蒸发传热特性的影响。实验结果表明：在表面蒸发区，水平管降膜蒸发传热系数随热流密度的增加而提高，随喷淋密度增大先降低后升高，冷凝例传热系数基本保持不变。总传热系数对操作条件变化很不明显，表面阳极氧化膜使传热系数略有下降，但由于其优良的抗垢时蚀性能，非常有必要再进行深入地研究。     

Experimental study of dropwise condensation on plasma-ion implanted stainless steel tubes

Plasma-ion implantation was used to achieve stable dropwise condensation of saturated steam on stainless steel tubes. For the investigation of the efficiency of plasma-ion implantation regarding the condensation process a condenser was constructed in order to measure the heat flux density and the heat transfer coefficient h_c for the condensation of steam on the outside surface of a single horizontal tube. For tubes implanted with a nitrogen ion dose of 10¹⁶ cm⁻², the heat transfer coefficient h_c was found to be larger, by a factor of 3.2, in comparison to values theoretically calculated by the corrected Nusselt film theory. The heat flux density and the heat transfer coefficient h_c were found to increase with increasing ion dose and steam pressure. The heat transfer coefficient decreases with increasing surface subcooling as it has been found in former work for dropwise condensation on ion implanted vertical plates.     

真空状态下等离子注入铜管管束的换热特性

进行了不同真空度下单管的凝结试验，研究了真空状态下等离子注入铜管管束的换热特性，以及其换热系数随Re数的变化情况，并与普通铜管管束的换热性能进行比较。研究表明：两种管束的凝结换热系数和总换热系数都随着真空度的提高而增大，而且真空状态下等离子注入铜管表面较普通铜管有更好的换热性能。在较高真空度（0．05MPa）和较大Re数（34755）下，等离子管束的总换热系数约为普通铜管管束的1．43倍，凝结换热系数约为普通铜管管束的2倍。图6表1参11     

Dropwise condensation of steam on ion implanted titanium surfaces

Stable dropwise condensation of steam was achieved by ion beam implantation of N⁺ on titanium surfaces stabilized by a preoxidation procedure. It is pointed out that dropwise condensation can be adjusted by ion implantation in spite of increased wettability indicated by contact angle and surface free energy measurements. Our results suggest a nucleation mechanism possibly caused by interactions of nano-scale surface roughness and surface chemistry effects connected with precipitation of nitrides. Measured heat transfer coefficients for dropwise condensation were up to 5.5 times larger than values calculated by Nußelt film theory. No significant influence of the applied ion implantation parameters was found.     

Condensation of organic binary mixtures flowing horizontally in a horizontal tube bundle

Both heat and mass transfer in the gas phase and heat transfer in the liquid phase are examined experimentally for film condensation of organic binary mixtures such as ethanol-water and methanol-water. Experimental results on the average heat flux, vapor-liquid interface temperature and liquid-phase Nusselt number are compared with analytical solutions based on stagnant film theory and heat-transfer relationships for film condensation from a pure vapor. Experimental heat transfer results agree well with the analytical solutions, except that the experimental liquid-phase Nusselt numbers under conditions of low mass fraction of water are considerably higher than predicted by the analytical solutions. This high value of the liquid-phase Nusselt number is considered to be caused by dropwise condensation in the liquid phase. However, its effect on the tube bundle is not so remarkable compared with that in gravity-controlled condensation on a vertical surface. This is considered to be caused by the condensate inundation effect. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25(6): 342–361, 1996     

Experimental research and theoretical analysis of anti-fouling characteristics on the surface of Ni-based implanted tube

The surface fouling of different tubes in boiling conditions are studied through experiments, which include Carbon steel tube, stainless steel tube, brass tube, and Ni-based implanted tube. Their anti-fouling characteristics and induction period are analyzed by the theory of surface energy. The experiments show that the boiling time is asymptotic regarding to fouling mass in unit area to Ni-based implanted tube, while it is linear in terms of fouling mass in unit area to other three tubes. Furthermore, the induction period of Ni-based implanted tube is two times of the others'. It is concluded that Ni-based implanted tube has better anti-fouling characteristic and longer induction period, which makes it good surface for anti-fouling purpose.     

气汽混合流体凝结液膜传热特性研究

液膜厚度对凝结传热具有较大影响,且传热管管型影响着凝结液膜形成及排除。为了通过改变管型降低液膜厚度达到强化传热的目的,对圆管、椭圆管及滴形管等三种管型凝结液膜建立了相应的物理及数学模型,并计算了液膜沿管壁的厚度分布及传热系数;分析了三种管型对液膜传热的影响。结果表明:在气汽混合流体凝结传热过程中,不同管型其凝结液膜厚度差别较大;壁面温度和混合流体速度对液膜传热均有影响;相同条件下滴形管管壁上所形成的液膜,其平均厚度较薄,传热系数较高,因此滴形管传热性能优于其他管型。     

管外冷凝强化换热管的结构及发展趋势

介绍了各种类型的管外冷凝强化换热管,分析了其强化机理及结构特点,并总结得出:管外冷凝强化管的换热系数与管型有关,且各管型的结构参数对强化传热具有重要的作用.对国内外管外冷凝强化技术研究工作进行分析,结果表明,目前管外冷凝强化换热管的研究主要集中于翅片形状、翅片密度、翅片高度等结构参数对换热性能的影响.强化换热管的冷凝传热性能不仅与翅片结构参数有关,而且也与管材的表面特性和导热系数有关.管外冷凝强化换热管的研究重点是开发新型三维结构翅片的双侧强化管并研究其传热关联式,以及研究不锈钢等低成本材料制造的强化管换热管的传热性能和强化结构的优化.     

The effective use of heat transfer additives for steam condensation

Reported herein are the results of the experimental investigations on the effect of small amounts of heat transfer additives (C_add<1000 ppm) on steam condensation in a bundle of horizontal tubes that are internally cooled. The condenser was designed with 12 tubes in an array of three horizontals and four verticals with a 2-in. horizontal and 1.5-in. vertical in-line pitch. By using effective additives, the condensation heat transfer rate can be augmented as much as 30% as compared with heat transfer that operated without additives under the same operating condition. The steam condensation, which occurred in our experiments while using effective additives, was mostly dropwise like. When heat transfer additives function effectively, the condensate-droplets become more dispersed and have a smaller shape than those produced without additives. These droplets, unlike traditional turbulence, start at the top portion of the condenser tubes and cover most of the tubes. Such a flow behavior can be explained by the Marangoni effect (in terms of thermodynamic equilibrium) in connection with obtained surface tension data. Furthermore, we extend our discussion to the dynamic effect of surface tension in the steam condensation process and propose a general criterion for selecting effective additives. In our experiments, we noted that the use of heat transfer additives such as 2-ethyl-1-hexanol for steam condensation was highly effective.