共查询到19条相似文献,搜索用时 750 毫秒
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研究了混合气体在垂直圆管内的对流凝结传热。利用修正的膜模型与Nusselt凝结理论建立了换热数学模型,预测了壁面温度对膜厚度和界面温度的影响,计算了凝结液膜厚度,并与报相热阻法进行比较,研究结果表明该模型更接近实验果,提出了混合气体对流凝结换热与Nusselt凝结的不同。 相似文献
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本研究基于VOF算法编写用户UDF(自定义函数),采用FLUENT软件建立了椭圆横管外降膜流动和换热的计算模型。根据CFD(计算流体力学)模型计算和分析了在不同长短轴比下管外降膜速度分布、压力分布、液膜内温度分布和管外换热分布的变化规律。研究结果表明:长短轴比的变化影响了管外液膜速度分布、压力分布和膜内温度分布;相比圆管,椭圆管的管外膜内液体流速更快。壁面压力沿周向逐渐减少并在X=0.9附近快速回升;随长短轴比e的增加,周向压力最小值位置逐渐向后推移。局部Nu数分布与压力分布在趋势上存在一致性。当e=1.65附近时,椭圆的换热性能最优;最后,通过对管形的研究分析,提出横管的传热分区模型。 相似文献
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由于制冷剂R11和R123对臭氧层有破坏作用,为完成环保新工质R245fa对R11和R123的替代工作,对R245fa在内螺纹外斜翅片的三维双侧强化管外的凝结换热性能进行试验。数据处理过程中,采用Wilson图解法获得管内水侧对流换热系数及其计算关联式,再利用热阻分离法获得管外凝结换热系数。研究表明:试验中管内对流换热系数高于管外冷凝换热系数,所以管外侧的传热热阻是占据主导地位的传热热阻;相对于光管,R245fa在三维双侧强化管管内换热强化换热倍率为3.58,管外强化换热倍率为2.48;对实验数据进行拟合,得到管外换热系数的变化规律和凝结换热关联式。 相似文献
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The mechanism of convective condensation heat transfer of moist mixed gas across a horizontal tube was studied in this paper. The models referring to how the liquid film flows and the heat transfers on the tube are set up by combining modified film model and Nusselt condensation theory. The effects of Re number, wall temperature, and water vapor concentration on condensation heat transfer are discussed. Results predict that the film thickness profile on the tube is influenced greatly by vapor shear force on liquid film. Local Nusselt number depends remarkably on gas phase heat resistance, which is different from pure vapor and very similar to single‐phase gas. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(6): 324–333, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20169 相似文献
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De-Yi Shang Liang-Cai Zhong 《International Journal of Heat and Mass Transfer》2008,51(17-18):4300-4314
The dimensionless velocity component method was successfully applied in a depth investigation of laminar free film condensation from a vapor–gas mixture, and the complete similarity transformation of its system of governing partial differential equations was conducted. The set of dimensionless variables of the transformed mathematical model greatly facilitates the analysis and calculation of the velocity, temperature and concentration fields, and heat and mass transfer of the film condensation from the vapor–gas mixture. Meanwhile, three difficult points of analysis related to the reliable analysis and calculation of heat and mass transfer for the film condensation from the vapor–gas mixture were overcome. They include: (i) correct determination of the interfacial vapor condensate saturated temperature; (ii) reliable treatment of the concentration-dependent densities of vapor–gas mixture, and (iii) rigorously satisfying the whole set of physical matching conditions at the liquid–vapor interface. Furthermore, the critical bulk vapor mass fraction for condensation was proposed, and evaluated for the film condensation from the water vapor–air mixture, and the useful methods in treatment of temperature-dependent physical properties of liquids and gases were applied. With these elements in place, the reliable results on analysis and calculation of heat and mass transfer of the film condensation from the vapor–gas mixture were achieved.The laminar free film condensation of water vapor in the presence of air was taken as an example for the numerical calculation. It was confirmed that the presence of the non-condensable gas is a decisive factor in decreasing the heat and mass transfer of the film condensation. It was demonstrated that an increase of the bulk gas mass fraction has the following impacts: an expedited decline in the interfacial vapor condensate saturation temperature; an expedited decrease in the condensate liquid film thickness, the condensate liquid velocity, and the condensate heat and mass transfer. It was found that an increase of the wall temperature will increase the negative effect of the non-condensable gas on heat and mass transfer of the film condensation from the vapor–gas mixture. 相似文献
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Li Jia Xiaoping Li Jindong SunDeparment of Urban Construction Engineering Beijing Institute of Civil Engineering Architecture Beijing Xiaofeng PengDepartment of Thermal Engineering Tsinghua University Beijing 《热科学学报(英文版)》2002,11(2):144-147
The experiment was conducted to investigate the heat transfer performance of wet flue gas in a vertical tube. The factors influencing the convective condensation of wet flue gas were experimentally investigated. The measured results indicate that the convective heat transfer of bulk flow and condensation heat transfer of vapor have significant contribution to the total heat transfer and the dominant transport mechanism is dependent upon the vapor fraction in mixture. 相似文献
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The convection‐condensation heat transfer of vapor‐gas mixtures in a vertical tube was studied theoretically and experimentally. The effects of the condensation of a small amount of water vapor (8 to 20%) on heat transfer in a vertical tube were discussed. Comparisons show that theoretical solutions obtained through modified film model and experimental results are in good agreement. The results show that the condensation heat transfer of a small amount of water vapor and single‐phase convection heat transfer in the vapor‐gas mixtures are of the same order of magnitude, and these two modes of heat transfer could not be neglected. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 531–539, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10055 相似文献
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An experimental system investigating condensation heat transfer of wet flue gas was set up, and the heat transfer performance of vapor‐gas mixture with vapor condensation was discussed. The experimental results of laminar flow in a plastic longitudinal spiral plate heat exchanger were obtained and are in good agreement with the modified classical film model. It is shown that the plastic air preheater can avoid acid corrosion in the low‐temperature field for the boiler using fuel containing sulfur and recover latent heat of the water vapor of the wet flue gas. Also some SO2 was scrubbed during the vapor condensing process in the heat exchanger. © 2001 Scripta Technica, Heat Trans Asian Res, 30(7): 571–580, 2001 相似文献
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Kwangkook Jeong Edward K. Levy 《International Journal of Heat and Mass Transfer》2012,55(25-26):8010-8019
Prediction of acid condensation has a critical role in designing heat exchangers to recover water vapor from power plant flue gas. Rates of mass transfer for condensation of sulfuric acid vapors onto heat exchanger tubes were theoretically investigated and a computer program for numerical simulations of sulfuric acid (H2SO4) condensation in a flue gas condensing heat exchanger was developed. Governing equations based on mass and energy balances for the system were derived to predict variables such as flue gas exit temperatures, cooling water outlet temperatures, and molar fractions and condensation rates of water and sulfuric acid vapors. The associated equations were solved using an iterative solution technique and a one dimensional finite difference method with forward differencing. The Controlled Condensation Method (EPA Method 8B) was applied to experimentally obtain concentration profiles of sulfuric acid vapor in flue gas along downstream in the system. Predicted results of sulfuric acid vapor condensation were compared with empirical data for model validation, and the discrepancy is analyzed in terms of measurement and computation uncertainties. It is found that from both modeling and test results sulfuric acids as well as water vapors are reduced and separated in condensing heat exchanger due to mass transfer with condensation in flue gas. The modeling methodology described here is applicable to theoretical prediction of sulfuric acid and water condensation in full scale flue gas condensing heat exchanger applications. 相似文献
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Xue-Hu Ma Xing-Dong Zhou Zhong Lan Yi-Ming LI Yu Zhang 《International Journal of Heat and Mass Transfer》2008,51(7-8):1728-1737
Heat transfer characteristics of dropwise condensation (DWC) were experimentally studied on a vertical plate for a variety of non-condensable gas (NCG) concentration, saturation pressure, and surface sub-cooling degree. As the heat transfer performance was dominated by the vapor diffusion process near the interface of the gas–liquid within the gas phase, the additional thermal resistance of the coating layer may not be strictly limited, a fluorocarbon coating was applied to promote dropwise condensation mode. Compared with the traditional filmwise condensation (FWC), heat and mass transfer with NCG can be enhanced with the dropwise condensation mode. In the present paper, the effect of condensate liquid resistance should not be regarded as the most vital factor to explain the results, but the vapor diffusion process. This is attributed to the liquid–vapor interfacial perturbation motion caused by coalescence and departure of condensate droplets. The results also demonstrated that the feature of droplets departure is the dominant factor for the steam–air condensation heat transfer enhancement. 相似文献
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Non-condensable gases greatly influence vapor condensation, resulting in a substantial reduction in the condensation heat transfer coefficient. Although extensive analytical and numerical investigations of condensation heat transfer in the presence of non-condensable gases have been done, most of the solutions are quite complicated. Based on a thermodynamics analysis, when the vapor is not close to its critical state and the mass fraction of the non-condensable gas in the main stream is less than 0.1, an equation which relates the vapor/gas-liquid interface parameters and the main stream parameters was developed in the present work. For forced convection film condensation heat transfer on the outside surface of a horizontal tube, the present equation combining with an existing analytical solution as well as a heat transfer correlation given by previous investigators, gives the heat flux and the interfacial parameters of the water vapor-air mixture. The results show that the predicted heat flux is in good agreement with experimental data available in the literature and that even a small amount of air substantially reduces the heat flux. An algebraic equation set is given to calculate free convection film condensation on a vertical flat surface, which associates the interfacial and main stream parameters, an integral solution and an analytical solution given by previous investigators. The calculated results are in good agreement with experimental data in the literature. 相似文献
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Numerical Simulation of Laminar Film Condensation in a Horizontal Minitube with and Without Non-Condensable Gas by the VOF Method 总被引:1,自引:0,他引:1
Zhan Yin Yanling Guo Bengt Sunden Qiuwang Wang 《Numerical Heat Transfer, Part A: Applications》2013,63(9):958-977
Based on the volume of fluid (VOF) method, a steady three-dimensional numerical simulation of laminar film condensation of water vapor in a horizontal minitube, with and without non-condensable gas, has been conducted. A user-defined function defining the phase change is interpreted and the interface temperature is correspondingly assumed to be the saturation temperature. An annular flow pattern is to be expected according to a generally accepted flow regime map. The heat-transfer coefficient increases with higher saturation temperature and a smaller temperature difference between the saturation and wall temperatures, but varies little with different mass flux and degree of superheat. The existence of a non-condensable gas will lead to the generation of a gas layer between vapor and liquid, resulting in a lower mass-transfer rate near the interface and higher vapor quality at the outlet. In consequence, the heat-transfer coefficient of condensation with a non-condensable gas drops sharply compared with that of pure vapor condensation. Meanwhile, the non-condensable gas with a smaller thermal conductivity would cause a stronger negative effect on heat flux as a result of a higher thermal resistance of heat conduction in the non-condensable gas layer. 相似文献