水汽增强温室效应的探讨

从研究水汽的热力学特性出发,通过定性分析和定量计算,着重探讨水汽对全球气候变暖的影响。通过研究,推测水蒸气凝结是通过辐射放热的,其凝结过程中向地面的辐射可能是大气温室效应的一个主要原因。     

Extensive study on laminar free film condensation from vapor–gas mixture

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.     

Coupled Conduction,Convection, Radiation Heat Transfer with Simultaneous Mass Transfer in Ice Rinks

ABSTRACT

This article presents numerical predictions of velocity, temperature, and absolute humidity distributions in an indoor ice rink with ventilation and heating. The computational fluid dynamics (CFD) simulation includes the effects of radiation between all inside surfaces of the building envelope, turbulent mixed convection, and vapor diffusion, as well as conduction through the walls and condensation on the ice. The net radiative fluxes for each element of the envelope's inside surfaces are calculated with a modified version of Gebhart's method. This modification reduces the calculation time and the memory required to store the radiation view factors for the discretized elements of the inside surfaces of the envelope. The predicted temperatures show very good agreement with measured data. The CFD code also calculates the heat fluxes toward the ice due to convection from the air, to condensation of vapor, and to radiation from the walls and ceiling. It is shown that a low emissivity ceiling reduces the sum of these fluxes and the risk of vapor condensation on the ceiling.     

Heat transfer research on vapor‐gas mixture with condensation in a vertical tube

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     

Absorption of solar radiation in ponds

Analysis is presented to predict the local rate of solar energy absorption in a pond using the radiative transfer theory. The physical model considers absorption and scattering by the water and internal reflection of radiation from the air-water interface as well as the bottom. A forward scattering approximation and a discrete-coordinate approximation of the radiative transfer equation are discussed. Numerical results for the local volumetric rate of solar energy absorption in the water are presented in the paper for a range of parameters of physical interest. The effects of the directional distribution of solar radiation incident on the water surface, the attenuation of solar radiation by the atmosphere during the diurnal cycle and the modification of the spectral radiation characteristics of water by impurities and additives on the absorption and distribution of the absorbed energy in the pond are investigated.     

Theoretical prediction of sulfuric acid condensation rates in boiler flue gas

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.     

Prediction and Measurement of Apparent Heat Transfer Coefficient by Condensation in Finned-Tube Heat Exchangers

The apparent heat transfer coefficient by condensation in finned-tube heat exchangers is determined experimentally and numerically in this paper. The film method is used to predict the partial or total condensation of the water vapor contained in the humid air over the smooth or finned tube-heat recuperators. Based on this method, a computer code is developed here. The mathematical formulation is validated by our experimental results, using tube bundles in staggered and aligned arrangements. The determination of the fin portion, which functions in wet regime, is carried out by the prediction of thermal field over a circular fin. The condensation of the water vapor contained in the humid air is done preferentially with the last rows of the heat exchanger. The heat flux is predicted in a range of 20% and 5% in wet and dry regimes, respectively. The apparent heat transfer coefficient by condensation can exceed 10 times the value of the air-side heat transfer coefficient in dry regime.     

径向错列翅片管内凝结对流换热数值模拟分析

采用数值模拟方法,对径向错列翅片管内含不凝结气体水蒸气的凝结对流换热及阻力特性进行了综合分析。将编写的自定义函数(UDF)导入ANSYS FLUENT软件,对新型强化管传热性能和阻力性能进行了数值模拟,并根据管长方向壁面上蒸汽质量分数的变化情况,讨论分析了凝结过程中翅片管传热性能的变化规律。分析结果表明:与光管相比,内翅片管的强化传热效果随翅数增多、翅片换热接触面积增大而更加显著;另一方面,翅片管的流动阻力相应增大,对管路换热产生不良影响。在所研究翅型范围内16翅y=2x~2型翅片管综合强化换热效果更优;此外随着换热过程的持续,蒸汽凝结逐渐放缓;入口速度增大导致水蒸气凝结不充分,对换热效果的提升有一定制约。     

增压富氧煤燃烧烟气凝结换热的计算

针对含有少量水蒸气的增压富氧煤燃烧产生的烟气在竖直管内的对流凝结换热进行了分析研究.利用修正的膜模型与Nusselt凝结理论建立了换热数学模型,并对不同壁面温度、不同雷诺数和不同水蒸气份额下烟气的凝结换热进行了计算.结果表明:壁面温度升高时,烟气的凝结速率、换热流率和凝结液膜的厚度均减小;混合气体的雷诺数增大时,烟气的凝结速率和换热流率增大,凝结液膜的厚度减小;烟气中水蒸气的份额减小时,烟气的凝结速率和换热流率减小,凝结液膜的厚度减小不明显.     

New correlations for the weighted-sum-of-gray-gases model in oxy-fuel conditions based on HITEMP 2010 database

Oxy-fuel combustion is one of the promising options for carbon dioxide capture in future coal power plants. Radiative properties of combustion gases and heat transfer characteristics inside oxy-fuel furnaces are different from those found in air-fired furnace. Nowadays, few publications provide appropriate radiation property correlations for oxy-fuel conditions. The available correlations are based on previous versions of HITRAN database, which is not accurate for prediction of spectral intensities at high temperature in combustion applications or above 1000 K. This paper considers the determination and evaluation of new correlations for the weighted-sum-of-gray-gases model to predict the radiative transfer in gases under oxy-fuel conditions. The new correlations are fitted from emittance charts calculated from the up-to-date HITEMP 2010 database for molar ratios of water vapor to carbon dioxide between 0.125 and 4, temperature range of 400–2500 K, and pressure path-length varying from 0.001 to 60 bar m. The new correlations are validated by comparing the radiative source term with line-by-line calculations from HITEMP 2010 database for a one-dimensional slab system. The radiative transfer equation is solved with the discrete ordinate method.     

Modeling aspects of vapor bubble condensation in subcooled liquid using the VOF approach

Direct contact condensation of a vapor bubble during subcooled boiling flow scenario has diverse applications in many fields such as thermal, chemical, and nuclear energies. The present work aims at the exploration of the underlying physics of single vapor bubble condensation in subcooled water following the volume of fluid method approach using ANSYS FLUENT 14.5. This work emphasizes on the modeling of the mass transfer rate using interfacial jump conditions for investigating the effect of various parametric conditions on the collapse phenomenon. A comparative study is also performed between the interface jump approach and the models based on the proposed empirical correlations to assess the condensation heat transfer (in terms of the collapse rate and Nusselt number) and bubble shape. The mass transfer model based on the interfacial jump condition is found to be the more realistic among all models for capturing the collapse rate as well as its shape.     

Water Mist and Radiation Interactions: Application to a Water Curtain Used as a Radiative Shield

A numerical simulation has been carried out on a water curtain used as a radiative shield against a source of thermal radiation. An Eulerian-Lagrangian code for the dynamics of the mist is combined to a Monte Carlo simulation of the radiative transfer in the air-droplet medium. Various attenuation efficiencies have been evaluated through comparisons on an academic reference case where multiple injections are generated on a line in order to build a water curtain. The aim of this study is to pinpoint more efficient configurations improving the radiative attenuation in saving the used water amount.     

水平单管外混合气体对流冷凝换热的理论研究

采用修正的膜模型与Nusselt凝结理论结合的方法,对含湿混合气体自上而下横掠水平管外时的对流冷凝换热机理进行研究,建立了液膜流动和传热模型,进行数值求解并分析了雷诺数、壁面温度及水蒸汽浓度等因素对混合气体冷凝换热的影响。计算结果表明:水管外壁液膜厚度分布很大程度上受气体边界层对液膜剪切力的影响。而局部努谢尔数不同于纯蒸气的的冷凝换热,它受气相热阻的影响很大,其分布状况类似于单相气体管外的对流换热。     

Steady and unsteady 3D non-isothermal modeling of PEM fuel cells with the effect of non-equilibrium phase transfer

A 3D model that fully couples multi-species and multi-phase transport, electrochemical kinetics, and heat transfer processes has been developed. The non-equilibrium membrane water absorption/desorption processes along with non-equilibrium condensation/evaporation processes have been investigated utilizing this comprehensive model. In addition, the fallacious assumption that water is produced in vapor phase during the half cell electrochemical reaction is addressed for the first time. The difference and relationship of the cell output current density among three water production mechanisms are exhibited to show the potential error induced by vapor or liquid production assumptions. The present model is capable of predicting transient phenomena within the cell as well. Our results show that compared to the liquid production modeling the dynamic response of PEM fuel cells in vapor production modeling is significantly overestimated owing to the sluggish condensation process.     

Total and partial condensation of binary mixtures under gravity driven film flow

Laminar film condensation at a vertical flat plate is considered under the aspects of total and partial condensation. With the aid of a perturbation approach the heat and mass transfer correlations for total condensation are derived from the governing balance equations, thus avoiding the complex iterative procedure of numerically solving the balance equations. This approach is particularly valuable during partial condensation where the more volatile component accumulates in the vapor phase. If this vapor is also the lighter component, which is most often the case, buoyancy forces lead then to an upward directed vapor flow, whereas the liquid flows down. Only the buoyancy forces of such vapor mixtures, as for example methanol–water, where the more volatile component is specifically heavier, are downward directed as is the liquid film. Buoyancy forces of the vapor reach a maximum for a given suction rate depending on the relevant properties of the mixture. These effects are illustrated on the basis of practical examples.     

Research on Marangoni condensation heat transfer for water and ethanol mixture vapor

A brass block was constructed as a test block to study the Marangoni condensation in this paper. The maximal temperature difference of the block surface on which pure steam condensed was 11^°C when the block was cooled by the normal temperature water. Regulations and modes of Marangoni condensation for mixture vapor with different mass fractions were studied when the speed of vapor was 0.3 m/s. As both temperature gradients and concentration gradients exist on the condensing surface, the experimental results indicate that the maximal heat transfer coefficient of mixture vapor can be 2.8 times that of pure steam when the Marangoni condensation of mixture vapor appears. The heat transfer coefficient of mixture vapor increases with the decrease of surface subcooling, and it appears a steep increase when the surface subcooling is small enough; the heat transfer flux has a maximum value as the surface subcooling rises; and the different modes of condensation are confirmed when the different ethanol concentration and different surface subcooling exist. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(8): 505–514, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20035     

Characteristic curves and the promotion effect of ethanol addition on steam condensation heat transfer

Although most previous studies concerning the condensation of binary vapor mixtures report the condensation rates to be less than that for pure vapor, heat transfer enhancement can be realized by using additives to form a positive system (solutal Marangoni condensation). The objective of the present study was to clarify the effect of mixing ethanol into steam on condensation heat transfer. Precise measurements of the ethanol concentration in the vapor of water-ethanol mixtures were performed over a wide range of ethanol concentrations, and the condensation behavior was observed. The maximum heat transfer coefficients in the condensation characteristic curves were determined to be 0.12 and 0.18 MW/m² K for vapor velocities of 0.4 and 1.5 m/s, respectively, and appeared at an ethanol vapor mass fraction of approximately 1%. The mixing was demonstrated to be extremely effective, particularly in the low-ethanol concentration range. The condensation heat transfer was enhanced approximately 2-8 times compared to pure steam.     

An experimental study on vapor condensation of wet flue gas in a plastic heat exchanger

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     

Approximate equations for film condensation in the presence of non-condensable gases

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.     

Large-cell model of radiation heat transfer in multiphase flows typical for fuel–coolant interaction

The visible and near-infrared radiative properties of water containing polydisperse steam bubbles and core melt particles are analyzed. The alternative approximate models of radiation heat transfer in the range of water semi-transparency are considered. A new approach based on radiation balance equations for a large computational cell of size about several centimeters is suggested. This approach called large-cell model takes into account not only water heating but also radiation heat transfer between the particles of different temperatures. The latter is important for problems of fuel–coolant interaction in possible severe accident of light water nuclear reactors. The model problems considered in the paper give estimates of nonlocal effects of thermal radiation and confirm the applicability of the large-cell model at parameters typical for fuel–coolant interaction.