具有凝结的混合气体传热理论研究

采用理论分析研究方法研究了含湿混合气体在竖向管内对流凝结换热，讨论了含有少量水蒸汽（8%～20%）的混合气体中水蒸汽凝结对换热的影响，利用修正的膜模型与Nusselt凝结理论得到了理论解。研究结果表明，对于含有少量水蒸汽的混合气体的凝结换热，其换强度与混合气体的单相对流换热处于同一数量级，换热方式均不能忽略，壁面温度与水蒸汽质量成份对凝结换热的影响十分显。     

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.     

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。     

混合气体横向冲刷水平管时对流冷凝换热的机理研究

采用修正的膜模型与Nusselt凝结理论相结合的方法，对含湿混合气体以一定速度冲刷水平管外时对流冷凝换热进行研究，在考虑气相边界层分离的情况下讨论了液膜流动和换热的情况，同时研究了气体来流冲刷角度对总体换热的影响。结果表明，冷凝液膜是一个相当薄的膜层，液相导热热阻在整个换热的过程中基本可以忽略。     

水平翅片管外混合气体对流冷凝传热的理论研究

以Nusselt理论为基础,结合修正的膜理论,对含湿混合气体横掠水平翅片管外时的翅片表面对流冷凝传热机理进行研究。建议了分区处理方法,建立了翅片侧壁和光管上的液膜流动和传热模型。得到了管壁温度、烟气进口温度和雷诺数对总凝结液量的影响,以及翅片侧壁液膜的厚度分布。     

烟气在陶瓷膜管内对流凝结换热实验研究

为了研究烟气中水蒸气在陶瓷膜管内的热、质传递规律,通过烟气在单根陶瓷膜管内的对流凝结换热实验,研究了不同烟气参数(流量、温度和相对湿度)和冷却水流量对烟气对流凝结Sherwood数、烟气对流凝结Nusselt数和烟气显热、潜热换热量的影响。实验结果表明:烟气对流凝结Sherwood数、Nusselt数随烟气流量、温度和相对湿度的增加而上升,Sherwood数随冷却水流量增加基本不变,Nusselt数随冷却水流量增加显著上升;烟气潜热换热量与烟气对流凝结Sherwood数呈相同变化趋势,烟气显热换热量随烟气相对湿度、冷却水流量增加而上升,烟气流量大于10 L/min时,烟气显热换热量增长趋于平缓,烟气温度对烟气显热换热量没有影响。研究结果对于陶瓷膜在电厂实际烟气中应用具有指导意义。     

水平管外混合气体平均传热系数及气膜厚度的计算

以水蒸气空气混合物为例,利用努谢尔特修正膜模型和传热传质比拟原理通过简单迭代,得到混合气体水平管外凝结换热的一种总平均传热系数和平均气膜厚度的计算模型。在已知初始条件下,计算得到总传热系数随不凝结气浓度、主流流速及壁温的变化规律,并与已有计算式进行了比较,验证了结果的正确性。该计算模型适用于不同混合气体种类及不同表面形状。     

Marangoni效应对圆管内膜状凝结的影响

对圆管内膜状凝结换热过程进行了理论分析.探讨了重力、表面张力梯度引起的Marangoni力以及气液界面剪切力对凝结换热Nusselt数的影响.建立了含有凝结液膜的物理模型和基于边界层方程组的数学模型,应用相似理论进行了无量纲变换.结果表明,表面张力梯度对凝结换热过程的影响不可忽略,梯度越大,液膜越薄,Nu数越大,换热越好.     

低温烟气凝结换热及对天然气热效率的影响实验研究

为研究烟气露点附近及以下的低温烟气对流凝结换热规律与烟气换热对天然气利用热效率的影响,建立了烟气在翅片管换热器内对流凝结换热实验系统,研究了不同烟气温度、水蒸气含量对烟气凝结换热的影响,得出了烟气凝结换热实验准则关联式,分析对比了天然气利用热效率实验值与理论值。实验结果表明:当被加热水温度为23℃,烟气出口温度为73℃,比烟气露点53℃高20℃时已开始冷凝;过量空气系数1.3,烟温由54.1~73.4℃降到28.7~57.8℃,被加热水进口温度21~25℃的条件下,烟气中水蒸气质量含量降低了27%~76%,潜热换热占总换热51%~63%;天然气利用低热值热效率实验值比理论值高1.04%~8.74%。为低温烟气对流凝结换热规律研究与烟气余热回收利用技术开发及应用提供理论依据和数据资料。     

剪切层流蒸发液膜的传热特性

为克服理论分析中气液界面对流换热难以计算的问题,基于气相传热模型,建立了在同向或反向切应力作用下层流饱和蒸发液膜的传热模型,推导出无量纲液膜厚度和壁面对流换热系数与流动长度、界面切应力和初始雷诺数间的理论关系式.研究表明,受液膜蒸发的影响,液膜厚度沿流动长度不断减小,换热传热系数不断增加;同向切应力具有减薄液膜厚度和增大传热系数的作用;反向切应力则具有相反的作用,其影响更为明显.这一理论模型可以反映层流饱和蒸发液膜的传热特性.     

Research on convective condensation heat transfer for a gas mixture in a vertical tube

This paper discusses the convective condensation of a gas mixture in a vertical tube. A mathematical model was derived by combining a modified film model and Nusselt's condensation theory. The effect of wall temperature on film thickness and interfacial temperature was predicted and film thickness was calculated. When compared with the gas phase resistance method, the film model is better. The phenomenon of SO₂ absorption into condensate is illustrated and discussed. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(4): 219–228, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20011     

Theoretical research of convective condensation heat transfer for mixture gas on a horizontal tube

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     

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.     

Mechanism research of convective condensation heat transfer for a gas mixture flowing over a horizontal tube

The modified film model combined with Nusselt's condensation theory are used for the study of convective condensation heat transfer on a horizontal tube with moist mixed horizontal gas flows at a given speed. A theoretical model considering gas boundary layer separation was set up. The liquid film flows and the heat transfer on the tube are presented. The effects of the flow direction on condensation heat transfer are discussed. The results predict that the condensate film is so thin that the liquid phase heat resistance can be ignored. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20238     

Two-phase modelling of laminar film condensation from vapour–gas mixtures in declining parallel-plate channels

A two-phase model is presented that analyzes laminar film condensation from mixtures of a vapour and a non-condensing gas in parallel-plate channels. The channel is declining (inclined downward from the horizontal) and has an isothermal cooled bottom plate and an insulated upper plate. The model uses a finite volume method to solve the complete two-phase boundary-layer equations including inertia forces, energy convection, interfacial shear, and axial pressure change. Results are presented for steam–air mixtures in terms of axial variation of film thickness and local Nusselt number for various Froude numbers, inlet Reynolds numbers, inlet gas mass fractions, and inlet temperature differences. Profiles of axial velocity, temperature, and gas mass fraction are also presented. Increasing the angle of declination (decreasing the Froude number) produces thinner, faster moving films. The change in local Nusselt number with Froude number was not as substantial as the change in film thickness. The detrimental effect of the noncondensable gas on the heat transfer rate was observed to be more pronounced at higher Froude numbers. An exact analytical solution for the liquid and mixture axial velocity profiles under end of condensation conditions is also presented and compared with the numerical results.     

Numerical solution of film condensation from turbulent flow of vapor–gas mixtures in vertical tubes

A complete two-phase model is presented for film condensation from turbulent downward flow of vapor–gas mixtures in a vertical tube. The model solves the complete parabolic governing equations in both phases including a model for turbulence in each phase, with no need for additional correlation equations for interfacial heat and mass transfer. A finite volume method is used to form the discretized mean flow equations for conservation of mass, momentum, and energy. A fully coupled solution approach is used with a mesh that automatically adapts to the changing film thickness. The results of using three turbulence models involving combinations of mixing length and k–ε models in the film and mixture regions are compared. This new model is extensively compared with previous numerical and experimental studies. In the experimental comparisons, it was found that a model consisting of a k–ε turbulence model for both the film and the mixture flows produced the best agreement. Results are also presented for a parametric study of condensation from steam-air mixtures. The effects of changes to the inlet Reynolds number, the inlet gas mass fraction, and the inlet-to-wall temperature difference on the film thickness and heat transfer are presented and discussed. Local profiles of axial velocity, temperature, and gas mass fraction are also presented.     

Free convective condensation heat transfer with noncondensable gas on a vertical surface

Free convective condensation with noncondensable gas on an isothermal vertical surface is studied under the condition of thermal equilibrium. An analysis is made by use of equations of a liquid film and boundary layars adjoining the liquid film and including small droplets generated by condensation, condensable and noncondensable gases. Tha calculation is made in the range of 1–99 per cent weight fractions of condensable gas. The result shows that Nusselt number asymptotically approaches those of free convection and film condensation at both extremes, and the phenomenon treated here intermediates free convection and film condensation along a vertical plate.     

Fully coupled solution of a two-phase model for laminar film condensation of vapor-gas mixtures in horizontal channels

Detailed results are presented for laminar film condensation of vapor-gas mixtures in horizontal flat-plate channels using a fully coupled implicit numerical approach that achieves excellent convergence behavior. These results correspond to steam-air and R134a-air mixtures over wide ranges of the independent parameters, and they include velocity, temperature, and gas concentration profiles, as well as axial variations of film thickness, pressure gradient and Nusselt number. Effects of the four independent variables (inlet values of gas concentration, Reynolds number and pressure, and the inlet-to-wall temperature difference) on the film thickness, pressure gradient, and the local and average Nusselt numbers are carefully examined. It was found that the condensation of R134a-air corresponds to thicker liquid films, lower heat transfer rates, and lower algebraic values of the pressure gradient when compared with steam-air at the same operating conditions.