喷管内水蒸气凝结相变的数值模拟研究

引入水蒸气真实气体物性,考虑相变、速度滑移等影响因素,建立了超音速水蒸气非平衡凝结流动数值模型,通过与实验数据对比,验证了模型的正确性。随后对喷管内水蒸气超音速流动进行了数值模拟研究,清晰的捕捉到了"X型"凝结激波,并对其形态形成原因进行了分析。通过与理想蒸汽模拟对比,结果表明:水蒸气的非平衡凝结会形成凝结激波现象,导致流场中压力和温度突然升高,速度骤然降低;随后对非平衡凝结现象发生后,液滴半径、液滴数和湿度的增长趋势进行了计算分析。发现在喉部下游0.1 m处,液滴数由0突跃至1014数量级,表明水蒸气发生非平衡凝结,极短时间内产生了大量凝结核,液滴半径和湿度也在短时间内迅速增加。     

水蒸气跨音速流动中非平衡现象的数值模拟

研究了水蒸气跨音速流动过程中的非平衡相变及凝结冲波现象,揭示了气动激波与非平衡凝结流动之间相互作用的非定常流动规律。采用欧拉形式的控制方程求解两相凝结流动,建立了二维可压缩守恒型计算模型对水蒸气跨音速非平衡流动进行了数值模拟．捕获到了凝结冲波现象,揭示了凝结冲波的非平衡热力学机理,并探讨了尺度效应对两相非平衡流动的影响。研究了凝结冲波与气动激波之间的不同热力学特性和形成机理,并探讨了气动激波与非平衡凝结流动之间相互作用的非定常流动规律。     

水蒸气核化及单体生长过程的分子动力学研究

通过分子技术模拟水蒸气凝结过程,采用非平衡分子动力学(NEMD)方法模拟并分析了凝结相变中核化及单体生长过程.计算相变过程中的凝结速率,并研究了核化和单体生长初期液态水的直径与凝结速率之间的关系,同时分析了模拟压力对凝结速率的影响.结果表明:在相同过冷度下,凝结速率随压力的降低而减慢;在核化及单体生长过程中,相同的压力和温度条件下,凝结相变的传热传质效果随着液体直径的增长而改善.     

核电汽轮机高压级内非定常流动对自发凝结的影响

为研究核电汽轮机高压级内非定常流动对湿蒸汽凝结流动液滴直径、压力以及湿汽损失等参数分布的影响,采用湿蒸汽非平衡凝结相变模型,对定常与非定常流动状态下湿蒸汽自发凝结流动进行三维数值分析。结果表明:非定常凝结流动趋于稳定时,各项参数的分布呈现周期性变化规律。非定常流动的动静干涉现象导致周向流场具有非均匀性,湿蒸汽级内的轴端功率下降0.266%。非定常流动过程中的静叶尾迹现象致使下游动叶通道内自发凝结的液滴直径减小。非定常凝结流动的压力位势作用使得动叶出口平均压力提高。非定常流动引起湿蒸汽凝结的热力学损失与制动损失分别升高62.27%和1.88%,疏水损失降低0.233%。     

转炉高温烟气除尘文氏管喷淋降温流动与传热性能分析

文氏管内部的流动与传热较为复杂,不仅涉及喷淋液滴与烟气在流动过程中的相互作用,而且涉及喷淋液滴遇到高温烟气发生相变进行的传热传质。视烟气为连续相,喷射液滴为离散相,考虑连续相与离散相在质量、动量和能量的相互作用,建立了转炉烟气文氏管喷淋降温三维瞬态数学模型。通过数值模拟,获得了文氏管内部压力、温度、速度和水蒸气摩尔分数分布,得到了不同烟气量、不同喷射水量条件下,文氏管压力损失和出口温度与挡板开度之间的关系。     

不凝气对池内亚音速蒸汽直接接触凝结影响

利用VOF多相流模型和修正的热相变凝结模型对含不凝气蒸汽亚音速射入池内的直接接触凝结过程进行了数值模拟。主要研究了不同不凝气含量对蒸汽直接接触凝结过程中气羽形态、温度和压力分布的影响。研究结果表明：随着凝结的进行不凝气在气液界面处集聚成为一层不凝气层，随着不凝气含量的增加，不凝气层的厚度也增加，气羽不再呈现周期性的变化；不凝气的存在使得池内温度高温区域增大，温度分布相对均一；同时随着不凝气含量的升高，压力振荡的强度减弱，凝结形成的负压值升高。     

催化裂化烟气轮机级叶栅内气固两相运动特性的数值研究

基于欧拉-拉格朗日模型,对催化裂化烟气轮机叶栅内烟气流动特性进行数值研究.采用可压缩流体的k-ε双方程湍流模型,对流道内压力、温度、速度以及水蒸气含量变化进行研究;研究结果表明:级叶栅流道低压区会导致超音速流动,增加流动的不均匀性;二次流使5μm颗粒向动叶压力面底部移动,且压力面处分子黏度大、温度低,易造成颗粒沉积结垢;10μm以上大颗粒容易造成叶顶出气边磨损.模拟结果为烟气轮机长周期安全运行提供理论基础.     

变马赫数喷管中湿蒸汽流动数值模拟

Fluent Wet-Steam Model是基于经典成核理论,在全欧拉坐标系下描述自发凝结过程的。本文基于该模型对变马赫数槽式喷管内伴随自发凝结的水蒸汽的跨音速流动过程进行了数值模拟,研究水蒸汽在跨音速流动过程中的自发凝结、液滴生长、凝结冲波等现象以及背压变化对流场参数的影响。对蒸汽透平的通流部分设计提供参考,以及为湿蒸汽风洞设计提供参考。     

湿蒸汽非均质高速凝结流动的数值研究

基于冠状成核机理,建立了湿蒸汽两相非均质凝结流动数值模型,对缩放喷管、汽轮机叶栅和汽轮机级内湿蒸汽两相非均质凝结流动进行了数值模拟．结果表明：与自发凝结相比,非均质凝结流动中杂质颗粒改变了凝结过程;杂质颗粒减小了喷管中凝结激波强度,改变了汽轮机叶栅中的压力分布,降低了蒸汽过冷度,减少了不平衡热力学损失;在汽轮机级内,非均质凝结流动的动、静叶进、出口汽流角接近过热蒸汽流动的动、静叶进、出口汽流角,其动叶前压力高于过热蒸汽的动叶前压力,但级反动度偏离过热蒸汽流动数值．     

关于汽轮机内湿蒸汽两相凝结流动的研究进展

汽轮机内的湿蒸汽非平衡凝结流动会导致机组效率降低和叶片水蚀损坏。由于两相流动远比单相流动复杂,通过实验难以测量其凝结特性。对此,通过分析国内外湿蒸汽两相流理论和实验研究现状,总结了现有的3种研究方法,并指出研究中存在的问题,讨论了两相流凝结机理及其求解方法,阐述了几种湿蒸汽测量方法优点及不足之处。分析指出:凝结相变过程十分复杂,传热传质在微米级水平发生,因此,有必要发展一种考虑两相间耦合的凝结模型。结合两相流数值研究的现状,对汽轮机两相流数值的求解方法进行阐述。湿蒸汽两相流研究为汽轮机通流部分设计和改造提供了理论参考。     

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.     

基于高压电晕除湿的烟气消白技术参数优化设计研究

为了探究离子风除湿机理,获得影响除湿效率的关键参数,基于ANSYS CFX均质成核理论建立了除湿过程的三维空间多场耦合多相流数值模型,分析了集电极筒结构、流动、物性等参数变化对除湿效果的影响。基于可视化后处理软件,分析了电/流场耦合作用下流场速度、液滴的分布规律,并进一步阐明离子风除湿机理和除湿过程。结果表明：渐扩形集电极筒效果最佳但难以并联组合;随着进口流速的增加除湿效果迅速减弱;温差的增大显著提高了饱和蒸汽凝结的效率;相比于入口饱和蒸汽,气液两相入口明显改善了圆柱形集电极筒内的整体凝结程度,且核心凝结区域不仅限于出口处的U形区域;而电压的增强可以促进气相凝结,但是电压超过38 kV之后,液相平均质量分数变化幅度很小;增加电极长度可以提高除湿效果,但当电极长度达到0.37 m时,继续增加电极长度液相平均质量分数没有显著变化。     

水平管外TFE/NMP部分膜状冷凝热、质耦合传递过程研究

通过对水平管外双组分(TFE／NMP为三氟乙醇／氮甲基吡咯烷酮)部分膜状冷凝过程特点的分析，建立起部分膜状冷凝过程中热质传递过程的物理模型。以双膜理论为基础，利用部分膜状冷凝的特点，通过对界面传质、液膜内质量平衡、界面相平衡、界面能量平衡和汽膜截面能量平衡的分析计算，得到汽相温度和界面温度分布、汽相及液相NMP质量分数分布，由此进一步计算出冷凝膜厚分布、液膜传热系数分布和热流密度的分布。计算的热流密度与相关实验作了比较，发现与实验能较好的吻合。     

中空纤维膜管内LiBr溶液除湿过程的数值研究

为研究中空纤维膜和吸湿溶液结合进行制冷预除湿过程的传质机理,建立了膜管外的水蒸气通过膜孔最后被溴化锂溶液吸收传质过程的数值模型,研究了液体进口流速、进口浓度和进口温度对管内溶液的温度分布、质量分数分布和膜孔内水蒸气质量分数分布的影响,并比较了这三种因素对传质的影响程度。在一般预除湿用疏水性膜组件的内部溶液压力条件下(小于10kpa),溶液与水蒸气的接触面在疏水性膜内壁表面。溶液流速的增大,溶液进口质量分数的增大以及溶液进口温度的降低均有利于传质进行,其中,提高溶液进口质量分数对加强传质最为有效。     

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.     

Direct contact condensation in packed beds

A diffusion driven desalination process was recently described where a very effective direct contact condenser with a packed bed is used to condense water vapor out of an air/vapor mixture. A laboratory scale direct contact condenser has been fabricated as a twin tower structure with two stages, co-current and countercurrent. Experiments have been operated in each stage with respective saturated air inlet temperatures of 36, 40 and 43 °C. The temperature and humidity data have been collected at the inlet and exit of the packed bed for different water to air mass flow ratios that vary between 0 and 2.5. A one-dimensional model based on conservation principles has been developed, which predicts the variation of temperature, humidity, and condensation rate through the condenser stages. Agreement between the model and experiments is very good. It is observed that the countercurrent flow stage condensation effectiveness is significantly higher than that for the co-current stage. The condensation heat and mass transfer rates were found to decrease when water blockages occur within the packed bed. Using high-speed digital cinematography, it was observed that this problem can occur at any operating condition, and is dependent on the packing surface wetting characteristics. This observation is used to explain the requirement for two different empirical constants, depending on packing diameter, suggested by Onda for the air side mass transfer coefficient correlation.     

Numerical simulation of water and heat transport in the cathode channel of a PEM fuel cell

Cathode channel of a PEM fuel cell is the critical domain for the transport of water and heat. In this study, a mathematical model of water and heat transport in the cathode channel is established by considering two-phase flow of water and air as well as the phase change between water and vapor. The transport process of the species of air is governed by the convection-diffusion equation. The VOSET (coupled volume-of-fluid and level set method) method is used to track the interface between air and water, and the phase equilibrium method of water and vapor is employed to calculate the mass transfer rate on the two-phase interface. The present model is validated against the results in the literature, then applied to investigate the characteristics of two-phase flow and heat transfer in the cathode channel. The results indicate that in the inlet section, water droplets experience three evolution stages: the growing stage, the coalescence stage and the generation stage of dispersed water drops. However, in the middle and outlet sections of the channel, there are only two stages: the growth of water droplets, and the formation of a water film. The mass transfer rate of phase change in the inlet section of the channel varies over time, exhibiting an initial increase, a decrease followed, and a stabilization finally, with the maximum and stable values of 1.78 × 10^?4 kg/s and 1.52 × 10^?4 kg/s for Part 1, respectively. In the middle and outlet sections, the mass transfer rate increase firstly and then keeps stable gradually. Furthermore, regarding the distribution of the temperature and vapor mass fraction in the channel, near the upper surface of the channel, the temperature and vapor mass fraction first change slightly (x < 0.03 m) and then rapidly decrease with fluctuations (x > 0.03 m). In the middle of the channel, the temperature and vapor mass fraction slowly decrease with fluctuation.     

A two-phase non-isothermal mixed-domain PEM fuel cell model and its application to two-dimensional simulations

In this paper, a two-phase non-isothermal PEM fuel cell model based on the previously developed mixed-domain PEM fuel cell model with a consistent treatment of water transport in MEA has been established using the traditional two-fluid method. This two-phase multi-dimensional PEM fuel cell model could fully incorporate both the anode and cathode sides, properly account for the various water phases, including water vapor, water in the membrane phase, and liquid water, and truly enable numerical investigations of water and thermal management issues with the existence of condensation/evaporation interfaces in a PEM fuel cell. This two-phase model has been applied in this paper in a two-dimensional configuration to determine the appropriate condensation and evaporation rate coefficients and conduct extensive numerical studies concerning the effects of the inlet humidity condition and temperature variation on liquid water distribution with or without a condensation/evaporation interface.     

High speed bubbly nozzle flow with heat, mass, and momentum interactions

The characteristics of high speed bubbly flows through convergent-divergent nozzles are studied theoretically. A steady, one-dimensional flow is considered. The liquid phase is water, whereas the gaseous phase consists of a mixture of both non-condensable (air) and condensable (water vapor) components. The comprehensive physical model allows for momentum and thermal lags as well as mass transfer between the gaseous and liquid phases due to evaporation and condensation. The parametric analysis reveals that choked flow with supersonic speeds along the diverging section of the nozzle, similar to the behavior of a compressible gas flow, may be obtained under appropriate conditions. Effects of flow parameters such as wall friction, interphase heat transfer, initial bubble size and void fraction are demonstrated.     

Thermodynamic analysis of the effects of atmospheric conditions on the performance of a precooled gas turbine fueled with liquid hydrogen

The effects of varying atmospheric conditions such as temperature, humidity and pressure on the performance of a precooled gas turbine cycle fueled with liquid hydrogen are analyzed. Since the hydrogen temperature at the precooler inlet is very low, the condensation and freezing of water vapor contained in suction air is supposed to occur within the precooler. Due to the condensation of water vapor, the precooling process requires more cryogenic hydrogen. Therefore, the temperature-drop ratio of suction air ? within the precooler decreases. Thermodynamic analysis has revealed that the thermal efficiency and specific output per unit mass flow rate considerably decrease with the increase of humidity ψ, the performance degradation of gas turbine due to atmospheric temperature rise is augmented with the increase of humidity. The humidity ratio between precooler inlet and outlet is also made clear.