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。     

微细通道内蒸汽直接接触凝结汽液界面演变特征的实验研究

为探究微细尺度条件下蒸汽直接接触凝结汽液界面瞬时特征及演变行为，开展了圆形截面T型微细通道内蒸汽直接接触凝结的可视化实验研究。利用高速摄像系统获取了饱和蒸汽体积流量630μL/min、过冷水温度60℃及过冷水体积流量12650μL/min工况下直接接触凝结过程的瞬时图像信息。研究发现，汽液界面演变历程不能简单的用一种凝结流型进行表述，其过程中包含了柱状、环状和间歇凝结流型的特征。蒸汽自支管入射至主管后其长度变化曲线经历了相对较长时间（108.2ms）地振荡增长，而消失过程则相对短暂（16.2ms）。此外，水平主管内的蒸汽柱先后发生了脱离和断裂现象，且其与支管正下方的蒸汽泡的凝结机理有所不同。     

蒸汽和空气在下流水柱表面接触冷凝的压力影响分析

含不凝气体的蒸汽直接接触冷凝在工业中应用广泛,冷凝参数对设备设计至关重要。数值模拟了蒸汽和空气在下流水柱表面的接触冷凝换热;分析了压力对下流水柱表面温度、空气质量分数的影响,以及凝结热流和凝结传热系数的变化规律。结果发现,下流水柱表面温度、凝结热流和凝结传热系数沿着流动方向下降,下流水柱表面空气质量分数沿着流动方向升高。随着压力增加,下流水柱凝结长度增加,在凝结长度末端,下流水柱表面空气质量分数高达90%以上,凝结热流和凝结传热系数均增加。下流水柱初始温度的升高会降低接触凝结传热系数和凝结热流。将计算结果和Celeta等的实验数据做了比较分析。     

蒸汽直接加热冷水的汽液界面压力振荡特性仿真研究

为探究蒸汽直接加热冷水过程中汽液界面瞬态压力振荡及两相流动特性,选用基于热平衡原理的凝结模型、大涡模拟湍流模型和流体体积分数多相流模型,对管道内蒸汽直接加热冷水过程进行了数值模拟,并将数值模拟获取的压力振荡与实验值进行了对比分析。结果表明,数值模拟获取的压力振荡时域特征与实验测量值吻合较好,汽液界面呈现显著的瞬态压力梯度波动,界面压力波动导致附近冷水流场结构发生瞬态剧烈变化。     

有/无多孔介质的矩形T型管道内蒸汽 直接接触凝结流型的实验研究

采用高速摄影仪,对填充和未填充球形多孔介质的矩形T型管道内蒸汽直接接触凝结流型进行了可视化实验研究。研究发现未填充多孔介质条件下存在五种典型凝结流型:间歇凝结、振荡射流、稳态射流、尾部振荡射流和发散射流;填充多孔介质条件下则存在三种典型凝结流型:振荡射流、弹状稳定射流和雾状稳定射流。研究表明:多孔介质对凝结流型及汽羽形状有显著影响;由于多孔介质内部的黏滞阻力和惯性阻力,填充多孔介质的凝结流型类型减少;随着蒸汽流量的增加,凝结流型较快的进入到稳定状态,削弱了蒸汽直接接触凝结汽羽的振荡。     

考虑进水温度的蒸汽喷射泵一维理论模型

蒸汽喷射泵(SDJP)是利用蒸汽在Laval喷嘴中形成的超音速气流来引射过冷水的动力装置,研究了水温及喷射泵结构参数对其性能的影响,结果表明:随着进水温度的升高,蒸汽喷射泵的喷射性能明显下降.由于已有理论模型不能确定进水温度与喷射性能的关系,提出用直接接触凝结(DCC)理论求解汽液两相流控制方程的一维理论模型,根据该模型分析了进水温度升高使蒸汽喷射泵性能下降的原因.     

三维湿蒸汽自发凝结流动的数值模拟

采用完全欧拉坐标系下的数值模型描述湿蒸汽两相自发凝结流动过程，对某汽轮机低压部分三维静叶栅中内复杂的自发凝结流动进行了模拟和分析。与忽略自发凝结的计算结果相比，叶片表面的压力分布有所不同，吸力面侧尤为明显；自发凝结的发生引起气流角的变化，直接导致作功能力的变化；由于流动参数沿径向分布不均，自发凝结引起的湿度沿径向分布也明显不同；计算结果同时显示边界层对自发凝结有很大的影响。     

考虑蒸汽热力行为的凝汽器喉部流动模拟

应用蒸汽凝结核心形成理论,从描述蒸汽凝结核心形成的微观机理的角度出发,分别对自发凝结核心的分布规律和形成率进行了分析,建立了蒸汽分子凝结的概率模型.在此基础上,应用直接模拟蒙特卡罗方法对汽轮机凝汽器喉部内蒸汽的热力行为进行了模拟计算,研究凝汽器喉部内蒸汽湿度对流动阻力和流动均匀性的影响.计算结果表明:考虑蒸汽热动力行为时,可以降低蒸汽的流动阻力,同时也使喉部内蒸汽的流场分布状况有了一定程度的改善.     

平面叶栅中湿蒸汽两相凝结与流动问题的数值分析

汽轮机末级中存在大量汽液两相混合流动现象。通过对湿蒸汽流动过程的分析,以经典成核理论为基础,对平面叶栅中的湿蒸汽流动进行了模拟。计算结果显示了蒸汽凝结主要发生的区域及在整个流场中湿蒸汽的流动特征。通过对流场的分析及与实验数据的对比,表明所用计算模型及方法可以正确模拟湿蒸汽流动过程,为优化设计和减小冲击损害提供依据。     

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．     

Modeling of steam–water direct contact condensation using volume of fluid approach

Direct contact condensation (DCC) of steam in subcooled water has paramount importance in many heat transfer devices in different industrial areas like nuclear, thermal, chemical plants. This work aims at the exploration of underlying physics of steam–water DCC in two dimensions using ANSYS FLUENT 14.5. The volume of fluid method is utilized for performing direct simulation of the phenomenon at the phase interface. In this work, thrust is given on the modeling of the interphase heat transfer using interfacial jump approach instead of proposed empirical correlations which have different applicability limits. User-defined functions in the FLUENT software are used for evaluating the interfacial mass transfer rate, thermal gradient across the phase interface, and interface curvature. This study also emphasizes on the prediction of transient temperature field and interface characteristics under different parametric conditions (e.g., variation of water injection velocity and water temperature). Observation reveals that the present condensation model is capable of capturing the transient temperature history as well as the flow regime transition (stratified to slug flow) induced by the interfacial instability.     

Three-dimensional condensation regime diagram for direct contact condensation of steam injected into water

Direct contact condensation (DCC) of steam in water occurs in many industrial devices. Crucial to obtain an appropriate design of the device is an accurate understanding and prediction of the condensation regimes. The two-dimensional regime maps presently available are able to predict different condensation regimes for limited flow conditions, but fail to predict regimes accurately if a different injector size is considered. In this paper, a new three-dimensional regime diagram is presented, and validated against experiments. Furthermore, corrections necessary to adopt the regime diagram for steam injection into a water flow are discussed in the paper.     

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.     

Experimental study on stable steam condensation in a quenching tank

Experimental study on direct contact condensation (DCC) of a stable steam discharging into a quenching tank with sub‐cooled water has been performed for five different sizes of horizontal nozzles over a wide range of steam mass flux and pool temperature conditions. Two different steam jet shapes (conical and ellipsoidal) were typically observed, depending on the steam mass flux and the pool temperature. The steam jet expansion ratios, the dimensionless steam jet lengths, and the average heat transfer coefficients were determined and the effects of steam mass flux, pool temperature, and nozzle diameter on these parameters were discussed. Empirical correlations for the dimensionless steam jet length and the average heat transfer coefficient as a function of steam mass flux and condensation driving potential were established. The axial and radial temperature distributions in the steam jet and in the surrounding pool water were measured and the effects of steam mass flux, pool temperature, and nozzle diameter on these parameters were also discussed. Copyright © 2001 John Wiley & Sons, Ltd.     

An investigation of direct condensation of steam jet in subcooled water

The direct contact condensation phenomenon, which occurs when steam is injected into the subcooled water, has been experimentally investigated. Two plume shapes in the stable condensation regime are found to be conical and ellipsoidal shapes depending on the steam mass flux and the liquid subcooling. Divergent plumes, however, are found when the subcooling is relatively small. The measured expansion ratio of the maximum plume diameter to the injector inner diameter ranges from 1.0 to 2.3. By means of fitting a large amount of measured data, an empirical correlation is obtained to predict the steam plume length as a function of a dimensionless steam mass flux and a driving potential for the condensation process. The average heat transfer coefficient of direct contact condensation has been found to be in the range 1.0∼3.5 MW/m²−°C. Present results show that the magnitude of the average condensation heat transfer coefficient depends mainly on the steam mass flux. By using dynamic pressure measurements and visual observations, six regimes of direct contact condensation have been identified on a condensation regime map, which are chugging, transition region from chugging to condensation oscillation, condensation oscillation, bubbling condensation oscillation, stable condensation, and interfacial oscillation. The regime boundaries stable condensation, and interfacial oscillation condensation. The regime boundaries are quite clearly distinguishable except the boundaries of bubbling condensation oscillation and interfacial oscillation condensation.     

Numerical investigation on steam jet submerged in subcooled water under different ambient pressures

This paper aims to investigate the effect of ambient pressure on the thermal hydraulic behavior of stable steam jet during the process of direct contact condensation (DCC). Three-dimensional steady CFD simulation was conducted. A thermal equilibrium phase change model was inserted as a user defined function (UDF) to simulate the process of steam jet condensed into subcooled water. The shape of steam plume and axial temperature distribution were compared between experimental and numerical results, and good agreements were obtained. The steam plume shapes under different ambient pressures were obtained. The transformation of flow pattern from annular flow to bubble flow along axial direction was observed from the cross-sectional slices at different axial locations. Then the axial parameters such as velocity, temperature and static pressure under different ambient pressures were investigated. The existence of expansion and compression wave was verified due to the existence of the fluctuation of axial temperature. Besides, the average heat transfer coefficient ranges from 0.97 to 1.08 MW/m² K when ambient pressure ranges from 80 to 200 kPa.