功能表面降膜蒸发传热特性的实验研究

研究了处理表面镀铬铝管、PTFE铜管和纯铝氧化管水平管降膜蒸发传热，研究了喷淋密度、热流密度、管内蒸汽速度和管表面处理对降膜蒸发传热特性的影响。实验结果表明：在表面蒸发区，水平管降膜蒸发传热系数随热流密度的增加而提高，随喷淋密度增大先降低后升高，冷凝例传热系数基本保持不变。总传热系数对操作条件变化很不明显，表面阳极氧化膜使传热系数略有下降，但由于其优良的抗垢时蚀性能，非常有必要再进行深入地研究。     

含高浓度CO2的水蒸气在竖直平板上的凝结换热实验研究

对含有高浓度CO2的水蒸气在竖直平板上冷凝传热特性进行了实验研究,得到了混合蒸气流速为0．8 m／s、CO2质量分数为60．0％～94．0％工况下的凝结传热特性曲线。结果表明,CO2的加入会极大恶化蒸汽的凝结传热,随过冷度的增加凝结传热系数逐渐降低,且随着CO2质量分数的增大,凝结传热系数受过冷度的影响越来越小;随着CO2质量分数的增加,凝结传热系数继续降低,但降低幅度逐渐减缓。对比高CO2质量分数下膜状和珠状凝结传热特性发现,珠状凝结并没有明显强化传热优势。     

水平管内分层湿润角与周向传热分布

实验研究了不同当地蒸汽质量流速Gs和凝结液质量流速下,水平管内凝结液湿润角和管圆周方向传热系数的变化规律。研究发现,在一定蒸汽入口流量下,随着凝结速的增加,湿润角增加速度先快后慢,然后在低蒸汽流量下又变快;形成波状分层流时所需的Gs随着凝结液质量流速的增加而增加;波状分层流时,凝结液侧的传热系数平滑的变化到蒸汽侧传热系数。     

T型圆管内蒸汽直接接触凝结数值模拟

选用流体体积分数两相流模型和大涡模拟湍流模型,并引入基于热平衡原理的凝结模型——双阻力模型的自定义函数(user defined function,UDF),在FLUENT平台上对T型圆管内低速蒸汽在缓慢流动冷水中的直接接触凝结进行了数值模拟。结果表明,该工况下的蒸汽直接接触凝结为典型的间歇(Chugging)凝结且凝结主要发生在界面附近很小的区域,同时蒸汽直接接触凝结导致压力瞬间急剧降低从而发生水锤现象。     

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。     

R1234ze(E)与R134a在水平双侧强化管外的凝结换热对比实验

搭建了水平单管降膜蒸发试验台,以第四代制冷剂R1234ze(E)和第三代制冷剂R134a作为工质,在新型水平双侧强化管管外分别进行了改变管内水速、热流密度和冷凝温度条件的凝结换热实验。使用Wilson-Gnielinski图解法计算得到管内表面传热系数h_i,进一步采用热阻分离法分离出两种制冷剂的管外表面传热系数,并分析了管内冷却水水速、冷凝温度和壁面过冷度的变化对其换热性能的影响。实验结果表明:同根实验管下不同制冷剂凝结换热性能的差异与制冷剂物性与强化管结构之间的匹配特性有关,实验管型下,R1234ze(E)的管外凝结换热性能高于R134a。     

不可凝气体对大扁管空冷凝汽器性能的影响分析

为了研究不可凝气体(non-condensable gases, NCG)对火电与光热发电机组上广泛使用的大扁管空冷凝汽器性能的影响,以工程机组凝汽器上普遍应用的通流面积220 mm×20 mm的大扁管为研究对象,针对汽轮机典型工况下的实际蒸汽流量,基于Lee相变方程、VOF方法以及组分扩散模型,对蒸汽与NCG混合气体管内两相流凝结换热进行数学建模与数值计算。结果表明：由于大扁管的狭窄通流几何结构与高蒸汽流量,NCG对管内蒸汽凝结的抑制效果要远低于预期;当入口空气质量分数按2%增加时,凝结管凝结换热系数仅下降2%左右,这与NCG导致低流量圆管凝结性能急剧下降的结论不同;空气正常泄漏不会导致空冷凝汽器性能下降而影响发电机组效率。     

水平强化通道内蒸汽凝结的实验研究

为探讨水平强化通道(HECC)和水平圆形通道(HCC)内蒸汽凝结传热性能,对蒸汽-氮气混合物在HECC和HCC内的凝结过程进行了实验研究,分析了蒸汽压力、冷却水质量流量、不凝气体质量分数对蒸汽凝结传热性能的影响,获得了平均凝结传热系数、出口平均凝液质量流量和汽侧平均压损的变化情况。结果表明:与HCC相比,水平多头螺旋通道(HMHSC)和水平多头直通道(HMSSC)平均凝结传热系数的增强因子分别为2.35和1.45,出口平均凝液质量流量的增强因子分别为1.25和1.12,汽侧平均压损的扩大因子分别为1.29和1.16,即HECC能有效增强凝结传热性能,但汽侧平均压损也相应增大。     

水平肋片管外自然凝结换热特性研究

基于Nusselt凝结传热理论,沿肋片管圆周方向划分有限个微元角,建立了每个微元角内肋侧壁、肋间基管及肋顶三个区域的凝结传热模型,通过求解非淹没区和淹没区总传热量,推导管外平均传热系数计算式。计算不同肋片高度、肋密度时,R134a饱和蒸汽的管外平均凝结传热系数。结果表明:随肋密度的增加,平均传热系数先增大后减小,肋密度为25fpi时传热最佳;高肋片管的平均凝结传热系数大于低肋片管的,肋片高度达到一定值时,平均传热系数几乎不随肋高增加而增加。当R134a饱和蒸汽为20℃时,两种不同翅片密度的管外平均凝结传热系数随温差的增大而减小,并通过所建模型得到的计算值与Beatty-Kate模型进行了比较,平均误差分别为约16.1%和8.3%,故所建模型基本反映肋片管外蒸汽凝结传热机理。     

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%。     

Condensation from gas–vapour mixtures in small non-circular tubes

Careful measurements have been made during condensation of steam from steam–air mixtures flowing in a small, flattened, horizontal tube. The ranges of the relevant variables covered (inlet temperature, pressure, air mole fraction and mixture mass flow rate) were chosen to simulate those occurring in an exhaust heat-exchanger tube of a proposed fuel-cell engine. The experimental tube was cooled by water in laminar counter flow to simulate the external heat-transfer coefficient (air flowing over fins) in the application. The total heat-transfer rate was found from the mass flow rate and temperature rise of the coolant. The tube wall temperature was measured by thermocouples attached in grooves along its length. Special arrangements were made to ensure good mixing of the coolant (in laminar flow) prior to measuring the inlet and outlet temperatures. The condensate was separated using a cyclone at exit from the tube. A simple model was developed to predict local and total heat-transfer and condensation rates and local bulk vapour composition, temperature and pressure along the tube in terms of the inlet parameters and the wall temperature distribution. The measured heat-transfer and condensation rates for the tube were found to be in good agreement with the calculated values without having recourse to empirical adjustment.     

Convection Condensation Heat Transfer of Steam-Air Mixture With Heat Pipe Heat Exchanger

A heat pipe heat exchanger (HPHE) was used to investigate the heat transfer performance of steam-air mixture condensation, analogous to the dew condensation of flue gas, when the steam volume fraction ranged from 0 to 20%, and the inlet temperature of steam-air mixture varied from 70 to 120°C. Self-assembled monolayers (SAMs) treatment with n-octadecyl mercaptan was adopted to modify the condensation surface of the heat pipe. The comparisons were conducted to examine the influence of SAMs on condensation heat transfer of steam-air mixture vapor. The results indicate that the convection condensation heat transfer coefficient increases with the increase of steam volume fraction and Re number of the steam-air mixture. As the inlet temperature increases, the heat transfer coefficient decreases accordingly. The heat transfer performance can be improved by the SAMs surface, with a heat transfer enhancement ratio up to 1.6 at a condition of 20% of the steam volume fraction and 1500 Re number.     

Complete condensation in a vertical tube passive condenser

An experimental study is performed for the steam condensation in a vertical tube where steam is completely condensed. A condenser tube is submerged in a water pool where the heat from the condenser tube is removed through boiling heat transfer. The experiment data showed that the operating pressure is uniquely determined by inlet steam flow rate for the complete condensation. The condensation heat transfer rate increases and the condensation heat transfer coefficient decreases with the system pressure. For the condenser submerged in a saturated water pool, strong primary pressure dependency was observed on the condensation heat transfer.     

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.     

Prediction of condensation characteristics of alternatives to R-502 inside air–refrigerant enhanced surface tubing

In this paper, an experimental study on the heat transfer characteristics of two-phase flow condensation of alternative azeotropic refrigerant mixtures to R-502, on air/refrigerant horizontal enhanced surface tubing, is presented. The condensation data indicated that the heat transfer coefficient on the blend R-408A has the highest heat transfer rate among the blends under investigation. The condensation data also showed that R-502 and R-407B have similar heat transfer rates when plotted against the refrigerant mass flow rate. It also can be observed that, as the mass flux increases, heat transfer coefficient increases. Correlations were proposed to predict the heat transfer characteristics such as average heat transfer coefficients, as well as pressure drops of alternatives to R-502; such as R507, R404A, R407B and R408A in two-phase flow condensation inside enhanced surface tubing. In addition, proposed correlations were found to fairly predict the two-phase flow heat transfer condensation data.     

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.     

Measuring the Overall Volumetric Heat Transfer Coefficient in a Vapor-Liquid–Liquid Three-Phase Direct Contact Heat Exchanger

An experimental investigation of the volumetric heat transfer coefficient in a three-phase direct contact condenser was carried out. A 75-cm-long cylindrical Perspex column with a 4 cm diameter was used. Only 48 cm of the column was utilised as the active direct contact condensation height. Pentane vapor at three different initial temperatures (40°C, 43.5°C, and 47.5°C), with differing mass flow rates, and tap water at a constant initial temperature (19°C) with five different mass flow rates were employed as the dispersed phase and the continuous phases, respectively. The results showed that the volumetric heat transfer coefficient increased with increasing mass flow rate ratio (variable dispersed phase mass flow rate per constant continuous phase mass flow rate), the continuous phase mass flow rate and holdup ratio. An optimal value of the continuous phase mass flow rate is shown for an individual dispersed phase mass flow rates. This value increases with increasing vapor (dispersed) phase mass flow rate. Furthermore, it was observed that the initial driving temperature difference had no effect on the volumetric heat transfer coefficient. While, the temperature gained by the continuous phase has a considerable effect.     

Experimental Study of Condensation in a Shell and Tube Heat Exchanger in the Presence of a Noncondensable Gas

In this paper, an experimental study of the condensation of water vapor from a binary mixture of air and low‐grade steam has been depicted. The study is based upon diffusion heat transfer in the presence of high concentration of noncondensable gas. To simplify the study, experimental analysis is supported by empirical solutions. The experimental setup is custom designed for testing a new shell and tube type heat exchanger supplied by the manufacturer. Air–vapor mixture at 80 °C (max) and 20.2% relative humidity enters the heat exchanger at a mass flow rate of 480 kg/h and condenses 27 kg/h vapor using cooling water at an inlet temperature of 7 °C to 10 °C and mass flow rate of 3500 kg/h. By using the experimental data of constant inlet air mass fraction, mixture gas velocity, and different volumetric flow rate of the cold fluid, the local heat transfer coefficients are obtained. The main objective of this work is to establish an approximate value for surface area and overall heat transfer coefficient of a horizontal shell and tube condenser used in process space. Under designed working conditions, the condenser is found to work efficiently with 90% vapor condensation by mass.     

Prediction of two‐phase condensation characteristics of some alternatives to R‐22 inside air/refrigerant enhanced surface tubing

The results of an experimental study on the heat transfer characteristics of two‐phase flow condensation of some azeotropic refrigerant mixtures, proposed as alternatives to R‐22, on air/refrigerant horizontal enhanced surface tubing are presented. The condensation data indicated that the heat transfer coefficient of the blend R‐408A has the highest heat transfer rate among the blends under investigation. The condensation data also showed that R‐507 and R‐404A have similar heat transfer rates to that of R‐22 when plotted against the refrigerant mass flow rate. It can also be observed that, as the mass flux increases, the heat transfer coefficient increases. Correlations were proposed to predict the heat transfer characteristics such as average heat transfer coefficients as well as pressure drops of alternatives to R‐22 such as R‐507, R‐404A, R‐407C and R‐408A, as well as R‐410A in two‐phase flow condensation inside enhanced surface tubing. In addition, proposed correlations were found to fairly predict the two‐phase flow heat transfer condensation data. Copyright © 2000 John Wiley & Sons, Ltd.