Numerical Study on Flow Heat Transfer Characteristics of Horizontal Tube Falling-Film Evaporator

As an efficient and energy-saving heat exchange technology, horizontal tube falling film evaporation has a great application prospect in refrigeration and air conditioning. The three-dimensional models of falling film flow evaporation outside horizontal single tube and inside evaporator were established, and the accuracy of flow and heat transfer simulation process was verified by comparison. For horizontal single tube, the results showed that total heat transfer coefficient was low and increased with larger spray density and evaporation temperature. The thickness of liquid film outside tube decreased gradually with the increase of tube diameter, and the total heat transfer coefficient of small tube diameter was significantly greater than that of the large tube diameter. The total heat transfer coefficient presented an increasing trend with larger liquid distribution height and density. In addition, the fluctuation of tube axial liquid film thickness distribution decreased with larger liquid distribution density. For evaporator, the results indicated that part of liquid refrigerant was carried into the vapor outlet. The temperature of tube wall and fluid presented a gradually rising trend in vertical downward direction, while tube wall temperature within the same horizontal and transverse row had little difference. The high-temperature zone on the outer wall of heat exchange tube moved towards the inlet and gradually decreased, and the outlet temperature of water in the tube also gradually decreased with the increase of refrigerant spray density. The local heat transfer coefficient of heat exchanger tube in the vertical direction presented a downward trend which was more obvious with the smaller spray density and it was obviously higher located in the middle of upper tube row and both sides of lower tube row for horizontal tube rows.

Mathematical Simulation of Lithium Bromide Solution Laminar Falling Film Evaporation in Vertical Tube

For utilization of the residual heat of flue gas to drive the absorption chillers,a lithium-bromide falling film in vertical tube type generator is presented.A mathematical model was developed to simulate the heat and mass coupled problem of laminar falling film evaporation in vertical tube.In the model,the factor of mass transfer was taken into account in heat transfer performance calculation.The temperature and concentration fields were calculated.Some tests were conducted for the factors such as Re number,heating flux,the inlet concentration and operating pressure which can affect the heat and mass transfer performance in laminar falling film evaporation.The heat transfer performance is enhanced with the increasing of heat flux.An increasing inlet concentration can weaken the heat transfer performance.The operating pressure hardly affects on heat and mass transfer.The bigger inlet Re number means weaker heat transfer effects and stronger mass transfer.The mass transfer obviously restrains the heat transfer in the falling film solution.The relation between dimensionless heat transfer coefficient and the inlet Re number is obtained.

Interfacial Evaporation of Falling Liquid Films with Wall Heating

The interfacial evaporation of falling water films with wall heating was experimentally studied and analyzed. The results presented in this paper showed that the capillary induced interfacial evaporation played an important role in heat transfer of a falling liquid film. It would be independent of the wall heat flux and somewhat lower than that without wall heating for impure fluids such as water air system. The thermodynamic analysis conducted gave a theoretical basis for the experimental observations. The effective capillary radius was correlated with the mass flow rate. The experimental results and analysis showed that the interfacial evaporation should be taken into account in the study of falling liquid film heat transfer.

The effect of interfacial evaporation on heat and mass transfer of falling liquid film

Analysis of experimental data and estimation of the order of magnitude for interfacial mass diffusion have demonstrated that considerable excess evaporation exists on the free interface of falling liquid film, and that the capillary pressure caused by surface tension is the driving force of this excess interfacial evaporation, which we called the “capillarity-induced interfacial evaporation”. By correlating the experimental data, an empirical expression of the effective capillary radius, r\-e, is obtained with which the evaporative rate formula we derived and reported previously has been modified to improve the prediction of the critical heat flux for film breakdown. Comparisons with the available predicting models show that our modified equation can predict the experimental results with much lower relative deviation.

钛椭圆低肋横槽管外降膜蒸发流热特性

为提高钛椭圆低肋横槽管外降膜蒸发器传热效率,在试验验证基础上对椭圆系数E在1.0~2.0内管外液膜流热特性进行多相流数值模拟。结果表明:管外液膜分布均存在不同程度干斑,且液膜在临近干斑处较其他区域增厚明显,当E为1.2时液膜覆盖率最高;液膜沿轴/周向均减速铺展,当E为1.0和1.8时管上端液膜速度在无量纲轴向长度Z*为-0.25~-0.375及0.25~0.375之间骤减,液膜厚度沿轴向从喷淋口正下方位置向喷淋口远端变化趋势为先减小后增大;最大传热系数随着E增大而增大,管壁在液膜与干斑交界区域附近传热系数最高;平均传热系数先增大后减小,相较圆管,E为1.2和2.0时平均传热系数分别增大34.5%和15.7%;槽内汽含率分布与液膜分布呈相反规律,液膜整体汽含率随E的增大先增大后减小,槽内/外平均汽含率差值随E增加而增大。

板管型降膜式蒸发器换热特性数值模拟

降膜式蒸发器兼有满液式蒸发器换热效果好及干式蒸发器制冷剂充灌量小的特点。对板管型降膜式蒸发器进行了数值模拟,得出了其在使用R410A为制冷工质时,条缝宽度、喷淋密度及板面温度对蒸发器换热性能的影响。结果表明,当工质入口条缝宽度为1.5mm时,板面的液膜覆盖率及换热系数最大,条缝宽度小于或大于1.5mm时,板面的液膜覆盖率及换热系数均有所降低;板面的液膜覆盖率及其换热系数随着液膜雷诺数的增加而增加,但当液膜雷诺数达到10000以上时,增加幅度明显减小并趋于零;板片温度升高可以增强蒸发器的换热性能,但雷诺数较小时的影响很小,只有雷诺数足够大时板片温度的影响才得到明显的体现。

Improved film evaporator for mechanistic understanding of microwave-induced separation process

Microwave-induced film evaporation separation process has been reported recently to separate the polar/nonpolar mixture.However,the efficiency of the separation is still too low for practical applications,which requires further enhancement via different strategies such as optimization design of evaporator structure.In addition the depth understanding of the separation mechanisms is great importance for better utilization of the microwaveinduced separation process.To carry out these investigations,a novel microwave-induced falling film evaporation instrument was developed in this paper.The improvement of the enhancement effect of microwave-induced separation was observed based on the improved film evaporator.The systematic experiments on microwave-induced separation with different binary azeotropic mixtures(ethanol-ethyl acetate system and dimethyl carbonate(DMC)-H20 system)were conducted based on the new evaporator.For the ethanol-ethyl acetate system,microwave irradiation shifted the direction of evaporation separation at higher ethanol content in the starting liquid mixture.Moreover,for DMC-H20 system microwaveinduced separation process broke through the limitations of the traditional distillation process.The results clearly demonstrated the microwave-induced evaporation separation process could be commendably applied to the separation of binary azeotrope with different dielectric properties.Effects of operating parameters are also investigated to trigger further mechanism understanding on the microwave-induced separation process.

机械蒸汽再压缩系统性能分析与运行策略优化

为了降低能耗,对机械蒸汽再压缩系统(MVR)进行优化研究,利用Matlab软件建立MVR系统的LiBr溶液再生的稳态模型,通过改变蒸发压力、流量和压缩比等参数,分析不同参数对系统单位质量能耗(SEC)的影响。结果表明,蒸发器换热量是影响该系统SEC的决定性因素;利用改进的遗传算法,确定相应工况下的最小SEC。在再生溶液浓度为10%,蒸发速率为3 kg/s,产出溶液浓度为45%的情况下,SEC降低了36%.

重力热管技术研究与应用概述

重力热管是一种高效的传热元件,因其结构简单、等温性良好、换热性能优异、不含有能动部件等特点,被广泛应用于各个领域。本文全面总结讨论了重力热管的传热机制,包括膜状冷凝和努塞尔理论、降膜蒸发和降膜沸腾等机理,并对相关的理论模型进行了讨论分析,给出了比较合适可靠的理论模型选择建议。此外还介绍了重力热管在工程领域的应用现状,指出了重力热管技术应用所面临的问题和发展方向。

基于分布参数模型的降膜式蒸发器性能智能仿真研究

为对大型水平管降膜式蒸发器进行高精度的设计计算和优化分析,建立了降膜式蒸发器的智能换热仿真模型,并分析研究了蒸发器的性能。首先,采用分布参数法建立了单出口水平管降膜式蒸发器的换热仿真模型;其次,通过贝叶斯优化算法利用蒸发器实验数据对模型进行参数辨识,使得换热仿真模型的预测精度和适应性有所提高;最后,探究了传热系数和热流密度沿管长方向的变化情况,以及满液区换热面积所占比例对蒸发器换热量的影响规律。仿真结果表明:结合分布参数法和贝叶斯优化算法的智能换热仿真模型对大型水平管降膜式蒸发器19个工况换热量的预测误差不超过±6%;在较大的换热量范围内,单出口水平管降膜式蒸发器的最优池沸腾换热面积比例维持不变;从管内对流换热和二次布液两个角度优化降膜式蒸发器能有效提升其换热性能。

氯代碳酸乙烯酯降膜纯化方法研究

以工业级氯代碳酸乙烯酯(CEC)为原料,利用Aspen Plus软件模拟计算CEC、HCl在不同压力、不同温度下的饱和分压,根据模拟计算结果以降膜刮板蒸发器为主要设备闪蒸纯化CEC,去除CEC中HCl、活性氯及其他活性杂质。根据实验结果调整纯化参数,并以碳酸亚乙烯酯(VC)合成收率作为纯化效果评价标准,确定降膜刮板蒸发器纯化CEC最佳工艺条件为闪蒸温度70℃、5 kPa,CEC气相纯度由82%提高至85.3%,VC合成收率由65.2%提高至75.5%。

苯乙烯生产工艺的节能优化和经济分析

在中石化自主研发的苯乙烯生产工艺中,乙苯脱氢反应需要加入大量过热蒸汽作为脱氢介质,水蒸气和乙苯流量的比值是影响脱氢反应的一个重要因素。随着低水比催化剂工业应用取得进展,可以减少进入反应器的蒸汽量,而降低水比的关键就在于减少乙苯/苯乙烯塔顶冷凝器共沸蒸出的乙苯和蒸汽的量。控制恒沸蒸发量后,会导致乙苯/苯乙烯塔塔顶物料携带的热量不能完全回收。为合理利用这部分热量,对原工艺流程进行优化、热量进行耦合。此次节能优化主要涉及9台换热器,其中2台为新增。优化过程中还降低了预分塔塔压,提高了尾气压缩机出口温度。经过优化,可减少综合能耗11.364 kg标油(以1 t苯乙烯计),静态投资回收期约为2个月。

新型旋流布液器成膜性能实验研究

布液器是降膜蒸发器的关键部件。文章针对现有布液器存在的不足,开发了一种新型旋流布液器。该布液器为切向开槽结构,槽一侧与管内壁相切,另一侧延伸一定长度与管内壁形成导流结构,下端与换热管通过螺纹连接。在有机玻璃实验装置内对新型旋流布液器和切向槽型布液器形成液膜等性能进行了系统的对比研究。实验结果表明:只要合理控制流量,两种布液器均能连续稳定成膜,不会出现“干管”现象;新型旋流布液器的最小成膜流量更小,成膜的流量范围更广,说明其操作弹性更大;在相同流量下,新型旋流布液器比切向槽型布液器形成的液膜厚且流速小,因此液膜稳定性更好,同时液膜在换热管内的停留时间更长,换热效果更好。

降膜再沸器液体布膜器性能研究

对降膜再沸器上常用的双切口液体布膜器进行单管布膜试验研究,测得最小成膜液体量和不同流量工况下的液膜厚度,为降膜再沸器工程设计提供了基础数据。进一步,利用有限元软件模拟该布膜器的流场分布情况,发现在切口附近液体分布不均;针对这个问题,提出在切口处增加引流挡板,并模拟改进后液体布膜器的流场分布情况,发现切口处液体局部汇聚情况得到明显改善,为后续降膜再沸器优化设计提供指导。

微正压条件下板式降膜MVR蒸发装置的工艺设计与应用

传统负压蒸发技术存在传热效率低、设备投资大、真空稳定性差、水质不稳定、最终乏汽冷凝成本高等问题。深入研究微正压蒸发理论依据及应用条件,提出热能平衡因子Δqz是控制微正压蒸发系统稳定运行的关键因素,不凝气外排所携蒸汽热能及冷凝水、外排浓水热能的回收是保障微正压工况下MVR系统平稳运行的重要措施。MVR板式降膜蒸发工艺在微正压条件下运行,装置产水矿化度小于等于30 mg/L,降膜蒸发板换热系数相较于微负压蒸发提高10%以上,总体设备投资减少约6%~8%,消泡剂、pH调节剂加药量大幅减少。通过对比微负压与微正压两种工况下的运行数据表明:微正压控制蒸发工艺参数具有产水水质更优、换热系数更大、投资更省、能耗更低、降低药剂耗量等优势。

水平管降膜蒸发器管外液体流动研究及膜厚的模拟计算

针对应用于空调和制冷系统的水平管降膜式蒸发器,建立了FLUENT数值模拟计算的物理模型。以制冷剂R134a为研究对象,对不同流量、不同布液器开孔孔径、不同管束结构下管外制冷剂液体的流动情况进行了模拟计算;并实现了绕管周方向不同角度液膜厚度的读取。

制冷用水平降膜式蒸发器研究进展

降膜式蒸发器目前在牛奶、制药、化工、海水淡化等行业取得了较为广泛的商业应用,它从上世纪90年代以后开始应用于制冷空调系统。目前针对制冷用水平降膜式蒸发器的研究目的主要是明确各个参数对蒸发器传热性能的影响以及他们之间的联系,由于降膜式蒸发器的独特结构和内部传热传质的复杂性,这些研究尚处于初步阶段。在介绍其工作原理的基础上,针对水平降膜式蒸发器,着重讨论了布液器的结构与高度、管束(包括管径、管排数、管道表面形状)、制冷剂(流型、流量)等参数对其性能的影响。由于经验关联式在实际应用中的巨大优势,还总结了前人提出的关联式及其适用的范围,文章将为制冷用降膜式蒸发器的进一步研究和应用提供参考。

水平管薄膜蒸发传热系数

采用智能化薄膜厚度测试仪实验研究了液体负荷、蒸发侧沸点、传热总温差和传热管管径等因素对水平光滑传热管外液膜厚度、波动状况以及因此而导致的蒸发侧传热膜系数的影响 ,得到了传热膜系数的关系式 ,突出了液膜波动的影响 .还讨论了多种操作参数对两侧传热膜系数和总传热系数的影响 ,为工业设计提供实验依据 .

机械蒸汽再压缩热泵技术研究进展

介绍了国内外关于机械蒸汽再压缩热泵系统的研究应用情况,分析了其系统流程原理与节能特性。通过总结其系统流程工艺、蒸汽压缩机和换热器等三方面的研究现状,指出了其目前存在的问题及今后发展的方向。

横管降膜蒸发闭式循环太阳能海水淡化装置的实验

基于盘形与竖式降膜太阳能海水淡化装置的优、缺点和横管降膜蒸发的理论分析 ,针对现有装置存在的问题 ,设计了横管降膜蒸发闭式循环太阳能海水淡化装置 ,并在不同的运行参数下进行了实验研究 ,分析了影响该装置性能的各种因素 .实验结果表明 ,在供能相同的情况下 ,该装置的产水率比单级盘式太阳能蒸馏器提高了 2～ 2 5倍左右 ;提高运行温度 ,有利于提高产水率及单位能耗产水率 ;热水流率取为 30 0～ 6 0 0kg/h ,海水流率取为 8～ 15kg/h ,有利于提高产水率 ,改善该装置的性能 .