Numerical and experimental study on the falling film flow characteristics with the effect of co-current gas flow in hydrogen liquefaction process

Liquid hydrogen storage and transportation is an effective method for large-scale transportation and utilization of hydrogen energy. Revealing the flow mechanism of cryogenic working fluid is the key to optimize heat exchanger structure and hydrogen liquefaction process(LH2). The methods of cryogenic visualization experiment, theoretical analysis and numerical simulation are conducted to study the falling film flow characteristics with the effect of co-current gas flow in LH2spiral wound heat exchanger.The results show that the flow rate of mixed refrigerant has a great influence on liquid film spreading process, falling film flow pattern and heat transfer performance. The liquid film of LH2mixed refrigerant with column flow pattern can not uniformly and completely cover the tube wall surface. As liquid flow rate increases, the falling film flow pattern evolves into sheet-column flow and sheet flow, and liquid film completely covers the surface of tube wall. With the increase of shear effect of gas-phase mixed refrigerant in the same direction, the liquid film gradually becomes unstable, and the flow pattern eventually evolves into a mist flow.

Residence time distribution of high viscosity fluids falling film flow down outside of industrial-scale vertical wavy wall: Experimental investigation and CFD prediction

The flow behavior of gravity-driven falling film of non-conductive high viscosity polymer fluids on an industrial-scale vertical wavy wall was investigated in terms of film thickness and residence time distribution by numerical simulation and experiment.Falling film flow of high viscosity fluids was found to be steady on a vertical wavy wall in the presence of the large film thickness.The comparison between numerical simulation and experiment for the film thickness both in crest and trough of wavy wall showed good agreement.The simulation results of average residence time of falling film flow with different viscous fluids were also consistent with the experimental results.This work provides the initial insights of how to evaluate and optimize the falling film flow system of polymer fluid.

Numerical study on falling film flowing characteristics of R113 inside vertical tube under different structural conditions

To develop an appropriate falling film evaporation device for organic fluid cogeneration, a numerical study on the gas–liquid two-phase counter-current flow characteristics of R113 inside a vertical tube under different structural conditions was conducted using the Fluent software. The effects of the tube length, tube diameter, and annular gap on the falling film flow characteristics were determined, respectively. The results indicated that under a certain spray density, the falling film thickness in the region of the steady section was almost constant with different structural parameters for the tube diameter, tube length, and annular gap. In addition, a smaller tube diameter resulted in a steadier film flow. When the tube diameter decreased to a specific value, the film thickness showed a uniform distribution along the wall surface. This indicated that a best falling tube diameter exists. Meanwhile, the film fluctuation was enhanced with an increase in the tube length. When the tube length was greater than 1.2 m, the falling film splashed and could not completely wet the wall surface. The film fluctuation was enhanced by decreasing the annular gap, and the film could not be formed when the annular gap was smaller than 1.2 mm.

Co-pyrolysis characteristics of typical components of waste plastics in a falling film pyrolysis reactor

Waste plastics mainly come from MSW and usually exist in the form of mixed plastics. During the co-pyrolysis process of mixed plastics, various plastic components have different physicochemical properties and reaction mechanisms. Considering the high viscosity and low thermal conductivity of molten plastics, a falling film pyrolysis reactor was selected to explore the rapid co-pyrolysis process of typical plastic components(PP, PE and PS).The oil and gas yields and the compositions of pyrolysis products of the three components under different ratios at pyrolysis temperatures were analyzed to explore the co-pyrolysis characteristics of PP, PE, and PS. The study is of great significance to the recycling of waste plastics.

Liquid film falling behaviour on horizontal circular cylinder

The flow pattern behaviour of falling liquid film over three horizontal cylinders was evaluated.These flows can take three forms:discrete droplets,individual jets,and continuous sheet,and special attention is paid to the effects of the physical properties and geometrical parameters of the first two forms(droplets and jets) because these forms are more important in heat-transfer behaviour and less research has been published for these forms,The flow modes and experimental results were successfully compared with previous experimental literatures,and also the effects of liquid flow rate,tube diameter,and tube spacing on departure site spacing,in both drop and jet modes,were evaluated in the low Galileo number and high viscosity fluid(cooking oil),to help developing criteria for determining falling film modes and their transitions,and to understand the heat transfer characteristics associated with each mode.

Simulation of the hydrodynamics and mass transfer in a falling film wavy microchannel

The flow in a liquid falling film is predominantly laminar,and the liquid-side mass transfer is limited by molecular diffusion.The effective way to enhance the mass transfer is to improve the liquid film flow behavior.The falling film behaviors of water,ethanol and ethylene glycol in nine different wavy microchannels were simulated by Computational Fluid Dynamics.The simulation results show that the falling film thickness exhibits a waveform distribution resulting in a resonance phenomenon along the wavy microchannel.The fluctuation of liquid film surface increases the gas-liquid interface area,and the internal eddy flow inside the liquid film also improves the turbulence of liquid film,the gas-liquid mass transfer in falling film microchannels is intensified.Compared with flat microchannel,the CO_(2) absorption efficiency in water in the wavy microchannel is improved over 41%.Prediction models of liquid film amplitude and average liquid film thickness were established respectively.

Research on the falling film flow and heat transfer characteristics of FLNG spiral wound heat exchanger under sea conditions

As the key equipment of floating liquefied natural gas(FLNG)process,the performance of spiral wound heat exchanger(SWHE)influences operation costs and reliability of the whole system.The sea conditions destroy the falling film flow state of the refrigeration and then affect the heat transfer performance of FLNG SWHE.In order to design and optimize the SWHE,a cryogenic experimental device of FLNG process and a numerical model of falling film flow have been constructed to study the effects of sea conditions on the falling film flow and heat transfer characteristics of SWHE.The cryogenic experimental results show that the pitching conditions have larger effects on the heat transfer performance than yawing.Under the pitching angle of 7°,the natural gas temperature and gaseous refrigerant temperature increase by 3.22°C and 7.42°C,respectively.The flow rates of refrigerant and feed natural gas have a great impact on the heat transfer performance of SWHE under pitching and compound sloshing conditions.When the tilt angle increases to 9°,the tube structure with outer diameter D=8 mm and pipe spacing S=4 mm is recommended to reduce the drying area of the pipe wall surface.

Study on heat transfer for falling liquid film flow with consideration of interfacial evaporation

The interfacial evaporative heat transfer was included in the semi-empirical study of the heat transfer for the falling liquid film flow. The investigations showed that, the inclusion of the interfacial evaporative heat transfer in the turbulent model would lower the predicted convective heat transfer coefficient. Predictions of the new model resulted in a prominent deviation from that predictions of the normal model in the case of large mass flow rate and low wall heat flux. This deviation will be decreased with increasing wall heat flux, such that it will be asymptotic zero at very high wall heat flux. Predictions of the new model agreed well with the current experimental measurements. This study has verified that the Reynolds number is not the sole crucial parameter for heat transfer of falling liquid film flow, and wall heat flux will be another important independent parameter. This result is consistent with our previous studies.

Separation and Purification of Dodecanedioic Acid from Its Homologous Compounds by Falling Film Crystallization

Separation and purification of dodecanedioic acid (DDDA) from its homologous compounds were studied experimentally by falling film crystallization (FFC). The influences of various operation parameters, including crystallizing time, flow rate of melt and temperature of glycerine bath, on purity of DDDA and crystallizing rate were investigated. Over 99% (by mole) DDDA was obtained for a feed composition of 96% (by mole). The main factors affecting the separation efficiency are flow rate of melt and temperature of glycerine bath. The crystallizing layer of DDDA was further purified by sweating and blasting. A set of optimized operation data are provided for better understanding the mechanism of heat and mass transfer in FFC, and for further industrial application of DDDA purification process.

Heat transfer characteristics of molten plastics in a vertical falling film reactor

The heat transfer efficiency during the pyrolysis process is a key factor to be considered in the design of pyrolysis reactors. In this study, the average apparent heat transfer characteristics of molten plastic pyrolysis in a vertical falling film reactor were explored by experiments and numerical simulation and the apparent heat transfer coefficients were determined. In addition, the temperature distribution and the thickness of the liquid film in the reactor were predicted and the influences of pyrolysis temperatures on the average apparent heat transfer coefficients were discussed.

水平管降膜换热器性能规律研究进展

水平管降膜换热器具有热质传递效率高、阻力小、结构简单等优点,被广泛应用于化工等传统领域及能源利用的节能减排领域。降膜换热器内部发生复杂的流动及传热传质相互耦合过程。介绍了实验及模拟研究手段的进展,综述了不同操作参数(气体温度、流向及流量,溶液流量、温度及浓度,内部媒介流量及温度等)与结构参数(管径、管间距等)对水平管降膜管间流型、液膜厚度与润湿性等流动特性的影响规律,以及对蒸发传热特性、吸收传热传质特性等换热器性能的影响规律,包括整体性能和局部微细特征,为水平管降膜换热器的性能优化提供理论支撑。指出在不同气流特征以及多因素相互作用下多维度的局部流动与传热传质性能的耦合影响规律以及强化换热手段会是水平管降膜换热器未来研究的重点方向。

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。

纳米TiO_(2)对NH_(3)-H_(2)O-LiBr工质降膜吸收性能的影响

为了探讨纳米TiO_(2)对三元NH_(3)-H_(2)O-LiBr工质降膜吸收的影响,本文利用数值分析建立了三元工作流体薄膜吸收的连续方程、能量方程和组分质量平衡方程,采用Matlab软件进行编程和计算纳米流体的降膜吸收性能,将其与实验数据进行对比验证,并进一步分析了纳米TiO_(2)质量分数、初始氨浓度、初始温度、冷却水进口温度、吸收压力和下降薄膜管长度对薄膜吸收性能的影响。研究表明:添加纳米TiO_(2)可以增强降膜吸收的传质速率,主要原因为液膜中氨的扩散系数增加。当纳米TiO_(2)质量分数从0%增加到0.1%、0.3%和0.5%时,扩散系数分别增加了3.44倍、6.42倍和11.76倍。此外,增加初始氨浓度、降低初始温度、提高冷却水进口温度或降低吸收压力都可以提高最终溶液的饱和度。

湍流状态下垂直降膜波动特性影响因素研究

针对液膜的稳定性和均匀性会直接影响气液两相和气液固三相传热的问题,对影响液膜流动的因素进行研究。从液膜厚度、液膜速度和湍流强度3个参数定量分析槽宽(2~5 mm)、管径(73~113 mm)、气液接触角(0°~90°)、摩擦系数(0.1~0.9)和气速(2~10 m·s^(-1))对高雷诺数下垂直降膜波动特性的影响机理。结果表明,当槽缝宽度大于3.5 mm时,液膜厚度基本不变,湍流强度变化相对稳定。液膜的厚度、速度和湍流强度随管径的增大而显著变化。随着气液接触角的增大,液膜厚度和速度变化不大,但液膜的湍流强度在出口处略微降低。液膜厚度随着摩擦系数的增加略有增加。随着气体速度加快,液膜厚度先急剧减小,然后缓慢增加。气体速度对入口区域的液膜速度具有抑制作用。

浮式氢能储运过程中FLH2通道管外降膜流动的海上适应性强化机理

随着海上风电和天然气制氢技术的发展,以氢能作为媒介的深海能源互联互通体系正不断完善。液态氢储运是实现海上氢能大规模运输和利用的有效方法,揭示海况条件下降膜流动机理是实现浮式氢能高效储运的关键。本文基于高速摄像系统和晃荡平台系统,同时基于耦合的VOF和Level Set两相流模型和动网格技术,采用浮式微观可视化实验和数值模拟相结合的方法,研究氢能储运过程中FLH_(2)(浮式氢气液化装置)换热通道管外降膜流动的海上适应性强化机理。研究结果表明,与常规光圆管、螺纹波管和方波管相比,波纹管管外降膜流动的液膜均匀性较好。在海况下波纹管间降膜流动流型稳定,并且均未出现管壁表面干涸的现象,因此在浮式氢气液化装置FLH2中推荐采用波纹管以强化其海上适应性。基于可视化实验和数值模拟结果,得到了降膜流动的流型演化规律,并拟合了波纹管外降膜流动的液膜厚度计算关联式。

高雷诺数下洗涤冷却管内垂直环状降膜流动传热

为了研究高雷诺数(R_(e)=1.81×10^(4)~4.23×10^(4))下洗涤冷却管内降液膜的传热特性与机理,对管内液膜厚度分布与传热过程进行模拟计算。研究结果表明,槽缝宽度与液膜Re越大,液膜越厚;液相出口温度随壁面温度而上升,最大传热系数约为3.7×10^(3)W·(m^(2)·K)^(-1);传热系数随进口温度上升而下降,最大传热系数约为8.3×10^(3)·W(m^(2)·K)^(-1);液膜Re的增加使传热系数上升,最大传热系数约为9.4×10^(3)W·(m^(2)·K)^(-1);表征传热系数h+与液膜Re近似于正比例关系;通过研究气液两相传热发现,传热过程在液膜入口段0~0.1 m处最剧烈;传热系数随气相温度上升而先升高后降低,最大传热系数约为899 W×(m^(2)·K)^(-1);计算数据与试验关联式的最小误差约在10%之内。

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。

CO_(2)化学吸收系统的垂直管式降膜再沸器传热传质研究

针对CO_(2)化学吸收系统中存在的高再生耗能和吸收剂长时间高温受热产生热降解的问题,提出了一种新型管式降膜再沸器。通过对降膜流动特性分析,开展了液体分布器结构优化设计研究,并且在200m^(3)/h烟气CO_(2)化学吸收中试平台进行了系统热工实验,以验证降膜再沸器的传热传质性能并寻找最优操作参数。通过降膜流动实验发现,优化接管开孔和分布槽孔径后的液体分布器不均匀度可显著降低至0.026。结果表明,随着管内液相流速的增加及层流与湍流状态的转变,降膜再沸器总传热系数呈先减小后增大的趋势。同时,随着蒸汽质量流量由60kg/h增加至70kg/h和80kg/h,再生率可由10%左右上升至20%左右,最高可达到24.7%,从而促进了胺溶液的二次解吸。增加蒸汽流量和胺溶液进口温度,降膜再沸器热流量逐渐增大,整体传热性能逐渐增强,总传热系数随之增加。所开发的CO_(2)化学吸收管式降膜再沸器,实现了降低吸收剂在再沸器内停留时间,同时提升了再沸器传热效率及胺溶液CO_(2)再生率,有望显著降低碳捕集再生热耗。

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。

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%。