水平管外降膜蒸发传热特性的数值模拟

为深入研究液膜内的微观传热机理,对水平管外降膜蒸发的传热特性进行了数值模拟,获得了液膜厚度、液膜流动速度和传热系数等热力参数在液膜内的分布特性。通过与实验数据的对比验证了数学模型的准确性。研究结果表明:在饱和蒸发温度62℃、传热温差2.8℃、管外径25.4mm和液膜入口速度0.071~0.15 m/s条件下,沿圆周方向,液膜厚度减小,传热系数增加,直至达到液膜热力发展区,膜厚和传热系数趋于稳定;受液膜内温度变化的影响,液膜内的粘度、表面张力和导热系数的变化对液膜传热特性产生显著影响。     

海水淡化系统水平管降膜蒸发器传热系数研究

针对海水淡化系统水平管降膜蒸发器,总结和分析管内冷凝侧与管外蒸发侧的换热系数关联式,比较管内径、入口蒸汽流速、蒸汽冷凝温度、出口蒸汽干度对管内蒸汽冷凝侧换热系数的影响;研究传热温差以及喷淋密度对管外蒸发侧换热系数的影响。结合不同的污垢系数,进行了总传热系数的影响因素分析,为海水淡化系统的工程设计提供依据。     

水平螺旋槽管降膜流动和传热特性研究

对水平螺旋槽管壁面降膜形成及传热特性进行了理论和实验研究,得到了液膜厚度及速度的解析及数值解.结果表明,降膜液膜特性主要受槽道结构和液膜表面张力控制.管壁温度沿周向向下逐渐升高,而且在定热流密度下保持不变,而液膜温度则沿周向逐渐上升.相比光管,螺旋槽管降膜具有更高的传热系数.     

水平管降膜蒸发器传热传质实验研究

水平管降膜蒸发器因传质传热系数高而被广泛应用于淡化水处理中。搭建了水平管降膜蒸发传热实验台,通过对实验结果的归纳,得到了水平管降膜蒸发器的蒸发量随喷淋密度、蒸发温度、热通量的变化规律及热量利用率随蒸发温度的变化规律。结果表明,热通量范围不同时,蒸发量随喷淋密度的变化规律不同;蒸发量随热通量的增大而增大,随蒸发温度的增大而增大;热量利用率随蒸发温度的增大而增大。     

海水淡化中降膜蒸发过程的实验研究

由于横管降膜蒸发具有传热系数高、热耗低、可利用低品位能源等优点,横管降膜蒸发技术成为低温多效海水淡化技术中应用最广泛的方法.搭建了单管横管降膜试验台,观测了横管降膜蒸发的流动过程,研究了横管降膜蒸发过程中的流动特性和传热特性,归纳了管间距、喷淋密度对横管降膜蒸发过程中流动特性和传热特性的影响.实验结果表明:喷淋密度的增...     

水平管降膜蒸发中强迫对流传质过程实验研究

对水平横管束降膜吸收器中,溴化锂溶液表面自然对流和强迫对流传质现象,分别在常压和负压下进行了实验研究。以惠特曼提出的双膜模型为基础,对实验结果做出了分析;以图线的方式,直观的比较了传质系数与压力,溶液表面对流情况的关系。得出了强迫对流对传质系数有很大提升的结论。     

Heat and mass transfer of ammonia-water in falling film evaporator

To investigate the performance of heat and mass transfer of ammonia-water during the process of falling film evaporation in vertical tube evaporator, a mathematical model of evaporation process was presented, the solution of which that needed a coordinate transformation was based on stream function. The computational results from the mathematical model were validated with experimental data. Subsequently, a series of parameters, such as velocity, film thickness and concentration, etc., were obtained from the mathematical model. Calculated results show that the average velocity and the film thickness change dramatically at the entrance region when x<100 mm, while they vary slightly with the tube length in the fully developed region when x>100 mm. The average concentration of the solution reduces along the tube length because of evaporation, but the reducing tendency becomes slow. It can be concluded that there is an optimalβrelationship between the tube length and the electricity generated. The reason for the bigger concentration gradient in the y direction is that the smooth tube is chosen in the calculation. It is suggested that the roll-worked enhanced tube or other enhanced tube can reduce the concentration gradient in the film thickness direction and enhance the heat and mass transfer rate.     

椭圆横管外降膜流动和换热特性分析

本研究基于VOF算法编写用户UDF(自定义函数),采用FLUENT软件建立了椭圆横管外降膜流动和换热的计算模型。根据CFD(计算流体力学)模型计算和分析了在不同长短轴比下管外降膜速度分布、压力分布、液膜内温度分布和管外换热分布的变化规律。研究结果表明:长短轴比的变化影响了管外液膜速度分布、压力分布和膜内温度分布;相比圆管,椭圆管的管外膜内液体流速更快。壁面压力沿周向逐渐减少并在X=0.9附近快速回升;随长短轴比e的增加,周向压力最小值位置逐渐向后推移。局部Nu数分布与压力分布在趋势上存在一致性。当e=1.65附近时,椭圆的换热性能最优;最后,通过对管形的研究分析,提出横管的传热分区模型。     

Numerical analysis on heat transfer enhancement by waves on falling liquid film

IntroductionLiquid films flowing on a vertical or inclined wall bythe gravitational force are encountered in the wideindustrial and engineering fields['1, such as condensatefilm, evaporating falling film, gas absorb by liquid film,etc. In the case of Indnar film flow with smooth surface,heat transfer through the liquid film is mainly carried outby conduction, and it is sufficiently explained by theNusselt's theory. On the other hand, the heat transfer isfairly enhanced for films generating su…     

Numerical study on heat and mass transfer in falling film crystallization by an integrated model

In this paper, a numerical study of the falling film crystallization process for purifying material uses an integrated model to solve unsteady coupled heat and mass transfer problems with phase change. The model divides the melt continuously coming down from the top of the tube into numbers of flow bundles according to their entering time sequences (steps). The heat and mass transfer equations in the coordinate system are integrated from the top of the tube to the bottom for each fluid bundle, so that the transient crystallizing amount and crystal concentration can be calculated along the tube, using conventional formulas for fully developed heat and mass transfer of falling film liquid between the liquid and tube wall. The concentrations on both liquid-solid sides of the interface are kept in equilibrium according to their temperature-concentration phase diagram so that the solving procedure must be iterated at each time step. The calculated results of some important parameters, such as crystallization rate and naphthalene concentration, coincide qualitatively with our experiments. Finally, several factors that influence the process are studied as a good reference for designing and operating the separator. © 1999 Scripta Technica, Heat Trans Asian Res, 28(4): 307–321, 1999     

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.     

横管降膜过程中液膜内流动和传热特性分析

横管降膜流动过程中,液膜速度和温度及其分布是影响传热传质的关键因素,由于实验研究方法的局限性,实验研究结果一般只是液膜内各参数的平均值,而液膜内部的速度和温度具体分布特性却很难得到。借助FLUENT软件,利用VOF模型研究了橫管外液膜速度和温度及其分布特性。通过建立三维数理模型,模拟研究了常温常压下,橫管外液膜无相变条件下横管液膜的传热过程,并从边界层的角度解释了液膜波动对传热过程的影响。     

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。     

制冷系统水平管降膜蒸发器换热性能的影响因素分析

在理论上对制冷系统中降膜蒸发器进行基本分析,介绍降膜蒸发器的发展背景、发展进程以及工作原理和结构,分析降膜蒸发器的蒸发管上方制冷剂分布器的分配效果和结构、液膜流动特性、制冷剂流量、蒸发器内部的管阵布置方式、蒸发管的表面特性、管束规格和几何排列、气流扰动情况和润滑油等影响因素对降膜蒸发器传热性能的影响,比较降膜蒸发器与其他蒸发器的特点,介绍降膜蒸发的某些技术难题。     

基于有限元方法的溴化锂降膜吸收传热传质的数值研究

吸收器是吸收式制冷系统的重要部件.溴化锂溶液的降膜吸收是吸收器中最常见的传质传热形式之一.通过对溴化锂溶液在降膜吸收过程中传质和传热特性的分析,使用基于有限元法的COMSOL Multiphysics软件,建立了溴化锂溶液和水蒸汽降膜吸收的物理数学模型,计算了液膜内部温度和质量分数的分布、界面处传质通量、界面处传热通量...     

Numerical investigation of effective parameters of falling film evaporation in a vertical-tube evaporator

Wastewater treatment is one of the most effective solutions to manage the problem of water scarcity. Falling film evaporators are excellent technology in wastewater treatment plants. These wastewater evaporators provide high heat transfer, short residence time in the heating zone, and high-purity distilled water. In the present study, the mechanism of turbulent falling film evaporation in a vertical tube has been investigated. A model has been developed for symmetrical two-dimensional pure and saline water flow in a vertical tube under constant wall heat flux. The numerical simulation has been carried out by a commercial computational fluid dynamics code. The evaporation of saturated liquid film is simulated utilizing a two-phase volume of fluid method and Tanasawa phase-change model. The main objective of this study is to evaluate the effects of water salinity, liquid Reynolds number, wall heat flux, and liquid film thickness on the two-phase heat transfer coefficient and vapor volume fraction. The numerical heat transfer coefficients are compared with the obtained results by Chen's empirical correlation. With a MAPE ≤ 11%, this study proves that the numerical method is highly effective at predicting the heat transfer coefficient. Moreover, the empirical coefficient of the Tanasawa model and the minimum thickness of the falling film are determined.     

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。     

水平管降膜蒸发器中不同形式的布液器对内部流场流动影响的数值模拟

应用Fluent两相流VOF模型,数值模拟水平管降膜蒸发器内部流场流动的过程。提出3种不同结构的布液器,分别对蒸发器内部流动特性进行了比较分析,得到气液两相图、速度云图、XY图等。模拟结果表明:中间开孔布液器使两部分蒸发管阵产生的蒸汽通过中间蒸汽排气口疏导出去,换热面大,使液体分配较均匀,更为合理,有利于蒸发器内部整体流场的优化设计。研究结果可为降膜蒸发器的优化设计提供理论依据。     

Numerical simulation of horizontal tube bundle falling film flow pattern transformation

It is important to study the falling-film pattern of a horizontal tube bundle in order to set up a heat and mass transfer model accurately. The falling-film pattern of a horizontal tube bundle is simulated in this paper. The technique is based on computational flow dynamics (CFD) for the two-phase flow of gas and water. The experimental results were used to validate the mathematical model. It indicates that the simulation results accord with experimental data well. The simulated results show that the flow pattern varies with different flow rates. Under the different flow rates, it observes the droplet, droplet-columnar, columnar, columnar-sheet and sheet flow patterns. The critical value is 0.0125 kg/s between droplet and columnar, and the critical value is 0.02 kg/s between columnar and sheet.     

Effect of fins on forced convection heat transfer around a tube in an aligned‐arranged tube bundle

The effect of fins on heat transfer around a tube in an aligned‐arranged tube bundle was investigated experimentally, and the obtained results were compared for three arrangements, i.e., single tube, single tube row, and staggered‐arrangement. It was found from the experiment that the effect of fins begins to appear in an aligned‐arrangement with larger fin spacing than in a staggered‐arrangement. The degradation in the local heat transfer coefficient due to fins can be recognized not only on the rear region of the tube, as observed in other arrangements, but also on the frontal region. As a result of this phenomenon, the degradation in the average heat transfer coefficient in an aligned‐arrangement becomes larger than in other arrangements with the same fin spacing. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(8): 555–563, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20091