横管降膜低温多效蒸发海水淡化系统的优化设计

随着横管降膜蒸发技术在低温多效蒸发海水淡化系统中的应用和推广,低温多效蒸发海水淡化技术以其传热系数高、热耗量小、要求供热的温位低等优点成为未来第二代海水淡化技术的主流技术.本文开展了横管降膜低温多效蒸发海水淡化系统的优化设计,比较了不同流程下低温多效蒸发海水淡化系统的热力特性,串并结合流程以其传热面积小、设备制造成本低...     

低温地热双循环式发电系统横管喷淋降膜蒸发器传热性能实验研究

以R245fa为工质,搭建有机朗肯循环（ORC）发电系统横管喷淋降膜蒸发器传热测试平台,研究有机工质喷淋密度,地热水初温及流率等因素对管外换热系数的影响。实验结果表明：随着有机工质喷淋密度、地热水初温、地热水流率的增大,传热系数均先增大后减小。最后,根据实验结果,对现有横管喷淋降膜蒸发器的管外传热系数经验公式的参数进行修正。     

竖直圆管内R113的降膜换热特性数值模拟

为开发适用于低温热源的高效降膜蒸发换热装置,本研究采用FLUENT软件对低沸点有机工质氟利昂(R113)在竖直管内汽液两相逆流降膜蒸发进行模拟研究。汽液界面捕捉选用VOF模型,并通过udf编程模拟汽液两相蒸发传热,研究了喷淋密度、热流密度及入口温度对R113降膜蒸发换热的影响。结果表明:在一定结构参数下,存在降膜换热最佳喷淋密度;在一定喷淋密度下,热流密度对降膜换热影响显著,且热流密度越高换热效果越好;随着入口温度升高,降膜换热效果削弱,且高于某温度后其对降膜换热几乎没有影响。     

R410A/R404A/R407C在T型翅内螺纹水平管外的降膜蒸发换热研究

为分析R410A、R404A、R407C在T型翅内螺纹水平强化管外降膜蒸发的换热特性,分别在变喷淋密度(0. 047～0. 113 kg/(m·s))、变蒸发温度(0～16℃)以及变热流密度(10～40 k W/m~2)条件下进行了实验,采用"Wilson"图解法以及热阻分离法进行实验数据处理,得到了3种制冷工质在管外降膜蒸发时的换热特性。结果表明:随着喷淋密度的增加,R410A、R404A和R407C 3种制冷工质的管外降膜蒸发传热系数先增加后减少,存在最佳喷淋密度,分别为0. 092、0. 088和0. 095 kg/(m·s);随着蒸发温度的升高,R410A和R404A的管外降膜蒸发传热系数先减小后增大,而R407C的管外降膜蒸发传热系数则一直在增大,但均小于R410A和R404A;随着热流密度的增加,3种制冷工质的管外降膜蒸发传热系数也随之增大,其中,R410A的换热性能最好,R404A次之,R407C最差。通过传热分析以及实验数据拟合,得到了3种制冷剂的降膜蒸发传热关联式。     

MVR降膜蒸发器液膜流动特性数值模拟

随着能源价格的上涨,蒸发浓缩过程的节能降耗工作显得非常重要。MVR技术节能潜力巨大,将成为蒸发浓缩技术发展的方向。利用传热学及流体力学的基本理论,在进行合理简化的基础上,建立了适用于降膜蒸发器竖管内蒸发过程的传热模型,模型同时考虑了重力和二次蒸汽剪切力的影响。利用VB语言开发了用于计算蒸发器降膜蒸发管流动状态、传热特性的应用软件。利用所开发的软件定量分析了液膜厚度、传热系数等参数沿蒸发管流动方向的变化规律,比较了两种模型对计算结果的影响。     

横管降膜蒸发太阳能海水淡化装置的性能分析

在分析横管降膜蒸发机理的基础上，设计了一台降膜蒸发太阳能海水淡化装置，并对影响该装置性能的因素进行了实验研究。结果表明，该装置比传统的叠盘式太阳能海水淡化装置瞬态时间短，蒸发效果好，运行温度低。保持合理的海水喷淋密度，使海水在管外降膜蒸发，可使装置在较宽的温度范围内(70～9012)运行，且具有较高的淡水产量和能源利用率(7096以上)。     

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。     

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。     

水平强化管管外R407C降膜蒸发换热特性实验研究

搭建降膜蒸发实验台,对水平布置的强化管单管外的降膜蒸发换热特性进行了实验研究。实验强化管外径为19 mm,有效长度为2 500 mm。实验采用一种新型布液器,布液采用滴淋方式,以R407C为管外降膜蒸发工质,与管内热水进行热交换,分别在变蒸发管管内流速(1、1.5、2、2.5、3m/s)、变喷淋量(0.08~0.16 kg/(m·s))、变蒸发温度(2.5~16℃)和变热流密度(15~40 k W/m~2)的情况下进行实验,得到了R407C在管外降膜蒸发时的特性:随着热流密度的增加,传热系数不断增大;随着喷淋量的增加,传热系数先增大后减小,降膜蒸发存在一个最佳喷淋量;随着蒸发温度的升高,传热系数不断增大。同时分析了强化传热的原理。     

转角正方形管束有降膜流动时的压降实验

为了深入研究大型海水淡化装置中流动阻力对装置性能的影响,建立了大型水平管束降膜流动特性实验台,模拟了水平管降膜蒸发器内蒸汽的流动过程,分析了饱和蒸汽温度和喷淋密度对管束流动阻力的影响,引入新的参数(喷淋雷诺数)对实验数据进行了拟合,得出蒸汽横掠有降膜流动的转角正方形管束的压降系数公式.结果表明:在相同的蒸汽质量流量和喷淋密度下,压降随饱和蒸汽温度的升高而降低;压差预测值与实验值的误差小于±15％.     

Evaporation heat transfer of falling water film on a horizontal tube bundle

A numerical simulation and experimental study were carried out for evaporation heat transfer of a falling water film on a smooth horizontal tube bundle evaporator. A laminar model and a turbulence model were respectively adopted to calculate the heat transfer coefficients of falling water film on horizontal heated tubes. The calculation zone on the heated tube was divided into the top stagnation zone and the lateral free film zone. The initial boundary conditions for the free film zone were determined from the calculated results of the stagnation zone. The modified wall function method was used for the turbulent flow. Comparisons between the experimental data and the numerical solutions by use of two flow models show that the experimental data lie between the laminar model solutions and the latter turbulence model solutions and that they are closer to the latter solutions. Finally, a simple dimensionless correction based on the numerical simulations is proposed for predicting the evaporation heat transfer of falling water film for actual engineering applications. © 2001 Scripta Technica, Heat Trans Asian Res, 31(1): 42–55, 2002     

Experimental Study on the Heat Transfer Characteristics of an Evaporating Falling Film on a Horizontal Plain Tube

In this paper, an experimental investigation on the heat transfer of saturated water falling film on a single horizontal plain tube is presented. The water film falling on the outside of the tube has been heated by the condensing steam flowing in the tube, and the heat transfer coefficient between the water film and the steam has been measured. Experiments were performed at saturation temperatures of liquid film and steam as 58°C and 61°C, and 61°C and 65°C, a tube pitch of 57.16 mm, heat fluxes from 10 to 50 kW m^-2, and film flow rate per unit of length of the tube up to 0.12 kg m^?1 s^?1. Brass plain tubes with external diameters of 25.4 mm and lengths of 950 mm were used in the experiments. The experimental results show that the heat transfer coefficient increases with the increasing film flow rate and heat flux, and the quality of vapor has an obvious influence on the heat transfer performance of falling film evaporation. The coupling of condensation and evaporation heat transfer inside and outside the tube is investigated qualitatively in this paper.     

Falling film evaporation heat transfer of water/salt mixtures from roll-worked enhanced tubes and tube bundle

An experimental study is carried out for enhancement of falling film evaporation heat transfer of pure water and water/salt mixtures on horizontal smooth tube and two kinds of structured tube bundles under atmospheric pressure. The experimental results show that the low-cost roll-worked tube can greatly enhance the evaporation heat transfer performance of the falling film, and make it comparable to that of expensive commercial enhanced tubes such as GEWA-T tubes, TE tubes and HF tubes, even at low and moderate heat flux levels. The average evaporation heat transfer coefficients for the roll-worked tube bundle are basically independent from the parameters tested such as flow and heating conditions, salt-concentrations, as well as geometries of the tube bundles. The present experimental data result in a constant heat transfer coefficient; α≈20 kW/m² K, in the convective heat transfer range of the heat fluxes <10⁵ W/m².     

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。     

Experience on horizontal‐tube falling‐film evaporation simulation by lattice‐Boltzmann method

Complex heat and mass transfers through falling‐film or spray‐film evaporation are widely used in chemical, refrigeration, petroleum refining, desalination, and food industries. Considering that microscopic effects, like surface tension, flow, mass, and heat transfers, are interdependent phenomena, the high‐precision simulation of falling‐film evaporation through a mesoscopic method is of great importance. In the current study, the lattice‐Boltzmann method and the phase‐field model with a proper source term are used for evaporation simulation in a horizontal‐tube falling film. Here, the curvature of the tube is captured by appropriate boundary conditions. Nondimensional numbers and the geometry of the model are determined in a range of practical values. By comparing the film thickness, mass, and heat transfer with valid references in the literature, an acceptable agreement is observed, which reveals the effectiveness of this method in understanding the details and predictions. Overall, the time evolution of temperature contours and streamlines during falling‐film evaporation approves the superiority of this method in keeping details along with lower difficulty and cost compared with the classical methods.     

Heat Transfer and Crisis Phenomena at the Film Flows of Freon Mixture over Vertical Structured Surfaces

Results on experimental investigation of heat transfer in the liquid films dichlorofluoromethane R21 and dichlorotetrafluoroethane R114 Freon mixture over the vertical tubes are presented. We have studied the film flow over the outer surface of tubes with 50-mm diameter and different configurations: smooth surface, horizontal ribs, and diamond-shape knurling. Heat transfer coefficients were measured under the conditions of evaporation and nucleate boiling together with wave characteristics of the falling film, binary mixture composition, and critical heat fluxes corresponding to dry spots formation. The film Reynolds number at the inlet to the test section was varied from 15 to 250. At evaporation regime the heat transfer coefficient for a smooth surface decreases classically with an increase of Reynolds number. Dependence of heat transfer coefficient on irrigation density for the surface with diamond-shape knurling is similar to dependence for the smooth surface with insignificant heat transfer intensification. The heat transfer coefficients at nucleate boiling for the studied structured surfaces are close to those obtained for the smooth tube. Development of critical phenomena is determined by regularities of dry spots formation typical for evaporation of the wavy liquid film.     

波纹管降膜蒸发器传热性能数值模拟

为了理解波纹管降膜蒸发过程中涉及的液膜传热过程,本文采用VOF法建立了二维气-液两相分层流动CFD模型,考虑了液相流量,传热温差,蒸发温度,液相粘度等参数对传热效果的影响,根据模拟结果给出了波纹管降膜蒸发器的流量可操作范围。模拟结果和实验数据比较吻合。     

Cross Vapor Stream Effect on Falling Film Evaporation in Horizontal Tube Bundle Using R134a

The falling film evaporation of R134a with nucleate boiling outside a triangular-pitch (2-3-2-3) tube bundle is experimentally investigated, and the effects of saturation temperature, film flow rate and heat flux on heat transfer performance are studied. To study the effect of cross vapor stream on the falling film evaporation, a novel test section is designed, including the tube bundle, liquid and extra vapor distributors. The measurements without extra vapor are conducted at the saturation temperature of 6, 10 and 16°C, film Reynolds number of 220 to 2650, and heat flux of 20 to 60 kWm^?2. Cross vapor stream effect experiments are operated at three heat fluxes 20, 30, and 40 kWm^?2 and two film flow rates of 0.035 and 0.07 kgm^?1s^?1, and the vapor velocity at the smallest clearance in the tube bundle varies from 0 to 2.4 ms^?1. The results indicate that: film flow rate, heat flux and saturation temperature significantly influence the heat transfer; the cross vapor stream either promote or inhibit the falling film evaporation, depending on the tube position, film flow rate, heat flux and vapor velocity.     

Falling water film evaporation on newly-designed enhanced tube bundles

Experimental studies are presented on falling film evaporation of water on 6-row horizontal enhanced tube bundles in a vacuum condition. Turbo-CAB (19 fpi and 26 fpi), Korodense, and smooth tubes were tested in a range of film Reynolds number from about 10 to 110 and in the condition of only convective evaporation, without nucleate boiling. The flow modes and heat flux will affect the transition Reynolds numbers. Tubes with enhanced inner surface provide better heat transfer performance. Hotter heating water may lead to better heat transfer performance mainly due to higher heat fluxes. Correlations were also derived to predict the heat transfer coefficients and the enhancement ratio.