Measurement of Falling Film Thickness Around a Horizontal Tube Using a Laser Measurement Technique

A new optical method for the non-intrusive measurement of falling film thickness on the perimeter of horizontal tubes is described. The technique uses a high-speed digital video camera to capture the images of the liquid interface that are illuminated by a laser sheet, with the contrast enhanced by a fluorescent powder in the fluid. The results are compared to those predicted by the Nusselt falling film theory, showing relatively good agreement around the upper perimeter of the tube but much poorer agreement on the lower perimeter. The corresponding effects on heat transfer have also been estimated.     

Falling Liquid Film Evaporation in Subcooled and Saturated Water Over Horizontal Heated Tubes

ABSTRACT

This paper presents an experimental study of heat transfer and film thickness behavior in falling liquid water film evaporation technology over horizontal tubes. Liquid distribution systems have also been evaluated. The experimental setup consisted of two horizontal 0.019-m OD stainless-steel tubes, 0.194 m in length. Reynolds numbers in the 160–940 range were tested in both subcooled and saturated liquid regimes. For the liquid distribution system study, several distributor geometries were tested in order to develop the least disturbed film over the tubes. An intrusive method was used for measuring the liquid film thickness in the laminar regime and the measured values were compared with the theoretical prediction computed from the Nusselt equation. An experimental heat transfer correlation was obtained and compared with previous ones obtained by other authors. In addition, the local heat transfer coefficient was observed to be always higher at the horizontal tube top region for all operational conditions (on the order of 14 kW/m²-°C). Finally, the use of a liquid storage distribution system along with the installation of a wire mesh to obtain an uniform liquid distribution for all Reynolds numbers tested.     

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 numbe...     

环形布膜器垂直管内R113降膜蒸发换热特性数值研究

通过建立垂直管内降膜蒸发物理数学模型,对环形插头型布膜器管内R113的气液两相逆流降膜蒸发换热特性进行二维非稳态数值研究,分析了管内液膜流动分布以及壁面温度和液膜表面温度分布,对比了加热前后液膜厚度的变化.结果表明:随着降膜蒸发过程的进行,液膜下端开始出现液滴飞溅,且不断向上端发展;R113在管内降膜蒸发过程中壁面温度和液膜表面温度沿流动方向逐渐升高,气相温度变化趋势则相反;从壁面到管中心,温度沿径向逐步降低,在近壁面1mm前后其分布趋势相反;加热后液膜厚度明显减小,且下游液膜厚度变得相对均匀.     

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.     

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。     

倾斜降膜蒸发特性实验研究

以电加热作为供热热源来模拟太阳能,研究了不同工况下倾斜降膜蒸发特性,通过对蒸馏器吸热面和冷凝面划分等间距小区段,根据液膜和冷凝面的温度分布,利用Dunkle模型预测了蒸馏器的产水速率.结果表明:热流密度、单位长度给水质量流量、倾斜角度和单位长度冷却水质量流量是影响蒸馏器产水速率的主要因素;产水速率随着热流密度的增大呈线性增加;在单位长度给水质量流量为5.5～10.0kg/(h.m)时,产水速率随着单位长度给水质量流量的减小呈线性增大,单位长度给水质量流量为0.7～5.5kg/(h.m)时,产水速率波动较小;在倾斜角度为15°～60°时,产水速率随着倾斜角度的增大而增大;冷却水均匀地流过冷凝面上表面有助于增大蒸馏器的产水速率;蒸馏器吸热面和冷凝面划分的区段越多,模型预测值与实验值吻合越好.     

Forced Convection in Film Condensation on a Horizontal Elliptical Tube

The aim of the present study is to investigate film condensation on an isothermal elliptical tube employing the model of Sarma et al. (1998). The study also considers the transition from laminar to turbulent film. The paper then presents analytical analysis for the local dimensionless film thickness and heat transfer characteristics for different degrees of tube eccentricity. Finally, the results developed in this study are compared with those generated by previous theoretical and experimental research. It is noted that the correlation between the two sets of data is quite satisfactory.     

降膜蒸发过程的传热性能研究

对圆管和平壁的降膜蒸发过程的传热机理及影响因素进行了分析,并对已有的实验结果进行了比较;已有的实验研究主要是围绕圆管或平壁来进行,不利于找出最佳加热元件;提出在平壁表面加圆管型肋的复合加热元件,对其进行研究有利于找出最佳加热元件,对提高降膜蒸发传热效率极具实用价值。     

Experimental Study on Pool Boiling Heat Transfer for R22, R407c,and R410a on a Horizontal Tube Bundle With Enhanced Tubes

An experimental test rig for study of the pooling-boiling heat transfer performance of pure and mixed refrigerants was designed and established. The test section is a horizontal tube bundle evaporator with nine mechanically fabricated porous surface tubes in a triangular layout. With this test system, the heat transfer coefficients of the nucleate boiling in the evaporator were measured for R22, R407c, and R410a. Extensive experimental measures were made for those pure and mixed refrigerants at different heat fluxes from 10 kW m^?2 to 43 kW m^?2 at saturation temperature of 9°C. Comprehensive measured data are presented in this paper. From experimental results, it is found that the pool boiling heat transfer coefficient increases with increasing the heat flux. It is also found that boiling heat transfer coefficients for R410a are 1.25–1.81 times and 6.33–7.02 times higher than that for R22 and R407c, respectively. The experimental correlations for the pool boiling heat transfer coefficients of R22, R407c, and R410a on the present enhanced tubes bundle are developed. The thermal resistance analysis reveals that the thermal resistance of the water side is a controlling factor for the evaporator for R22 and R410a. However, for R407c, the thermal resistance of the refrigerant side is slightly higher than that of the water side. To further improve the overall heat transfer coefficient in the evaporator of R22 and R410a, the enhancement for both the inside and outside is equally important, and the effectively enhanced boiling surface must be developed for the evaporator of R407c.     

Curvature Ratio Effect on Two-Phase Pressure Drops in Horizontal Return Bends: Experimental Data for R134a

This article presents 154 pressure drop data points measured during two-phase flow of R-134a in horizontal return bends. The tube diameter is constant at 10.85 mm and the curvature ratio is either 7.74 or 5.53. Saturation temperature varies from 15 to 20°C, vapor quality from 0.05 and 0.95, and mass velocity ranges from 300 to 600 kg m^?2 s^?1. Return bend pressure drops are calculated by subtracting the straight tube pressure drop from the total measured pressure drop along the bend. The perturbations induced up- and downstream of the singularity are taken into account in the measurements. The comparison of the pressure drops for the two configurations (curvature ratio of 5.53 and 7.74) showed that they are greater (about 10%) for the larger curvature ratio. This can be attributed to the effect of the developed length on the pressure drop; on the other side the pressure gradients are larger for the lower curvature ratio, which can be explained by the effect of the centrifugal force and the perturbations up- and downstream of the return bend. The experimental data are compared against four prediction methods available in the literature. The Domanski and Hermès correlation is the best at predicting the present data.     

Performance Evaluation of R134a/DMF-Based Vapor Absorption Refrigeration System

This article describes an experimental investigation to measure performances of a vapor absorption refrigeration system of 1 ton of refrigeration capacity employing tetrafluoro ethane (R134a)/dimethyl formamide (DMF). Plate heat exchangers are used as system components for evaporator, condenser, absorber, generator, and solution heat exchanger. The bubble absorption principle is employed in the absorber. Hot water is used as a heat source to supply heat to the generator. Effects of operating parameters such as generator, condenser, and evaporator temperatures on system performance are investigated. System performance was compared with theoretically simulated performance. It was found that circulation ratio is lower at high generator and evaporator temperatures, whereas it is higher at higher condenser temperatures. The coefficient of performance is higher at high generator and evaporator temperatures, whereas it is lower at higher condenser temperatures. Experimental results indicate that with addition of a rectifier as well as improvement of vapor separation in the generator storage tank, the R134a/DMF-based vapor absorption refrigeration system with plate heat exchangers could be very competitive for applications ranging from –10°C to 10°C, with heat source temperature in the range of 80°C to 90°C and with cooling water as coolant for the absorber and condenser in a temperature range of 20°C to 35°C.     

Condensation of R-134a Vapor over Single Horizontal Circular Integral-Fin Tubes with Trapezoidal Fins

An experimental investigation has been carried out for the condensation of R-134a vapor on four single horizontal circular integral-fin tubes (CIFTs) made of copper. The fins are of trapezoidal shape and the fin density of these tubes has been varied systematically from 934 fpm to 1,875 fpm. All the experimental data have been acquired at the saturation temperature of 312.4 +/- 0.5 K. The CIFT with 1,560 fpm fin density has been found to be the best-performing tube, as it has provided an enhancement factor (EF) of the order of 5.6. The experimental results are in the best agreement with Honda and Nozu model. This model has predicted 80% of the experimental results in a range of 0-30%.     

竖直螺旋槽管壁面液膜在蒸发/冷凝时的传热特性的研究

研究竖直螺旋槽管壁面液膜在传热条件下的液膜形成及流动特性,建立了单组分流体的物理和数学模型并得出解析解,且分析了壁面液膜在蒸发,冷凝及无热传输时的液膜厚度分布及速度分布,结果表明,液膜的形状主要受表面张力影响,在表面内弯处流膜较厚,而在槽道起始部液膜较薄,相对于光滑直管,竖直螺旋槽管壁面液膜具有均匀的厚度分布和更好的传热传质性质,特别在冷凝时壁面液膜更薄且分布更加均匀。     

Experimental Investigation on Ejector Performance Using R134a as Refrigerant

Ejector refrigeration has the advantage of low capital cost, simple design, reliable operation, long lifespan and almost no maintenance. The only weakness is the low efficiency and its intolerance to deviations from design operation condition. R134 a used in ejector refrigeration system gives better performance in comparison with many other environmental friendly refrigerants as the generation temperature is from 75°C to 80°C. The present work experimentally investigated the on-design and off-design performance of the ejector with fixed geometry using R134 a as refrigerant, and cycle performance of the ejector refrigeration system. The experimental prototype was constructed and the effects of primary flow inlet pressure, secondary flow inlet pressure and ejector back pressure on ejector performance and cycle performance were investigated respectively. The operation conditions are: primary flow inlet pressure from 2.2 MPa to 3.25 MPa, secondary flow inlet pressure from 0.36 MPa to 0.51 MPa, ejector back pressure from 0.45 MPa to 0.67 MPa. Conclusions were drawn from the experimental results, and the experimental data can be used for validation of theoretical model for both critical and subcritical mode.     

基于水平圆管外加装排液板降膜流动的数值模拟

为了研究排液板对水平圆管外降膜流动与传热的影响,建立了水平圆管底部加装排液板的物理模型,采用流体体积函数(VOF)模型对其管外降膜流动进行了数值模拟,并将数值模拟结果与文献中的实验数据进行了对比.结果表明:加装排液板的水平圆管的平均液膜厚度比未加装排液板的水平圆管薄,管壁局部Nu大,排液板起到加速排液及减薄液膜的作用,有利于强化传热;排液板高度越大,管外同一周向角位置处的液膜厚度越薄,管壁局部Nu越大;排液板厚度较小或者较大都不能有效发挥加速排液的作用.     

The Effect of the Electrohydrodynamic on the Two-Phase Flow Pressure Drop of R-134a during Evaporation inside Horizontal Smooth and Micro-Fin Tubes

This article concerns the pressure drop caused by using the electrohydrodynamic (EHD) technique during evaporation of pure R-134a inside smooth and micro-fin tubes. The test section is a counter-flow concentric tube-in-tube heat exchanger where R-134a flows inside the inner tube and hot water flows in the annulus. A smooth tube and micro-fin tube having an inner diameter of 8.12 mm and 8.92 mm, respectively, are used as an inner tube. The length of the inner tube is 2.50 m. The outer tube is a smooth copper tube having an inner diameter of 21.2 mm. The electrode, which is a cylindrical stainless steel wire having diameter of 1.47 mm, is placed in the center of the inner tube. The electrical field is established by connecting a DC high voltage power supply of 2.5 kV to the electrode while the inner tube is grounded. Experiments are conducted at saturation temperatures of 10–20°C, mass fluxes of 200–600 kg/m²s, and heat fluxes of 10–20 kW/m². The experimental results indicate that the application of EHD introduces a small pressure drop penalty. New correlations for the pressure drop are proposed for practical applications.     

混合工质R134a/R23替代R22的可行性研究

叙述了基于新型环保型混合制冷剂R134a/R23替代制冷剂R22的问题,以及通过REFPROP7.5,对混合工质R134a/R23从物性和热力学特性进行的理论计算分析,指出,由质量分数为70%的R134a和质量分数为30%的R23组成的混合制冷剂与R22性能最为接近,在变工况运行条件下,其COP值比R22高8%左右,其冷凝压力比同条件下用R22作为循环工质低21%～36%,理论上完全具有替代R22的可能性。     

Impact of Tube Surface Properties on Crystallization Fouling in Falling Film Evaporators for Seawater Desalination

ABSTRACT

Crystallization fouling on heat transfer surfaces is a severe problem and a complex phenomenon in multiple-effect distillation plants with horizontal tube falling film evaporators for seawater desalination. The choice of tube material affects the wettability, the adhesion forces between surface and deposit, and the induction time of crystallization fouling. The effects of surface properties on crystallization fouling from seawater have been investigated in a horizontal tube falling film evaporator in pilot plant scale. Experiments were performed with artificial seawater and various tube materials. The tube surfaces were characterized by measuring surface roughness and contact angles and by determining surface free energies. The tube materials show qualitative and quantitative differences with respect to scale formation. The interfacial defect model was applied to the system. Spreading coefficients of CaCO₃ scale on the aluminum alloys 5052 and 6060 and stainless steel grade 1.4565 were calculated to be higher than those on copper–nickel 90/10 and aluminum brass, but the quantities of CaCO₃ scale measured on the tube surfaces were much lower compared to CuNi 90/10 and aluminum brass. The application of advanced approaches such as the interfacial defect model depends on the precise knowledge of interfacial free energies, which are very difficult to find. However, results suggest that more similar values of the interfacial free energies of heat transfer surface and deposit lead to increased scale formation.     

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％.