Film Condensation of R-134a and R-236fa,Part 2: Experimental Results and Predictive Correlation for Bundle Condensation on Enhanced Tubes

Local test results for two enhanced condensing tubes (next-generation versions of the Wieland Gewa and Wolverine Turbo enhanced condensing tubes) are obtained for refrigerants R-134a and R-236fa on the center row of a three row-wide tube bundle. The “bundle effect” on the heat transfer performance of the test section is observed and described. New predictive methods for falling film condensation on bundles are proposed, based on a modification of the previous vertical single-row method with condensate slinging. The modifications performed to the experimental setup to allow for bundle tests are described. For two types of enhanced tubes and two refrigerants, the local heat flux is correlated as a function of condensation temperature difference, the film Reynolds number, the tube spacing, and liquid slinging effect.     

Effect of vapour velocity on film condensation of R-113 on horizontal tubes in a crossflow

Film condensation of downward flowing R-113 vapour at near atmospheric pressure on single horizontal tubes was studied experimentally over wide ranges of vapour velocity and condensation temperature difference. The flow of condensate was visualized by injecting a dye tracer. Three flow regimes : smooth surface, two-dimensional waves and three-dimensional waves, were observed. In the last flow regime an abrupt thickening of the condensate film was seen at an angular position about 1.75 rad from the tube top. Turbulent mixing of condensate was observed in the thick film region. The present and earlier heat transfer results for R-113 and R-21 were compared with the laminar two-phase boundary-layer theory. The point at deviation from the theoretical prediction was found to be dependent on a dimensionless number which gave a transition criterion between smooth and wavy condensate surfaces. A correlation equation for the average heat transfer coefficient is proposed, where an equivalent Reynolds number is introduced for the high vapour velocity region.     

Recent Progress in Enhancing Film Condensation Heat Transfer on Horizontal Tubes

An assessment of several promising techniques that use surface tension forces to enhance film condensation heat transfer on horizontal tubes is made. Recent progress on integral-fin tubes is stressed, including experimental findings on fin spacing, geometry, and material and the latest theoretical developments for predicting performance. Condensation heat transfer enhancement on wire-wrapped tubes is also examined, as well as the use of nonwetting strips and porous drainage devices. The effect of condensate inundation on plain and enhanced tubes is reviewed briefly, and future research directions are discussed.     

Condensation of R-113 on Pin-Fin Tubes: Effect of Circumferential Pin Thickness and Spacing

New experimental data are reported for condensation of R-113 at near atmospheric pressure and low velocity on five three-dimensional pin-fin tubes. The only geometric parameters varied were circumferential spacing and thickness, since these have been shown to have a strong effect on condensate retention on pin-fin tubes. Heat transfer enhancement was found to be strongly dependent on the active-area enhancement, i.e., on the parts of the tube and pin surface not covered by condensate retained by surface tension. For all the tubes, vapor-side heat transfer enhancements were found to be approximately 2.5 times the corresponding active-area enhancements, and this finding was in line with earlier data for R-113. An increase in the vapor-side heat transfer enhancement is noticed with the decreasing values of pin spacing. The best performing pin-fin tube gave a heat transfer enhancement about 14% higher than the “equivalent” two-dimensional integral-fin tube (i.e., with the same fin root diameter, longitudinal fin spacing, and thickness and fin height).     

Condensation of R-134a on horizontal integral-fin titanium tubes

Experimental research was conducted to evaluate the condensation of R-134a on horizontal smooth and integral-fin (32 fpi) titanium tubes of 19.05 mm outer diameter. Experiments were carried out at saturation temperatures of 30, 40 and 50 °C and wall subcoolings from 0.5 to 9 °C. The results show that the condensation heat transfer coefficients (HTCs) on the smooth tubes are well predicted by the Nusselt theory with an average error of +2.38% and within a deviation between +0.13% and +5.42%. The enhancement factors provided by the integral-fin tubes on the overall condensation HTCs range between 3.09–3.94, 3.27–4 and 3.54–4.1 for the condensation temperatures of 30, 40 and 50 °C, respectively. The enhancement factors increase by increasing the wall subcooling and with the rise of the condensing temperature. The condensate flooded fraction of the integral-fin tubes perimeter varies from 25% to 20% at saturation temperatures of 30 °C and 50 °C, respectively. The correlation reported by Kang et al. (2007) [1] predicted the experimental data with a mean deviation of ?5.5%.     

水平螺旋管外冷凝换热的理论分析与实验研究

用改进的Nusselt—Rohsenow方法分析了水平螺旋管外的层流膜状冷凝换热。考虑了粘性和重力的影响以及离心力对液膜流动和换热的影响。通过分析,得到了较通用的无量纲冷凝方程,并导出适合于其它几何形状换热元件的冷凝换热方程。经数值计算得到了在不同条件下的局部Nusselt数(Nu)和平均Nusselt数(?)。理论结果得到了实验数据的验证,并推荐了实用的传热计算公式。     

电场对管内凝结液膜厚度的影响

以R123为工质,对圆柱形电极进行了电场强化管内凝结换热的试验研究,提出并计算了当量液膜厚度。试验换热管为垂直套管式,外层为冷却水路,内层为工质回路。电压范围为0～30 kV,热流密度范围为4～6 kW/m2。实验观察到了凝结液膜的减薄,在给定的实验条件下,当电压超过5 kV后,随着电压的增加,凝结换热系数增加,当量液膜厚度减小。实验结果表明,液膜厚度的减薄是电场强化凝结换热的主要原因。     

Condensation in a vertical tube bundle passive condenser – Part 2: Complete condensation

An experimental study of the tube bundle effect on heat removal capabilities in complete condensation mode of a passive condenser was performed. A full scale test section, with four condenser tubes, was designed and constructed to simulate operating conditions of a passive containment cooling system. For complete condensation analysis, pure steam was supplied to the test section and heat transfer properties were measured for pressure from 100 to 280 kPa. The condensation heat transfer results were similar to the findings from single tubes, except for a slightly higher condensate mass flux. This was determined to be a result of turbulent mixing in the secondary boiling water caused by the tube bundle.     

Extensive study on laminar free film condensation from vapor–gas mixture

The dimensionless velocity component method was successfully applied in a depth investigation of laminar free film condensation from a vapor–gas mixture, and the complete similarity transformation of its system of governing partial differential equations was conducted. The set of dimensionless variables of the transformed mathematical model greatly facilitates the analysis and calculation of the velocity, temperature and concentration fields, and heat and mass transfer of the film condensation from the vapor–gas mixture. Meanwhile, three difficult points of analysis related to the reliable analysis and calculation of heat and mass transfer for the film condensation from the vapor–gas mixture were overcome. They include: (i) correct determination of the interfacial vapor condensate saturated temperature; (ii) reliable treatment of the concentration-dependent densities of vapor–gas mixture, and (iii) rigorously satisfying the whole set of physical matching conditions at the liquid–vapor interface. Furthermore, the critical bulk vapor mass fraction for condensation was proposed, and evaluated for the film condensation from the water vapor–air mixture, and the useful methods in treatment of temperature-dependent physical properties of liquids and gases were applied. With these elements in place, the reliable results on analysis and calculation of heat and mass transfer of the film condensation from the vapor–gas mixture were achieved.The laminar free film condensation of water vapor in the presence of air was taken as an example for the numerical calculation. It was confirmed that the presence of the non-condensable gas is a decisive factor in decreasing the heat and mass transfer of the film condensation. It was demonstrated that an increase of the bulk gas mass fraction has the following impacts: an expedited decline in the interfacial vapor condensate saturation temperature; an expedited decrease in the condensate liquid film thickness, the condensate liquid velocity, and the condensate heat and mass transfer. It was found that an increase of the wall temperature will increase the negative effect of the non-condensable gas on heat and mass transfer of the film condensation from the vapor–gas mixture.     

Condensation of organic binary mixtures flowing horizontally in a horizontal tube bundle

Both heat and mass transfer in the gas phase and heat transfer in the liquid phase are examined experimentally for film condensation of organic binary mixtures such as ethanol-water and methanol-water. Experimental results on the average heat flux, vapor-liquid interface temperature and liquid-phase Nusselt number are compared with analytical solutions based on stagnant film theory and heat-transfer relationships for film condensation from a pure vapor. Experimental heat transfer results agree well with the analytical solutions, except that the experimental liquid-phase Nusselt numbers under conditions of low mass fraction of water are considerably higher than predicted by the analytical solutions. This high value of the liquid-phase Nusselt number is considered to be caused by dropwise condensation in the liquid phase. However, its effect on the tube bundle is not so remarkable compared with that in gravity-controlled condensation on a vertical surface. This is considered to be caused by the condensate inundation effect. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25(6): 342–361, 1996     

Condensation Heat Transfer Inside a Tube in a Microgravity Environment

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Effects of surface roughness and tube materials on the filmwise condensation heat transfer coefficient at low heat transfer rates

The laminar filmwise condensation heat transfer coefficient on the horizontal tubes of copper and stainless steel was investigated. The outside diameter of the tubes was 15.88 mm, and the tube thickness ranged from 1.07 to 1.6 mm. The polished stainless steel tube had an RMS surface roughness of 0.37 μm, and commercial stainless steel tubes had maximum surface roughness of 15 μm. The tests were conducted at saturation temperatures of 20 and 30 °C, and liquid wall subcoolings from 0.4 to 2.1 °C. The measured condensation heat transfer coefficients were significantly lower than the predicted data by the Nusselt analysis when the ratio of the condensate liquid film thickness to the surface roughness, δ / R_p–v, was relatively low. When the condensate liquid film was very thin, tube material affected the condensation heat transfer coefficient in the filmwise condensation.     

Multi-Objective Optimization of R-404A Vapor Condensation in Swirling Flow Using Genetic Algorithms

Applying twisted tape inserts as a passive improvement technique increases both pressure drop and heat transfer coefficient. In the design of heat exchangers, decreasing of pressure drop and increasing of heat transfer coefficient simultaneously comprise an important aim. In this study, multi-objective optimization is used to find optimum combinations of heat transfer coefficient and pressure drop during condensation of R404A vapor inside twisted-tape-inserted tubes. At first, Pareto-based multi-objective optimization is used to find the proper artificial neural networks based on the experimental data for prediction of heat transfer coefficient and pressure drop. In the next step, Pareto-based multi-objective optimization and previously obtained artificial neural networks are used to find optimal operation conditions that lead to optimum combinations of heat transfer coefficient and pressure drop. The corresponding optimal set of design variables, namely, mass velocity, vapor quality, and dimensional parameters of tubes, show the important design aspects.     

Condensation heat transfer studies for stratified,cocurrent two-phase flow in horizontal tubes

Condensation heat transfer inside horizontal tubes is investigated for a stratified, cocurrent two-phase flow of vapor and liquid. The analysis takes into account the effects of interfacial shear, axial pressure gradient, saturation temperature level, driving temperature difference and the development of the stratified angle associated with the accumulated condensate layer at the bottom of the tube. The influence of these parameters is evaluated with respect to the peripheral condensate film heat transfer performance for the practically interesting laminar flow range of operating conditions of water-vapor flow. A theoretical predictive method is developed to obtain the overall heat transfer coefficient along the tube length. Results of the theoretical predictions are found to agree favorably with the reported experimental data which cover a variety of fluids with a relatively wide range of operating conditions. A simple, predictive heat transfer coefficient correlation is proposed from the numerical solution by means of regression analysis.     

Temperatures and the condensate heat resistance in dropwise condensation of multicomponent mixtures with inert gases

The temperature variations occurring in dropwise condensation at condenser plates of a compact, polymer heat exchanger are studied using instantaneous infrared temperature field recordings. An averaging procedure in time and an assessment of extreme values is proposed and carried out. With the results, the heat resistance of the condensate is quantified. It is found that mixing and convection in the condensate, caused by coalescence and drainage of drops, reduces the condensate heat resistance by a factor 4 as compared with purely conductive heat transfer. This reduction is comparable, both in nature and in magnitude, to the effect of enhanced mixing due to turbulence in the liquid film of filmwise condensation. A second condensable species has been added to the gas mixture in order to study the contribution of Marangoni convection due to concentration gradients to the condensate heat transfer resistance. No contribution is found.     

制冷剂R-134a在螺旋环形通道内凝结换热的实验研究

制冷剂R-134a在螺旋管内的凝结换热和压力降特性数据对于制冷空调和热泵等系统的设计改造及运行都具有重要的理论意义和工程应用价值。本文对R-134a在螺旋环形通道内的凝结换热和压力降特性进行了实验研究．得到了平均的凝结换热系数和压力降特性的实验数据,并与文献报导的R-134a在直管和螺旋管内凝结换热的实验结果进行了比较,所得实验数据可望为新型螺旋管换热器的开发设计和工程应用提供参考数据。     

Unified Modeling Suite for Two-Phase Flow,Convective Boiling,and Condensation in Macro- and Microchannels

This paper focuses on the unified modeling suite for annular flow that the authors have developed and continue to develop. The annular flow suite currently includes models to predict the void fraction, the entrained liquid fraction, and the wall shear stress and pressure gradient, and a turbulence model for momentum and heat transport inside the annular liquid film. The turbulence model, in particular, allows prediction of the local average liquid film thicknesses and the local heat transfer coefficients during convective evaporation and condensation. The benefit of a unified modeling suite is that all the included prediction methods are consistently formulated and are proven to work well together, and provide a platform for continued advancement based on the other models in the suite. First the unified suite of methods is presented, illustrating in particular the most recent updates. Then results for the pressure drop and the heat transfer coefficient during convective evaporation and condensation are presented and discussed, covering both water and refrigerants flowing through circular tubes and noncircular multi-microchannel configurations for microelectronics cooling.     

Numerical heat transfer analysis of laminar film condensation on a vertical fluted tube

The purpose of this study is to find a convenient and practical procedure for calculating heat transfer of laminar film condensation on a vertical fluted tube. The condensate film on the tube surface along the axial direction was divided into two portions: the initial portion and the developing portion. The developing portion was analyzed in details. The film thickness equation of condensate film over the crest and the momentum equation of condensate film in the trough were established respectively after some simplifications and coupled with two-dimensional thermal conduction equation. The relationship between the heat transfer rate and the length of the flute was obtained through solving the equations numerically. The present procedure was tested on a sinusoidal fluted tube. The amount of heat transfer rates Q_t of the tube were calculated at different temperature differences by using this procedure. The calculation was compared with the experimental data quoted. The results were in good agreement with a maximum deviation of 18%. So the present procedure is reliable and can be used in the parameter design of sinusoidal fluted tubes.     

Dropwise Condensation Heat Transfer on Plasma-Ion-Implanted Small Horizontal Tube Bundles

Stable dropwise condensation of saturated steam was achieved on stainless-steel tube bundles implanted with nitrogen ions by plasma ion implantation. For the investigation of the condensation heat transfer enhancement by plasma ion implantation, a condenser was constructed in order to measure the heat flow and the overall heat transfer coefficient for the condensation of steam on the outside surface of tube bundles. For a horizontal tube bundle of nine tubes implanted with a nitrogen ion dose of 10¹⁶ cm^{? 2}, the enhancement ratio, which represents the ratio of the overall heat transfer coefficient of the implanted tube bundle to that of the unimplanted one, was found to be 1.12 for a cooling-water Reynolds number of about 21,000. The heat flow and the overall heat transfer coefficient were increased by increasing the steam pressure. The maximum overall heat transfer coefficient of 2.22 kW · m^?2· K^?1 was measured at a steam pressure of 2 bar and a cooling-water Reynolds number of about 2,000. At these conditions, more dropwise condensation was formed on the upper tube rows, while the lowest row received more condensate, which converted the condensation form to filmwise condensation.     

Non-iterative condensation modeling for steam condensation with non-condensable gas in a vertical tube

Based on a heat and mass transfer analogy, an iterative condensation model for steam condensation in the presence of a non-condensable gas in a vertical tube is proposed including the high mass transfer effect, entrance effect, and interfacial waviness effect on condensation. A non-iterative condensation model is proposed for easy engineering application using the iterative condensation model and the assumption of the same profile of the steam mass fraction as that of the gas temperature in the gas film boundary layer. It turns out that the Nusselt number for condensation heat transfer is expressed in terms of air mass fraction, Jakob number, Stanton number for mass transfer, gas mixture Reynolds number, gas Prandtl number and condensate film Nusselt number. The comparison shows that the non-iterative condensation model reasonably well predicts the experimental data of Park, Siddique, and Kuhn.