Liquid film and droplet flow behaviour and heat transfer characteristics of herringbone microfin tubes

Cross-sectional liquid flow rate distribution of vapour liquid two phase flow of R123 in different herringbone microfin tubes has been measured. Droplet and liquid film flow rates are calculated with the measured data and assumptions for droplet distribution and slip ratio. Heat transfer coefficients of evaporation and condensation in herringbone microfin tubes have been measured using R22. Heat transfer enhancement mechanism by the herringbone microfins is discussed by using the measured data and numerically obtained cross-sectional flow field of a single phase flow. Flow rate of thin liquid film flowing on tube sides is affected by the helix angle and fin height. Larger helix angle and higher fin give thinner film. Liquid film flow rates in tube top and bottom are higher than tube sides. Droplet flow rate is increased with increase of helix angle and fin height, although the effect of fin height is not as pronounced as helix angle. Droplet radial mass velocity to tube side walls is increased with helix angle.     

Experimental investigation of a cooled thick fin in dry and humid air flows

The results of a study are reported in which a cooled, thick vertical fin was tested in a closed loop tunnel with and without condensation from the air flowing over it. In particular, the temperature distributions for the dry and wet fin cases, together with the condensate film thickness in the wet fin case, were investigated. From a flow visualization investigation, it was found that the boundary layer separates at the leading edge, resulting in a higher air heat transfer coefficient. The wet fin test results also indicated that the mode of condensation was dependent on fin surface characteristics and that the wet fin performance was governed by the air flow parameter. Within the laminar air flow range, the condensate film flowed downward under the action of gravity. However, at higher air velocities, both gravity and shear forces affected the condensate flow, a variation in the condensate film in the direction of air flow being noticed.     

Condensation on curvilinear fins (effect of groove flooding): EMERALD experiment of ESA

Some aspects of the ESA space program EMERALD are presented. A problem of the film type condensation on curvilinear fin is considered with taking into account the essential influence of the capillary forces and non uniform temperature of the fin. Disjoining pressure effect on condensate film flow is studied numerically, because of very thin film of condensate on the fin tip. Value of the groove flooding depth changes significantly the condensate outflow from the condenser. Calculations for water condensation predict a very high value of the heat transfer coefficient, more than 50 kW/m²K, on a quite important part of the surface area and confirm the idea of proposed condenser and Double Capillary Pumped Loop.     

Effects of fin shape on condensation in herringbone microfin tubes

Effects of fin height and helix angle on condensation inside a herringbone microfin tube have been experimentally investigated with five types of herringbone microfin tubes. Heat transfer coefficients are about 2–4 times higher than that of the helical microfin tube under high mass velocity conditions. In the low mass velocity, they are equal to that of the helical microfin tube. The heat transfer enhancement increases with fin height up to 0.18 mm; higher fin heights show enhancement values similar to the 0.18 mm results. Pressure drop increases with the fin height. Larger helix angle yields higher heat transfer and higher pressure drop. For the lowest fin and/or smallest helix angle, the pressure drop is comparable with that of the helical microfin tube, while the heat transfer enhancement is higher. The enhancement mechanism is discussed from flow pattern observations. Effect of mass transfer resistance for R410A is estimated and negligible effects have been proved.     

Optimum configurations of vertical fins under condensation of saturated vapor

In the present study, an optimum fin shape envelope was determined via a theoretical procedure that minimized the fin volume subject to constrained heat transfer rate. A unified analysis is presented for both longitudinal and pin fins with vertical orientation of the fin surface. The analysis was carried out on the assumption of a power law fin temperature distribution. Under certain simplified conditions, this optimization model has been validated. Results obtained from the model are also compared with previously published results for the specific set of designed conditions. From optimization study, practical fin profiles are found under the condensation of quiescent vapor atmosphere.     

Development of disk fin type compact current leads

Compact current leads have been developed for a superconducting thin solenoid magnet at KEK. A disk fin of a special type (straight-cut disk fin) is devised which is suited to be cut straight with numerically controlled machines such as a wire cutter or a milling machine. Since the straight-cut disk fins increase both the heat exchange efficiency and the thermal inertia of current-carrying conductor, they enable us to construct the current lead compactly. The length of the current lead fabricated is as short as 60 cm for its operating current of 4.1 kA. Good results have been achieved experimentally on the heat leakage into the cold region and on the pressure drop of cooling gas. The reliability of this current lead is estimated to be equivalent to that of the current lead twice in length without the cooling fin.     

Experimental Study of Film Condensation of FC-72 Vapour on Disk-Shaped Fin

The results of activity made in the framework of preparation of the experiment SAFIR (Single fin condensAtion: FIlm local measuRements) of European Space Agency are presented. First prototype of the test cell has been developed and tested. Confocal technique has been adopted for condensate film thickness measurements. Experiments on condensation of the liquid FC-72 have been carried out. Average heat transfer coefficient has been measured in the range of 880–1440 W/m² K and compared to the Nusselt theory.     

Condensation heat transfer coefficients of R22, R407C, and R410A on a horizontal plain, low fin, and turbo-C tubes

In this study, condensation heat transfer coefficients (HTCs) were measured on a horizontal plain tube, low fin tube, and Turbo-C tube at the saturated vapor temperature of 39 °C for R22, R407C, and R410A with the wall subcooling of 3–8 °C. R407C, a non-azeotropic refrigerant mixture, exhibited a quite different condensation phenomenon from those of R22 and R410A and its condensation HTCs were up to 50% lower than those of R22. For R407C, as the wall subcooling increased, condensation HTCs decreased on a plain tube while they increased on both low fin and turbo-C tubes. This was due to the lessening effect of the vapor diffusion film with a rapid increase in condensation rate on enhanced tubes. On the other hand, condensation HTCs of R410A, almost an azeotrope, were similar to those of R22. For all refrigerants tested, condensation HTCs of turbo-C tube were the highest among the tubes tested showing a 3–8 times increase as compared to those of a plain tube.     

An approximate analytical prediction about thermal performance and optimum design of pin fins subject to condensation of saturated steam flowing under forced convection

An approximate analytical method has been suggested for solving the governing equation for horizontal pin fins subject to condensation while saturated steam flowing over its under laminar forced convection. Adomian decomposition method is used for determination of the temperature distribution, performance and optimum dimensions of pin fins with temperature dependent thermal conductivity under the condensation of steam on the fin surface. From the results, a significant effect on the temperature distribution in the fin and its performances are noticed with the variation in fin-geometric parameters and thermo-physical properties of saturated vapor. Next, a generalized scheme for optimization has been demonstrated in such a way that either heat-transfer duty or fin volume can be taken as a constraint. Finally, the curves for the optimum design have been generated for the variation of different thermo-physical and geometric parameters, which may be helpful to a designer for selecting an appropriate design condition.     

Vapor condensation on nonisothermal curvilinear fins

An analytical expression is obtained for the optimum curvature of a nonisothermal fin featuring stationary condensation of motionless vapor under the conditions of a significant influence of the surface tension on the motion of a condensed liquid. An algorithm is proposed and realized that finds the optimum surface shape for an unknown temperature distribution in the nonisothermal fin. The algorithm is based on a joint solution of the equations of heat conduction and condensed liquid film flow on the fin surface. Allowance for the thermal conductivity of a material in optimization of the fin shape provides for a significant increase in the condensate outflow as compared to the case of the optimum isothermal fin shape and a finite thermal conductivity of the material.     

Optimum plate-fin heat sinks by using a multi-objective evolutionary algorithm

This article demonstrates the practical applications of a multi-objective evolutionary algorithm (MOEA) namely population-based incremental learning (PBIL) for an automated shape optimization of plate-fin heat sinks. The computational procedure of multi-objective PBIL is detailed. The design problem is posed to find heat sink shapes which minimize the junction temperature and fan pumping power while meeting predefined constraints. Three sets of shape design variables used in this study are defined as: vertical straight fins with fin height variation, oblique straight fins with steady fin heights, and oblique straight fins with fin height variation. The optimum results obtained from using the various sets of design variables are illustrated and compared. It can be said that, with this sophisticated design system, efficient and effective design of plate-fin heat sinks is achievable and the best design variables set is the oblique straight fins with fin height variation.     

Study of the Attachment Stage of a Welding Arc Discharge of Direct-Current Straight Polarity on Aluminum Surface

The results of a study on aluminum welding by direct-current straight polarity arc in a protective gas environment (argon, helium) are presented. The welding arc burns in aluminum vapor; the condensation products consequently lower the temperature of the welding column in the anode region. The condensation products of aluminum are formed by the cluster mechanism with the formation of fractal thread-like structures. The clustering mechanism is characterized by the release of nondissociated molecular blocks of aluminum into the vapor state. They form a morphologically complex composition of the alumina film on the surface of the weld pool during condensation.     

水平管外二氧化碳膜状凝结传热分析

综述了水平低翅片管外凝结传热的基本模型,阐述了二氧化碳制冷剂的物性特点,讨论了表面张力与凝液滞留角及二氧化碳管外凝结换热系数的关系,分析了翅片密度、环形翅片管尺寸对翅片效率、滞留角、凝结换热系数以及传热增强比的影响,优化了外翅片管的齿高与齿距,并求得相应的强化传热增强比.结果表明,对于根径为20mm的低翅片管,最佳翅片密度为每米435个翅片,最佳齿高为5.1mm,最佳齿距2.3mm.     

Approximate analytical method for prediction of performance and optimum dimensions of pin fins subject to condensation of quiescent vapor

An approximate analytic method is proposed to solve the governing equation of horizontal pin fins subject to the condensation of saturated vapor under laminar free convection. The temperature distribution and performance parameters are determined by using Adomian decomposition method. A numerical scheme followed by the finite difference method has been used to validate the present analytical result. From the results, a significant effect on the temperature distribution and performances with the variation of fin dimensions and thermophysical parameters of saturated vapor is noticed. Next, a scheme for fin optimization has been carried out in such a way that either heat transfer duty or fin volume can be taken as a constraint selected according to the requirement of a design. Finally, dependency of design variables on an optimum condition has been established which may help the designer to adopt physical conditions in a practical application.     

Experimental Investigations on Condensation in the Framework of ENhanced COndensers in Microgravity (ENCOM-2) Project

We report preliminary results on experimental investigations on condensation in the framework of the European Space Agency funded programme Enhanced Condensers in Microgravity (ENCOM-2) which aims at better understanding underlying phenomena during condensation. The first experiment is a study on condensation of HFE on external curvilinear surface of 15 mm height during reduced gravity experiments. It is found that the local minimum of the film thickness exists at the conjugation area of condensed film and the meniscus at the bottom of the fin; this leads to the local maximum of the heat transfer coefficient, which we also found moves towards the fin tip. The second experiment is a study of falling films hydrodynamics inside a vertical long pipe. In particular, characteristics of wavy falling films produced employing intermittent liquid feed are examined in order to assess wave effects on film condensation. Preliminary results suggest that intermittent feed simply divides the film in two autonomous regions with the wave feature of each one depending only on its flow rate. The processing of registered film thickness data can lead to the estimation of the transverse velocity profile in the film, which is mainly responsible for heat transfer during condensation. The third experiment looks at in-tube convective condensation at low mass fluxes (typical of Loop Heat Pipes and Capillary Pumped Loops) of n-pentane inside a 0.56 mm diameter channel. The results show that the mean heat transfer in the annular zone when it is elongated may be less than the mean heat transfer when it is shorter, due to the interface deformation involved by surface tension effect. When the length of this annular zone reaches a critical value, the interface becomes unstable, and a liquid bridge forms, involving the release of a bubble. The heat transfer due to the phase-change in this isolated bubble zone appears to be very small compared to the sensible heat transfer: the bubbles evolve and collapse in a highly subcooled liquid. The last experiment concerns in-tube condensation of R134a inside a square channel of 1.23 mm hydraulic diameter at mass fluxes of 135 kg m^?2 s^?1 and 390 kg m^?2 s^?1 for three different configurations: horizontal, vertical downflow and vertical upflow. For the calculated heat transfer coefficient it is found that gravity has no effect on condensation in downflow configurations at 390 kg m^?2 s^?1 and in upflow conditions at both values of mass velocity. The effect of gravity on the condensation heat transfer coefficient becomes noteworthy in downflow at mass velocity G = 135 kg m^?2 s^?1 and vapour quality lower than 0.6.     

Condensation of downward-flowing HFC134a in a staggered bundle of horizontal finned tubes: effect of fin geometry

Experimental results are presented that show the effect of fin geometry on condensation of refrigerant HFC134a in a staggered bundle of horizontal finned tubes. Two types of conventional low-fin tubes and three types of three-dimensional fin tubes were tested. The refrigerant mass velocity ranged from 8 to 23 kg/m²s and the condensation temperature difference from 1.5 to 12 K. The effect of condensate inundation was more significant for the three-dimensional fin tubes than for the low-fin tubes. In most cases, the highest performance was obtained by the tube with a three-dimensional structure at the tip of low fins. In the case of high mass velocity and high condensate inundation rate, however, the highest performance was obtained by one of the low-fin tubes. The results were compared with previous results for bundles of smooth tubes and low-fin tubes.     

A Theoretical Study of Steady State and Transient Condensation on Axisymmetric Fins Under Combined Capillary and Gravitational Forces

The present work attempts to model the case of combined gravitational and capillary motion of condensate for an axisymmetric fin under steady and transient fin operation conditions. The focus here is to examine the structure of the mathematical problem and to develop suitable numerical techniques rather than yield information on the macroscopic condensate flow rate and fin efficiency. The problem is formulated starting from general conditions and is simplified step by step by introducing corresponding assumptions. The particular fin shape of a paraboloid from revolution is chosen and the equations are properly non-dimensionalized. A vast reduction of the number of problem parameters is achieved in this way. The cases of isothermal fin, steady state operation and dynamic operation are treated separately using specialized numerical solution techniques developed for each case in order to improve computational efficiency and accuracy. Typical results of fin temperature and condensate film thickness are presented and discussed.     

Performance and optimum design analysis of convective fin arrays attached to flat and curved primary surfaces

A model has been developed analytically to carry out the performance and optimum design analysis of four fin arrays, namely, longitudinal rectangular fin array (LRFA), annular rectangular fin array (ARFA), longitudinal trapezoidal fin array (LTRA) and annular trapezoidal fin array (ATFA) under convective cooling conditions. The performance parameters such as fin efficiency, fin effectiveness and augmentation factor are evaluated for a wide range of design variables. It has been observed that the conduction through the supporting structure and the convection from the interfin spacing have a pronounced effect on the performance of a fin array. The optimum fin dimensions in a fin assembly have been determined by consideration of the constant total height of the fin assembly and interfin spacing. From the results, it can be highlighted that the optimum fin dimensions in fin arrays differ from that of the individual fins.     

Refined method of calculating heat exchange in the condensation of stationary steam on finned horizontal tubes

A relation is obtained for calculating the heat-transfer rate in the condensation of pure vapors on finned tubes with allowance for fin efficiency.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 4, pp. 597–602, October, 1980.     

Analysis of Condensation Heat Transfer Performance in Curved Triangle Microchannels Based on the Volume of Fluid Method

Numerical simulations of condensation heat transfer of R134a in curved triangle microchannels with various curvatures are proposed. The model is established on the volume of fluid (VOF) approach and user-defined routines which including mass transfer at the vapor-liquid interface and latent heat. Microgravity operating condition is assumed in order to highlight the surface tension. The predictive accuracy of the model is assessed by comparing the simulated results with available correlations in the literature. Both an increased mass flux and the decreased hydraulic diameter could bring better heat transfer performance. No obvious effect of the wall heat flux is observed in condensation heat transfer coefficient. Changes in geometry and surface tension lead to a reduction of the condensate film thickness at the sides of the channel and accumulation of the condensate film at the corners of the channel. Better heat transfer performance is obtained in the curved triangle microchannels over the straight ones, and the performance could be further improved in curved triangle microchannels with larger curvatures. The minimum film thickness where most of the heat transfer process takes place exists near the corners and moves toward the corners in curved triangle microchannels with larger curvatures.