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.     

Condensation heat transfer and pressure drop in flattened microfin tubes having different aspect ratios

In this study, condensation heat transfer coefficients and pressure drops of R-410A are obtained in flattened microfin tubes made from 7.0 mm O.D. round microfin tubes. The test range covers saturation temperature 45 °C, mass flux 100–400 kg m⁻² s⁻¹ and quality 0.2–0.8. Results show that the effect of aspect ratio on condensation heat transfer coefficient is dependent on the flow pattern. For annular flow, the heat transfer coefficient increases as aspect ratio increases. For stratified flow, however, the heat transfer coefficient decreases as aspect ratio increases. The pressure drop always increases as aspect ratio increases. Possible reasoning is provided based on the estimated flow pattern in flat microfin tubes. Comparison with existing round microfin tube correlations is made.     

Heat transfer, pressure drop, and flow pattern recognition during condensation inside smooth, helical micro-fin, and herringbone tubes

This paper presents a study of flow regimes, pressure drops, and heat transfer coefficients during refrigerant condensation inside a smooth, an 18° helical micro-fin, and a herringbone tubes. Experimental work was conducted for condensing refrigerants R-22, R-407C, and R-134a at an average saturation temperature of 40 °C with mass fluxes ranging from 400 to 800 kg m⁻² s⁻¹, and with vapour qualities ranging from 0.85 to 0.95 at condenser inlet and from 0.05 to 0.15 at condenser outlet. These test conditions represent annular and intermittent (slug and plug) flow conditions. Results showed that transition from annular flow to intermittent flow, on average for the three refrigerants, occurred at a vapour quality of 0.49 for the smooth tube, 0.29 for the helical micro-fin tube, and 0.26 for the herringbone tube. These transition vapour qualities were also reflected in the pressure gradients, with the herringbone tube having the highest pressure gradient. The pressure gradients encountered in the herringbone tube were about 79% higher than that of the smooth tube and about 27% higher than that of the helical micro-fin tube. A widely used pressure drop correlation for condensation in helical micro-fin tubes was modified for the case of the herringbone tube. The modified correlation predicted the data within a 1% error with an absolute deviation of 7%. Heat transfer enhancement factors for the herringbone tube against the smooth tube were on average 70% higher while against the helical micro-fin tube it was 40% higher. A correlation for predicting heat transfer coefficients inside a helical micro-fin tube was modified for the herringbone tube. On average the correlation predicted the data to within 4% with an average standard deviation of 8%.     

Friction factor correlation and pressure loss of single-phase flow inside herringbone microfin tubes

Pressure drop of single-phase turbulent flow inside herringbone microfin tubes of different fin dimensions has been measured experimentally to develop a general correlation of single-phase friction factor for the herringbone tubes. Water has been used as a working fluid and the mass flow rate has been varied from 0.03 to 0.2 kg/s, where the Reynolds number range is 10⁴ to 6.5 × 10⁴. Comparison of experimental data of the herringbone microfin tubes with those of helical microfin and smooth tubes shows that pressure drop of the herringbone tube is significantly higher than the helical and smooth tubes depending on the fin geometric parameters and mass velocity of the working fluid. Through semi-analytical approach and using the present experimental data, a new correlation of single-phase friction factor for the herringbone microfin tubes has been proposed incorporating the effects of fin geometric parameters. The proposed correlation can predict the experimental data within ±10%.     

Condensation of refrigerants in horizontal microfin tubes: comparison of correlations for frictional pressure drop

This paper presents a comprehensive comparison of eight previously proposed correlations with available experimental data for the frictional pressure drop during condensation of refrigerants in helically grooved, horizontal microfin tubes. Calculated values are compared with experimental data for seven refrigerants (R11, R123, R134a, R22, R32, R125 and R410A) and eight tubes and with mass velocity from 78 to 459 kg/m² s. The tubes had inside diameter at the fin root between 6.41 and 8.91 mm; the fin height varied between 0.15 and 0.24 mm; the fin pitch varied between 0.34 and 0.53 mm and helix angle between 13 and 20°. The results show that the overall r.m.s. deviations of relative residuals of frictional pressure gradient for all tubes and all refrigerants taking together decreased in the order of the correlations of Nozu et al. [Exp. Therm. Fluid Sci. 18 (1998) 82], Newell and Shah [Refrigerant heat transfer, pressure drop, and void fraction effects in microfin tubes. In: Proc. 2nd Int. Symp. on Two-Phase Flow and Experimentation, vol. 3. Italy: Edizioni ETS; 1999. p. 1623–39], Kedzierski and Goncalves [J. Enhanced Heat Transfer 6 (1999) 161], Cavallini et al. [Heat Technol. 15 (1997) 3], Goto et al. (b) [Int. J. Refrigeration 24 (2001) 628], Choi et al. [Generalized pressure drop correlation for evaporation and condensation in smooth and microfin tubes. In: Proc. of IIF-IIR Commision B1, Paderborn, Germany, B4, 2001. p. 9–16], Haraguchi et al. [Condensation heat transfer of refrigerants HCFC134a, HCFC123 and HCFC22 in a horizontal smooth tube and a horizontal microfin tube. In: Proc. 30th National Symp. of Japan, Yokohama, 1993. p. 343–5], and Goto et al. (a) [Int. J. Refrigeration 24 (2001) 628], i.e., this final correlation (Goto et al. (a)) gives the best overall representation of the data.     

Refrigerant charge, pressure drop, and condensation heat transfer in flattened tubes

Horizontal smooth and microfinned copper tubes with an approximate diameter of 9 mm were successively flattened in order to determine changes in flow field characteristics as a round tube is altered into a flattened tube profile. Refrigerants R134a and R410A were investigated over a mass flux range from 75 to 400 kg m⁻² s⁻¹ and a quality range from approximately 10–80%. For a given refrigerant mass flow rate, the results show that a significant reduction in refrigerant charge is possible. Pressure drop results show increases of pressure drop at a given mass flux and quality as a tube profile is flattened. Heat transfer results indicate enhancement of the condensation heat transfer coefficient as a tube is flattened. Flattened tubes with an 18° helix angle displayed the highest heat transfer coefficients. Smooth tubes and axial microfin tubes displayed similar levels of heat transfer enhancement. Heat transfer enhancement is dependent on the mass flux, quality and tube profile.     

Convective boiling pressure drop of refrigerant R-134a in horizontal smooth and microfin tubes

Present study deals with the pressure drop of refrigerant R-134a under convective boiling conditions in horizontal smooth and microfinned (‘grooved’) copper tubes. Experiments have been carried out in an experimental set up developed for change of phase studies with a test section made out of 7.0, 7.93, and 9.52 mm external diameter, 1.5 m long copper tubes, electrically heated by tape resistors wrapped on the external surface. Mass velocities and refrigerant qualities varied in the following ranges: 70–1100 kg s⁻¹ m⁻² and 5–95%. The annular flow pattern has been observed to occur over most of the operational conditions. For smooth tubes, the Jung and Radermacher correlation for the liquid two phase flow multiplier fits with reasonable precision the experimental data. As for grooved tubes, a correlation of the two phase flow multiplier in terms of the Martinelli's parameter has been developed which fits the data with an average absolute deviation of the order of 6.3%. The proposed correlation fits with good precision data obtained elsewhere for grooved tubes of different diameter and microfin geometry.     

Convective boiling performance of refrigerant R-134a in herringbone and microfin copper tubes

This paper reports an experimental investigation of convective boiling heat transfer and pressure drop of refrigerant R-134a in smooth, standard microfin and herringbone copper tubes of 9.52 mm external diameter. Tests have been conducted under the following conditions: inlet saturation temperature of 5 °C, qualities from 5 to 90%, mass velocity from 100 to 500 kg s⁻¹ m⁻², and a heat flux of 5 kW m⁻². Experimental results indicate that the herringbone tube has a distinct heat transfer performance over the mass velocity range considered in the present study. Thermal performance of the herringbone tube has been found better than that of the standard microfin in the high range of mass velocities, and worst for the smallest mass velocity (G=100 kg s⁻¹ m⁻²) at qualities higher than 50%. The herringbone tube pressure drop is higher than that of the standard microfin tube over the whole range of mass velocities and qualities. The enhancement parameter is higher than one for both tubes for mass velocities lower than 200 kg s⁻¹ m⁻². Values lower than one have been obtained for both tubes in the mass velocity upper range as a result of a significant pressure drop increment not followed by a correspondent increment in the heat transfer coefficient.     

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.     

Flow condensation heat transfer coefficients of R22, R134a, R407C, and R410A inside plain and microfin tubes

Flow condensation heat transfer coefficients (HTCs) of R22, R134a, R407C, and R410A inside horizontal plain and microfin tubes of 9.52 mm outside diameter and 1 m length were measured at the condensation temperature of 40 °C with mass fluxes of 100, 200, and 300 kg m⁻² s⁻¹ and a heat flux of 7.7–7.9 kW m⁻². For a plain tube, HTCs of R134a and R410A were similar to those of R22 while HTCs of R407C are 11–15% lower than those of R22. For a microfin tube, HTCs of R134a were similar to those of R22 while HTCs of R407C and R410A were 23–53% and 10–21% lower than those of R22. For a plain tube, our correlation agreed well with the present data for all refrigerants exhibiting a mean deviation of 11.6%. Finally, HTCs of a microfin tube were 2–3 times higher than those of a plain tube and the heat transfer enhancement factor decreased as the mass flux increased for all refrigerants tested.     

R404A condensing under forced flow conditions inside smooth, microfin and cross-hatched horizontal tubes

Two-phase heat transfer coefficient characteristics of R404A condensing under forced flow conditions inside smooth, microfin and cross-hatched horizontal tubes are experimentally investigated. Experimental parameters include a lubricating polyol ester oil concentration varied from 0 to 4%. The test runs were done at average inlet saturated condensing temperatures of 40 °C. The inlet vapor was kept at saturation (quality=1.0). The mass fluxes were between 200 and 600 kg/m² s, and the heat fluxes were selected to obtain a quality of 0.0 at the outlet of the test section, varying from 5 to 45 kW/m². The heat transfer enhancement factor varied between 1.8 and 2.4 for both microfin and cross-hatched tubes. The larger values applied for larger mass fluxes for the cross-hatched tube and smaller mass fluxes for the microfin tube. Enhancement factors increased as oil concentration increased up to oil concentrations of 2%. For higher oil concentrations the enhancement decreased especially at high mass fluxes, the cross-hatched tube being less sensitive to oil contamination. Pressure drop in the test section increased by approximately 25% as the oil concentration increased from 0 to 4%. The results from the experiments are compared with those calculated from correlations reported in the literature. Moreover, modified correlations for the condensation heat transfer coefficient are proposed for practical applications.     

Heat transfer and pressure drop during condensation of refrigerants inside horizontal enhanced tubesTransfert de chaleur et chute de pression lors de la condensation de frigorigènes à l'intérieur de tubes horizontaux à surface augmentée

This paper presents a critical review of correlations to compute heat transfer coefficients and pressure drop, for refrigerants condensing inside commercially available tubes with enhanced surfaces of various types, and a theoretical analysis of the condensation phenomenon. Predictions from some of the above equations are compared with experimental data. In addition, information is presented about the influence of small amounts of compressor oil on the condensation of refrigerants in enhanced tubes.     

Experimental investigation of heat transfer with drop condensation inside horizontal tubes

The authors describe the method and present results of an investigation of heat transfer with drop condensation of water vapor, generated by injecting fluoridecontaining bisulphide into the condensate of a closed system.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 58, No. 3, pp. 488–492, March, 1990.     

Evaporation of R407C and R410A in a horizontal herringbone microfin tube: heat transfer and pressure drop

Based on experimental data for R134a, the present work deals with the development of a prediction method for heat transfer in herringbone microfin tubes. As is shown in earlier works, heat transfer coefficients for the investigated herringbone microfin tube tend to peak at lower vapour qualities than in helical microfin tubes. Correlations developed for other tube types fail to describe this behaviour. A hypothesis that the position of the peak is related to the point where the average film thickness becomes smaller than the fin height is tested and found to be consistent with observed behaviour. The proposed method accounts for this hypothesis and incorporates the well-known Steiner and Taborek correlation for the calculation of flow boiling heat transfer coefficients. The correlation is modified by introducing a surface enhancement factor and adjusting the two-phase multiplier. Experimental data for R134a are predicted with an average residual of 1.5% and a standard deviation of 21%. Tested against experimental data for mixtures R410A and R407C, the proposed method overpredicts experimental data by around 60%. An alternative adjustment of the two-phase multiplier, in order to better predict mixture data, is discussed.     

Condensation heat transfer and pressure drop characteristics of CO2 in a microchannel

The condensation heat transfer coefficient and pressure drop of CO₂ in a multiport microchannel with a hydraulic diameter of 1.5 mm was investigated with variation of the mass flux from 400 to 1000 kgm⁻²s⁻¹ and of the condensation temperature from −5 to 5 °C. The heat transfer coefficient and pressure drop increased with the decrease of condensation temperature and the increase of mass flux. However, the rate of increase of the heat transfer coefficient was retarded by these changes. The gradient of the pressure drop with respect to vapor quality is significant with the increase of mass flux. The existing models for heat transfer coefficient overpredicted the experimental data, and the deviation increased at high vapor quality and at high heat transfer coefficient. The smallest mean deviation of ±51.8% was found by the Thome et al. model. For the pressure drop, the Mishima and Hibiki model showed mean deviation of 29.1%.     

Modeling and experimental validation of pressure drop for pneumatic plug conveying

This paper indicates that a variety of models exist to calculate the pressure drop of pneumatic plug conveying. Unfortunately these models are limited in their flexibility and calculate pressure drop only for two extremes, the active and passive case of bulk solid. Until now the stress state within the plug cannot be determined. Therefore a sensor was built to detect wall shear stress, normal stress and the stress state along a moving plug. Beginning with vertical pneumatic plug conveying, the previous models were verified and modified to get new perceptions about formation, stability and pressure drop of plugs.The DFG (German Research Community) is gratefully appreciated for the financial support to this work.Received October 2003     

Prediction of heat transfer coefficient during condensation of water and R-134a on single horizontal integral-fin tubes

This paper presents a few salient features of an investigation carried out to study the heat transfer augmentation during condensation of water and R-134a vapor on horizontal integral-fin tubes. The experimental investigation was performed on two different experimental set-ups for water and R-134a. The test-sections were manufactured by machining fins over plain copper tubes of 24.4 ± 0.6 mm outside diameter. The performance of two types of finned tubes viz. circular integral-fin tubes (CIFTs) and spine integral-fin tubes (SIFTs) was studied for the condensation of water and R-134a. These tubes were positioned one by one inside the test-condenser to perform the experiments. All together the experiments were conducted for the condensation on 10 different test-section tubes. With the help of the experimental results, authors have developed an empirical equation. This equation predicts the condensing heat transfer coefficient from their own experimental data for the condensation over CIFTs and SIFTs within a range of ± 15% and experimental data of other thirteen investigators in a range of ± 35% for condensation of water and different refrigerants.     

Evaporating heat transfer of R22 and R410A in horizontal smooth and microfin tubes

An experimental investigation of condensation heat transfer in 9.52 mm O.D. horizontal copper tubes was conducted using R22 and R410A. The test rig had a straight, horizontal test section with an active length of 0.92 m and was cooled by the heat transfer fluid (cold water) circulated in a surrounding annulus. Constant heat flux of 11.0 kW/m² was maintained throughout the experiment and refrigerant quality varied from 0.9 to 0.1. The condensation test results at 45 °C were reported for 40–80 kg/h mass flow rate. The local and average condensation coefficients for seven microfin tubes were presented compared to those for a smooth tube. The average condensation coefficients of R22 and R410A for the microfin tubes were 1.7–3.19 and 1.7–2.94 times larger than those in smooth tube, respectively.     

Refrigerant R134a condensation heat transfer and pressure drop inside a small brazed plate heat exchanger

This paper presents the experimental tests on HFC-134a condensation inside a small brazed plate heat exchanger: the effects of refrigerant mass flux, saturation temperature and vapour super-heating are investigated.A transition point between gravity controlled and forced convection condensation has been found for a refrigerant mass flux around 20 kg/m² s. For refrigerant mass flux lower than 20 kg/m² s, the saturated vapour heat transfer coefficients are not dependent on mass flux and are well predicted by the Nusselt [Nusselt, W., 1916. Die oberflachenkondensation des wasserdampfes. Z. Ver. Dt. Ing. 60, 541–546, 569–575] analysis for vertical surface. For refrigerant mass flux higher than 20 kg/m² s, the saturated vapour heat transfer coefficients depend on mass flux and are well predicted by the Akers et al. [Akers, W.W., Deans, H.A., Crosser, O.K., 1959. Condensing heat transfer within horizontal tubes. Chem. Eng. Prog. Symp. Ser. 55, 171–176] equation. In the forced convection condensation region, the heat transfer coefficients show a 30% increase for a doubling of the refrigerant mass flux. The condensation heat transfer coefficients of super-heated vapour are 8–10% higher than those of saturated vapour and are well predicted by the Webb [Webb, R.L., 1998. Convective condensation of superheated vapour. ASME J. Heat Transfer 120, 418–421] model. The heat transfer coefficients show weak sensitivity to saturation temperature. The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on the refrigerant mass flux.     

氮浆在水平圆管中流动压降的数值模拟

氮浆因其密度高、温度低、热容大等特性,可作为高温超导器件等的潜在冷却剂.氮浆的流动特性(尤其是压降)是其在应用中很重要的特征参数.建立了氮浆在水平圆管中流动时压降的二维计算模型,并应用双流体模型、Syamlal-O'Brien曳力模型的CFD模拟方法,对系统中跟流动压降相关的关键因素进行了分析,得出了固氮体积分数、氮浆流速、管道直径和固氮颗粒大小等参数对氮浆液固两相流在水平圆管内流动压降的影响情况.