Numerical simulation of R410A condensation in horizontal microfin tubes

The heat transfer characteristics of condensation for R410A inside horizontal microfin tubes with 0° and 18° helical angles were investigated numerically. The numerical data fit well with the experimental results and with the empirical correlations. The results indicate that local heat transfer coefficients increase with increasing mass flux, vapor quality, and helical angle. The heat transfer enhancement in the helical microfin tubes is more pronounced at higher mass flux and vapor quality. The centrifugal force induced by the microfin with a 18° helical angle tends to spread the liquid from the bottom to the top, leading to a nearly symmetrical liquid–vapor interface during condensation. Swirling flows in the liquid phase are observed in the tube with the 18° helical angle, but the liquid phase tends to flow to the bottom due to gravity in the tube with the 0° helical angle.     

Experimental study of the thermal hydraulic performance of sub-cooled refrigerants flowing in smooth,micro-fin and herringbone tubes

The present paper reports an investigation of the thermo-hydraulic performance of two standard microfin tubes with different number of fins and a herringbone microfin tube. As a reference for comparison purposes a smooth tube of equal external diameter and wall thickness has been tested for similar operational conditions. Copper tubes of 9.52 mm external diameter, electrically heated, have been used in the investigation. Most of the reported data has been gathered with refrigerant R134a though refrigerant R22 was also used in some of the tests with both the smooth and one of the standard microfin tubes. The test tube entrance state of the working fluid was kept constant at a temperature of the order of −2.5 °C and a pressure of 500 kPa, whereas its mass velocity varied in the range between 100 kg/s m² and 1350 kg/s m². It has been found that the thermal performance of the herringbone tube is superior to that of the two standard microfin tubes though the pressure drop obtained with the former is clearly higher. All the microfin tubes present significant heat transfer enhancement with respect to the smooth tube. According to the present results, the enhancement ratio of the microfin tubes attains an asymptotic value for Reynolds numbers of the order of 20,000, the herringbone tube presenting the higher asymptotic value. A figure of merit parameter has been introduced to qualify each of the microfin tubes tested, with the both standard microfin tubes being the best qualified with respect to the herringbone one, especially for applications that operate at the intermediate Reynolds numbers range.     

Experiment of flow regime map and local condensing heat transfer coefficients inside three dimensional inner microfin tubes

INTRODUCTIONSince1980s,thestudyofthein-tubecondensingheattransferenhancementhasbecomeimportant.Thetwodimensionalinnerfintubes,twodimensionalinnermicrofintubes(inthefollowingsimplycalled2Dtubes)andinserterswerepaidattentiontoforthein-tubecondensinghea...     

Study of local condensing heat transfer coefficients for stratified flow inside horizontal three‐dimensional inner microfin tubes

This paper presents a new type of three‐dimensional inner microfin tube. The flow patterns, the flow pattern criteria, and the heat transfer enhancement performances for horizontal condensation inside these tubes were obtained by experiment. The correlation of the local condensing heat transfer coefficient for stratified flow inside a two‐dimensional inner microfin tube was obtained on the basis of analysis and experimental data. The calculated results with the correlation are consistent with the experimental data within ±30%. © 2000 Scripta Technica, Heat Trans Asian Res, 29(8): 623–633, 2000     

水平三维内微肋管在局部蒸干区的沸腾换热及其关联式

为了得到不同流型下的换热性能 ,以 R1 3 4a为实验工质在一种水平三维内微肋管内进行了流动沸腾换热实验研究 ,通过可视化等措施对得到的主要流型及其转换曲线表示在 G-x图上。对局部蒸干区的沸腾换热特点进行了讨论 ,并根据此区域换热的特点 ,沿周向管壁分成两个部分 ,即 :蒸干部分和非蒸干部分。对于非蒸干部分又分为淹没微肋的底部液体 ,且认为同环状流换热机理相同 ,而另一部分认为液休带领在沟槽中 ,从而得到了此区域的换热实验关联式 ,此换热关联式与实验值的最大偏差在± 1 6%以内     

Experimental analysis of the single phase pressure drop characteristics of smooth and microfin tubes

In this study, the single phase pressure drop characteristics of smooth and microfin tubes are investigated experimentally. The horizontal test section is a counter flow double tube heat exchanger with water flowing in the inner tube and cooling water flowing in the annulus. By means of experimental setup, required temperature and pressure measurements are recorded and friction factor coefficient and pressure drop of smooth and microfin tubes are determined. Experiments are conducted for mass flow rates in the range between 0.023 kg/s and 0.100 kg/s and effect of Reynolds number on pressure drop is investigated. By using experimental results, Blasius type friction factor equations are developed for both smooth and microfin tubes. Experimental results for both smooth and microfin tubes are compared with correlations given in the literature.     

Review of In-Tube Condensation Heat Transfer Correlations for Smooth and Microfin Tubes

This article provides a comprehensive review of the published literature on inside-tube condensation heat transfer correlations for smooth and microfin tubes with an emphasis on correlations used in air conditioning and refrigeration. The correlations presented are discussed and evaluated with experimental data from different authors for different fluids and flow conditions. This review is divided in two main parts: condensation inside smooth tubes and condensation inside microfin tubes. According to the comparison between empirical correlation and experimental data for smooth tubes, correlations proposed by Dobson et al., Dobson and Chato, and Cavallini et al. appear to be the most accurate ones to be used for different fluids and boundary conditions. In the case of microfin tubes, much additional work is needed to develop more general and accurate correlations.     

Visual study of flow patterns during condensation inside a microfin tube with different tube inclinations

In this paper, flow patterns and their transitions for refrigerant R-134a condensing in a microfin tube are visually observed and analyzed. The microfin tube has been provided with different tube inclination angles of the direction of fluid flow from horizontal, α. The experiments were performed for seven different tube inclinations, α, in a range of − 90° to + 90° and refrigerant mass velocities in a range of 53 to 212 kg/m²s for each tube inclination angle during condensation of R-134a vapor. From analysis of acquired data, it was found that the tube inclination strongly influenced the vapor and condensate liquid distribution. Annular flow was the dominant flow pattern for vertical downward flow, α = − 90°. Annular flow, semi annular flow and stratified flow were observed for α = − 60°and − 30°. Annular flow, wavy-annular flow and stratified-wavy flow exist in sequence for horizontal tube. Annular flow and wavy-annular flow were observed for α = + 30°and + 60°. Annular flow, annular-wavy flow, churn flow and slug flow occurred for α = + 90°.     

Experimental study on condensation heat transfer enhancement and pressure drop penalty factors in four microfin tubes

Heat transfer and pressure drop characteristics of four microfin tubes were experimentally investigated for condensation of refrigerants R134a, R22, and R410A in four different test sections. The microfin tubes examined during this study consisted of 8.92, 6.46, 5.1, and 4 mm maximum inside diameter. The effect of mass flux, vapor quality, and refrigerants on condensation was investigated in terms of the heat transfer enhancement factor and the pressure drop penalty factor. The pressure drop penalty factor and the heat transfer enhancement factor showed a similar tendency for each tube at given vapor quality and mass flux. Based on the experimental data and the heat-momentum analogy, correlations for the condensation heat transfer coefficients in an annular flow regime and the frictional pressure drops are proposed.     

An empirical study on heat transfer and pressure drop properties of heat transfer oil-copper oxide nanofluid in microfin tubes

In this paper, the convective heat transfer of the heat transfer oil-copper oxide nanofluid flow in horizontal smooth and microfin tubes is investigated experimentally. Using a flow control system, the flow regime is always laminar and the wall temperature is constant by using a steam tank. Pure heat transfer oil and nanofluid with the weight concentrations of 0.5%, 1% and 1.5% are used as working fluids. The results are in good agreement with the classic correlations for the pure fluid flow. Based on the results, combination use of nanoparticles and the microfin tube leads to the heat transfer enhancement up to 230%, in comparison with the base fluid flow in the smooth tube. The results are useful in the prediction of the heat transfer rate and the pressure drop in nanofluid flows.     

Condensation heat transfer of R-134a inside a microfin tube with different tube inclinations

Experimental heat transfer studies during condensation of pure R-134a vapor inside a single microfin tube have been carried out. The microfin tube has been provided with different tube inclination angles of the direction of fluid flow from horizontal, α. The data are acquired for seven different tube inclinations, α, in a range of −90 to +90° and three mass velocities of 54, 81, and 107 kg/m²-s for each inclination angle during condensation of R-134a vapor. The experimental results indicate that the tube inclination angle of, α, affects the condensation heat transfer coefficient in a significant manner. The highest heat transfer coefficient is attained at inclination angle of α = +30°. The effect of inclination angle, α, on heat transfer coefficient, h, is more prominent at low vapor quality and mass velocity. A correlation has also been developed to predict the condensing side heat transfer coefficient for different vapor qualities and mass velocities.     

超临界CO2倾斜螺旋槽管内冷却换热特性研究

为了提高气体冷却器内换热效率,对不同倾斜角下(-90°,-45°,0°,45°,90°)螺旋槽管内超临界CO2冷却对流换热特性进行了数值模拟,分析了各槽管内的湍动能和速度分布随倾斜角的变化趋势,并研究了不同螺旋角下倾斜角对换热特性的影响。结果表明:浮升力沿流动方向分量和垂直于流动方向分量对流动特性的影响并不相同;在类气区,流体速度对流动特性起主要作用,且换热系数随倾斜角的减小而增大;在类液区,流动特性的主要影响因素是速度梯度,此时换热系数随倾斜角的变化与类气区相反;螺旋角越大即螺旋程度越小,当流体倾斜向上流动时浮升力效应越为显著;当螺旋角为 0.70 rad时,最优倾斜角度为-45°,当螺旋角为0.94 rad时,最优倾斜角为45°。     

The flow characteristics of an upward gas‐liquid two‐phase flow in a vertical tube with a wire coil: Part 2. Effect on void fraction and liquid film thickness

An experimental study has been carried out to clarify the characteristics of the void fraction and the liquid film thickness of the air‐water two‐phase flow in vertical tubes of 25‐mm inside diameter with wire coils of varying wire diameter and pitch. The flow pattern in the experiment on the average void fraction and the local void fraction distribution in cross section was a bubble flow, and the liquid film thickness was in the region of semiannular and annular flows. It is clarified from these experiments that the average void fraction in tubes with wire coils is lower than that in a smooth tube and decreases with the wire diameter owing to the centrifugal force of the swirl flow which concentrates bubbles at the center of the tube, that the local liquid film thickness becomes more uniform with a decrease in the pitch of the wire coil, and that the liquid film becomes thicker after the passage through the wire coil with an increase in the wire diameter. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(8): 652–664, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10067     

Heat transfer characteristics for evaporation of R417A flowing inside horizontal smooth and internally grooved tubes

The experimental study on evaporation heat transfer of R417A (R125/R134a/R600) flowing inside horizontal smooth and two internally grooved tubes with different geometrical parameters was conducted with the mass flow rate range from 176 to 344 kg m⁻² s⁻¹, heat flux from 11 to 32 kW m⁻², evaporation temperature from 0 to 5.5 °C and vapor quality from 0.2 to 1. Based on the experimental results, the mechanism and role of the mass flow rate, heat flux, vapor quality and enhanced surface influencing the evaporation heat transfer coefficients were analyzed and discussed. In comparison to R22, the evaporation heat transfer coefficients for R417A were lower and much lower in the internally grooved tubes than in the smooth tube. The present experimental results are also compared with the existing correlations, and the modified Kattan model is found to be in much better agreement with the experimental results than the Kattan model. The Koyama and Wellsandt microfin models all tend to over predict the evaporation heat transfer coefficients rather strongly for R417A inside internally grooved tubes.     

三维微肋螺旋管内R134a沸腾环状流区的传热与关联式

首次实验研究了制冷剂R134a在三维微肋螺旋管内流动沸腾环状流区的流动与传热性能。对流型的可视化观察发现：当质量流速大于100kg／(m^2s)时。螺旋管内才开始出现环状流。环状流的起始干度为0．3、0．4。在流型图上给出了环状流区与其它主要流型的分区。回归了实验环状流区的传热实验数据，得到的传热关联武计算值与实验值的平均绝对误差为9．1％。     

Effect of helix angle of spirally corrugated tube on condensation heat transfer characteristics

Spirally corrugated tubes with different helix angles have been used in experiments on the characteristics of condensation heat transfer and flow resistance. The result of the experimentation shows that the helix angle plays a vital part in the spirally corrugated tube during the process of condensation heat transfer. This paper analyzes the mechanism of how the helix angle influences the characteristics of condensation heat transfer and flow resistance, and presents an experimental correlation formula of condensation heat transfer which takes the helix angle into account as a factor. ©2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(5): 275–282, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20208     

Pressure drop and thermal characteristics of CuO-base oil nanofluid laminar flow in flattened tubes under constant heat flux

An experimental investigation has been carried out to study the heat transfer and pressure drop characteristics of nanofluid flow inside horizontal flattened tubes under constant heat flux. The nanofluid is prepared by dispersion of CuO nanoparticle in base oil and stabilized by means of an ultrasonic device. Nanofluids with different particle weight concentrations of 0.2%, 0.5%, 1% and 2% are used. Copper tubes of 11.5 mm I.D. are flattened into oblong shapes and used as test sections. The nanofluid flowing inside the tube is heated by an electrical heating coil wrapped around it. Required data are acquired for laminar and hydrodynamically fully developed flow inside round and flattened tubes.The effect of different parameters such as flow Reynolds number, flattened tube internal height and nanofluid particle concentration on heat transfer coefficient and pressure drop of the flow is studied. Observations show that the heat transfer performance is improved as the tube profile is flattened. Flattening the tube profile resulted in pressure drop increasing. In addition, the heat transfer coefficient as well as pressure drop is increased by using nanofluid instead of base fluid. Furthermore, the performance evaluation of the two enhanced heat transfer techniques studied in this investigation shows that applying flattened tubes instead of the round tube is a more effective way to enhance the convective heat transfer coefficient compared to the second method which is using nanofluids instead of the base liquid.     

Experimental study on combined buoyant-thermocapillary flow along with rising liquid film on the surface of a horizontal metallic mesh tube

Temperature distribution and variation with time has been considered in the analysis of the influences of the initial level of immersion of a horizontal metallic mesh tube in the liquid on combined buoyant and thermo-capillary flow. The combined flow occurs along with the rising liquid film flow on the surface of a horizontal metallic mesh tube. Three different levels of immersion of the metallic mesh tube in the liquid have been tested. Experiments of 60 min in duration have been performed using a heating metallic tube with a diameter of 25 mm and a length of 110 mm, sealed outside with a metallic mesh of 178 mm by 178 mm, and distilled water. These reveal two distinct flow patterns. Thermocouples and infrared thermal imager are utilized to measure the temperature. The level of the liquid free surface relative to the lower edge of the tube is measured as angle q. The results show that for a smaller q angle, or a low level of immersion, with a relatively low heating power, it is possible to near fully combine the upwards buoyant flow with the rising liquid film flow. In this case, the liquid is heated only in the vicinity of the tube, while the liquid away from the flow region experiences small changes in temperature and the system approaches steady conditions. For larger q angles, or higher levels of immersion, a different flow pattern is noticed on the liquid free surface and identified as the thermo-capillary (Marangoni) flow. The rising liquid film is also present. The higher levels of immersion cause a high temperature gradient in the liquid free surface region and promote thermal stratification; therefore the system could not approach steady conditions.     

平行流蒸发器内气液两相流分配均匀性实验研究

平行流蒸发器内气液两相(特别是液相)在各扁管间的分配对其传热性能影响较大,如果各扁管间的气液分配不均匀其传热性能将显著地下降.在不同气-液流量下实验研究了6种不同形式的平行流蒸发器的分支管液体流量分配情况,实验中观察到流型以环状流为主.研究发现,对于竖直向下流动和竖直向上流动,用通过增加管径的方法不能改善液体流量在各分支管的分配,而主管中气液入口的位置对于流量分配均匀性影响较大.     

Numerical–Theoretical Analysis of Heat Transfer,Pressure Drop,and Fouling in Internal Helically Ribbed Tubes of Different Geometries

A numerical analysis of heat transfer and pressure drop for turbulent flow in a series of 15.54-mm inside diameter helically ribbed tubes has been performed. The ranges of geometric parameters were number of rib starts (10 to 40), helix angle (25 to 55 degrees), and rib height (0.3 to 0.6 mm). The effect of grid independence was extensively examined. The computational results match well with the experimental data to validate the accuracy of the numerical model. The effect of each main parameter, rib starts, helix angle, and rib height, on heat transfer and pressure drop is investigated. Considering fouling in practical situations, the ratio of pitch over rib height is an important parameter to select the tubes. It is advisable to select tubes with pitch over rib height ratio greater than 3.5, which have better heat transfer and lower fouling potential.