Enhancement of condensation on a vertical plate (2nd report,prediction of condensation on a dispersed finned surface)

In this study, a prediction model for condensation heat transfer on a vertical dispersed finned surface was proposed, utilizing the Adamek‐Webb model for condensation heat transfer outside a horizontal finned tube. The prediction model was based on two main experimental observation results. One is the phenomena of the condensate retention at the bottom of each row of the dispersed fin. Another is the offset phenomena of the condensate flow between each row of the dispersed fin. Given the results by the present model, it is predicted that the dependence of the condensation heat transfer coefficient for the dispersed finned surface on the fin pitch is controlled mainly by the dispersed fin length, not the total fin length. On the contrary, for a different fin pitch, the effect to the condensation heat transfer by dispersing the fin is different. From comparison with the experiment results, it is confirmed that the present model was able to predict the condensation with extremely good precision when the fin pitch was larger. Further, when the fin pitch was smaller, the predicted values were higher than the experimental values, but the tendency of the condensation heat transfer with dispersing the fin was nearly predicted. In addition, this condensing model can predict the experimental values with an error of 25% at the maximum in a range of fin pitch 0.6 mm to 1 mm. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20288     

Optimized electrode arrangement in solar air heater

Laminar forced convection inside the solar air heater with various wire electrode arrangements are numerically examined for heat transfer enhancement using electrohydrodynamic technique. The electric field is generated by the wire electrodes charged with DC high voltage ranging from 7.5 to 17.5 kV. Reynolds number corresponding to the flow considered is between 100 and 2000. The numerical modeling of computational fluid dynamics includes the interactions among electric field, flow field, and temperature field. It is found that the enhancement of heat transfer coefficient with the presence of electric field increases in relation with the supplied voltage but decreases when the Reynolds number and the distance between electrode and channel surface are augmented. The optimized electrode arrangement, which obtains the best heat transfer enhancement is investigated incorporating with the pressure drop consideration. The heat transfer enhancement is also depended on the number of electrodes per length and the channel dimension.     

径向错列翅片管内凝结对流换热数值模拟分析

采用数值模拟方法,对径向错列翅片管内含不凝结气体水蒸气的凝结对流换热及阻力特性进行了综合分析。将编写的自定义函数(UDF)导入ANSYS FLUENT软件,对新型强化管传热性能和阻力性能进行了数值模拟,并根据管长方向壁面上蒸汽质量分数的变化情况,讨论分析了凝结过程中翅片管传热性能的变化规律。分析结果表明:与光管相比,内翅片管的强化传热效果随翅数增多、翅片换热接触面积增大而更加显著;另一方面,翅片管的流动阻力相应增大,对管路换热产生不良影响。在所研究翅型范围内16翅y=2x~2型翅片管综合强化换热效果更优;此外随着换热过程的持续,蒸汽凝结逐渐放缓;入口速度增大导致水蒸气凝结不充分,对换热效果的提升有一定制约。     

油浸式自冷变压器用三维肋管散热器换热特性研究

为研究三维肋管散热器的自然对流换热特性,分析不同温升下三维肋管散热器的强化换热性能,搭建了模拟油浸式自冷变压器工作过程的实验平台,在不同功率下分别测试了片间距为45 mm的片式散热器、排间距为45和55 mm的三维肋管散热器的进口温升,拟合得到散热器的温升-功率曲线,计算出3个散热器在相同温升下的散热量.研究表明:相比...     

Experimental studies on heat transfer enhancement of the inside and outside spirally triangle finned tube with small spiral angles for high‐pressure preheaters

A new heat transfer enhanced tube—the inside and outside spirally triangle finned tube with small spiral angles (IOSTF tube)—was developed and manufactured for improving the performance of high‐pressure preheaters. The triangle flutes with small spiral angle on the outside surface of the IOSTF tube perform like the vertically fluted tube, and the triangle flutes with small spiral angle on the inside surface of the IOSTF tube perform like the spirally fluted tube. The experiments show that the total heat transfer coefficient of the vertical IOSTF tube is 63–95 per cent larger than that of the smooth tube with only a slight increase in the inside flowing friction and the field results show that a 43 per cent increase in the total heat transfer coefficient of the high‐pressure preheater with the IOSTF tubes can be obtained. Copyright © 2000 John Wiley & Sons, Ltd.     

Numerical study of the intensified heat transfer of an internally longitudinal ridged finned tube under pulsating flow

The turbulent pulsating flow and heat transfer in an internally longitudinal protuberant finned tube was numerically investigated by solving unsteady three‐dimensional elliptical Navier–Stokes equations. The realized k–? turbulent model was adopted. The dynamic behaviors of velocity field, average Nusselt number, and friction number of the internally longitudinal protuberant finned tube were numerically analyzed in a pulsating period, and it was further investigated by changing the frequency of the pulsating flow. It was found that the intensity of heat transfer enhancement increases with an increase of pulsating frequency, while the pressure drop will be increased simultaneously, the intensification of heat transfer in internally longitudinal protuberant finned tubes are gradually better than the pressure drop with an increase of pulsating frequency. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20253     

Heat Transfer Performance of an Edge-Shaped Finned Tube

The third-generation heat transfer technologies, such as three-dimensional fin and dimple, are still important means of improving energy efficiency and will continue to be challenging issues. This paper presents condensation heat transfer performance of an edge-shaped finned tube fabricated by a ploughing–extruding process. The edge-shaped finned tube integrates more than one heat transfer enhancement technology and can enhance the heat transfer capacity greatly. It is seen that the overall heat transfer coefficient and heat flux increase with inlet velocity of cold water increasing, and decrease with inlet temperature of cold water increasing, whereas the shell-side heat transfer coefficient decreases with inlet velocity of cold water increasing and increases with inlet temperature of cold water increasing. At the same inlet velocity, the shell-side heat transfer coefficient for the edge-shaped finned tube is improved by 5–7 times compared to that of a smooth tube. At the same temperature difference between wall and vapor, the shell-side heat transfer coefficient is also higher than what had been reported in the literature. The shell-side heat transfer coefficient of the edge-shaped finned tube decreases with the increase of fabrication parameter feed at the same inlet velocity or inlet temperature of cold water.     

内展翅片换热器应用在压缩空气系统中的节能效益分析

文中通过实验数据分析以及实际工程案例,就内展翅片换热器在压缩空气系统的相同工况下,采用光管换热器和外翅片换热器的换热效果及阻力特性进行了比较,结果表明,内展翅片换热器不仅具有较好的强化传热效果,而且单位压降损耗低,节能效果显著。     

内展翅片换热器在压缩空气系统中的应用分析

通过实验数据分析以及实际工程案例,对内展翅片换热器在压缩空气系统的相同工况下采用光管换热器和外翅片换热器的换热效果及阻力特性进行了比较。结果表明,内展翅片换热器不仅具有较好的强化传热效果,而且单位压降损耗低,节能效果显著。     

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。     

不可凝气体对大扁管空冷凝汽器性能的影响分析

为了研究不可凝气体(non-condensable gases, NCG)对火电与光热发电机组上广泛使用的大扁管空冷凝汽器性能的影响,以工程机组凝汽器上普遍应用的通流面积220 mm×20 mm的大扁管为研究对象,针对汽轮机典型工况下的实际蒸汽流量,基于Lee相变方程、VOF方法以及组分扩散模型,对蒸汽与NCG混合气体管内两相流凝结换热进行数学建模与数值计算。结果表明：由于大扁管的狭窄通流几何结构与高蒸汽流量,NCG对管内蒸汽凝结的抑制效果要远低于预期;当入口空气质量分数按2%增加时,凝结管凝结换热系数仅下降2%左右,这与NCG导致低流量圆管凝结性能急剧下降的结论不同;空气正常泄漏不会导致空冷凝汽器性能下降而影响发电机组效率。     

Characterization of heat transfer modes of tube side convective condensation under the influence of an applied DC voltage

An experimental study was performed to determine the modes of heat transfer in horizontal tube side convective condensation under an applied DC high voltage. Experiments were performed with R134a in a horizontal, single-pass, counter-current heat exchanger. A 8 kV DC voltage was applied through a central rod electrode with the tube wall grounded for mass flux in the range 45–156 kg/m² s. The average quality across the test section was 45%, which corresponds to an initially stratified flow with the liquid level below the central electrode. The high-voltage electric field results in an interfacial electric body force which extracts the liquid from the bottom stratum towards the vapour core. This results in a phase redistribution with a resultant increase in heat transfer. The increase in heat transfer can be attributed to a change of the heat transfer mode in the lower section of the tube from convective condensation to film condensation.     

Enhancement of condensation heat transfer by means of EHD condensate drainage

The problem of electrohydrodynamic (EHD) condensation enhancement has been studied very recently. The electric field is commonly generated in such case by means of rod and mesh electrodes. The EHD method is suitable for dielectric media used in refrigeration and heat pump devices. The strong positive EHD effect has been obtained so far for vertical and horizontal bare tubes. However it was found that this effect for horizontal integral-fin tubes, commonly used in heat exchangers, is negligible small. The mechanism of the EHD condensation enhancement is discussed in the paper and a novel arrangement of the tube-electrode system is proposed. It consists of a horizontal finned tube with a rod type electrode placed beneath the tube. The experimental investigations have been carried out for this tube-electrode arrangement using HCFC-123 as a working medium. The obtained results confirmed the expectations since the application of EHD method for the new tube-electrode configuration increased the heat transfer coefficient from 27% to 110%, depending on the electrode potential. A simple model of the EHD condensate drainage has been proposed for the new tube-electrode arrangement.     

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     

Effect of inundation for condensation of steam on smooth and enhanced condenser tubes

The paper presents new measurements on the effect of inundation during condensation of steam in tubes banks. Most of the data relate to wire-wrapped enhanced tubes but measurements are also reported for low-finned and smooth tubes. The technique of artificial inundation has been used where liquid is supplied above a single horizontal test condenser tube to simulate condensate draining from higher tubes. Inundation rates have been used to simulate a column of up to almost 30 tubes. The surface temperature of the condenser tube was measured at four locations around the tube using buried thermocouples. The heat transfer and hence condensation rate was determined from the mass flow rate and temperature rise from coolant. The temperature and flow rate of the simulated inundation was carefully controlled. All tests were carried out at atmospheric pressure with constant vapour downflow approach velocity and constant coolant flow rate. For the given coolant and vapour flow rates and temperatures (same for all tests), and in the absence of inundation, the vapour-side heat-transfer coefficient for the finned tube was around four times that of the smooth tube while the heat-transfer coefficient for the wire-wrapped tubes was independent of winding pitch and around 30% higher than for the smooth tube. For inundation conditions the smooth tube data are in line with the widely used Kern equation relating the heat-transfer coefficient to the depth of a tube in the bank. The heat-transfer coefficient for the finned tube was virtually unaffected by inundation up to the maximum used which was equivalent to a depth of about 20 finned tubes in a bank. At this depth level the heat-transfer coefficient for the finned tube was around six times that of the smooth tube. For the wire-wrapped tubes the deterioration in performance with increasing inundation was least for the smallest winding pitch used for which the heat-transfer coefficient fell by around 9% at an equivalent depth in a bank of 25 tubes. At this depth level the heat-transfer coefficient for the wire-wrapped tube was almost twice that of the smooth tube.     

Condensation enhancement on a vertical plate (1st report,heat transfer characteristic on a dispersed finned surface)

In this study, a condensation heat transfer experiment on vertical continuous and dispersed finned surfaces using FC5312 was carried out. Experimental parameters were the pitch and height of the fin, and the dispersed fin length. In the results, the phenomena of condensate retention were observed in the bottom of each row of the dispersed fin. The condensate flow from the upper row was concentrated into the valley of the fin and then flowed down into the valley of the next fin. Moreover, it was found from the experiment that the heat transfer coefficient on the dispersed finned surface was lower than the one on the continuous finned surface as the fin pitch was smaller, but was larger than that of the continued finned surface for a larger fin pitch. Furthermore, the heat transfer enhancing effect became more significant for the higher fin with the larger fin pitch, and the heat transfer reducing effect became more significant for the lower fin with the smaller fin pitch. These special characteristics of condensation mentioned above were caused by the phenomena of condensate retention in each row of the fin and the flow pattern of the condensate between two adjacent fins on the dispersed finned surface based on experimental observations. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20221     

Experimental and theoretical study of waste heat recovery from a refrigeration system using a finned helical coil heat exchanger

This paper is an experimental and theoretical study that aimed at conserving energy by utilizing the waste heat generated from a refrigeration system by calculating the range of condensation after the compression stage for the refrigerant (R410A). A helical coil tube‐shell heat exchanger was designed as a heat recovery unit to use the waste heat from an air conditioner 1TR (split type) in the outdoor unit between the compressor and the condenser to produce hot water and increase the coefficient of performance (COP) of the refrigeration cycle. Two experimental types of the helical coil heat exchanger (conventional and finned) were used in attempts to induce absorption of the rejected heat into tap water. The increase in the COP ranges from 12.5% to 40%, an increase in the water outlet temperatures difference reaches 12°C. A cost–benefit analysis in terms of the net present value and the payback period (PP) has been performed. From the analysis, it has been observed that the use of the designed heat recovery unit will save electrical consumption to produce the required hot water with a PP of about 11.7 months for the conventional heat exchanger and 17.5 months for the finned helical coil heat exchanger.     

Optimized mass flux ratio of double-flow solar air heater with EHD

The numerical study of laminar forced convection inside double-flow solar air heater with electrohydrodynamic technique is investigated by finite difference method. The electric field is generated by the wire electrodes charged with DC high voltage. The mathematical modeling of computational fluid dynamics includes the interactions among electric field, flow field, and temperature field. It can be perceived that augmented heat transfer with presence of an electric field increases with the supplied voltage but decreases with the total mass flux. The optimized mass flux ratio is expressed incorporating with concerning parameter comprising of the electrode arrangement, the number of electrodes, the total heat flux at an absorbing plate, and the channel geometry.     

椭圆钎焊与双金属轧制翅片管传热与阻力性能的试验对比

对椭圆钎焊翅片管和双金属轧制翅片管换热器的传热及阻力性能进行试验对比研究,试验得到了一系列工况下的传热数据与管外空气流动阻力数据,给出了相应的传热系数、流动阻力曲线。从总传热系数中分离出管外空气侧的对流换热系数,得到了具有一定应用价值的管外换热的计算关联式;拟合得到了管外阻力计算关联式。结果表明:椭圆钎焊翅片管比双金属轧制翅片管的传热系数约高9%,管外换热系数约高17%,且管外空气流动阻力约低11%。     

Mechanism of tonal noise generation from circular cylinder with spiral fin

The pitch of the spiral finned tube influences seriously to the acoustic resonance in the heat exchanger.In this research,the flow characteristics in relating to the aeolian tone from the finned cylinder are studied by the numerical simulation.It is observed that the tonal noise generated from the finned tube at two pitch spaces.The ratio of the fin pitch to the cylinder diameter is changed at 0.11 and 0.27.The tone level increases and the frequency decreases with the pitch shorter.The separation flow from the cylinder generates the span-wise vortices,Karman vortices,and the separation flow from the fin generates the stream-wise vortices.When the fin pitch ratio is small,the stream-wise vortices line up to span-wise and become weak rapidly.Only the Karman vortices are remained and integrate in span.So the Karman vortex became large.This causes the low frequency and the large aeolian tone.