单向开缝翅片管换热器传热与阻力性能的数值模拟及试验研究

对一种单向开缝翅片管换热器进行了数值模拟及试验研究,分析了不同翅片间距及管径下单向开缝翅片管换热器的传热与阻力性能的变化规律。数值模拟和试验结果的对比表明,采用数值模拟方法研究单向开缝翅片管换热器的传热与阻力性能是可行的。     

翅片参数对开缝翅片管换热器换热与阻力性能的影响

在模化试验验证的基础上,通过数值模拟,获得了翅片间距P_f及开缝数量n_s对开缝翅片管换热器性能的影响规律:n_s≤6时,翅片侧Nu和流动阻力均随着P_f增大而减小;n_s6时,翅片侧Nu随P_f增大先减小后增大,而阻力逐渐降低;P_f=3.51～3.97 mm时,随n_s增大,阻力逐渐增大,n_s=4～6时,翅片侧Nu逐渐增大,n_s=6～8时,翅片侧Nu变化较小;P_f=3.97～4.43 mm时,n_s由4片增加至8片,翅片侧Nu和阻力均逐渐增大。根据不同结构的开缝翅片管换热器的综合流动换热性能,提出了P_f与n_s的最佳组合。     

管间距对开缝翅片管换热器传热与阻力特性的影响

为了获得管间距对开缝翅片管换热器传热与阻力特性的影响规律,对5种不同翅片管换热器进行了数值模拟研究,并进行了模化试验验证。结果表明:开缝翅片管束的传热和阻力特性与翅片侧气体的Re数有关,随着Re数增大,翅片侧Nu数增大,摩擦因子f逐渐减小;纵向间距S2对开缝翅片管换热器的综合流动传热性能的影响较大。数值模拟与试验结果偏差较小,采用数值模拟方法能够比较准确地分析开缝翅片管换热器的传热与阻力特性。     

百叶窗翅片管换热器空气侧传热和流动特性数值模拟

采用数值模拟方法对百叶窗翅片管换热器空气侧传热和流动特性进行研究,分析管排数、开窗角度和翅片间距对百叶窗翅片管换热器空气侧性能的影响。结果表明:空气侧传热系数随管排数增多而降低,最大降幅约12.5%,压降随管排数增多而增大;低雷诺数下百叶窗角度为20°时换热器具有较好的综合性能,较大雷诺数下25°为最佳百叶窗角度;随着翅片间距的减小,换热器传热因子j和阻力因子f均逐渐增大,但低雷诺数时翅片间距较小的换热器综合性能较差。     

低气压下翅片管换热器空气侧换热特性的实验研究

为研究低气压环境下翅片管换热器空气侧的换热特性,对不同气压环境下空气侧流速和翅片间距对平翅片管换热器空气侧换热特性的影响进行了实验分析。实验环境气压范围为40～100 kPa,换热器迎面风速为1.0～3.5 m/s,翅片间距2～3 mm。研究表明:实验工况下环境气压40 kPa时空气侧传热因子仅为常压下的30.42%～46.41%;低气压环境空气侧流速和翅片间距对空气侧换热的影响趋势与常压数据基本保持一致;不改变换热器结构,环境气压的变化仅影响空气物性,而对空气的流动状态的影响不大;翅片间距影响随Re的减小和环境气压的降低而减弱,两种翅片间距模型空气侧传热因子平均差异在环境气压为100 kPa时为12.07%,40 kPa时缩小为3.00%。     

圆弧型开缝翅片管空气冷却器传热与阻力特性试验研究

对3种不同翅片间距的圆弧型开缝翅片管空气冷却器进行试验研究,得到迎面风速在1.0~3.0 m/s空气侧传热与阻力特性变化规律,分析了迎面风速、翅片间距对换热器传热与阻力特性的影响;雷诺数Re在1200~3800,综合性能指标随着Re的增大而增大;当Re1800时,Pf=1.7 mm的综合流动传热性能最好,当Re1800时,Pf=2.5 mm的综合流动传热性能最好;圆弧型开缝翅片管的综合流动传热性能比平直翅片管高。     

汽轮发电机气体冷却器性能试验研究

气体冷却器是汽轮发电机的重要设备之一。其传热与阻力性能将直接影响汽轮发电机的运行经济性和可靠性。为实现汽轮发电机气体冷却器的优化设计,对不同翅片间距的翅片管冷却器的传热和阻力性能进行了试验研究,得到了Re在3 000~190 000之间换热器翅片侧的传热和阻力特性,并分析了风速和翅片间距对气体冷却器传热和阻力性能的影响规律。研究成果对汽轮发电机气体冷却器的结构与性能优化具有重要的指导作用。     

螺旋翅片管束传热和阻力特性的试验研究

对13个不同翅片间距、翅片高度、横向管间距、纵向管间距的螺旋翅片管束换热器在不同雷诺数条件下的传热和阻力特性进行了试验研究,得出了翅片间距、翅片高度、横向管间距、纵向管间距及雷诺数与换热特性Nu和阻力特性Eu的准则关系式,并对准则关系式进行了分析.结果表明:随着横向管间距和翅片间距的增大,螺旋翅片管的传热得到强化,但随着纵向管间距和翅片高度的增加,螺旋翅片管的传热有所减弱;随着横向管间距、纵向管间距和翅片间距的增大,螺旋翅片管的阻力减少,但随着翅片高度的增加,螺旋翅片管的阻力增加.     

平直翅片换热器空气侧换热与阻力特性研究

以平直翅片管式换热器为研究对象,利用计算流体力学(CFD)软件进行流动与传热模拟计算。采用正交试验方法确定模型工况,对换热负荷15 kW,设计气温30℃,入口风速为1～5 m/s,管壁温度为40～60℃,翅片间距为1～5mm、翅片厚度0.5～4 mm、管纵向间距为0.5～2.5倍外管径,管排数为1～5排的计算工况进行努塞尔数、阻力因子的计算分析。参数敏感性分析结果表明:在1 mm≤翅片间距δ≤5 mm,444≤雷诺数Re≤3 405时,翅片间距δ是对努塞尔数Nu及阻力因子f影响最大的结构参数。在该范围内提出了由翅片间距与特征长度比值组成的无量纲参数对努塞尔数Nu与阻力因子f的计算关联式。该关联式参数图表明:翅片间距δ越小、雷诺数Re越大,对提高平直翅片努塞尔数、降低阻力因子越有利。     

开缝布置方式对开缝翅片管换热器 传热与阻力特性的影响

为了获得开缝布置方式对开缝翅片管换热器传热与阻力特性的影响规律,对5种不同翅片管换热器进行了数值模拟研究,并进行了模化试验验证。结果表明：增加开缝会提高翅片管换热器的传热性能,但阻力也随之增加;与开缝位置相比,开缝数量对开缝翅片管换热器传热与阻力特性的影响更大;在Re=4800~7500日时,开缝翅片管换热器综合流动传热性能 随着Re数的增大而增大;在5种翅片中,开缝翅片的综合流动传热性能高于普通平直翅片;数值模拟与试验结果偏差较小,采用数值模拟方法能够比较准确地分析开缝翅片管换热器的传热与阻力特性。     

Fin Pattern Effects on Air-Side Heat Transfer and Friction Characteristics of Fin-and-Tube Heat Exchangers with Large Number of Large-Diameter Tube Rows

Air-side heat transfer and friction characteristics of nine kinds of fin-and-tube heat exchangers, with a large number of tube rows (6, 9, and 12, respectively) and large diameter of tubes (18 mm), are experimentally investigated. The test samples consist of three types of fin configurations: plain fin, slit fin, and fin with delta-wing longitudinal vortex generators. The working fluid in the tube is steam. Results show that when the number of tube is larger than 6, the heat transfer and friction performance for three kinds of fins is independent of the number of tube rows, and slit fin provides higher heat transfer and pressure drop than the other two fins. The heat transfer and friction factor correlations for all the heat exchangers were acquired with Reynolds numbers ranging from 4000 to 10000. The air-side performance of heat exchangers with plain fin, slit fin, and longitudinal vortex-generator fin were evaluated under three sets of criteria, and the results showed that the heat exchanger with slit fin has better performance than that with vortex-generator fin, especially at high Reynolds numbers.     

Experimental Study on Heat Transfer and Pressure Drop Characteristics of Four Types of Plate Fin－and－TUbe Heat Exchanger Surfaces

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Numerical study of local heat transfer coefficient and fin efficiency of wavy fin-and-tube heat exchangers

Three-dimensional numerical simulations were performed for laminar flow of wavy fin-and-tube heat exchangers by using body-fitted coordinates (BFC) method with fin efficiency effect accounted. The prediction results of average Nusselt number, friction factor and fin efficiency were compared with the related experimental correlations [R.C. Xin, H.Z. Li, H.J. Kang, W. Li, W.Q. Tao, An experimental investigation on heat transfer and pressure drop characteristics of triangular wavy fin-and-tube heat exchanger surfaces, J. Xi'an Jiaotong Univ. 28 (2) (1994) 77–83] and Schmidt approximation [T.E. Schmidt, Heat transfer calculations for extended surfaces, Refrigerating Engineering (April 1949) 351–357]. For Reynolds numbers based on the tube outside diameter ranging from 500 to 4000, the mean deviation is 3.3% for Nusselt number, 1.9% for friction factor and 3.6% for fin efficiency. The distributions of local Nusselt number and fin efficiency on fin surface were studied at wavy angle equal to 0° (plain plate fin), 10° and 20° respectively. The local Nusselt number decreases along the air flow direction, but fin efficiency increases in general. The wavy angle can greatly affect the distributions of local Nusselt number and fin efficiency, and make the distributions present fluctuation along the flow direction. The result also shows that the fin efficiency at the inlet region of wavy fin is larger than that of plain plate fin at the same region. With the increase of Reynolds number, the effects of wavy angle on the distributions of local Nusselt number and fin efficiency are more and more significant.     

On the heat transfer characteristics of heat sinks: Influence of fin spacing at low Reynolds number region

This study examines the thermal–hydraulic performance of heat sinks having plate, slit, and louver fin patterns. Comparison of the associated heat transfer performance and the effect of fin spacing are made. The results indicate that the enhanced fin patterns like louver or slit fin operated at a higher frontal velocity and at a larger fin spacing is more beneficial than that of plain fin geometry. The heat transfer performance of louver fin is usually better than that of slit fin but accompanies with higher pressure drops. However, it is found that the pressure drops for slit fin is comparable to the louver fin geometry when the fin spacing is reduced to 0.8 mm. This is associated with the appreciable rise of entrance/exit loss (form drag) caused by the slit fin geometry. The test results also reveal a significant drop of heat transfer performance at a low Reynolds number and at a small fin spacing, or the so-called “maximum” phenomenon of Colburn j factor. This is applicable to all the tested geometries. By a careful examination of the test results, it is concluded that this phenomenon is related to the developing/fully developed flow characteristics. In fact, the maximum point occurred roughly at x⁺ = 0.1 where fully developed and developing flow is separated.     

Three-dimensional numerical study of wavy fin-and-tube heat exchangers and field synergy principle analysis

In this paper, 3-D numerical simulations were performed for laminar heat transfer and fluid flow characteristics of wavy fin-and-tube heat exchanger by body-fitted coordinates system. The effect of four factors were examined: Reynolds number, fin pitch, wavy angle and tube row number. The Reynolds number based on the tube diameter varied from 500 to 5000, the fin pitch from 0.4 to 5.2 mm, the wavy angle from 0° to 50°, and the tube row range from 1 to 4. The numerical results were compared with experiments and good agreement was obtained. The numerical results show that with the increasing of wavy angles, decreasing of the fin pitch and tube row number, the heat transfer of the finned tube bank are enhanced with some penalty in pressure drop. The effects of the four factors were also analyzed from the view point of field synergy principle which says that the reduction of the intersection angle between velocity and fluid temperature gradient is the basic mechanism for enhance convective heat transfer. It is found that the effects of the four factors on the heat transfer performance of the wavy fin-and-tube exchangers can be well described by the field synergy principle.     

Three-dimensional performance analysis of plain fin tube heat exchangers in transitional regime

Three-dimensional CFD simulations are carried out to investigate heat transfer and fluid flow characteristics of a four-row plain fin-and-tube heat exchanger using the Commercial Computational Fluid Dynamics Code ANSYS CFX 12.0. Heat transfer and pressure drop characteristics of the heat exchanger are investigated for Reynolds numbers ranging from 400 to 2000. Fluid flow and heat transfer are simulated and results compared using both laminar and turbulent flow models (k-ω) with steady and incompressible fluid flow. Model validation is carried out by comparing the simulated case friction factor (f) and Colburn factor (j) with the experimental data of Wang et al. [1]. Reasonable agreement is found between the simulations and experimental data. In this study the effect of geometrical parameters such as fin pitch, longitudinal pitch and transverse pitch of tube spacing are studied. Results are presented in the form of friction factor (f) and Colburn factor (j). For both laminar and transitional flow conditions heat transfer and friction factor decrease with the increase of longitudinal and transverse pitches of tube spacing whereas they increase with fin pitches for both in-line and staggered configurations. Efficiency index increases with the increase of longitudinal and transverse pitches of tube spacing but decreases with increase of fin pitches. For a particular Reynolds number, the efficiency index is higher in in-line arrangement than the staggered case.     

Correlations for wet surface ratio of fin-and-tube heat exchangers

Correlations are proposed for the wet surface ratio of a fin-and-tube heat exchanger with plain and wavy fin geometry under dehumidifying conditions. The ‘Finite Circular Fin Method’ (FCFM) is used for data reduction. It is found that the percentage of wet surface area increases with increasing fin spacing or number of tube rows and decreasing Reynolds number. Despite the addition of tube rows or reduced fin spacing the effective surface area is increased, and its influence on a partially wet surface is apparently the opposite. This is because adding tube rows will provide a more effective temperature drop in air flow than adding fins, Moreover, the heat and mass transfer characteristics of j_h and j_m increase with an increase in the area of wet surface. Correlations for prediction of the percentage of wet surface area are proposed. These correlations can describe 83.81% of the area of wet surface to within ±10%.     

Comparative studies on performance of plain,perforated, threaded,and threaded–perforated pin fin: A numerical approach

A heat sink is a thermal management system for electrical and electronic appliances whose performance is a function of fin geometry, arrangement, and flow field. Earlier research addressed the enhancement in the heat dissipation capacity of the sink with a change in the geometry of the fin. However, the change should increase the heat transfer rate per unit weight and per unit volume. One such attempt is made in the present work, which deals with numerical forced convection heat transfer simulation over a pin fin with three different surface modifications, namely, threads, equilateral triangular perforation, and threads with perforations. A numerical investigation is performed for 0.5773–2.5574 mm pitch of threads, 3–4.8 mm size of perforation, and 2–8 m/s velocities of air. To describe the flow pattern around the fin and its variation with surface modification, streamline profiles are drawn which reveals that the fluid–solid interaction is improved either with threaded or perforated surface and is maximum for threaded–perforated fin. The enhanced convection rates bring down the local fin temperature and the maximum fin temperature, where the drop is more for the threaded surface than that of the perforated surface because of turbulence. A 10° drop in maximum fin temperature is achieved by replacing a plain pin fin with a threaded–perforated pin fin, and the drop is 8° with threads alone and 6° only with perforations. The increased fineness of threads and size of perforation further bring down the maximum fin temperature.     

Numerical investigation for improved heat transfer characteristics in micro‐fin tubes

Generally, internal micro‐fin tubes are used for increasing the life and performance of electronic devices. The micro‐fins enhance the heat transfer rate by increasing the surface area with an increase of the pressure drop. In this study, heat transfer and pressure drop are analyzed by varying Reynolds number with the increase in the number of fins in tubes. Heat transfer and pressure drop, together with turbulence kinetic energy of micro‐fin tubes (helical and straight) and a smooth tube, have been evaluated for different Reynolds numbers (60 000, 40 000, 20 000, and 2000) at a constant temperature of 350 K, which clearly establishes laminar to turbulent flow. It is observed that the helical micro‐fin tube has a better result compared with the straight micro‐fin tube and smooth tube at Reynolds numbers 60 000, 40 000, and 20 000 at velocity 2, 1, and 0.5 m/s, respectively. This study is an attempt to establish a comparison of different micro‐fin geometries with varying Reynolds numbers, concluding that a high Reynolds number is suitable for the same.