相变换热器烟气侧流动及传热特性的数值模拟

对壁温均匀的相变换热器换热管外烟气的流动状态与翅片管的换热过程进行数值模拟,分析烟气入口速度、翅片间距及翅片管横向间距、纵向间距对流动传热特性的影响。结果表明:随着烟气入口速度的增加,换热量和烟气流动阻力均增加;一定范围内增大翅片间距,能够强化传热性能,降低烟气流动阻力;翅片管横向间距的增大能强化传热,而纵向间距的增大会减弱传热性能,二者均能够降低烟气的流动阻力。     

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。     

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。     

新型轴对称内翅管换热过程的数值模拟

采用数值模拟的方法,对新型整体式轴对称不等高内展翅片换热管的管内流动、换热及阻力特性进行了研究,并将模拟结果分别与实验测量值和光管经验公式计算值进行了比较,结果表明:数值模拟结果与实验测量结果和经验公式计算结果符合得较好;新型内翅片管的强化换热效果最好,其换热系数约为光管的1.67~2.27倍,为中心对称翅片管的1.17~1.35倍;内翅片管的阻力系数是光管的1.44~2.20倍,中心对称和新型内翅片管具有基本相同的阻力系数;8翅和10翅新型内翅片管的综合性能参数较优.     

湿烟气在开孔翅片管外凝结换热数值模拟

为高效回收湿烟气全热并对烟气冷凝换热设备进行优化设计,以设计的紧凑式开孔翅片管换热器为对象,采用欧拉壁膜(EWF)模型与组分输运模型耦合研究低温烟气在翅片管换热器中的凝结换热规律。数值模拟得到的凝结速率及对流凝结换热系数与实验结果最大偏差分别为13.4%和10.9%。结果表明：对流凝结换热系数随入口水蒸气质量分数和烟气流速增大以及管壁温降低而增大,翅片开孔可以起到均压、破坏温度边界层、截断液膜、加快凝结液排出进而强化传热的作用,基于模拟拟合的关于改进雅各布数J的烟气对流凝结关联式与实验数据平均相对误差为13.1%,模拟关联式对于90%的实验结果预测误差在-20%～+20%以内。     

空冷凝汽器U型翅片管换热特性研究

以U型翅片单排椭圆管为研究对象,给出其传热过程的物理模型,通过流固界面传热耦合,应用计算流体力学(CFD)软件,对不同迎面风速下翅片换热特性进行数值模拟;并利用"场协同理论"进行分析,得出了翅片管换热量与迎面风速的关系以及最佳翅化比,为U型翅片管的设计提供参考.     

恒肋重鳍片管换热与流动特性的三维数值模拟

利用FLUENT软件对省煤器横截面积恒定的4种矩形鳍片管的传热和流动特性进行了三维数值模拟,将数值模拟结果与试验结果进行了比较,并编程计算分析了四种鳍片的换热效率。结果表明:恒重时,随鳍片厚度肋效率增大,其对流换热效果增加,但流动阻力也增大;5mm×20mm矩形鳍片管的换热效果较高并且流动阻力系数较小,其综合强化传热效果好。     

波壁管几何尺寸对流动与传热特性的影响

基于数值模拟的方法,分析在低雷诺数下波壁管波形变化对流体流动与传热特性的影响,并分析了相同功耗下波壁管的综合传热性能。结果表明:波幅和波长变化对波壁管传热均有影响,强化效果与波幅成正比,与波长成反比;当功耗相同时,小波幅的波壁管有较好的综合换热效果,大波幅的波壁管强化传热以较大能量消耗作为代价;雷诺数大于2 000时,增大波长能达到较好的综合换热效果。     

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。     

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。     

Effect of lateral fin profiles on turbulent flow and heat transfer performance of internally finned tubes

Heat transfer performance of internally finned tubes with blocked core-tube was numerically investigated by the realizable k–ε turbulence model with wall function method using a commercial software FLUENT. Three kinds of lateral fin profiles, that is, S-shape, Z-shape and V-shape, were studied and compared. The corresponding correlations of Nusselt number and friction factor for different-shape internally finned tubes were obtained. The comprehensive performances of the studied tubes were compared under identical mass flow rate, identical pumping power and identical pressure drop conditions. It was found that tubes with S-shape fins and Z-shape fins were superior to that with V-shape fins, and moreover, tube with Z-shape fins had the best performance. The fin outer curvature radius R near the inner surface of out-tube for the S-shape finned tube had appreciable effect on heat transfer, whereas the fin inner curvature radius r near the outer surface of blocked core-tube had little impact on heat transfer. Hence, when manufacturing the internally finned tube with S-shape fins, it is better to select the outer curvature radius R as smaller as possible.     

Laminar Flow Forced Convection Heat Transfer Behavior of a Phase Change Material Fluid in Finned Tubes

The heat transfer behavior of phase change material fluid (PCM) under laminar flow conditions in circular tubes and internally longitudinal finned tubes was studied. An effective specific heat technique was used to model the phase change process. Heat transfer results for a smooth circular tube with PCM fluid were obtained under hydrodynamically and thermally fully developed conditions. Results for the finned tube were obtained using the H2 and T boundary conditions. It was determined that the Nusselt number was strongly dependent on the Stefan number, fin thermal conductivity value, and height of the fins.     

Analysis of an air cooled ammonia–water vertical tubular absorber

This paper presents a detailed analysis of an ammonia–water vertical tubular absorber cooled by air. The absorption process takes place co-currently upward inside the tubes. The tubes are externally finned with continuous plate fins and the tube rows are arranged staggered in the direction of the air flow. The air is forced over the tube bank and circulates between the plain fins in cross flow with the ammonia–water mixture. The analysis has been carried out by means of a mathematical model developed on the basis of mass and energy balances and heat and mass transfer equations. The model takes into account separately the churn, slug and bubbly flow patterns experimentally forecasted in this type of absorption processes inside vertical tubes and considers the simultaneous heat and mass transfer processes in both liquid and vapour phases, as well as heat transfer to the cooling air. The model has been implemented in a computer program. Results based on a representative design and nominal operating conditions of an absorber for a small capacity ammonia–water absorption refrigeration system are shown. A parametric analysis was realised to investigate the influence of the design parameters and operating conditions on the absorber performance. The noteworthy results that have effect on practical design of the absorber are presented and commented.     

Performance investigation of plain and finned tube evaporatively cooled heat exchangers

The performance of two evaporatively cooled heat exchangers is investigated under similar operating conditions of air flow rates and inlet hot water temperatures. The heat exchangers are plain and plate-finned circular tube types which occupy the same volume. Spray water, which is circulated in a closed circuit, is injected onto the exposed surfaces of the tubes and fins. The contact between air and spray water results in evaporative heat transfer. The tubes are copper, 10 mm o.d. The finned configuration is constructed by introducing 0.5 mm thick copper plates between the tubes, with a total area ratio of four. A substantial increase in heat transfer takes place for the plate-finned tubes. The increase is 92–140% for air velocities from 1.66 to 3.57 m s⁻¹. A model is used to calculate the thermal performance of the plain and finned tubes assuming a constant spray water temperature in the heat exchanger. The wet-finned surfaces show low fin efficiency compared with dry surfaces. An energy index defined as the ratio of volumetric thermal conductance to air pressure drop per unit length is found to be close for the two heat exchangers. This reveals higher thermal utilisation of the occupied volume by the finned tubes with the same energy index.     

椭圆翅片管空冷器流动传热特性的研究

用稳态的恒壁温法对3个椭圆翅片管空冷器和1个圆翅片管空冷器的传热和阻力特性进行了研究,得到空冷器空气侧的传热与阻力性能,在相同的迎风面流速下,椭圆翅片管比圆翅片管空气侧换热系数约大3－7倍;在相同的换热系数下,椭圆翅片管比圆翅片管的压降低。     

Numerical computation of flow and heat transfer in finned and unfinned tube banks

A finite volume numerical scheme is utilized to predict fluid flow and heat transfer characteristics in inline tube banks. The effect of equipping the tubes with longitudinal fins on the pressure drop and heat transfer is studied. The governing equations for fluid flow and heat transfer are numerically solved, with the assumption of periodic, fully developed flow. The numerical methodology utilizes the stepped boundary technique to approximate the tube surface. The tubes are maintained at a constant temperature, and the calculations are carried out for laminar flow and for a large range of Reynolds and Prandtl numbers. The results for the unfinned tube case are compared with previously published experimental data. The numerical results agree well with the experimental measurements. Representative results for the case of the finned tubes indicate, surprisingly, a decrease in the heat transfer rate, and small changes in the pressure drop, as a result of finning. The decrease in the heat transfer rate probably occurs because the fins are placed in the stagnation regions at the front and rear of the tubes, and thus do not increase the heat transfer.     

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.     

不同材质花瓣形扁通道内翅换热管的实验研究

通过实验的方法研究了一种花瓣形扁通道内翅换热管的对流换热和阻力特性,拟合了所测Re范围内对流换热和阻力实验关联式,并运用相同质量流量、相同泵功率和相同阻力降这3种准则比较了采用不同材质时翅片管与普通光管之间的传热效果。结果表明,翅片材质对换热强化效果有较大影响,无论采用哪种材质,花瓣形扁通道内翅管均有较强的换热效果,特别是在低盅条件下,强化效果更加明显。     

Forced Convective Condensation of Nonazeotropic Refrigerant Mixtures in Horizontal Annulus with Petal Shaped Fin Tubes

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