A Numerical Study on Fully Developed Fluid Flow and Heat Transfer in a Spiral Finned Tube

In this paper, the standard k-ε two-equation model is adopted to numerically simulate fully developed fluid flow and heat transfer in a spiral finned tube within a cracking furnace for ethylene manufacturing. By variable transformation, the original 3-D problem is converted into a 2-D problem in spiral coordinates. The algorithm of SIMPLEC is used to study the fully developed fluid flow and heat transfer in the spiral finned tube at constant periphery temperature and constant axial heat flux. The computed results agree pretty well with the experimental data obtained from the industry. Further studies on the fluid flows and temperature profiles at different Reynolds numbers within straight and spiral finned tubes are conducted and the mechanisms involved are explored. It is found that with the spiral finned tube, pressure drop increases to a great extent whereas heat transfer tends to be decreased.

Numerical analysis of heat transfer enhancement on steam condensation in the presence of air outside the tube

In loss-of-coolant accidents,a passive containment heat removal system protects the integrity of the containment by condensing steam.As a large amount of air exists in the containment,the steam condensation heat transfer can be significantly reduced.Based on previous research,traditional methods for enhancing pure steam condensation may not be applicable to steam–air condensation.In the present study,new methods of enhancing condensation heat transfer were adopted and several potentially enhanced heat transfer tubes,including corrugated tubes,spiral fin tubes,and ring fin tubes were designed.STAR-CCM+was used to determine the effect of enhanced heat transfer tubes on the steam condensation heat transfer.According to the calculations,the gas pressure ranged from 0.2 to 1.6 MPa,and air mass fraction ranged from 0.1 to 0.9.The effective perturbation of the high-concentration air layer was identified as the key factor for enhancing steam–air condensation heat transfer.Further,the designed corrugated tube performed well at atmospheric pressure,with a maximum enhancement of 27.4%,and performed poorly at high pressures.In the design of spiral fin tubes,special attention should be paid to the locations that may accumulate high-concentration air.Nonetheless,the ring-fin tubes generally displayed good performance under all conditions of interest,with a maximum enhancement of 24.2%.

Heat transfer and friction factor of Therminol liquid phase heat transfer fluid in a ribbed tube

Experiments and simulations on flow and heat transfer behavior of Therminol-55 liquid phase heat transfer fluid have been conducted in a ribbed tube with the outer diameter and inner diameter 25.0 and 20.0 mm,pitch and rib height of 4.5 and 1.0 mm.respectively.Experimental results show that the heat transfer and thermal performance of Therminol-55 liquid phase heat transfer fluid in the ribbed tube are considerably improved compared to those of the smooth tube.The Nusselt number increase with the increase of Reynolds number.The increase in heat transfer rate of the ribbed tube has a mean value of 2.24 times.Also,the pressure drop results reveal that the average friction factor of the ribbed tube is in a range of 2.4 and 2.8 times over the smooth tube.Numerical simulations of three-dimensional flow behavior of Therminol-55 liquid phase heat transfer fluid are carried out using three different turbulence models in the ribbed tube.The numerical results show that the heat transfer of ribbed tube is improved because vortices are generated behind ribs,which produce some disruptions to fluid flow and enhance heat transfer compared with smooth tube.The numerical results prove that the ribbed tube can improve heat transfer and fluid flow performances of Therminol liquid phase heat transfer fluid.

Heat Transfer Characteristics of Square Micro Pin Fins under Natural Convection

In order to comply with the recent demand for downsizing of the electric equipment, the minia- turization and the improvement in heat transfer performance of a heat sink under natural air-cooling are increasingly required. This paper describes the experimental and numerical investigations of heat sinks with miniature/micro pins and the effect of the pin size, pin height and the number of pins on heat transfer characteristics of heat sinks. Five types of basic heat sink models are investigated in this study. The whole heat transfer area of heat sinks having the different pin size, pin height and the number of pins respectively is kept constant. From a series of experiments and numerical analyses, it has been clarified that the heat sink temperature rises with increase in the number of pins. That is, the heat sink with miniaturized fine pins showed almost no effect on the heat transfer enhancement. This is because of the choking phenomenon occurred in the air space among the pin fins. Reflecting these results, it is confirmed that the heat transfer coefficient reduces with miniaturization of pins. Concerning the effects of the heat transfer area on the heat sink performance, almost the same tendency has been observed in other three series of large surface area, that is, higher pin height. Furthermore as a result of studying non-dimensional convection heat transfer performance, it was found that the relation between the Nusselt number (Nu) and the Rayleight number (Ra) is given by Nu = 0.16 Ra0.52.

Validation of the TASS/SMR-S Code for the Core Heat Transfer Model on the Steady Experimental Conditions

The SMART （System-integrated Modular Advanced ReacTor） which is a 330 MWt advanced integral PWR was developed by the KAERI （Korea Atomic Energy Institute） for electricity generation and seawater desalination. To enhance its safety, the various design concepts were adopted such as the most containing of the RCS （reactor coolant system） components and a PRHRS （passive residual heat removal system）. To ensure the safety and performance of the SMART, a thermal hydraulic evaluation and safety analysis are performed by the TASS/SMR-S code. It uses a one dimensional node/path modeling and point kinetics for the core power simulation. The code also has specific models reflecting the design features of the SMART such as a helical tube and PRHRS heat transfer models. In this study, the validation of the core heat transfer model in the TASS/SMR-S code on the steady conditions was performed with the Bennett＇s heated tube tests and THTF （thermal hydraulic test facility） experiment. From the results of the TASS/SMR-S code calculation, the CHF （critical heat flux） point and the fuel rod surface temperature were predicted conservatively compared to the test results.

Fin formation model during pre-roll ploughing of copper 3D outside fin tube

The mechanism of pre roll ploughing for 3D fins on the outside surface of copper tube was studied systematically, and especially the process and conditions of 3D fin formation were analyzed. The right mathematical model was also established. Based on the volume of fin ploughed out is equal to the volume of the metal extruded up by the extruding face of the tool, the relations between fin height, pre roll ploughing feed and pre roll ploughing depth have been achieved. With the increase of pre roll ploughing depth which must be equal to groove depth, the fin height gradually becomes larger. There are different critical feeds with the various depths of pre roll ploughing. The pre roll ploughing feed is the critical one, the height of fin is largest. And when the feed is above the critical one, the fin height will reduce with the increase of feed. The theoretical analysis basically accords with experimental results.

Compound forming technology of outside 3D integral fin of copper tubes

Using rolling-ploughing-extrusion compound processing methods,a 3D integral-fin structure on outside surface of red copper tube with diameter of 16.0 mm and wall thickness of 1.5 mm was obtained. When both rolling depth and ploughing-extrusion(P-E) depth were 0.2 mm,rotating speed was 50 r/min,feed speed was 0.16 mm/r,3D fin structures with height of 0.25 mm were gotten. Two different fin structures were obtained in grooves formed with rolling-ploughing-extrusion compound forming technology and observed by scanning electron microscope(SEM). One is the compound structure with V-shaped groove and U-shaped groove,and the other is the single structure with V-shaped grooves. Two kinds of groove structures obtained by rolling processing and ploughing extrusion processing are restricted together by groove interval and rolling depth,and pitch and P-E depth,respectively. Based on the analysis of interaction of rolling and P-E processing,it is found from the result that the outside 3D integral-fin can be achieved by rolling-ploughing-extrusion compound processing when single V-shaped groove structures are formed by both rolling and P-E processing.

Surface treatment of 3D outside serrated integral-fin tube manufactured by rolling and extrusion

The outside serrated integral-fin tubes fabricated by rolling-plowing-extrusion processing were surface-treated through different processes of annealing in hydrogen atmosphere,electrochemical corrosion or sandblasting.The purpose was to eliminate residual stress,clear secondary micro-fins and enhance heat transfer performance.By comparing the surface characteristics,it is found that the finned tubes treated by electrochemical corrosion have the most glabrous surfaces where the fins are almost perfectly reserved.Clear layer cracks can be observed on the top of the fins.These structures are effective in enhancing heat transfer performance when being applied to flow heat exchange.Therefore,the finned tubes treated by electrochemical corrosion are proper for the tubular exchanger with water coolant.The finned tubes treated by sandblasting have rougher surfaces with layer cracks and micro gaps removed.As these structures are useful to clearing adhesive feculence,the tubes are more suitable for the tubular heat exchanger with oil coolant.

EXPERIMENTAL STUDY ON ENHANCED HEAT TRANSFER WITH NI-BASED IMPLANTED SPIRAL FINNED TUBES

The new type of heat exchanger elements-Ni-based Implanted Spiral FinnedTurbes (NISFT) was studied and tested, which can contribute to increase the efficiency andreliability. The relation of Nu, Eu and Re with different fin pitch, fin height, transverse pitchand longitudinal pitch were gained, which constituted the basic foundation for the engineeringapplication of NISFT.

Study on heat transfer enhancement of discontinuous short wave finned flat tube

The typical configuration adopted by air-cooled condenser(ACC) in coal-fired power generating unit is the wave finned flat tube. The development of boundary layer between wave fins along long axis of flat tube can suppress the air-side heat transfer enhancement to a great extent. It has been proved that the serrated fins can enhance heat transfer obviously by breaking the development of boundary layer periodically. In the present study,the discontinuous short wave fin was introduced to the flat tube to enhance the air-side heat transfer of ACC. Two different types of arrangements,i.e. staggered and in-line for discontinuous short wave fins on the flat tube,were designed. By numerical simulation,the heat transfer and flow performances of short wave fins were studied under different arrangements(in-line,staggered) ,and the influences on heat transfer and flow characteristics of rows of short wave fin and interrupted distance between discontinuous short wave fins were revealed numerically. The results indicated that,compared with the original continuous wave fin,the discontinuous short wave fin effectively improved the air-side heat transfer of flat tube under the air flow velocities in the practical application of engineering. Moreover,the increment of pressure loss of air-side flow was restricted for the discontinuous short wave fins because of the reduction of contact areas between the air flow and fin surface.

Forming mechanism of integral serrated high fins by plowing-extruding based on variational feed

Plowing-extruding tool was designed and plowing-extruding process was investigated.Then,a manufacturing method of integral serrated high-finned tube,plowing-extruding based on variational feed was proposed,in which plowing-extruding tool moved forward at two different feeds,f1 and f2,in turn.In this method,overlaps that are usually avoided in practical application were utilized to manufacture high fins and average height of fins was up to 1.58 mm.The critical feed(fc) of overlaps forming and terms of high fins forming were analyzed.The main technical parameters that affect the fins height were discussed.The experimental results show that the fins height increases with extruding inclination angle and plowing-extruding depth,and the fins height increases with f1 increasing when f1 is smaller than fc,and decreases with f1 increasing if f1 is larger than fc.

Enhancing Heat Transfer and Energy Storage Performance of Shell-and-Tube Latent Heat Thermal Energy Storage Unit with Unequal-Length Fins

Previous studies in literatures adequately emphasized that inserting fins into phase change material is among the most promising techniques to augment thermal performance of shell-and-tube latent heat thermal energy storage unit.In this study,the novel unequal-length fins are designed from the perspective of synergistic benefits of heat transfer and energy storage performance,and the effects of arrangement,number and total length of unequal-length fins are numerically investigated.Results show that utilization of fins with ascending length,when short and long fins are located in the inlet and outlet of heat transfer fluid respectively,can further promote the heat transfer and energy storage performance compared with equal length fins,and a maximum 6.17%and 0.43%increment of heat transfer performance and stored energy is achieved in full melting time,respectively.The number of unequal-length fins plays a major role in the energy storage,and 18.95%and 0.91%improvement of heat transfer performance and stored energy is realized when equipped with 2 unequal-length fins.A 21.17%improvement of the heat transfer performance is obtained when the total length of unequal-length fins is 18 mm.The present study is helpful to make further efforts to enhance heat transfer and energy storage of shell-and-tube latent heat thermal energy storage unit with unequal-length fins.

Heat transfer characteristics in micro-fin tube equipped with double twisted tapes: Effect of twisted tape and micro-fin tube arrangements

An experimental study was carried out to investigate the influence of double twisted-tape inserts （DTs） in micro-fin tubes （MFs） on heat transfer, friction factor and thermal performance factor characteristics of the compound devices in the following configurations： （1） twisted tapes acted in the same direction （for co-swirl） while MF and twisted tapes acted in the same （parallel） direction （MF-CoDTs：P）, （2） twisted tapes acted in the same direction （for co-swirl） while micro-fin tube and twisted tapes acted in opposite directions （MF-CoDTs：O） and （3） twisted tapes acted in opposite directions for counter swirl （MF-CDTs）. The MF alone and the MF equipped with a single twisted tape in parallel/opposite arrangement were also considered for comparison. The experiments were conducted for the flows with the Reynolds numbers between 5 650 and 17 000, under uniform heat flux condition. The experimental results indicate that MF-CDTs induce stronger swirl/turbulence flow, resulting in higher heat transfer rate, friction factor and thermal performance factor than other combined devices. The thermal performance factors associated with the use of MF-CDTs were found to be higher than those associated with the uses of MF-CoDTs：P, MF-CoDTs：O and MF alone up to 9.3%, 6.5% and 56.4%, respectively. The empirical correlations developed using the present experimental data for the Nusselt number, friction factor and thermal performance factor are also reported.

Heat Transfer Characteristics of Laminar Flow in Internally Finned Tubes under Various Boundary Conditions

Numerical solutions for fully developed laminar flow in internally finned tubes with trapezoidal and triangular fin profiles were given with Finite Element Method (FEM): The heat transfer characteristics were obtained and compared under the boundary conditions of uniform heat flux, uniform wall temperature, and the third boundary condition with finite wall thermal conductivity considered. The numerical results show that boundary conditions have pronounced effects on the temperature field.Furthermore, a new mechanism on the heat transfer augmentation of internally finned tubes is proposed.

基于响应面法的矿用翅片管空冷器参数优化

矿用空冷器是矿井降温系统常用的末端设备,优化矿用空冷器换热性能对提升矿井降温系统能效具有重要意义。采用Fluent建立了平直翅片管矿用空冷器模型,研究了翅片间距、翅片厚度、横向管间距和纵向管间距对传热因子、阻力因子和综合性能评价指标的影响规律;采用Box-Behnken响应面法,分析了双参数协同作用下传热因子、阻力因子和综合性能评价指标变化规律。结果表明:随着翅片间距、横向管间距的减小,以及翅片厚度增加,传热因子、阻力因子和综合性能评价指标逐渐增加;综合性能评价指标则随纵向管间距的增加而先增加后降低;横向管间距、翅片厚度是影响空冷器传热及流动性能最显著的两个参数,而翅片间距、纵向管间距的影响最小;得到了平直翅片管矿用空冷器的最优参数,传热因子提升16.3%、阻力因子增加26.3%、综合性能评价指标提升8.3%。研究结果可为矿用翅片空冷器设计参数优化提供指导。

扁平纵向翅片管换热器的传热特性分析及结构优化

为进一步优化传统纵向翅片管换热器的传热性能,提出了一种扁平纵向翅片管换热器。基于传热流动基础理论,建立扁平纵向翅片管的物理模型,给出相关传热计算公式,优化扁平管和翅片的结构参数。进一步将扁平翅片管和圆管翅片管以及组成换热器的换热性能进行比较分析,结果表明,等截面同翅片结构参数下单位体积内气侧换热面积增加1.5倍,同质热负荷下换热器的体积减少50%以上,结构紧凑,具有较好的综合优势,为该换热器的推广应用提供参考。

水平微肋管内R513A冷凝换热特性研究

采用实验方法研究了R513A在光管和不同齿密度微肋管内的冷凝换热性能,并通过对比冷凝换热系数实验值与关联式的预测值,探究了现有关联式对R513A的适用性,实验在冷凝温度33、35、38℃,质量流速50~150 kg/(m^(2)·s)的工况范围内进行。实验结果表明:冷凝换热系数随质量流速的增大而增大,在低质量流速区域,微肋管的冷凝换热系数增长幅度更大;随着齿密度的增加,管内换热面积增大,但管内表面张力作用增强,相应的流动阻力增强,强化传热作用被削弱;与R134a相比,微肋管对R513A的强化传热效果更优;对于光管,Bashar关联式的预测效果最佳,Cavallini关联式与Dobson关联式在低质量流速区域的预测误差偏大;对于微肋管,Yu关联式的预测效果最理想,Cavallini关联式与Kedzierski关联式均低估了管内冷凝换热特性。以Kedzierski关联式为基础,拟合了新的修正关联式,修正关联式的预测值与实验值的一致性较好,其平均绝对误差与标准差分别为7.8%、8.5%。

鱼鳍型涡发生器强化管翅式换热器传热

提出了一种鱼鳍型涡发生器,并将其应用在管翅式换热器中强化传热。采用数值模拟方法研究了鱼鳍型涡发生器安装方式对换热器中流体流动和传热的影响,分析了流体流经涡发生器前后的压力变化。结果表明:涡发生器以common-flow-up方式安装在圆管下游时传热j因子提高了46.6%,摩擦因子提高了24.4%,与以common-flow-down方式安装于圆管上游相比,圆管后方的负压区更小,流动阻力更小,冷热流体的混合效果更好,因而具有更好的强化传热效果。

管壳式换热器换热管强化传热研究

研究了油库夏季高温导致的油气回收工艺中吸附罐回收率低、吸收塔解吸效率低、油气回收系统能耗过高以及安全隐患较多等问题。将温度较低的地下水作为冷却水,从实践的角度构建多种强化传热结构的管壳式换热器并采用CFD的方法对其热力学与水力学性能进行多方面探究。结果表明当油气在管壳式换热器壳程中流动时,热流可以获得更好的冷却效果;油气在壳程流动时,CIRF型换热管具有最佳的热力学性能,RAW型换热管具有最佳的水力学性能。综合油气回收工况与计算结果配置CIRF型换热管的管壳式换热器在油气回收工艺中有着较强的适用性,其可以在有效提高油气的冷却效果的同时兼顾换热器的水力学性能。

倾斜微肋管内两相流动冷凝换热特性研究

用实验方法对倾斜微肋管内两相流动冷凝换热特性进行实验研究,主要分析管内传热系数在不同实验工况、微肋管中的变化趋势,以揭示各实验变量对管内换热特性的影响机理,获取管内传热系数理论模型。实验结果显示:微肋管内传热系数随质流密度和干度的增加而增大,随管倾斜角的增大呈现先升高后减小的变化趋势;相比于光滑管,40°微肋管和25°微肋管换热强化倍率随干度的增加而增大、随倾斜角的增加而减小。此外,现有关联式低估了大部分实验数据,其预测误差均随干度、肋片螺旋角等的升高而增大。基于实验数据所获取新关联式可对40°微肋管和25°微肋管表现出较好的预测效果,其平均预测误差小于7%。