新型扭带在层流中强化传热性能的数值模拟研究

提出一种新型的正反两个方向交替扭转的扭带模型,并基于ANSYS FLUENT软件在层流状态下(Re=400～18 00)对换热管中插入新型扭带的换热特性、流体阻力特性和综合性能指标进行数值模拟研究。对y=3.0,4.5,6.0三种扭率下的无缺口正反扭转扭带与传统单一方向扭带进行对比研究,并对扭率y=3.0的无缺口正反扭转扭带和带有三角、半圆及方形缺口的正反扭转扭带进行了对比计算。研究结果表明:插入不同扭率的扭带,换热管的努赛尔数Nu、摩擦系数f与综合性能PEC值均随着扭率的减小而增大;相同扭率下无缺口的正反扭转扭带在强化换热效果和综合性能表现上要优于传统扭带;扭率为3.0时,无缺口的正反扭转扭带比带三种缺口的正反扭转扭带换热效果好,而缺口的存在可以大幅度地降低插入扭带产生的摩擦阻力,且缺口的面积越大,摩擦和换热效果降低越多。     

不同宽度扭带传热与阻力特性试验研究

本文对管内扭带旋流装置在不同扭带宽度下的传热与阻力进行了试验研究,并得到了传热与阻力拟合式。     

圆形流道内置扭带强化传热机理分析

为了分析圆形管道内置扭带的强化传热机理,用Fluent软件建立了其三维模型,模拟得到了管内流场和温度场,运用场协同原理证实了扭带强化传热的主要机理是扭带引起的螺旋流动使流体产生二次流,促进了主体流体和边界层流体的混合,提高了流场和温度场的协同性。分析了传热系数、压降与雷诺数及湍流动能的关系,提出应使换热器的湍流动能限制在一定的范围内,以获得较好的综合性能。     

自转清洗扭带管对流传热强化机理的实验研究

具有传热强化功能的自转螺旋扭带清洗防垢技术发展较快。应用激光测速仪LDV(Laser Doppler Velocimeter)实验研究自转清洗扭带管内流体的湍流特性。结果表明：在自转扭带竹带动下,管内流体的流动结构发生了反常态的变化。在近管壁环形区域内流体的轴向分速度明显比管中心区域的高,轴向湍流度比无白转扭带时大;切向分速度随半径的增大而增大,并且存在很大的径向湍流度。这些结果初步说明了自转螺旋扭带管对流传热强化的机理是：管内由扭带带动形成的强制旋流和轴向平行流叠加而形成的螺旋流动,以及近管壁环形区域内流速的增大,不仅加强了边界层流体的扰动以及边界层流体与主流流体的混合,并且使边界层厚度减簿,从而才使管内的对流传热得以强化。本文试验研究的结果为自转螺旋扭带管内对流传热强化机理的深入理论研究提供实验基础。     

规则间隙内置扭带管的强化传热特性研究

圆管内置扭带能大大强化管内传热,利用周期边界对规则间隙扭带的6种结构形式采用CFD方法进行了研究,比较了各结构下管内传热能力Nu、阻力因子f、流动传热增强因子(f/f0、Nu/Nu0)和综合性能η。结果表明:扭带提高了管内流速,使高速区向壁面靠近,形成径向旋流冲刷管壁减薄边界层;规则间隙扭带加速了流体的扰动,使之形成间歇式的混合与分离;内置扭带结构的Nu/Nu0随Re增大成指数规律减小,最小值大于2.5;交替排列正反旋向扭带提升了Nu,但也使f/f0大幅升高,不同结构的f/f0变化规律各异,综合性能表明s=1的内置扭带总体性能最优。根据数值模拟结果拟合出了各结构流动传热关联式。     

管内换热元件基于数值模拟与实验对比的研究

利用流体计算软件Fluent 6.0分析自转式转子与螺旋扭带两种换热元件在强化传热方面的优劣,并按模拟条件进行实验对比。结果表明,相对于螺旋扭带径向扰流较弱的劣势,自转式转子扰动能力强,提高换热系数,为实际应用提供有力的指导。     

余隙率对内置扭带换热管传热性能影响的数值研究

为了研究管内插入扭带对换热的影响,建立了余隙率分别为0.1,0.15,0.2,0.25,0.3的内置扭带换热管流体流动的三维物理模型,采用RNGk-ε湍流模型对这几种余隙率的扭带换热管的流动和传热特性进行了数值模拟,分析了插入不同余隙率扭带换热管的换热效果和流动阻力。结果表明,Nu数和摩擦阻力系数均随着余隙率的增大而减小,在低Re数的时候内置扭带有明显的经济性,特别是余隙率为0.1的扭带换热管综合性能最高,强化传热效果最为明显。     

光管内插入扭带传热与流动阻力的试验研究

为了研究管内强化换热技术,对三根不同结构参数的扭带插入光管的换热特性和流体动力学特性进行了试验研究。试验以空气为工质,Re在8000～10^5之间,管外被水冷却。对大量实验数据用多元线性回归法得到了具有较高精度的扭带管的传热系数和摩擦系数的统计关联式,分析了扭带管的传热与流阻性能,为换热器的设计及改造提供了理论依据。     

泰勒涡流流动及强化传热数值研究

泰勒涡流为叠加于剪切流之上的二次流,其具有强化传热作用,在航空、水处理、制药工程和化工等领域都具有很大的应用价值。运用Fluent软件,建立长径比Γ = 30的模型并对同轴套管间的流态演变和传热特性进行了数值模拟。模拟结果显示了环隙内流体流态随着内筒转速增加的演变过程,表明在存在径向温差的情况下,涡流的存在强化了传热效率。对不同转速下的强化传热效果进行了对比分析,并确定了最佳状态点。     

管内复合强化传热技术及机理分析

文中对管内强化传热及复合强化传热技术进行了在紊流流动下的阻力和传热特性分析,并给出了几种复合强化传热技术的试验研究结果。提出了螺旋槽带中插入旋向相反的部分管长扭带是行之有效,效果明显的复合强化传热技术。     

Numerical study of heat transfer enhancement using perforated dual twisted tape inserts in converging‐diverging tubes

In this article, a 3D numerical simulation for investigating friction factor and heat transfer enhancements in case of inserting normal or perforated dual twisted tapes (TTs) in converging‐diverging tubes (CDTs) is presented. The effects of Reynolds number changing from 10,000 to 20,000 and a various number of holes (N = 0, 1, 2, and 3) in TTs, under constant uniform heat flux on tube outer wall and using water as working fluid circumstances, were examined. It was found that generally using dual TTs in CDTs improves the Nusselt number up to 9% compared to bare CDTs. Perforating TTs do not have a noticeable effect on heat transfer, but decrease the friction factor significantly, up to 396% compared to normal dual TT implantation, in higher Reynolds numbers. Following that, thermal performance factor considerably increases up to 57%, compared to CDTs with normal dual TTs. To show the mechanism of these effects, velocity streamlines are presented and will be discussed in this paper.     

Investigation of thermal flow structure and performance heat transfer in three-dimensional circular pipe using twisted tape based on Taguchi method analysis

In the current investigation, the twisted tape inserts are considered as the augmentation thermal technique, the influence of a variety of twisted tape configurations on pressure drop characteristics, temperature differences, thermal performance of fluid flow structure, heat transfer improvement, and friction factor are numerically evaluated. The changed geometrical parameters employed for this study comprise twisted tape width, twisted tape thickness, number of turns, and inward thickness are the input parameters. Design of experiments method is applied to analyze the influence of latter various types of geometrical parameters on hydraulic thermofluid pattern and heat transfer improvement in the twisted tube heat exchanger as the output variables. For the experimental design optimization Taguchi analysis is based on investigate of alterations and performs the orthogonal arrays (OA). Moreover, the OA L16 is chosen as the plan of experimental study. It is found the best design of twisted tape in this study by using computational fluid dynamics numerical methodology complained with Taguchi method the enhancement in heat transfer and hence the overall performance evaluation factor is higher than 1.2.     

Prediction of friction factors and heat transfer coefficients for turbulent flow in corrugated tubes combined with twisted tape inserts. Part 2: heat transfer coefficients

A simple mathematical model following the suggestion of Smithberg and Landis has been created to predict the heat transfer coefficients for the case of a fully developed turbulent flow in a spirally corrugated tube combined with a twisted tape insert. The heat transfer can be predicted from the combined effects of the axial and the tangential boundary layer flows coupled with an additional “vortex mixing” effect near the wall through the solution of the corresponding momentun and energy transfer equations. The “wall roughness” has an effect simultaneously on the axial velocity, secondary fluid motion and the resulting swirl mixing. The model reflects the influence of the “wall roughness” and the twisted tape on the thermal resistances of the helicoidal core flow, twisting boundary layer flow and the viscous sublayer near the wall. The calculated heat transfer coefficients have been compared to 544 experimental points obtained from 57 tubes tested. Four hundred thirty-eight points (80.5%) have a relative difference of less than ±15% and 106 points (19.5%) have a relative difference between ±(15-20)%.     

扭带强化管内单相对流换热的研究进展

能源与材料费用的不断增长推进了高效节能换热器的发展.在管壳式换热器中,为强化管内单相对流换热并有效清除污垢,制造工艺简单、拆装维修方便的管内插入物-扭带被广泛应用.介绍了多种扭带的结构和特点,分析了扭带强化管内单相对流换热的机理.从连续扭带、闻隔扭带、异型扭带、清洗扭带和复合强化技术五个方面,对扭带强化管内单相对流换热...     

Prediction of friction factors and heat transfer coefficients for turbulent flow in corrugated tubes combined with twisted tape inserts. Part 1: friction factors

A simple mathematical model following the suggestion of Smithberg and Landis has been created to predict the friction factors for the case of a fully developed turbulent flow in a spirally corrugated tube combined with a twisted tape insert. The flow field is divided into two principal regions--a helicoidal core flow modified by secondary circulation effects and a twisting boundary layer flow. The “wall roughness” has an effect simultaneously on the axial velocity, secondary fluid motion and the resulting swirl mixing. The model reflects the influence of the “wall roughness” on the axial and tangential velocity components. The calculated friction factors have been compared with 570 experimental points obtained from 57 tubes tested. Five hundred points (87.7%) have a relative difference of less than ±15%; fifty one points (8.9%) have a difference of within ±(15-20)% and only nineteen points (3.4%) have a relative difference greater than ±20%.     

Experimental investigation of twisted tape inserts performance on condensation heat transfer enhancement and pressure drop

A comprehensive experimental investigation is conducted on the augmentation of heat transfer coefficients and pressure drop during condensation of HFC-134a in a horizontal tube at the presence of different twisted tape inserts. The test section is a 1.04 m long double-tube counter-flow heat exchanger. The refrigerant flows in the inner copper and the cooling water flows in annulus. The experiments are performed for a plain tube and four tubes with twisted tapes inserts of 6, 9, 12 and 15 twist ratios. The pressure drop is directly measured by a differential pressure transducer. It is found that the twisted tape with twist ratio of 6 gives the highest enhancement in the heat transfer coefficient and the maximum pressure drop compared to the plain tube on a nominal area basis. For this case the enhancement in heat transfer and the pressure drop are increased by 40 and 240% in comparison with to the plain tube. It is observed that the twisted tape with the twist ratio of 9 has the best performance enhancing the heat transfer with the minimum pressure drop. Also empirical correlations are developed to predict smooth tube and swirl flow pressure drop. Predicted results are compared to experimental data and it is found that these correlations are reliable for pressure drop estimation.     

Heat transfer enhancement for laminar flow of ethylene glycol through a square duct fitted with increasing and decreasing orders of twisted tape

The isothermal friction factor and heat transfer enhancement through a square duct fitted with increasing and decreasing order of twist ratio sets have been studied under nearly uniform wall temperature conditions. The ethylene glycol flows under laminar flow (Re = 30–1200) through a square duct and hot water flows through an annular channel formed between a square duct and circular tube, in a counter current fashion. The hot water at a very high flow rate is circulated though the annular channel to ensure a nearly uniform wall temperature condition. There is not much change in the magnitude of the heat transfer coefficient enhancement with the increasing twist ratio and with the decreasing twist ratio set, as the intensity of the swirl generated at the inlet or at the outlet in the order of increasing twist ratio or decreasing twist ratio, is the same in both the cases. Performance evaluation analysis on constant pumping power was made and a maximum performance ratio was obtained for each twist insert corresponding to a Reynolds number of 680. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20410     

Heat transfer augmentation in a helical-ribbed tube with double twisted tape inserts

Turbulent convective heat transfer characteristics in a helical-ribbed tube fitted with twin twisted tapes have been investigated experimentally. The experiment was carried out in a double tube heat exchanger using the helical-ribbed tube having a single rib-height to tube-diameter ratio, e/D_H = 0.06 and rib-pitch to diameter ratio, P/D_H = 0.27 as the tested section. The insertion of the double twisted tapes with twist ratio, Y, in the range of 2.17 to 9.39 is to create vortex flows inside the tube. The inserted ribbed tube is arranged in similar directions of the helical swirl of the twisted tape and the helical rib motion of the tube (called co-swirl). Effects of the co-swirl motion of the ribbed tube and the double twisted tapes with various twist ratios on heat transfer and friction characteristics are examined. The results obtained from the ribbed tube and the twin twisted tape insert are compared with those from the smooth tube and the ribbed tube acting alone. The experimental results reveal that the co-swirling inserted tube performs much better than the ribbed/smooth tube alone at a similar operating condition. The co-swirl tube at Y ≈ 8 yields the highest thermal performance at lower Reynolds number (Re). In addition, the correlations of Nusselt number and friction factor as functions of Re, Pr and Y are also proposed.     

3-D Numerical simulation of swirling flow and convective heat transfer in a circular tube induced by means of loose-fit twisted tapes

The article presents the application of a mathematical model for simulation of the swirling flow in a tube induced by loose-fit twisted tape insertion. Effects of the clearance ratio defined as ratio of clearance between the edge of tape and tube wall to tube diameter (CR = c/D = 0.0 (tight-fit), 0.1, 0.2 and 0.3) on heat transfer enhancement (Nu), friction factor (f) and thermal performance factor (η) are numerically investigated for twisted tapes at two different twist ratios (y/w = 2.5 and 5.0). The simulation is conducted in order to gain an understanding of physical behavior of the thermal and fluid flow in the tube fitted with loose-fit twisted tape under constant wall temperature conditions in the turbulent flow regime for the Reynolds number ranging from 3000 to 10,000. The Navier–Stokes equation in common with a energy equation is solved using the SIMPLE technique with the standard k–ε turbulence model, the Renormalized Group (RNG) k–ε turbulence model, the standard k–ω turbulence model, and Shear Stress Transport (SST) k–ω turbulence model. The numerical results show that the predictions of heat transfer (Nu) and friction factor (f) based on the SST k–ω turbulence models are in better agreement with Manglik and Bergles [R.M. Manglik, A.E. Bergles, Heat transfer and pressure drop correlations for twisted-tape inserts in isothermal tubes, part II: Transition and turbulent flows, Transaction ASME, Journal of Heat Transfer, 115 (1993) 890–896.] than other turbulence models. The mean flow patterns in a tube with loose-fit twisted tapes in terms of contour plots of velocity, pathline, pressure, temperature and turbulent kinetics energy (TKE) are presented and compared with those in a tube fitted with tight-fit twisted tapes. It is visible that the twisted tape inserts for y/w = 2.5 with CR = 0.0 (tight-fit), 0.1, 0.2 and 0.3 can enhance heat transfer rates up to 73.6%, 46.6%, 17.5% and 20%, respectively and increase friction factors up to 330%, 262%, 189%, and 160%, respectively, in comparison with those of the plain tube. The tube with loose-fit twisted tape inserts with CR = 0.1, 0.2 and 0.3 provide heat transfer enhancement around 15.6%, 33.3% and 31.6% lower than those with CR = 0.0 (the tight-fit twisted tape). The heat transfer augmentation is expected to involve the swirl flow formation between the tape and a tube wall. In addition, the simulation for thermal performance factor (η) of a tube with the loose-fit twisted tape and the tight-fit twisted tape under the same pumping power is also conducted, for comparison.