正反扭转扭带层流换热及摩擦特性的实验研究

本文提出将一种正反扭转且带有双排半圆切口的新型扭带用在套管式换热器上在层流条件下对换热器其换热性能和流动特性进行实验研究并分析这种扭带的综合性能。实验工质为水,Re(雷诺数)变化范围为540~1 440。本实验采用4种不同长度扭带做对比,4种分别是全长、3/4长、半长和1/4长扭带。结果显示,插入全长、3/4长、半长和1/4长扭带的换热Nu(努塞尔数)分别是光管的1.92~3.80,1.69~3.20,1.60~2.62和1.48~2.23倍;f(摩擦阻力系数)分别是光管的3.69~6.30,3.30~5.51,2.72~4.67,2.33~3.55倍;对应的PEC(综合性能指标)分别为1.24~2.06,1.14~1.81,1.15~1.57和1.11~1.47。插入全长扭带在实验范围内达到了最优,插入其它长度扭带的PEC也都高于1,可以应用在压力较小的工况下。最后,本文还对该类型扭带拟合了实验关联式,通过对比实验值和预测值,Nu和f的误差均在10%以内,这为今后的研究提供了理论依据。     

扭带插入件强化圆管内换热的计算方法

通过对在等功率条件下,强化圆管内低Re数气体传热的扭带式插入件的研究。其扭转比Y是影响换热系数及沿程阻力的重要参数,恰当选择扭转比Y,以及正确计算换热系数和沿程阻力是影响换热设备计算准确的重要环节。     

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。     

管内强化对流换热的热力经济性分析

鉴于管内换热和阻力同步增长的事实,依据Ｗｅｂｂ指标对管内强化对流换热方式下传热和流阻的综合热力性能进行了推导,得到了热力性能指标Ｑ／ＱＳ、Ｐ／ＰＳ和Ｆ／ＦＳ与管内对流换热努氏数Ｎｕ和管内阻力系数λ之间的函数关系式。在此基础上,对螺旋槽管强化管内换热的热力性能进行了分析。     

关于管内单相对流换热强化的极限问题

从场协同理论出发,分析了通道内表面全部为射流冲击换热表面时的极限换热率;将全射流冲击管内换热与普通流动管内换热进行了比较。给出了层流和紊流工况下全射流冲击换热可能达到的最大强化比。针对相同R e,分析得出:在层流充分发展段,全射流冲击通道的强化极限是16.9倍;在紊流充分发展段是3.5倍。综合现有各种通道内强化换热的研究结果进行比较,其换热率均低于全射流冲击管内换热率,其中层流工况以折流翅片式通道和交叉缩放椭圆管的换热率与极限换热率最为接近;紊流工况以内插螺旋丝强化管最为接近。     

轧槽管内单相对流强化换热及阻力特性的实验研究

通过实验对不同规格的轧槽管进行了强迫对流换热和阻力特性的研究。根据工程需要，选用适当的准则对这些管子的强化换热性能进行了评价，在实验范围内得出了最佳管型。     

管内单相对流强化换热及阻力特性的实验研究

通过实验对光管、内覆丝网管、轨槽管及多头内肋管进行了强迫对流换热和阻力特性的研究。根据工程需要，选用适当的准则对这些管子的强化传热进行了评价，在实验范围内得出了最佳管型。     

换热管内插开圆孔扭带提高传热效率

文章简述传热设备的强化传热的节能技术;通过自制不同规格自旋开孔扭带置入换热管进行冷态实验和热态实验,分析换热管内插扭带后传热效果的改变;用实验数据解析扭带转速、压力降、传热系数与流速的内在关系,验证出开孔扭带比光滑扭带的强化传热效果更好,为企业在节能降耗改造现有换热器设备中提高其传热效率提供理论依据。     

换热管内旋向自交叉转子强化传热性能研究

利用综合传热性能实验台,研究对比旋向自交叉转子、同向转子的换热器和光管的传热性能。结果发现:通过Webb性能比较方法,在同样面积和同等泵功率情况下,同向转子比光管换热器的换热量最大可提高为6.99%,而旋向自交叉转子比光管的换热量最大可提高8.11%,并计算推导了旋向自交叉转子与同向转子、光管之间的准则关联式;同时研究了换热量与转子外径的关系,结果发现换热量随转子外径变大而增多。     

管壳式换热器的换热管强化传热技术浅述

介绍了管壳式换热器的换热管强化传热技术,分析了各自的原理、优缺点及推荐的使用场合。采用节能技术的换热器不仅提高了能源的利用率,而且减少了金属材料的消耗,对化工行业提高经济效益具有重要意义。     

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.     

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.     

扭带强化传热性能的数值与实验研究

通过模拟和实验的方法研究在湍流工况下(3 000Re10 000)换热器管内插入不同扭带模型后的传热特性和阻力特性。区别于传统螺旋扭带,提出一种顺时针与逆时针交替扭转的正反扭带。对不同扭率的传统扭带以及扭率为3的无缺口和半圆缺口正反转扭带进行模拟计算,并将模拟结果与实验结果进行对比验证。结果表明:在湍流流态下,雷诺数越小,扭带的强化换热效果表现越好;对不同扭率的扭带,其努塞尔数、摩擦系数和综合性能指标随扭率的减小而增大;扭率为3时,两种正反扭带的强化换热效果均优于传统扭带,无缺口正反扭带的换热效果最好;模拟计算的结果数据与实验结果数据比较,最大误差不超过8%。     

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。     

Decaying swirl flow in round tubes with short-length twisted tapes

This paper presents experimental and numerical results of the local heat transfer coefficient and flow characteristics of decaying turbulent swirl flow generated by short-length twisted tapes (STs). The STs with three different twist ratios (y/W = 3, 4 and 5) were applied at the entrance of the test section. The experiments were conducted under uniform heat flux conditions for water flow rates in the range of 5200 ≤ Re ≤ 15,300. The results of the tube without swirl generator as well as the ones with full-length twisted tape (TTs) are also reported as the reference cases. The experimental results reveal that the tube with STs consistently yields higher local Nusselt number than that the one without swirl generator. The local Nusselt numbers decrease with increasing axial distance (x/D) due to the decaying effect. Although, STs consistently provide poorer heat transfer than TTs over the range studied, the STs with y/W = 4 and 5 yield superior thermal performance factors to the TTs at the same twist ratios, for Reynolds numbers beyond 10,000 due to the prominent effect of heat transfer improvement over that of the increase of friction factor. For better understanding, the visualization of flow structure (pathline and vector plot) in the tubes with STs is also presented.     

Numerical investigation on the heat transfer characteristics in a circular pipe using multiple twisted tapes in laminar flow conditions

Introducing passive devices in the form of inserts helps to increase the heat transfer characteristics in a circular pipe. In the present work, the feasibility of using two twisted tapes has been numerically studied under laminar flow conditions. Two twisted tapes with configurations of co‐swirl and counter‐swirl conditions were simulated for Re ranging from 1000 to 2500. Heat transfer characteristics like Nusselt number, friction factor, and thermohydraulic performance index (THPI) were investigated. Even the effect of spacing of the tubes inside the pipe was numerically studied. Results indicated that the insertion of two tapes will improve the performance of the circular tube compared with a single tape in terms of a higher Nu and higher THPI. It is observed that for the co‐swirl condition for a Reynolds number of 1000, Nu and THPI is 38% and 29% higher than using a single tape, whereas, for the counter‐swirl condition, they are 43% and 34% higher than that of the single tape condition. Also, it is revealed that counter swirl positioning of the tape with an L/D ratio 0.56 outperformed all other configurations with the highest THPI.     

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)%.     

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.     

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     

Compound heat transfer enhancement of a dimpled tube with a twisted tape swirl generator

Friction and compound heat transfer behaviors in a dimpled tube fitted with a twisted tape swirl generator are investigated experimentally using air as working fluid. The effects of the pitch and twist ratio on the average heat transfer coefficient and the pressure loss are determined in a circular tube with the fully developed flow for the Reynolds number in the range of 12,000 to 44,000. The experiments are performed using two dimpled tubes with different pitch ratios of dimpled surfaces (PR = 0.7 and 1.0) and three twisted tapes with three different twist ratios (y/w = 3, 5, and 7). Experiments using plain tube and dimpled tube acting alone are also carried out for comparison. The experimental results reveal that both heat transfer coefficient and friction factor in the dimpled tube fitted with the twisted tape, are higher than those in the dimple tube acting alone and plain tube. It is also found that the heat transfer coefficient and friction factor in the combined devices increase as the pitch ratio (PR) and twist ratio (y/w) decrease. In addition, an empirical correlation based on the experimental results of the present study is sufficiently accurate for prediction the heat transfer (Nu) and friction factor (f) behaviors.