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。     

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。     

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...     

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。     

换热管内转子强化换热的数值模拟

本文概述了转子组合式强化传热装置的强化传热和自清洁原理。分别通过建立光管及内置多个转子换热管的三维流动模型，对换热管内流场、温度场、压力场以及换热过程进行了模拟，得到了管内流体的流动特性和传热特性。对比模拟结果表明，内置转子换热管管内的三维流动比较复杂，转子与管壁之间缝隙内的流体有着明显的环绕流动，切向速度和径向速度也增大到一定范围，转子旋转半径内的流体整体呈螺旋流动。总的来说，内置转子的换热管内较光管有较强的湍流度，尤其是近壁区域，因此强化了管内的对流换热，传热系数显著提高，从而验证了转子具有强化传热和自清洁的双重功能。     

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。     

管内插入扭带及螺旋线圈的传热与阻力特性实验研究

实验研究了以空气为工质的管内插入扭带与螺旋线圈的传热与阻力特性,在3000相似文献   

纳米流体在内置扭带外螺纹管内的流动和换热特性

为了研究纳米流体在内置扭带外螺纹管内的流动与传热特性,在Re(雷诺数)为2 000~12 000的范围内,分别对质量分数为0.1%、0.2%、0.3%、0.4%、0.5%和0.6%的Cu、Al、A1_2O3、Fe_2O_3、多壁碳纳米管和石墨纳米流体在内置扭带外螺纹管内的流动与对流换热特性进行了实验研究。实验结果表明:在相同Re下不同纳米流体都存在最佳浓度比0.5%,其中Cu-水纳米流体的换热性能最好但是摩擦阻力较大,石墨的换热性能和摩擦阻力方面的综合性能最好。内置扭带外螺纹管较光管在换热性能方面提高了50.32%,但摩擦阻力系数也相应增加。根据实验数据对热性能系数进行了综合分析,得到了石墨纳米流体内置扭带外螺纹管对流换热以及摩擦阻力系数关联式,其计算值和实验值有较好的吻合度。     

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%。     

强化传热管在能源节约中的应用

强化传热技术是利用各种型式的翅片管,多孔表面管、表面粗糙化管,螺旋槽管,管内插件等换热元件或在流动介质中附加电场,磁场、超声波,机械振动,添加剂等辅助设施,促使流过换热元件的介质产生湍流,减薄边界层使整个对流换热热阻减少,起到强化传热作用的技术。 由光管滚轧制成的内外表面螺旋槽管是促使流体形成边界层分离流和螺旋流,具有     

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)%.     

Heat transfer augmentation in a circular tube with perforated double counter twisted tape inserts

The present study explored the effects of perforated double counter twisted tapes on heat transfer and fluid friction characteristics in a heat exchanger tube. The twisted tapes with four different porosities of R_p = 1.2, 4.6, 10.4 and 18.6% were used as counter-swirl flow generators in the test section. The experiments were conducted in a circular tube in turbulent flow regime with Reynolds number ranging from 7200 to 50,000 using air as the working fluid under uniform wall heat flux boundary condition. The experimental results demonstrated that the Nusselt number, friction factor and thermal enhancement efficiency were increased with decreasing porosity except porosity of 1.2%. The results also revealed that the heat transfer rate of the tube fitted with tapes were significantly increased with corresponding increase in friction factor. In the range of the present investigation, heat transfer rate and friction factor were obtained to be around 80 to 290% and 111 to 335% higher than those of the plain tube values, respectively. Based on constant blower power, the highest thermal enhancement efficiency of 1.44 was achieved. In addition, the empirical correlations of Nusselt number, friction factor and thermal enhancement efficiency were developed based on the experimental data.     

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.     

Conjugated heat transfer in circular ducts with a power-law laminar convection fluid flow

This work deals with the study of the steady-state analysis of conjugated heat transfer process for the thermal entrance region of a developed laminar-forced convection flow of a power-law fluid in a circular tube. A known uniform heat flux is applied at the external surface of the tube. The energy equation in the fluid is solved analytically using the integral boundary layer approximation by neglecting the heat generation by viscous dissipation and the axial heat conduction in the fluid. This solution is coupled to the Laplace equation for the solid, where the axial heat conduction effects are taken into account. The governing equations are reduced to an integro-differential equation which is solved by analytical and numerical methods. The results are shown for different parameters such as conduction parameter, α, the aspect ratio of the tube, ε and the index of power-law fluid, n.     

分离式热管小螺旋管蒸发段换热特性的实验研究

将小螺旋管应用于分离式热管的蒸发段。通过采用玻璃管和不锈钢管模拟分离式热管的蒸发段，对不同充液率和热流密度下，小螺旋管管内流体的流动与换热特性进行了实验研究。通过可视化实验观察小螺旋管蒸发段管内流型，初步分析热流密度和充液率对流型转换的影响，讨论壁温分布与管内两相流流型的关系。提出螺旋管内的脉冲震荡和二次回流使得管内流体的紊动强化，从而使平均换热系数和临界热流密度得以提高，不会产生壁温飞升，具有较好安全性的结论。     

Experimental study of exergetic performance of Al2O3 nanofluid in a double‐pipe heat exchanger fitted by a new modified twisted tape

The use of nanofluids and surface enhancers today are among the new technologies used to increase heat transfer. In this study, heat transfer phenomena in heat exchanger were investigated using Al₂O₃ nanoparticles and modified spiral band as flow turbulator. Results are verified with well‐known correlations. The results show that the tube with cross‐hollow twisted tape inserts has the best exergetic performance for different hollow widths of the tape. Clearance, which is defined as the width between the tube and twisted tape, also affects the heat transfer performance. The smaller the clearance, the better is the exergetic performance. The tube can achieve the best exergetic performance when the number of unilateral twisted tapes is four. The results showed that increasing nanofluid concentration improves exergetic performance.     

管内强化传热结构及其性能分析

强化传热一直是工程中节能降耗的有效措施，本文阐述了各种管内强化传热结构及其性能特点，并用场协同原理和数值模拟方法分析了其强化传热机理。数值分析表明，管内壁面的凸起对流体产生扰流作用，减薄和破坏了液体边界层，管内插入物促进了流体的湍动，加速流体的径向混合，这都有利于提高流体的传热强度。     

Turbulent heat transfer and pressure drop in tube fitted with serrated twisted tape

This paper presents an original experimental study on compound heat transfer enhancement in a tube fitted with serrated twisted tape. The serrations on two sides of the twisted tape with twist ratio of 1.56, 1.88, 2.81 or ∞ are the square-sectioned ribs with the identical rib-pitch and rib-height. The local Nusselt number and Fanning friction factor increase as the twist ratio decreases in the tube fitted with smooth or serrated twisted tape. In the Re range of 5000–25 000, heat transfer augmentation attributed to the serrated twisted tape falls in the range of 250–480% of the plain-tube level. That is about 1.25–1.67 times the heat transfer level in the tube fitted with smooth twisted tape. Fanning friction factors are respectively decreased and increased in the tubes fitted with smooth and serrated twisted tapes as Re increases. Based on the same pumping power consumption, the thermal performances of the tubes with smooth and serrated twisted tapes are compared. A set of empirical correlations that permits the evaluation of the Nusselt number and the Fanning friction factor in the developed flow region for the tubes fitted with smooth and serrated twisted tapes is generated for engineering applications.