空气外掠波纹管束强化传热规律数值计算

采用低雷诺数湍流数值模型,对空气外掠8排波纹管束及光管管束的流动与传热性能进行了数值模拟,通过比较分析探讨了波纹管束强化传热机理.研究表明,波纹管束由于波纹凸起的存在,导致流场横截面内产生了二次纵向涡流.增强了流体的扰动及其湍动能,从而起到了传热强化作用.通过数值计算分析了波纹管束的几何参数对其流动与传热性能的影响规律,计算表明:存在一个临界雷诺数Recr =8000,当Re Recr时,传热因子η随着参数ξ的降低而增加;在所研究的雷诺数范围内,适当降低参数ξ取值及提高参数ψ取值有利于改善波纹管束的综合传热性能.     

波纹管流动和传热特性的数值模拟

基于计算流体力学软件Fluent模拟了三维波纹管的流动与传热特性,研究了波纹管结构参数及运行工况变化对阻力特性和传热特性的影响。结果表明:在所模拟的工况范围内,波纹管相比于光管传热特性最多提高1.83倍,1~#波纹管具有最佳的综合换热性能,当Re=5000时,综合换热因子η达到最大值1.33;波纹管波高相对于波纹管间距对强化换热影响较大,当Re15000时宜选用波高较小的波纹管。     

除湿条件下波纹通道内对流传热传质的数值研究

通过数值计算对紧凑换热器一种波纹翅片通道内除湿条件下周期性充分发展的对流传热传质情况进行数值研究。计算采用曲线坐标系下压力与速度耦合的SIMPLER算法,湿空气流动Re数的范围为100～1100,Pr数为0.71,Sc数为0.61。讨论了不同波纹高度、波纹间距对阻力与换热的影响,给出了不同Re数下的浓度场,并对动量、传热及传质进行了定量比较分析。计算结果表明,整体Nu数及fRe数随着波纹高度的增加或波纹间距的减小而增加;浓度随着Re数的增加沿着流动方向迅速降低;计算能较好的满足Chilton-Colburn相似,表明传热特性均可类推到传质特性中去。     

波纹板式空气预热器内流动换热过程的数值模拟

对波纹板式空气预热器内换热与流动过程进行数值模拟,分析了波纹板片的结构特征、波纹倾角β、波纹高度H、波纹节距以及板间距对空气预热器性能的影响.结果表明:当波纹倾角和波纹高度增大时,换热性能提高但是压降增大,在β=45°、H=10mm时换热性能最优;波纹间距的改变对换热性能影响较小,但板间距的影响较为明显,综合考虑换热器的紧凑性,选用较小板间距能达到较好的换热效果.     

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。     

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。     

波纹通道板间距对通道内流动与换热影响的数值研究

应用数值模拟方法，分析了流体在不同板间距的正弦型波纹通道内，周期性充分发展的稳态层流流动与换热的特性；探讨了板间距对流动与换热的影响，并对其综合性能进行评估。结果表明：在Re相同的条件下，通道内所形成旋涡的尺寸随相对间距A／H(波长／间距)的减小而增大。     

组合肋通道流动与传热特性的数值分析

本文采用数值模拟的方法研究了一种新型组合肋通道的流动与传热特性,主要对比了不同肋型通道的传热性能和阻力性能,考察了雷诺数、肋间距和肋高对通道壁面特征数的影响规律.结果 表明与矩形肋、半圆肋通道相比,组合肋通道的综合传热性能最好,且阻力损失小.     

内置异形等间距扭带的管内传热与流动特性对比

数值研究了空气在内置异形等间距扭带管内的传热与流阻特性,对模拟结果进行了实验验证,然后分析了结构对空气传热与流阻特性的影响;进一步通过对局部流场、温度场和绝对涡量的分析,揭示了其强化传热机制。结果表明:模型与实验值吻合较好,最大相对偏差约为8%,在大多数Re(雷诺数)范围内,异形等间距扭带相对传统等间距扭带提高了空气传热综合性能,在Re 10 000~20 000范围内,最大传热综合性能约为1.08,交错布置的等间距扭带对传热综合性能的提升没有其它等间距扭带明显。等间距扭带能够强化空气传热主要在于通过诱导二次流,加强了近壁区流体与主流区流体的置换程度,均化了流场,并且能够以自旋流的形式在下游维持较长的距离。     

人字形波纹板传热与阻力特性的数值模拟和试验研究

对板式低温多效海水淡化技术中的人字形波纹板式换热器进行了单相流换热试验和数值模拟,试验结果表明其换热系数较高,在低Re数下就能达到3000 W/(m~2·K)以上。通过建立三维模型,运用CFD软件对人字形波纹板内部的传热及阻力特性进行数值模拟,研究了波纹倾角β、波高h和波纹间距λ对人字板传热以及流动阻力的影响,模拟结果与试验值误差都在15%以内。数值模拟结果表明,波纹倾角从30°增大到60°,传热因子j约提高60%,继续增大波纹倾角,传热因子反而降低,摩擦因子f随着波纹倾角的增大而增大;波高从3 mm增大到6 mm,传热因子j约提高5%,但摩擦因子f增大1倍;波纹间距从5 mm增大到20 mm,传热因子j约降低30%,但同时摩擦因子f降低70%。建议板式低温多效海水淡化系统的板型参数:波纹倾角60°,波高3~4 mm,波纹间距15~20 mm。     

新型横纹螺旋盘管强化传热的数值模拟

为了增强螺旋盘管的传热性能,对现有的普通螺旋盘管进行优化设计,提出一种管壁向内凸起形成环肋的异型管,称为横纹螺旋盘管。通过数值模拟方法对横纹螺旋盘管和普通盘管内部流动和传热过程进行模拟。应用场协同原理对其速度场和温度场的协同作用进行分析。实验数据与仿真结果的误差在5%以内,验证了数值模拟方法的正确性。在不同Re(雷诺数)条件下,计算两种盘管的Nu(努塞尔数),进而与Gnielinski(格尼林斯基)修正公式计算结果进行比较,误差在10%以内。结果表明:环肋结构通过工质旋转流动破坏边界层厚度,改善了管内速度场与温度场的协同程度,从而实现了强化传热。在较高的Re范围内,横纹螺旋盘管的Nu为普通盘管的1.29~1.43倍。因此,横纹螺旋盘管具有更好的传热性能,为异型螺旋盘管的研究及工程应用提供一定的理论依据。     

波节管纵向逆流换热性能的三维数值模拟研究

本文基于k-ε模型,针对波节管高效换热元件中纵向逆流换热的传热特性和阻力特性进行三维数值模拟研究。传热工质在管程和壳程分别为氦气和氮气,管束采用三角形布置。本文首先分析了不同波距及雷诺数下对换热量影响。为了体现高效换热元件比光管的优越性,随后分析了不同波距及雷诺数对Q/Q0(波节管与光管的换热量比)与Δp/Δp0(波节管与光管的压力降比)。最后得出结论,波距L的增加使高效换热元件的传热性能和阻力性能有所降低,但提高了其综合传热性能。雷诺数的增加会大幅提高换热量,但同时综合传热效率也大幅降低。     

Turbulent heat transfer enhancement in a heat exchanger using helically corrugated tube

The augmentation of convective heat transfer in a single-phase turbulent flow by using helically corrugated tubes has been experimentally investigated. Effects of pitch-to-diameter ratio (P/D_H = 0.18, 0.22 and 0.27) and rib-height to diameter ratio (e/D_H = 0.02, 0.04 and 0.06) of helically corrugated tubes on the heat transfer enhancement, isothermal friction and thermal performance factor in a concentric tube heat exchanger are examined. The experiments were conducted over a wide range of turbulent fluid flow of Reynolds number from 5500 to 60,000 by employing water as the test fluid. Experimental results show that the heat transfer and thermal performance of the corrugated tube are considerably increased compared to those of the smooth tube. The mean increase in heat transfer rate is between 123% and 232% at the test range, depending on the rib height/pitch ratios and Reynolds number while the maximum thermal performance is found to be about 2.3 for using the corrugated tube with P/D_H = 0.27 and e/D_H = 0.06 at low Reynolds number. Also, the pressure loss result reveals that the average friction factor of the corrugated tube is in a range between 1.46 and 1.93 times over the smooth tube. In addition, correlations of the Nusselt number, friction factor and thermal performance factor in terms of pitch ratio (P/D_H), rib-height ratio (e/D_H), Reynolds number (Re), and Prandtl number (Pr) for the corrugated tube are determined, based on the curve fitting of the experimental data.     

A geometric study on shell side heat transfer and flow resistance of a six-start spirally corrugated tube

Heat transfer enhancement is of great importance for energy efficiency improvement. The utilization of spirally corrugated tubes is one of the efficient ways to strengthen heat transfer. In this article, based on a validated numerical model, the effects of geometric parameters of a six-start spirally corrugated tube, including the pitch p and the corrugation depth e, on the shell side heat transfer and flow resistance performance are numerically investigated, in high Reynolds number conditions ranging from 10,000 to 60,000. The shell side secondary flow velocity distribution, longitudinal vortex distribution, and temperature distribution of a six-start spirally corrugated tube are presented, respectively. In addition, the heat transfer and flow resistance characteristics are evaluated by comparing the Nusselt number and the flow resistance coefficient with these of smooth tubes. Results show that the utilization of six-start spirally corrugated tubes can enhance the heat transfer performance at the expense of an increase of the flow resistance. However, with the same geometric parameters, the Nusselt number increases and the flow resistance coefficient decreases as Reynolds number increases. With the pitch increasing, the Nusselt number and the flow resistance coefficient decrease at a fixed Reynolds number. In contrast, as the corrugation depth increasing, the Nusselt number changes irregularly, and the flow resistance coefficient increases. Finally, correlations for the shell side Nusselt number and flow resistance coefficient of the six-start spirally corrugated tube are established. This work is of significance for engineers and scientists focusing on the heat transfer and the flow resistance characteristics of spirally corrugated tubes and their applications.     

Heat Transfer and Pressure Drop Characteristics of Nanofluid Flows Inside Corrugated Tubes

An experimental investigation is carried out to study the heat transfer and pressure drop characteristics of multiwalled carbon nanotubes (MWCNTs)/heat transfer oil nanofluid flows inside horizontal corrugated tubes under uniform wall temperature condition. To provide the applied nanafluids, MWCNTs are dispersed in heat transfer oil with mass concentrations of 0.05, 0.1, and 0.2 wt%. The Reynolds number varies between 100 and 4,000. Three tubes with hydraulic diameters of 11.9, 13.2, and 15.5 mm are applied as the test section in the experimental setup. Tubes are corrugated four times on the cross section; that is, there are four different helices around the tube. Depths of the corrugations are chosen as 0.9, 1.1, and 1.3 mm, and pitch of corrugation is 14 mm. The acquired data confirm the increase of heat transfer rate as a result of utilizing nanofluids in comparison with the base fluid flow. However, corrugating the tubes decreases the heat transfer rate at low Reynolds numbers. The highest increase in heat transfer rate is observed for the Reynolds numbers for which the smooth tube is in the transition regime and the corrugated tube reaches the turbulent flow, that is, Reynolds number in the range of 1,000 to 3,000. Rough correlations are proposed to predict the Nusselt number and friction factor.     

Numerical study of an exhaust heat recovery system using corrugated tube heat exchanger with twisted tape inserts

The purpose of this work is to investigate gas to liquid heat transfer performance of concentric tube heat exchanger with twisted tape inserted corrugated tube and to evaluate its impact on engine performance and economics through heat recovery from the exhaust of a heavy duty diesel generator (120 ekW rated load). This type of heat exchanger is expected to be inexpensive to install and effective in heat transfer and to have minimal effect on exhaust emissions of diesel engines. This type of heat exchanger has been investigated for liquid to liquid heat transfer at low Reynolds number by few investigators, but not for gas to liquid heat transfer. In this paper, a detail of heat transfer performance is investigated through simulations using computer software. The software is first justified by comparing the simulation results with the developed renowned correlations. Simulations are then conducted for concentric tube heat exchanger with different twisted tape configuration for optimal design. The results show that the enhancement in the rate of heat transfer in annularly corrugated tube heat exchanger with twisted tape is about 235.3% and 67.26% when compared with the plain tube and annularly corrugated tube heat exchangers without twisted tapes respectively. Based on optimal results, for a 120 ekW diesel generator, the application of corrugated tube with twisted tape concentric tube heat exchanger can save 2250 gal of fuel, $11,330 of fuel cost annually and expected payback of 1 month. In addition, saving in heating fuel also reduces in CO₂ emission by 23 metric tons a year.     

Heat transfer enhancement by using CuO/water nanofluid in corrugated tube equipped with twisted tape

Heat transfer enhancement by using CuO/water nanofluid in corrugated tube equipped with twisted tape is presented. The investigated ranges are (1) three different CuO concentrations: 0.3, 0.5 and 0.7% by volume (2) three different twist ratios of twisted tape: y/w = 2.7, 3.6 and 5.3 (3) two different arrangements of twisted direction of twisted tape relative to spiral direction of corrugated tube: parallel and counter arrangements, and (4) Reynolds number from 6200 to 24000. The results achieved from the use of the nanofluid and twisted tape, are compared with those obtained from the uses of nanofluid alone and twisted tape alone. The experimental results reveal that at similar operating conditions, heat transfer rate, friction factor as well as thermal performance factor associated with the simultaneous application of CuO/water nanofluid and twisted tape are higher than those associated with the individual techniques. Evidently, heat transfer rate increases with increasing CuO/water nanofluid concentration and decreasing twist ratio. In addition, the twisted tape coupled with corrugated tube in counter pattern offer higher heat transfer performances than the ones in parallel pattern. Over the range studied, the maximum thermal performance factor 1.57 is found with the use of CuO/water nanofluid at concentration of 0.7% by volume in corrugated tube together with twisted tape at twist ratio (y/w) of 2.7 (in counter arrangement), for Reynolds number of 6200 where heat transfer rate and friction factor increase to 2.67 times and 5.76 times of those in the plain corrugated tube.     

Numerical study of flow and heat transfer characteristics in outward convex corrugated tubes

The flow and heat transfer characteristics in convex corrugated tubes have been investigated through numerical simulations in this paper. Two kinds of tube types named as symmetric corrugated tube (SCT) and asymmetric corrugated tube (ACT) are modeled and studied numerically based on the k-ε model. The heat transfer working fluid at shell and tube sides are nitrogen and helium gases respectively. 2D axisymmetric model is adopted to simplify 3D model in order to reduce the computation cost greatly. Numerical simulation results for flow and heat transfer performances in SCT and ACT with various geometrical parameters, including corrugation pitch, corrugation height and corrugation trough radii are systematically analyzed. The mechanisms behind the improvement of overall performances of the simulated outward convex corrugated tube are discussed through investigating the details of turbulent velocity fields at both tube and shell sides. Compared to SCT, ACT exhibits 8–18% higher overall heat transfer performance.     

Effect of corrugated plates in an in-phase arrangement on the heat transfer and flow developments

In the present study, the numerical results of the heat transfer and flow developments in the corrugated channel under constant heat flux conditions are presented. The test section is the channel with two opposite corrugated plates which all configuration peaks lie in an in-phase arrangement. The corrugated plates with three different corrugated tile angles of 20°, 40°, and 60° are tested with the height of the channel of 12.5 mm. The model was simulated for the Reynolds number and heat flux in the ranges of 400–1600 and 0.5–1.2 kW/m², respectively. The flow and heat transfer developments are simulated by using the k-ε standard turbulent model. A finite volume method with the structured uniform grid system is employed for solving the model. The predicted results are validated by comparing with the measured data. There is reasonable agreement from the comparison between the numerical data and experimental data. Effects of relevant parameters on the heat transfer and flow developments are discussed. Due to the breaking and destabilizing in the thermal boundary zone, the corrugated surface has significant effect on the enhancement of heat transfer and pressure drop.     

Correlations for evaporation heat transfer coefficient and two-phase friction factor for R-134a flowing through horizontal corrugated tubes

Correlations for the evaporation heat transfer coefficient and two-phase friction factor of R-134a flowing through horizontal corrugated tubes are proposed. In the present study, the test section is a horizontal counter-flow concentric tube-in-tube heat exchanger with R-134a flowing in the inner tube and hot water flowing in the annulus. Smooth tube and corrugated tubes with inner diameters of 8.7 mm and lengths of 2000 mm are used as the inner tube. The corrugation pitches are 5.08, 6.35, and 8.46 mm and the corrugation depths are 1, 1.25, and 1.5 mm, respectively. The outer tube is made from smooth copper tube with an inner diameter of 21.2 mm. The correlations presented are formed by using approximately 200 data points for five different corrugated tube geometries and are then proposed in terms of Nusselt number, equivalent Reynolds number, Prandtl number, corrugation pitch and depth, and inside diameter.