椭圆管束层流尾迹的流动过渡和分岔

运用有限容积法,在不同无量纲管间距下对空气横掠椭圆管束进行二维数值模拟,对尾迹的流动变化过程和相应的分岔现象进行了分析.结果表明:保持初始边界条件不变,在无量纲管间距为0.5～3.5时,随着雷诺数增大至相应的临界值,Hopf分岔的发生使得尾迹从稳定的对称流动进入准周期状态;管间距较小时系统还会发生音叉分岔,间隙流动的偏...     

直条缝翅片管换热器传热与流动阻力性能研究

为了解翅片间距、翅片厚度、翅片材料和基管材料对直条缝翅片管换热器的传热和流阻性能的影响,以及获得通用的换热与流动阻力计算关联式,对一种直条缝翅片管换热器进行了实验研究。通过风洞试验台共进行了7个试件的试验,试验过程中管内水的进口温度和速度保持60℃和1.5 m/s不变,进风温度保持21℃,入口风速为1.5～4.5 m/s。结果表明：在管外空气侧雷诺数Re_a为2647～8143范围内,随着翅片间距的增大,对流换热系数先增大后减小,存在一个峰值;翅片间距对阻力的影响与管外空气侧雷诺数有关,当Re_a≤5 000时翅片间距越小摩擦系数越大,当Re_a>5 000时,翅片间距越小,摩擦系数越小;翅片厚度的增加会增加对流换热系数和摩擦系数;紫铜(T2)翅片的对流换热系数高于8011铝合金(AL8011)翅片,但摩擦系数较低;T2基管的对流换热系数最高,铁白铜(B10)基管次之,316L不锈钢(316L)基管最低;不同的基管材料对摩擦系数没有影响。     

螺旋翅片管束传热和阻力特性的试验研究

对13个不同翅片间距、翅片高度、横向管间距、纵向管间距的螺旋翅片管束换热器在不同雷诺数条件下的传热和阻力特性进行了试验研究,得出了翅片间距、翅片高度、横向管间距、纵向管间距及雷诺数与换热特性Nu和阻力特性Eu的准则关系式,并对准则关系式进行了分析.结果表明:随着横向管间距和翅片间距的增大,螺旋翅片管的传热得到强化,但随着纵向管间距和翅片高度的增加,螺旋翅片管的传热有所减弱;随着横向管间距、纵向管间距和翅片间距的增大,螺旋翅片管的阻力减少,但随着翅片高度的增加,螺旋翅片管的阻力增加.     

波壁管几何尺寸对流动与传热特性的影响

基于数值模拟的方法,分析在低雷诺数下波壁管波形变化对流体流动与传热特性的影响,并分析了相同功耗下波壁管的综合传热性能。结果表明:波幅和波长变化对波壁管传热均有影响,强化效果与波幅成正比,与波长成反比;当功耗相同时,小波幅的波壁管有较好的综合换热效果,大波幅的波壁管强化传热以较大能量消耗作为代价;雷诺数大于2 000时,增大波长能达到较好的综合换热效果。     

并列双柱绕流的格子Boltzmann模拟

采用格子Boltzmann方法模拟研究了并列双圆柱的绕流现象,利用非平衡外推法进行边界条件处理,基于C++编制程序,获得了不同雷诺数Re和柱间距比H/D下的流速及回流区分布,并分析了并列双柱绕流的流动特性。结果表明,随Re的增大,流场由层流向卡门涡街转变,回流区长度先增大后减小,临界值为Re=50;随H/D的增大,可将流动模式分为单体模式、交替翻转模式、同相模式及反相模式四种,回流区长度在这四种模式下随H/D的增大而增大。     

水平外波纹管降膜管间流型转变

相比于光管,外波纹管的壁面凹凸结构可增加换热面积,从而提高水平管降膜蒸发器的传热特性。准确预测溶液管间流型是提高传热传质性能的关键步骤。建立了水平管降膜实验装置,研究管间距、溶液浓度、管型对降膜流动过程管间流型转变的影响。结果表明:随着雷诺数(Re)的增加,管间流动形态依次出现滴状流、滴柱状流、柱状流和柱片状流和片状流;对于每一种流型转变所对应的Re,外波纹管明显低于光管,并随管间距的增大而增大,随NaCl溶液质量分数的增大而减小;相较于光管,两种外波纹管更易在较低Re下获得各稳定流型。流型转变边界可以通过流体Re与伽利略数(Ga)之间的函数关系式描述。通过拟合实验数据,得到了3种管的流型转变关系式。     

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。     

进口参数变化对高压压气机性能影响研究

以某型高压九级轴流压气机为研究对象,利用NUMECA软件在不同进口条件下进行变工况性能模拟计算。研究了变转速下由于进口总温、总压的改变引起的雷诺数变化对压气机性能影响。结果表明:试验条件与设计条件下的试验结果相比,进口雷诺数由1.348×10~6下降到4.318×10~5,压气机设计点折合流量比减小0.008,效率下降0.72%,喘振裕度降低了8.11%,压气机性能曲线整体向左下方移动;在一定范围内升高进口总压或降低进口总温,将改善压气机的压比、效率以及折合流量比;当雷诺数高于一定临界值后,同一转速下的压气机的效率以及折合流量比基本保持不变;当转速降低至设计转速以下时,临界雷诺数将进一步增大,雷诺数效应影响增强;雷诺数降低会导致泄漏流损失增大,径向涡损失增强,加剧叶尖区域的流动分离,此时叶尖区域的流动阻塞成为引起流动失稳与整机性能恶化的主要原因。     

颗粒帘换热器中固体颗粒流动特性实验研究

提出了一种基于气体-固体颗粒快速热平衡的颗粒帘换热器。为揭示该换热器中气粒两相间的动力学特性,在颗粒帘换热器实验平台上开展了颗粒帘换热器中固体颗粒流动特性的冷态实验。实验结果表明:颗粒帘沿程厚度随进气速度的增加而减小,颗粒帘后沿水平偏移量随进气速度的增加而增大;颗粒帘沿程厚度及颗粒帘后沿水平偏移量随颗粒帘初始厚度的增加而增大,随颗粒粒径及颗粒质量流量的增加而减小;颗粒帘沿程厚度和颗粒帘后沿水平偏移量随着颗粒帘下落高度的增加而增大;颗粒帘落点水平偏移量随进气速度的增加而增大,随颗粒帘初始厚度及颗粒粒径及颗粒质量流量的增加而减小。这些结果为颗粒帘换热器结构的优化设计和气固两相流动、换热及积灰特性的研究奠定了良好的基础。     

低雷诺数下液体的微尺度流动与传热

针对低雷诺数下单相水流经矩形微通道时的流动特性和换热特性进行了试验和理论研究。试验工质为去离子水，微通道宽度为50μm，高度为200μm，雷诺数2．3～15．6。试验结果表明，在低雷诺数下的流动特性和传统理论存在偏差：矩形微通道实验压降值与阻力系数均大于传统理论值的预测，阻力系数的最大偏差为7．9％；Poiseuille数比传统理论预测值大6％；但当热流密度为2．5～15W／cm^2时，换热特性的试验结果与传统理论较为吻合。     

基于场协同理论的超声波对强化换热影响

本研究分别对圆管、波节管和横纹管在Fluent软件中进行数值模拟,模拟了3种管型在紊流工况下的换热效果并对数值模拟所得到的结果用场协同的理论分析。结果表明:从场协同理论得出加入超声波会增强场的协同程度,增强换热管的换热效果;圆管、波节管、横纹管的场协同数则随着雷诺数的增加而减小;而努塞尔数和表面传热系数随着雷诺数的增加而增加,综合性能系数随着雷诺数的增加而增加,而效能评价系数会随着雷诺数的增加而减小。     

Comparative analysis of heat transfer and pressure drop in helically segmented finned tube heat exchangers

Four different semi-empirical models of heat transfer and pressure drop for helically segmented finned tubes in staggered layout were analyzed. The performance of a Helically Segmented Finned Tubes Heat Exchanger on an industrial scale was obtained and the predictions were compared with experimental data. The method used for thermal analysis is the Logarithmic Mean Temperature Difference (LMTD). Comparisons between predictions and experimental data show a precision greater than 95% in heat transfer for a combination between the Kawaguchi and Gnielinski models at a flue gas Reynolds number, based on the outside bare tube, of about 10,000. In the case of pressure drop, there is a precision of approximately 90% for the Weierman model at a Reynolds number, based on the outside bare tube, of about 10,000. And so, the results show that the best flow regime in which heat transfer and pressure drop are optimum, is for a Reynolds number (based on the outside bare tube) of about 10,000.     

Numerical investigation for improved heat transfer characteristics in micro‐fin tubes

Generally, internal micro‐fin tubes are used for increasing the life and performance of electronic devices. The micro‐fins enhance the heat transfer rate by increasing the surface area with an increase of the pressure drop. In this study, heat transfer and pressure drop are analyzed by varying Reynolds number with the increase in the number of fins in tubes. Heat transfer and pressure drop, together with turbulence kinetic energy of micro‐fin tubes (helical and straight) and a smooth tube, have been evaluated for different Reynolds numbers (60 000, 40 000, 20 000, and 2000) at a constant temperature of 350 K, which clearly establishes laminar to turbulent flow. It is observed that the helical micro‐fin tube has a better result compared with the straight micro‐fin tube and smooth tube at Reynolds numbers 60 000, 40 000, and 20 000 at velocity 2, 1, and 0.5 m/s, respectively. This study is an attempt to establish a comparison of different micro‐fin geometries with varying Reynolds numbers, concluding that a high Reynolds number is suitable for the same.     

Visualization of R-134a Flowing on Tube Arrays with Plain and Enhanced Surfaces under Adiabatic and Condensing Conditions

Experimental studies were performed to investigate the flow patterns of liquid films of R-134a on vertical arrays of horizontal tubes under adiabatic test conditions at room temperature and during condensation at a saturation temperature of 304 K. Three arrays with tube pitches of 25.5, 28.6, and 44.5 mm and up to ten tubes of four commercially available copper tubes with a nominal diameter of 19.05 mm and 544 mm in length were tested: a plain tube, a 26 fpi/1024 fpm low finned tube (Turbo-Chil), and two tubes with three-dimensional enhanced surface structures (Turbo-CSL and Gewa-C). Flow pattern observations and transitions were obtained and compared to existing flow pattern maps. The effect of the flow pattern on heat transfer was also investigated. The ideal flow modes (droplet, column, and sheet) could strictly be observed on the tubes with 3D enhanced surface structures. On the low-finned tube and the plain tube, the flow was very unstable, causing a part of liquid to leave the array sideways. For the 3D enhanced tubes, a difference in the formation of the sheets could be seen. The Gewa-C tube tended to form small sheets, while the Turbo-CSL formed large sheets at low film Reynolds numbers. A cyclic forward and backward motion of the sheet was observed that increased in amplitude with an increasing film Reynolds number. At high film Reynolds numbers, part of the liquid left the array of tubes sideways due to this oscillation, the effect of which on heat transfer was found to be more significant than the effect of the flow modes themselves.     

膜片管通道流动与换热过程的数值模拟

采用SIMPLE算法模拟膜片管通道中的流动与换热,分析流场中出现的非线性现象以及不同管束排列方式对换热的影响。物理模型长度为185. 6 mm,高度为92. 8 mm,圆管直径为32 mm。烟气入口温度为400 K,上下两侧固体壁面温度为300 K。假设流动与换热进入充分发展阶段,雷诺数(Re)的取值范围是3 000～25 000,通入不同流速的烟气与两侧的壁面进行换热。结果表明:采用雷诺应力模型(RSM)所得的努塞尔数(Nu)与实验关联式结果最吻合,而且相对误差在5%～17%间;采用直接模拟(DNS)模拟时,稳态到非稳态的临界Re是100;在同一Re时,随着管间距减小,Nu是逐渐增加的,当Re取为25 000,管束水平间距和竖直间距均取为43. 2 mm时,通道换热能力达到最大且相应的Nu是195. 23。     

Experimental Study on Cu/Oil Nanofluids through Concentric Annular Tube: A Correlation

This study deals with an empirical investigation on the convective heat transfer of Cu/oil nanofluid flow inside a concentric annular tube with constant heat flux boundary condition and suggests a correlation to predict the Nusselt number. The average size of particles was 20 nm and the applied nanofluid was prepared by Electrical Explosion of Wire technique with no nanoparticle agglomeration during nanofluid preparation process and experiments. The nanofluid flowing between the tubes is heated by an electrical heating coil wrapped around it. The effects of different parameters such as the flow Reynolds number, tube diameter ratio, and nanofluid particle concentration on heat transfer coefficient are studied. Using the acquired experimental data, a correlation is developed for the estimation of the Nusselt number of nanofluid flow inside the annular tube. This correlation has been presented by using the exponential regression analysis and least‐squares method. The correlation is valid for Cu/base oil nanofluid flow with weight concentrations of 0.12, 0.36, and 0.72 in the hydrodynamically full‐developed laminar flow regime with Re <140, which is applicable in mini‐ and microchannel heat exchangers, and it is in good agreement with the experimental data.     

Influence of spiky twisted tape insert on thermal fluid performances of tubular air–water bubbly flow

An experimental study that comparatively examined the two-phase flow structures, pressure drops and wall-to-fluid heat transfer properties between the plain tube and the enhanced tube with the spiky twisted-tape insert (swirl tube), was performed to disclose their differential thermal-fluid performances with air–water flows. On-line and post-processed high-speed digital images of air–water two-phase phenomena in plain and swirl tubes were detected to ensure the bubbly flow pattern in plain tube and to visualize their characteristic interfacial structures. Superficial liquid Reynolds number (Re_L) and air-to-water mass flow ratio (AW), which were respectively controlled in the ranges of 5000–15,000 and 0.0004–0.01, were selected as the controlling parameters of heat transfer performances. The dispersed rising air bubbles in the plain tube and the centrifugal-force induced coherent spiral stream of coalesced bubbles in the swirl-tube core considerably modify the pressure-drop and heat-transfer performances from the single-phase conditions. Selected results of pressure-drop and heat-transfer measurements, flow images and tube-averaged void fractions detected from the plain and swirl tubes with air–water two-phase flows were cross-referenced to illustrate the mechanisms responsible for the modified thermal-fluid performance due to the spiky twisted-tape insert. Empirical heat transfer correlations which evaluate the Nusselt numbers over the developed flow regions of the plain and swirl tubes with air–water two-phase flows were generated for industrial 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.     

Experimental study of mixed convection and pressure drop in helically dimpled tubes for laminar and transition flow

This paper presents the experimental results carried out in dimpled tubes for laminar and transition flows and completes a previous work of the authors focused on the turbulent region. It was observed that laminar flow heat transfer through horizontal dimpled tubes is produced in mixed convection, where Nusselt number depends on both the natural convection and the entry region. Employing water and ethylene glycol as test fluids, the following flow range was covered: x^*=10⁻⁴–10⁻² and Ra=10⁶–10⁸.

The experimental results of isothermal pressure drop for laminar flow showed dimpled tube friction factors between 10% and 30% higher than the smooth tube ones. Moreover, it was perceived that roughness accelerates transition to critical Reynolds numbers down to 1400. Correlations for the laminar friction factor f=f(Re,h/d) and for the critical Reynolds Re_crit=Re_crit(h/d) are proposed. The hydraulic behaviour of dimpled tubes was found to depend mainly on dimple height.

In mixed convection, high temperature differences in the cross section were measured and therefore heat transfer was evaluated by a circumferentially averaged Nusselt number. Experimenal correlations for the local and the fully developed Nusselt numbers and are given. Results showed that at low Rayleigh numbers, heat transfer is similar to the smooth tube one whereas at high Rayleigh, enhancement produced by dimpled tubes can be up to 30%.     

Effect of turbulence on Taylor dispersion for oscillatory flows

Experiments were carried out to explore the effect of the turbulence on Taylor dispersion for oscillatory tube flow. We found from measured velocity profiles that, while the velocity fluctuations exist over the parameter range in the present investigation, periodic conditional turbulent flow occurs and the turbulence intensity increases rapidly when the Stokes layer-based Reynolds number exceeds 535. The effective axial mass diffusivity for oscillatory flow deviate from the theoretical predictions corresponding to laminar flow as Re_δ value increases, and the deviation becomes noticeable as Re_δ increases to 235. It was also found that an ratio of effective diffusivity to theoretical value for laminar flow as large as 100 is available during turbulent oscillatory flows.