波纹翅片通道内液化天然气流动沸腾换热特性分析

波纹翅片广泛应用于液化天然气板翅式换热器中;为了对板翅式换热器进行优化设计,必须明确翅片通道内的流动沸腾机理。首先对波纹翅片流道中流体的流动和传热传质机理进行分析,建立了稳态工况下汽化相变模型。然后进行了不同质流密度、热流密度和干度工况下波纹翅片流道内流体流动换热过程的模拟,分析了质流密度、热流密度和干度对波纹翅片传热特性的影响,并与平直翅片进行了对比。结果显示:随着干度增大,波纹翅片换热系数呈现先上升后下降的趋势,且在0.5干度左右达到最大值;随着干度增加,质流密度的增大对换热性能的提升越来越明显,热流密度的增大对换热性能的提升越来越小;波纹翅片比平直翅片换热系数提高30%~150%,在低干度工况下波纹翅片强化传热效果更明显。     

锯齿形翅片管

锯齿形翅片管是以普通光管为坯管,通过外表加工而成的一种异形翅片管,它适用于作为卧式壳管冷凝器的传热元件。与一般的低肋管相比,这种管子的结构特点是:翅片密;翅片距可小至0.5mm,且翅片外缘上开有锯齿形缺口,与低肋管相反,其翅片两侧是粗糙的而不是平滑的。     

冷氦换热器翅片结构仿真优化分析

应用商业软件Ansys CFX 11.0对冷氦换热器多种结构尺寸的翅片进行了数值仿真,分析了翅片高度、翅片间距、翅片内表面温度对翅片管自然对流换热的影响。结果表明:对于管径一定的换热器,翅片厚度一定时,有一个最佳的翅高翅距比,对应于最大的Nusselt数。     

穿孔翅片管的实验研究及翅片效率分析

在平翅片上开小圆孔是一种简单的强化翅片表面换热的措施,本文通过正交实验,研究了开孔大小和位置对换热性能和阻力性能的影响,得出了关联式。通过与普通平翅片管对比表明,最优穿孔型翅片管的换热系数可提高１２．５％,而阻力增加不超过７％,在同样的传热量的温差下,最优穿 形翅片管可以节约１０％的金属耗量,数值计算表明,最优穿孔形翅片的翅片效率比普通平翅片约低２％。     

穿孔型平翅片管的实验研究和数值分析

在平翅片上开小圆孔是一种简单的强化翅片表面换热的措施。本文通过正交实验,研究了开孔大小和位置对换热性能和阻力性能的影响,得出了关联式。通过与普通平翅片管对比表明,最优穿孔型翅片管的换热系数可提高12.5％,而阻力增加不超过7％。在同样的传热量和温差下,最优穿孔形翅片管可以节约10％的金属耗量。数值计算表明,穿孔形翅片效率比普通平翅片低1％-2％。穿孔型翅片的效率降低以及存在面积损失,将抵消部分强化作用。     

R134a单元式风冷冷风空调机蒸发器设计

以理论模型为基础，对R134a单元式风冷冷风机组翅片管式蒸发器进行设计。应用管内流动沸腾换热模型仿真分析R134a的质量流量对沸腾换热的影响，利用外掠翅片管束换热关联式计算管外翅片侧表面换热系数，进而得出翅片管蒸发器总传热系数，利用计算结果进行设计。     

单一金属高螺纹翅片管换热器的探索与实践

通过对管壳式换热器几种常见翅片管的结构特点、换热性能和制造经济性进行分析比较和研究,研制出传热效率更高、结构更紧凑、制造成本更低廉的新型换热元件——单一金属轧制的高螺纹翅片管;提出了传热学意义上最佳翅片截面轮廓线的通用解析式。通过生产实践与不断改进,形成了一组高螺纹翅片管系列产品,并成功地应用于为透平压缩机配套的大中型管壳式换热器。     

凸环翅片及其在热泵型空调器中的应用

提出一种应用于热泵型空调器的凸环翅片结构。通过CFD模拟,分析波纹翅片、开缝翅片和凸环翅片的传热性能,对翅片进行优化设计。换热器能力测试显示,凸环翅片管式换热器的换热能力,在冷凝工况和蒸发工况下,分别比波纹翅片管式换热器提升3%~5%和2.5%,比开缝翅片管式换热器降低0.8%和0.5%。空调整机测试结果显示,采用凸环翅片管式换热器可实现用单排换热器替代1.5排或2排换热器,降低换热器成本的效果。     

R134a在水平直齿外翅片管表面冷凝传热实验研究

在对R134a在水平直齿外翅片管表面冷凝传热理论研究的基础上,利用用计算机建立了传热数学模型,并在实验室中用5根紫铜外翅片铜管进行试验验证,结果表明该理论数学模型在一定范围内的预测值是准确的.     

星型翅片管换热器结霜过程的数值模拟

采用数值模拟的方法对星型翅片管换热器的结霜过程进行了研究。得到了霜层厚度和霜层密度等物性参数的变化规律,分析了其空气侧结霜过程中的传热传质特性,研究了翅片高度和翅片个数两个参数对结霜过程的影响。结果表明,结霜现象会造成换热器换热性能的恶化,并且翅片个数越多、翅片高度越小,传热恶化更严重。     

Effects of fin shape on condensation in herringbone microfin tubes

Effects of fin height and helix angle on condensation inside a herringbone microfin tube have been experimentally investigated with five types of herringbone microfin tubes. Heat transfer coefficients are about 2–4 times higher than that of the helical microfin tube under high mass velocity conditions. In the low mass velocity, they are equal to that of the helical microfin tube. The heat transfer enhancement increases with fin height up to 0.18 mm; higher fin heights show enhancement values similar to the 0.18 mm results. Pressure drop increases with the fin height. Larger helix angle yields higher heat transfer and higher pressure drop. For the lowest fin and/or smallest helix angle, the pressure drop is comparable with that of the helical microfin tube, while the heat transfer enhancement is higher. The enhancement mechanism is discussed from flow pattern observations. Effect of mass transfer resistance for R410A is estimated and negligible effects have been proved.     

Condensation of R-134a vapour over single horizontal integral-fin tubes: effect of fin height

An experimental investigation has been carried out to study the condensation of R-134 vapour over five single horizontal circular integral-fin tubes of 472 fpm fin density, 417 mm length and different fin heights of 0.45, 1.14, 1.47, 1.92 and 2.40 mm. The circular fins are rectangular in shape and the fin thickness of all tubes is 0.70 mm. The tube with the fin height of 0.45 mm has given the highest enhancement in heat transfer coefficient, h, of the order of 3.18 in comparison to that predicted by the Nusselt model for a plain tube.     

螺旋槽锯齿翅片管在空调机冷凝器上的应用

螺旋槽锯齿翅片管是一种新型高效冷凝换热管。试验表明，在额定工况下，以R22为制冷剂，螺旋槽锯齿翅片管冷凝器与通常使用的低肋管冷凝器相比具有较高的冷凝换热性能，在换热面积减少30％的情况下，空调机的制冷量增加3．3％，能效比提高6．9％。     

微肋表面强化液氮温区冷凝传热特性的数值模拟研究

开展了微肋表面强化氮蒸气凝结传热的机理研究,基于CFD数值方法建立了竖直方向平板和微肋板上氮蒸气冷凝的三维数值模型,对比分析了平板表面和微肋表面的冷凝换热效果,通过表面冷凝的液膜分布和速度分布等内在参数揭示平板和微肋表面的冷凝换热机理。结果表明,氮的冷凝传热性能在微肋表面上得到显著提升,在1.5 K的冷凝传热温差下,其平均传热系数提高1.6倍,初步揭示了微肋表面(肋高H=0.6 mm、节距p=1 mm)对强化氮冷凝的有效性。分析显示,微肋的弯曲结构改变了液膜的空间分布,通过减薄局部液膜厚度实现了冷凝的强化。     

Conjugate heat transfer in solar collector panels with internal longitudinal corrugated fins—Part I: Overall results

A computational study is conducted to examine the fully developed laminar flow and heat transfer characteristics in solar collector panels with internal, longitudinal, corrugated fins. The fins are integrally attached to the upper and lower panel walls. The objective of the study is to determine the effects of varying the fin pitch (or fin angle), the fin thickness, the ratio of the thermal conductivity of the panel walls and the fin to that of the fluid, and the thermal boundary condition on the panel heat transfer and pressure drop. The solutions of the momentum and energy equations are obtained by using a control-volume-based finite difference algorithm. The results of the study are also applicable to the design of internally-finned channels in compact heat exchangers. The overall panel heat transfer increases when the fin pitch (or the fin angle) is decreased, when the fin thickness is increased, and when the thermal conductivity ratio is increased. The streamwise pressure drop increases with decreasing fin pitch (or fin angle) and increasing fin thickness. For a fixed fin thickness, the selection of a small fin pitch (or fin angle) over the range studied results in a higher heat transfer enhancement per unit pumping power.     

In-tube cooling heat transfer of supercritical carbon dioxide. Part 2. Comparison of numerical calculation with different turbulence models

Heat transfer of supercritical carbon dioxide cooled in circular tubes was investigated experimentally. The effect of mass flux, pressure, and heat flux on the heat transfer coefficient and pressure drop was measured for four horizontal cooling tubes with different inner diameters ranging from 1 to 6 mm. The radial distribution of the thermophysical properties (i.e. specific heat, density, thermal conductivity and viscosity) in the tube cross-section was critical for interpreting the experimental results. A modified Gnielinski equation by selecting the reference temperature properly was then developed to predict the heat transfer coefficient of supercritical carbon dioxide under cooling conditions. This proposed correlation was accurate to within 20% of the experimental data.     

多元平行流冷凝器传热流动性能研究

平行流冷凝器空气侧采用间断型扩展表面的波纹型百叶窗翅片,制冷剂侧采用小水力直径的非圆截面微通道多孔铝制扁管,选用适合于该微尺度强化换热结构的传热和压降关联式,对某规格的平行流冷凝器建立数学模型并在一定工况下进行数值模拟.结果分析表明,制冷剂在非圆截面微通道内的冷凝过程中,表面张力对表面传热系数的强化效果明显;通过改变流程数和各流程管数来改变冷凝过程中的流通截面而达到调整流速的作用,从而可以保持较高的冷凝换热系数和较低的流动压降,与常规换热器相比具有显著的优越性.     

5mm强化管蒸发器中齿高对性能的影响及齿高优化

本文对齿高的变化对5mm内螺纹强化管蒸发器性能的影响进行了研究,结果表明齿高的增加总是增加蒸发器的总换热系数和降低换热器的平均换热温差。利用已有的换热和压降关联式,设计了齿高对系统性能影响的仿真算法,在蒸发器换热性能最优的条件下,定量得出5turn内螺纹强化管齿高设计的推荐值为0．09mm。     

Experimental studies on the evaporative heat transfer and pressure drop of CO2 in smooth and micro-fin tubes of the diameters of 5 and 9.52 mm

Carbon dioxide among natural refrigerants has gained a considerable attention as an alternative refrigerant due to its excellent thermophysical properties. In-tube evaporation heat transfer characteristics of carbon dioxide were experimentally investigated and analyzed as a function of evaporating temperature, mass flux, heat flux and tube geometry. Heat transfer coefficient data during evaporation process of carbon dioxide were measured for 5 m long smooth and micro-fin tubes with outer diameters of 5 and 9.52 mm. The tests were conducted at mass fluxes of from 212 to 656 kg m⁻² s⁻¹, saturation temperatures of from 0 to 20 °C and heat fluxes of from 6 to 20 kW m⁻². The difference of heat transfer characteristics between smooth and micro-fin tubes and the effect of mass flux, heat flux, and evaporation temperature on enhancement factor (EF) and penalty factor (PF) were presented. Average evaporation heat transfer coefficients for a micro-fin tube were approximately 150–200% for 9.52 mm OD tube and 170–210% for 5 mm OD tube higher than those for the smooth tube at the same test conditions. The effect of pressure drop expressed by measured penalty factor of 1.2–1.35 was smaller than that of heat transfer enhancement.     

Refrigerant charge, pressure drop, and condensation heat transfer in flattened tubes

Horizontal smooth and microfinned copper tubes with an approximate diameter of 9 mm were successively flattened in order to determine changes in flow field characteristics as a round tube is altered into a flattened tube profile. Refrigerants R134a and R410A were investigated over a mass flux range from 75 to 400 kg m⁻² s⁻¹ and a quality range from approximately 10–80%. For a given refrigerant mass flow rate, the results show that a significant reduction in refrigerant charge is possible. Pressure drop results show increases of pressure drop at a given mass flux and quality as a tube profile is flattened. Heat transfer results indicate enhancement of the condensation heat transfer coefficient as a tube is flattened. Flattened tubes with an 18° helix angle displayed the highest heat transfer coefficients. Smooth tubes and axial microfin tubes displayed similar levels of heat transfer enhancement. Heat transfer enhancement is dependent on the mass flux, quality and tube profile.