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本文以水蒸汽为工质对水平三维微肋管内凝结换热及阻力特性进行了实验研究.与光管和二维管相比,在相同条件下,实验中效果最好的T3管全长平均凝结换热系数分别提高了113%~410%和20%~65%,同时,与二维管相比流动阻力增加较小,最大值不超过6.3%.比较另两种管型(T1,T2管)也证明三维管以较小的流阻增加为代价换取了明显的强化效果. 相似文献
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FC—72圆形射流冲击模拟电子芯片单相局部对流传热的实验研究 总被引:8,自引:0,他引:8
以最新微电子设备冷却剂FC-72为工质,实验研究了自由和浸没情况下,圆形射流冲击5mm×5mm模拟电子芯片的局部对流换热情况。喷嘴直径d=0.987mm,Re数范围为3488~42644。测定了自由和侵没情况下驻点换热及换热系数的径向分布,讨论了雷诺数、喷嘴一传热面间距的影响。首次发现FC—72自由射流在高Re数下,随着喷射间距的增加,驻点换热得到加强,而浸没射流随喷射间距的增大,驻点换热出现非单调性变化;给出了喷射间距z/d=4时驻点换热系数的关联式,并与其它工质的已有结果进行了比较;对换热系数径向分布的研究表明,与自由射流不同,浸没射流分布曲线上明显出现由层流向湍流过渡所引起的二次峰值。 相似文献
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N─甲基吡咯烷酮两相闭式热虹吸管传热特性的实验研究 总被引:2,自引:0,他引:2
本文报道了用N—甲基吡咯烷酮(C5H9NO3)为工质以光滑管与三维内翅片管为管壳制成的两相闭式逆流热虹吸管的传热特性。实验结果表明在工作温度为220~350℃,热负荷为8.2~17.3kW/m2范围内光滑热虹吸管蒸发段与凝结段换热系数的积分平均值分别约为1700W/m2·℃和1600W/m2·℃。由于三维翅片的强化传热作用,使三维内翅片热虹吸管蒸发段与凝结段换热系数分别比光滑管增大了20%~40%和70%~160%。实验表明N—甲基吡咯烷酮综合传热性能优于萘,可望作为中温热管工质使用。 相似文献
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水平内微肋管局部凝结换热性能实验与数值求解 总被引:1,自引:0,他引:1
以R11为工质,蒸汽凝结压力为147-265kPa,质量流率4ty153kg/m2s,本文对二维内微肋管和三维内微肋管水平管内凝结分层流区局部换热系数进行了系统的实验。与光管比较,二维内微肋管和三维内微肋管局部凝结换热系数分别提高了147-783%和261-997%。本文首次从理论分析入手建立了二维内微肋管水平管内凝结分层流区局部换热系数分析模型并进行了数值求解。计算结果与本文实验相当吻合。 相似文献
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实验研究了有机工质R245fa在内径为10 mm的不锈钢光管和烧结电镀多尺度镀层强化管内的流动沸腾传热特性。实验工况为:饱和压力0.6 MPa,质量流速189.3~708.14 kg·s-1·m-2,热流密度4.94~44.74 k W·m-2,干度0.01~0.9。实验结果表明:实验条件下观测到分层流与环状流两种流型,发现管内多尺度强浸润镀层能够促进环状流的转变提前。与光管相比,烧结电镀多尺度镀层强化管具有明显的强化沸腾换热的效果,其传热强化因子平均为1.87,最大强化效果为3.15。随着热流密度以及干度的增大,传热强化因子先增加后降低。流型可视化对比发现,强化管壁面的强润湿性促进上壁面液相吸附以及快速再浸润,对流动沸腾换热具有积极作用。 相似文献
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Shyy-Woei Chang 《实验传热》2013,26(1):61-86
This article describes the results of an experimental study aimed at revealing the heat transfer physics of a twisted-tape insert in a uniformly heated tube for transitional and turbulent flow situations when the test tube remains stationary and reciprocates. Dry air is used as the test fluid, and this study is motivated by the need to understand the effectiveness of twisted-tape inserts on the heat transfer augmentation when applied to the piston cooling of a compression-ignition engine. Prior to the experimental tests, a parametric analysis was performed for which the dimensionless groups that characterized heat convection were identified and adopted to formulate the experimental program and procedures of data reduction. The parametric test matrix involves Reynolds, pulsating, and buoyancy numbers in the ranges 3,000-12,000, 0-0.9, and 0.003-0.009, respectively, with five different reciprocating frequencies tested, namely, 0, 0.83, 1.25, 1.67, and 2 Hz. The nonreciprocating experimental data confirmed considerable heat transfer enhancement due to the twisted-tape insert. When the test tube was reciprocated orthogonally to the main flow direction, both pulsating and reciprocating buoyancy forces arose, which modified the heat transfer from nonreciprocating situations. Initially, a range of heat transfer impediments relative to the nonreciprocating results that could reduce local Nusselt numbers to levels about 75% of nonreciprocating values took place when these reciprocating forces were relatively weak. Further increases of relative strengths of reciprocating forces resulted in subsequent heat transfer recovery and could lead to about 60% heat transfer enhancement relative to nonreciprocating values. 相似文献
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This experimental study performs heat transfer and pressure drop measurements in three test tubes fitted with single, twin, and triple twisted-tapes with Reynolds number (Re) varying from 1,500 to 14,000. Each element of these continuous twisted-tapes that formulates the single, twin, and triple twisted-tapes has identical width, pitch, and thickness. Heat-transfer augmentations from the plain-tube conditions in three test tubes decrease with the increase of Re, while the descending rate decreases with the increase of twisted-tape in the tube. These twisted-tapes generate the more effective heat transfer enhancements in laminar flow regime than those developed in turbulent flow regime. With 3,000 ≤ Re < 14,000, the local Nusselt numbers in the tubes fitted with single, twin, and triple twisted-tapes were, respectively, 1.5–2.3, 1.98–2.8, and 2.86–3.76 times of the Dittus-Boelter levels. The Fanning friction factors in three test tubes decrease with the increase of Reynolds number. Based on the same pumping power consumption, the tube fitted with single and triple twisted-tapes possess the highest performance factors for laminar and turbulent flows, respectively. Experimental correlations of heat transfer and Fanning friction factor were derived for three tubes fitted with single, twin, and triple twisted-tapes. 相似文献
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The CFD simulation of heat transfer characteristics of a nanofluid in a circular tube fitted with helical twist inserts under constant heat flux has been explained using Fluent version 6.3.26 in laminar flow. Al2O3 nanoparticles in water of 0.5%, 1.0% and 1.5% concentrations and helical twist inserts of twist ratios 2.93, 3.91 and 4.89 has been used for the simulation. All thermophysical properties of nanofluids are temperature dependent. The heat transfer enhancement increases with Reynolds number and decreases with twist ratio with maximum for the twist ratio 2.93. By comparing the heat transfer rates of water and nanofluids, the increase in Nusselt number is 5%–31% for different helical inserts and different volume concentrations. The maximum heat transfer enhancement is 31.29% for helical insert of twist ratio 2.93 and for the volume concentration of 1.5% corresponding to the Reynolds number of 2039. The data obtained by simulation match with the literature value of water with the discrepancy of less than ±10% for plain tube and tube fitted with helical tape inserts for Nusselt number. 相似文献
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Heat transfer was studied to two-phase flow in the post-CHF dispersed flow regime under swirl conditions. Data were used for post-CHF water flow at high pressure, 15.9 MPa, inside a vertical tube with twisted-tape inserts. Data used were obtained under test conditions typical of heat exchanger operation with low wall superheats of 5-40°C. The mass flux range of the data was 910-1878 kg/m2 s, and three tape-twist ratios were used: 2.55, 5.03, and 7.52. The low wall superheat and the relatively high mass fluxes resulted in higher than predicted drop-wall heat transfer. The only existing heat transfer correlation for this flow situation was modified to include this heat exchanger parameter range. 相似文献
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Accurate, repeatable heat transfer and pressure-drop measurements have been made for condensation of CFC-113 with downflow inside enhanced microfin tubes and tubes containing twisted-wire inserts. In the latter case measurements have also been made for CFC-113/air mixtures. The heat transfer rate was calculated from the coolant flow rate and temperature rise, the latter measured using a 10-junction thermopile with careful attention paid to adequate coolant mixing and isothermal immersion of the thermopile leads. The surface temperature was found from thermocouples embedded in the tube wall. One plain tube, nine microfin tubes (with different fin heights, helix angles, and number of fins), and four twisted-wire inserts (with different wire densities) were tested. Enhancement ratios (i.e., vapor-side heat transfer coefficient for the enhanced tube divided by that for a smooth tube at the same vapor-side temperature difference and vapor inlet velocity) between 1.6 and 5.6 for the microfin tubes and between 1.2 and 1.6 for the twisted-wire inserts were found, with values depending on vapor-side temperature difference, vapor inlet velocity, and air inlet mole fraction in the case of CFC-113/air mixtures. The microfin tubes showed moderate pressure-drop penalties of around 50% compared to the plain tube, while the twisted-wire inserts showed increasing pressure-drop penalty with increasing wire density. 相似文献
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The water/graphene oxide nanofluid effect in a pipe equipped by twisted tape inserts under air cross-flow is investigated and the optimal tape geometry is determined. The range of internal and external Reynolds numbers are: 3800<Reo<21500 and 550<Rei<2000. Heat transfer and pressure drop increase by increasing Re and inserts width and heat transfer performance coefficient increased up to 1.4, indicating enhanced heat transfer compared to undesirable pressure drop. On the other hand, the heat transfer coefficient is 26% higher when compared with water in a plain tube. According to the results, this method is a good alternative in heat exchangers. 相似文献
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本文提供了7个内径为15.54mm的内螺纹粗糙管单相流动的传热和阻力数据,为当前商用粗糙管常用尺寸提供了一系列新的内部强化数据.螺纹管结构参数为螺纹数(18~45),螺旋角(25°~45°)、螺纹高(0.33mm至0.55 mm).测试管由于螺纹处流动分离及表面积的显著增加而得到强化.测试数据范围为5.08≤Pr≤6.29.利用两种关联式来预测不同几何变量和雷诺数下St和摩擦系数.多重回归传热关联式的平均差分别为2.9%和3.8%.传热和摩擦关联式基于粗糙表面热量-动量传输比拟,标准差为分别是1.4%和5.4%.关联式能够合理地预测商用螺纹粗糙管的传热和摩擦. 相似文献