On the Performance of Ag/Oil Nanofluids in Heat Transfer Enhancement in a Sinusoidal Tube: Constant Heat Flux Boundary Condition

The present work provides an empirical investigation on the thermal characteristics of Ag/oil nanofluids flow inside a sinusoidal tube under a constant heat flux boundary condition. Ag/oil nanofluids have been prepared in low‐volume concentrations of 0.011%, 0.044%, and 0.171%. The average size of the nanoparticles was 20 nm. A heated coil was attached to the upper and lower surface of the tube that satisfied the constant thermal boundary condition of 204 W. The experiment has been pursued at low Reynolds numbers less than 160. A loop was designed to keep the flow hydrodynamically fully developed during the experiment. The test case was a sinusoidal tube. Upper and lower surfaces of the tube have been designed sinusoidally. Moreover, the width of the plates was long enough, so the problem was not considerably affected by the three‐dimensional releasing effect. Convective heat transfer coefficient and Nusselt number were calculated. It has been observed that based on the acquired data of the present work, convective heat transfer coefficient increased up to 23% for the best case (nanofluid with a volume concentration of 0.171%) compared to the base fluid. This happened while the rise of the friction factor was very low. In addition, a comparison between the new results and the previous work by authors showed the positive performance of sinusoidal tubes in increasing the convective heat transfer coefficient (the average increase was calculated to be about 82%) compared to the annular tube.     

波纹内翅片管内空气强化传热特性的研究

研究了一种波纹内翅片管的换热特性,得出了所测参数范围内沿程温度及换热系数分布特点,并比较了相同管径的内翅片波纹管和光管的综合换热效果,结果表明：高Re数时管子与翅片间的接触热阻对传热性能有较大影响;波纹内翅片管的综合换热效果强于光管,而且在低流速下换热强化效果更加明显。     

Performance Evaluation of Some Tube Inserts as Heat Transfer Enhancement Techniques

Different heat transfer enhancement techniques can be used successfully in the retrofit design of shell-and-tube heat exchangers that can lead to significant cost savings. Tube inserts are frequently used in such applications. Twisted tapes, wire coils, helically coiled ribbons (as tube inserts in smooth pipes), and rifled pipes have been evaluated as heat transfer enhancement techniques. To assess the benefit of using these kinds of techniques, extended performance evaluation criteria (PEC) have been implemented under different constraints. The decrease in entropy generation can be combined with the relative increase in the heat transfer rate or decrease of the heat transfer area to determine the geometrical parameters of the tubes for optimal thermodynamics performance.     

具有内插管的真空管太阳能热水器性能研究

为了提高真空管集热管效率,在相同环境下,对未插内管和插有内管的玻璃真空管太阳能热水器进行了三维数值模拟。分析表明,通过在真空管中插入内管可以改善真空管内冷热水的流动和换热,提高真空管集热器的效率。对插入不同长度内插管的三组实验做了比较,结果表明,应合理选择内插管的长度。     

变热流条件下木材点燃的实验研究

为研究木材在变化的热流条件下的点燃性能,对泡桐、榆木、红椿木和刺槐等4种木材进行了小尺寸的实验研究.测量了木块表面接受到的热辐射强度、点燃时间、质量损失和木块的内部温度等参数.在实验数据的基础上,得出木材点燃的临界热流增长率约为0.07kW/(m~2·s),临界质量损失速率约为20～30g/(m~2·s).利用PDE模型计算出木块温度变化情况,并与实验数据进行了比较.计算结果表明,在自然着火情况下,木块点燃时的表面温度约为500℃左右.     

Thermo-Hydraulic Performance Enhancement of Finned Elliptical Tube Heat Exchangers by Utilizing Innovative Dimple Turbulators

Abstract

As a new type of fin structure in finned tube heat exchangers, dimple turbulators exhibit excellent potential for thermo-hydraulic performance enhancement. A three-dimensional numerical simulation study was conducted to investigate the influences of five kinds of innovative concave dimple turbulators (CDTs), namely – elliptical dimple, conical frustum dimple, trapezoidal prism dimple, leeward triangular dimple and upward triangular dimple (UwTD) on the thermo-hydraulic performance enhancement in a plate fin-and-elliptical tube (PFET) heat exchanger, where CDTs are textured on the fin surface transversely between the elliptical tubes. The computational results are analyzed by considering the performance evaluation criterion for the PFET heat exchangers with different types of CDT shapes. The present investigation demonstrates that the heat transfer enhancement is intimately pertained to ejection with longitudinal counter-rotating flow, strengthened secondary flow and vortex structures at the downstream rim of CDT. A parametric study on the CDTs indicated that the UwTD vortex turbulators give better thermo-hydraulic performance under the present conditions. The numerical simulation results illustrated different secondary flow structures and heat transfer characteristics of the CDTs with various shapes, which disclosed the influential mechanisms of differently shaped dimple turbulators on the heat transfer augmentation in PFET heat exchangers.     

多孔表面管沸腾传热试验研究

针对烧制成多孔表面管,进行了传热性能研究,试验表明：多孔管可以显著地强化多孔侧沸腾传热,民同规格光滑管传热性能试验对比,其沸腾给热系数比光滑管提高５－６倍。     

Helmholtz共振腔强化换热的试验研究

为研究脉动流体对强化换热的影响,设计Helmholtz共振腔并分别在加装和不加装共振腔的情况下进行对比试验,发现水流经共振腔后变成了脉动流体,脉动的水经过单管换热器后强化了换热,在一定的共振腔参数的配合下,换热系数提高约10％～40％。     

扰流孔强化波纹板蓄热元件传热性能的实验研究

采用瞬态实验方法研究了3种排列角度(15°、30°和45°)下的扰流孔波纹板蓄热元件的传热性能,并比较了等效泵功下的Nu/Nu_o*值.结果表明:与未添加扰流孔的波纹板蓄热元件相比,3种排列角度下的带扰流孔的波纹板蓄热元件的平均Nu分别提高了17.1%、24.8%和34.9%;3种情况下的平均阻力因数f分别提高了24.7%、33.1%和38.4%;与排列角度为30°和15°相比,45°时的Nu/Nu_o*分别提高了53.3%和67.8%,表明扰流孔排列角度为45°时,波纹板蓄热元件具有最佳性能.     

管内插物强化换热性能分析及应用

管内插物的种类很多，扭带、螺旋线圈以及绕花丝是三种较常用的管内插物强化换热技术，对它们的强化换热性能以及应用进行分析比较，是非常有必要的。从综合强化性能来看，螺旋线圈内插物比扭带内插物效果好；绕花丝内插物是一种新型综合强化换热技术，它优于前两种内插物的特点是：可使流体在流动方向上做复杂的三维混合流动，并且所产生的阻力降非常小，所以综合强化换热性能最好。图7表1参10     

重力热管在真空管式热水器中的应用

本文从分析真空集热管的缺点入手,结合真空管太阳能热水器的特点,分析了重力热管的几种特性,并对其应用于真空管式太阳能热水器的可能性、必要性做了一些探讨。     

管中心填充两层多孔介质强化传热研究

采用圆管中心层流充分发展段中心流等截面分层填充金属多孔介质以实现强化传热,建立了流动与传热数学模型,并着重分析了三种新型强化传热管的速度、温度分布及传热综合性能.结果表明,与光管相比,填充金属多孔介质后,管中心流体温度更均匀,壁面附近流体的温度梯度更大,圆管中心流体速度分布趋于平坦,壁面附近流体速度梯度增大,壁面与流体间的换热显著增强;对三种强化传热管,在两区域内填充孔隙率相同的多孔介质,可望得到较高的综合性能指标值.     

Heat Transfer Enhancement of Spray Cooling on Flat Aluminum Tube Heat Exchanger

This study provides an experimental analysis on the heat transfer performance of a flat aluminum tube microchannel heat exchanger with spray cooling. The effects of water spraying rate, airflow rate, and relative humidity were investigated. The test results show that the heat transfer performance increased with increasing the water spraying rate but without the penalty of increased flow resistance at low spray conditions. This effect is further enhanced by increasing the water spraying rate. However, when the spraying rate is high, part of the nonevaporated drops attached to the fin surface and formed a liquid film, which caused the flow passage to become narrower. Further increase in the spraying rate resulted in part of the flow passages being blocked by the nonevaporated water drops and caused a region of poor heat transfer. The friction coefficient jumped drastically at this condition. This phenomenon deceased gradually with increasing airflow rate. High inlet air humidity resulted in the water accumulation phenomenon appearing at lower water spraying rates. The evaporative cooling effect decreased and flow friction increased. The test results just described show that the water spray is able to significantly improve the air-side heat transfer performance. The optimum spray rate for each airflow rate must be carefully determined.     

全玻璃真空集热管的使用寿命

使用寿命是全玻璃真空集热管最重要的指标之一。本文从应用角度分析了真空度、内璧导热能力、抗冻能力等对使用寿命的影响,并探讨了相应的对策。     

铜-R22分离式热管传热特性的试验研究

采用铜管模拟分离式热管蒸发段,以R22为工质,在热流密度1～5 kW/m2、蒸发温度30～60℃和蒸发段倾角10°～90°范围内,研究了这种分离式热管的传热特性.分析了热管的启动特性以及热流密度、蒸发段倾角和充液率对其传热特性的影响.结果发现:以R22为工质的分离式热管具有良好的启动性,蒸发段合理的充液率为80%～100%,在试验参数范围内,热流密度、蒸发段倾角和充液率对热管的传热性能没有明显影响.     

Helmholtz共振腔脉动流体强化换热的试验研究

本文采用试验研究的方法研究了流体动力学不稳定性对强化换热的影响。水流经Helmholtz共振腔时被转变为脉动流体,脉动的水经单管换热器时被加热,测量了不同条件下加装共振腔和不加共振腔时的换热系数。研究发现,加装了Helmholtz共振腔时换热系数明显提高约10%~40%。     

Helmholtz共振腔脉动流体强化换热的试验研究

采用试验研究的方法研究了流体力学不稳定性对强化换热的影响。水流经Helmholtz共振腔时被转变为脉动流体,脉动的水经单管换热器时被加热,测量不同条件下加装共振腔和不加共振腔时的换热系数。研究发现,加装了Helmholtz共振腔时换热系数明显提高约10％～40％。     

新V型内聚光CPC热管式真空管集热器的热性能

基于圆管型复合抛物聚光器(CPC)的研究,由于接受体和CPC反光板之间存在缝隙而造成大量漏光损失,设计了一种可以减少漏光损失和热损的新V型CPC反光板,并综合利用热管式真空管集热器的优越性,开发出一种新V型内聚光CPC热管式真空管集热器.首先给出了其设计结构,并对其进行传热分析,推导出该型集热器的集热效率、总热损系数的计算方法,再进行室外动态性能试验,测定了它的效率曲线,试验结果与理论计算值较符合.     

Experimental Study of Fouling Resistance in Twisted Tube Heat Exchanger

This paper discusses fouling of a twisted tube heat exchanger under different conditions of fluid velocity and heat input. The fluid velocity was varied from 0.5 to 2.0 m/s, whereas the heat input to the heat exchanger was varied from 200 to 800 W. The experimental results show that for low fluid velocity of 0.5 m/s, the fouling resistance showed noticeable variation with respect to heat input, whereas for high velocity ranges, that is, 1.0–2 m/s, the variation in fouling resistance is less. The fouling in twisted tube steadily increases with time for different values of heat input from 1000 min onward for fluid velocity in the range from 1.0 to 2.0 m/s. It is also observed that fouling resistance curves overlap for various values of heat input. During the initial 1000 min of the test duration, the maximum fouling in a twisted tube heat exchanger decreases with increase in fluid velocity from 1.0 to 2.0 m/s. This behavior of the fouling rate can be attributed to the fact that at higher fluid velocity, flow becomes turbulent, and this in turn flushes the fouling particles. The time-series correlations for the fouling resistance are found to be logarithmic in nature.     

纳米流体在车用机油冷却器中的强化换热试验研究

为了探求新型冷却介质--纳米流体的换热效果,制备了不同粒子体积分数的氧化铝有机纳米流体,并在车用机油冷却器中进行了换热性能的试验研究.研究结果表明:添加纳米粒子能够有效提高纳米流体基础液体的换热能力,且换热能力随着粒子体积分数的增加 而增高.在不同温度和温差条件下,粒子体积分数为5%的纳米流体的传热量和换热系数均超过常规冷却介质(水和防冻液).纳米流体的黏度和流动阻力亦随着粒子体积分数增加而增加.当冷、热介质的进口温差不变时,提高冷却介质的进口温度能在明显增强换热能力的同时大幅度降低流动阻力,并且纳米流体换热能力的增幅要高于防冻液和基础液体.