两相闭式热虹吸管换热特性的数值模拟

本文建立数学模型对两相闭式热虹吸管换热特性进行数值模拟。考察了几何尺寸、工质及加热条件对液膜流态、液膜厚度等的影响,从而提示了这三个部因素影响冷凝段换热特性的内在机理。     

两相闭式热虹吸管传热特性研究

本文通过实验研究在不同热流密度下,两相闭式热虹吸管传热特性,并通过计算比较了三个较为常用的换热准则系统,同时提出了改进意见,为热管换热器的设计提供了较为可靠的依据。     

Effect of Serpentine Grooves on Heat Transfer Characteristics of Microchannel Heat Sink with Different Nanofluids

An experimental and numerical investigation of the thermal performance of three different nanofluids ethylene glycol‐based CuO, water‐based CuO, and Al₂O₃ is done in a serpentine‐shaped micorchannel heat sink. The microchannels considered ranged from 810 μm to 890 μm in hydraulic diameter and were made of copper material. The experiments were conducted with the Reynolds number ranging from approximately 100 to 1300. The forced convective heat transfer coefficient of nanofluids shows that there is an improved heat transfer rate compared to base fluids water and ethylene glycol. The experimental results also confirm that there is an earlier transition from laminar to turbulent flow in microchannels. The results prove that as the hydraulic diameter decreases there is increased pressure drop and the heat transfer coefficient increases for both the base fluids and nanofluids. The flow characteristics are discussed based on the pressure drop. While investigating the heat transfer coefficient of the three different nanofluids the nanofluid CuO/EG has the highest heat transfer coefficient as a result of the material's property. This research also will encourage young researchers to work on nanofluids of varying nanoparticle size and concentration to discover new results.     

凹凸变化壁面强化传热机理与传热性能的研究

介绍了一种近年来开发的板式外流降膜蒸发器中的凹凸变化壁面的传热元件;分析讨论了其强化传热的机理及其传热性能;建立了凹凸变化壁面上降落液膜的传热系数准数关联式,为其应用于工业生产提供了参考数据.     

闭式循环喷雾冷却传热特性实验研究

分别以蒸馏水和无水乙醇为工质在闭式循环喷雾冷却系统上研究了喷雾流量、表面结构、喷雾工质对传热性能的影响。从对流换热和相变换热比例关系的角度对实验数据进行了整理。结果表明:随喷雾流量的增加,热流密度增加、相变换热份额降低。当水喷雾流量为227.1 g/min、表面温度为74.32℃时,热流密度达到6.8×105W/m2;表面温度对换热有较大影响,温度越高换热性能愈强,无水乙醇的换热始终强于蒸馏水,特别是表面温度较高时,强化换热效果更为明显;其相变份额同样始终大于蒸馏水,表面温度为50℃、流量同为27.52 g/min时,蒸馏水的相变换热份额仅为20.4%,而无水乙醇的达到55.5%。微结构表面虽然减弱了对流换热能力,但减薄了液膜厚度,增强相变换热能力,使换热效果大幅提升。给出了反映表面温度影响的无量纲准则方程。     

A Study on Enhancement of Boiling Heat Transfer by Mixed-Wettability Surface

ABSTRACT

The enhancement of pool boiling heat transfer in FC-72 on a novel mixed-wettability surface was experimentally investigated. On the mixed-wettability surface, the micro-pin-finned area and the smooth area were distributed in the form of fractal by using micromaching method (dry etching method). From the comparison with the smooth surface and the micro-pin-finned surface, the mixed-wettability surface could efficiently enhance the heat transfer performance in the nucleate boiling region, and the critical heat flux was also efficiently improved. From the boiling experiment result, it is discovered that a larger heat transfer area does not always lead to a better heat transfer performance. From the peculiar boiling phenomenon of the novel surface, it can be observed that large number of nucleation sites are formed in the micro-pin-finned area, and the small bubbles grow, collide, merge and move rapidly to the nearby smooth channel. When the bubble grows large enough, it will departure quickly under the effect of channel pressure. It can be concluded that the mixed-wettability surface can not only guarantee sufficient nucleation sites, but also facilitate the departure of bubbles and enhance the bubbles' interaction.     

椭圆翅片管空冷器流动传热特性的研究

用稳态的恒壁温法对3个椭圆翅片管空冷器和1个圆翅片管空冷器的传热和阻力特性进行了研究,得到空冷器空气侧的传热与阻力性能,在相同的迎风面流速下,椭圆翅片管比圆翅片管空气侧换热系数约大3－7倍;在相同的换热系数下,椭圆翅片管比圆翅片管的压降低。     

V形肋片型太阳能空气集热器换热特性的研究

本文运用数值模拟的方法,对带有多重V形肋片的平板型太阳能空气集热器的换热与流动特性进行了研究。在雷诺数5 000~20 000范围内,采用RNG k-ε湍流模型,研究了相对肋宽比(W/w)对集热器换热特性的影响规律,并对集热器内部流场进行了分析。研究结果表明,V形肋片增强了空气的对流换热。这是由于流体掠过肋片后,产生旋涡和回流,加强了流体扰动,从而使换热增强。相对其它肋宽比而言,W/w=6时,集热器具有较好的换热效果,努塞尔数为光滑壁面的2.54倍,但肋片的存在导致摩擦损失增加,换热性能因数最大为1.55。     

Experimental Study on the Heat Transfer Characteristics of an Evaporating Falling Film on a Horizontal Plain Tube

In this paper, an experimental investigation on the heat transfer of saturated water falling film on a single horizontal plain tube is presented. The water film falling on the outside of the tube has been heated by the condensing steam flowing in the tube, and the heat transfer coefficient between the water film and the steam has been measured. Experiments were performed at saturation temperatures of liquid film and steam as 58°C and 61°C, and 61°C and 65°C, a tube pitch of 57.16 mm, heat fluxes from 10 to 50 kW m^-2, and film flow rate per unit of length of the tube up to 0.12 kg m^?1 s^?1. Brass plain tubes with external diameters of 25.4 mm and lengths of 950 mm were used in the experiments. The experimental results show that the heat transfer coefficient increases with the increasing film flow rate and heat flux, and the quality of vapor has an obvious influence on the heat transfer performance of falling film evaporation. The coupling of condensation and evaporation heat transfer inside and outside the tube is investigated qualitatively in this paper.     

全氢罩式退火炉退火过程传热的研究

分析了全氢罩式退火炉退火过程各阶段的基本传热形式,着重介绍了影响炉内换热的两个重要参数－对流换热系数和钢卷径向等效导热系数,并对比了氮气和氢气气氛下以上两个参数的不同。最后通过数值模拟计算得到了钢卷退火曲线,并与实测值进行了对比。     

水平放置离散矩形直翅片强制对流的传热研究

实验了不同空气流速时水平放置离散矩形直翅片组的稳态传热着重探讨了翅片列间距对翅片组散热性能和阻力损失的影响,用离散翅片的长度作为定性尺寸,得到了计算直线排列的离散翅片组平均对流传热系数的无量纲传热准数关系式     

交错内肋微通道的流动和传热特性研究

为了研究带有交错内肋微通道的流动和传热特性,采用数值模拟的方法分析了肋片的形状对微通道热力性能的影响,对比了矩形肋、菱形肋、三角形肋和圆形肋4种不同形状内肋结构的微通道和光滑矩形微通道的热力性能。结果表明：矩形肋、菱形肋、三角形肋和圆形肋微通道的努塞尔数Nu都大于光滑矩形微通道的努塞尔数Nu,最大值分别为光滑矩形微通道的2.59,2.71,2.90和2.48倍;肋片对微通道的传热特性具有显著的强化作用,这是由于流体在交错内肋的后方产生涡流,实现整个流场的全局强化传热,极大提升微通道传热特性;交错内肋的应用也增大了通道的摩擦系数,矩形肋、菱形肋、三角形肋和圆形肋微通道摩擦系数的最大值分别为光滑矩形微通道的8.66,7.96,17.50和5.96倍。     

A Study on Characteristics of Cooling Heat Transfer of Supercritical Pressure Fluids in a Plate Heat Exchanger

For the development of industrial heat pump systems supplying a high-temperature heat source over 130°C, experiments were carried out on cooling heat transfer of supercritical pressure fluids flowing in a plate heat exchanger (PHE). Using two refrigerants of HFC134a and HCFC22 as the test fluids, heat transfer coefficient data were obtained at different pressure, flow rate, and heat load conditions. The heat transfer coefficient generally had a maximum in the vicinity of the pseudocritical point and showed seven- to ninefold values compared with tube flow. Based on the measurements, characteristics of cooling heat transfer of supercritical pressure fluids in the PHE were clarified and a correlation of heat transfer coefficient was developed.     

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.     

水蒸气在TA2改性表面冷凝传热特性实验研究

通过对纯钛(TA2)表面改性获得不同静态接触角的冷凝表面,并对水蒸气在各试样表面的传热特性进行了可视化实验研究。实验结果表明,蒸汽在原始表面为液滴和沟流状液膜共存的混合冷凝,在氢氟酸刻蚀表面为膜状冷凝,传热性能降低为原始表面的30%以下,而在双氧水氧化表面及氢氟酸刻蚀后再双氧水氧化表面则为明显的滴状冷凝,相应传热性能也有明显提高,约为原始表面的1.3~1.6倍,且经两步处理表面强化冷凝效果更为明显。通过对试样表面显微形貌的观察和分析指出,冷凝表面微观形貌的改变是导致各试样表面静态接触角和冷凝形貌存在差异的主要原因。     

板式换热器火用传递特性研究

文中运用推导逆流换热器对数平均温度的方法,得到了板式换热器中冷热流体及板上的温度分布.在此基础上运用火用传递理论,推导出了热流体至板、板至冷流体的局部火用传递系数.以某板式换热器为例进行了算例分析,从火用传递角度分析了该换热器的换热性能,此结果可以作为对板式换热器进行优化和改进的一种可行有效的重要参考.     

尾迹作用下叶片表面非定常传热特性的数值分析

基于非定常RANS方程,采用尺度自适应湍流模型,模拟分析了某透平级的非定常流动与传热,研究在不同尾缘冷气射流条件下上游静叶尾迹结构的变化及其对下游动叶表面传热的影响.结果表明:导叶尾缘冷气射流对动叶表面传热的作用主要来自湍动能尾迹的传播,动叶压力面后部的时均表面传热系数提升约20%,但是动叶吸力面传热的时均值基本没有受到影响;带有冷气射流的尾迹给下游带来的扰动强度与范围更大,下游叶片表面的传热幅值波动加剧,这一特征随着冷气射流的动量比与速度比的增大而增强,动量比为1时,波动幅值相较无冷气射流工况时提高1倍.     

两种用于HCPV/T的水冷换热器传热特性的对比实验研究

本文对两种适用于高倍聚光发电供热（HCPV/T）系统的多槽道和微通道水冷换热器进行了实验研究。利用模拟热源模拟了HCPV/T系统中光伏电池工作时的热流密度,分别研究了流量、壁面温度和输入电压对两种换热器传热特性的影响,并利用传热学理论对两种换热器的特点进行分析,获得了两种换热器努赛尔数Nu与雷诺数Re的拟合经验公式。实验结果表明,微通道换热器在低流量下有较强的换热能力,但在高流量下,换热能力无法随流量增大继续提高;多槽道换热器在低流量下换热能力不佳,但在高流量下仍可随流量增大继续提高。