相变微胶囊悬浮液强制对流换热实验研究

提出了一种可以同时作为储能介质与传热流体的新型相变微胶囊悬浮液(MPCS),设计和搭建试验台,分别在层流和湍流条件下在等热流密度的光滑圆管中对MPCS进行了强制对流换热实验,研究了悬浮液浓度、流量、泵送功率和加热速率对其流动及传热特性的影响。结果表明:对于质量分数为5%的MPCS,当微胶囊中相变材料分别处于固体、固体-液体和液体状态时,对应的努塞尔数平均增大了23.9%、20.5%和9.1%;与纯水相比,MPCS作为在热力系统应用的传热流体可以实现强化传热,但是需要在传热实验中控制好相变过程才能使MPCS的传热性能优于水。     

相变微胶囊悬浮液传热特性实验研究

相变微胶囊(microencapsulated phase change material,MPCM)在建筑节能领域应用广泛,为研究其传热特性,搭建了以水为换热流体(heat transfer fluid,HTF),微胶囊悬浮液为储能介质的潜热储能(latent thermal energy storage,LTES)系统。在实验过程中,通过改变换热流体的进口初始温度以及搅拌器的搅拌速率,获得了MPCM悬浮液的温度变化规律并计算了MPCM悬浮液的平均充放冷速率。实验结果表明:在充冷过程中,MPCM相变时温度变化速率减缓,相变温度区间较大,而在放冷过程中,MPCM相变时温度保持恒定,相变温度区间较小;未搅拌时,MPCM悬浮液中温度梯度较大,传热能力较差;搅拌时,MPCM悬浮液混合均匀,其温度梯度很小,传热能力较强;增加搅拌器的搅拌速率及水与相变微胶囊悬浮液的温差均可以提高MPCM的充放冷速率。     

圆管内相变微胶囊悬浮液换热特性实验研究

实验研究了质量浓度为2%和5%的相变微胶囊悬浮液在等热流加热圆管内的对流换热特性,结果表明:相变微胶囊颗粒的加入明显增强了溶液的冷却效果,有效降低了管道壁面温度,并且悬浮液浓度越高,管道壁面温度越低;相变微胶囊悬浮液使管道入口段的局部努塞尔数明显增大,但随着流程进行,对流换热增强的幅度减弱;和水相比,浓度2%的相变微胶囊悬浮液的平均努塞尔数提高11%,悬浮液具有较好的强化换热效果;但浓度5%的悬浮液的平均努塞尔数低于水,这是由于5%的悬浮液粘度增加明显,悬浮液的高粘度和低导热率减缓了圆管中心处微胶囊颗粒的吸热相变,降低了5%悬浮液的强化换热效果。     

TiO_2纳米颗粒对相变悬浮液流变和导热系数特性影响研究

通过实验方法研究了掺入TiO2纳米颗粒的相变悬浮液粘性和导热系数。研究表明,当纳米颗粒浓度不超过5%时,悬浮液仍可被视为牛顿流体,悬浮液的粘性随纳米颗粒浓度增加以非线性方式增加;当纳米质量颗粒浓度为5%时,相变悬浮液的粘性提高约23%。纳米颗粒的加入能够显著提高相变悬浮液的导热系数,当纳米颗粒质量浓度为5%时,相变悬浮液导热系数提高约7%。当纳米颗粒浓度较低时,纳米颗粒对相变悬浮液导热系数的提高幅度要高于对水的提高幅度。文中从不同方面分析了使用这种新型悬浮液作为传热工质的优势。     

相变微胶囊悬浮液层流强迫对流换热实验研究

对以溴代十六烷(C16H33Br)为相变材料的相变微胶囊悬浮液(MPCMS)作了管内层流恒定热流密度加热条件下的强化传热性能测试和分析.悬浮液质量浓度为5.0%、10.0%和15.8%(文中百分数若无特别指出均为质量分数),整体雷诺数和斯蒂芬数分别在400～1900和1.1～8.8范围内.实验结果表明:实验工况下与水相比,使用MPCMS可降低壁面温度3.9℃,Nux数可提高27%～42%,传热实验过程中MPCMS样品压降增加不大,压降可降低26.7%(um=1.3m/s),在其输传热综合效果较好.     

微胶囊相变悬浮液物理稳定性实验研究

通过实验研究确定制备稳定的微胶囊相变悬浮液必须采用的最优化表面活性剂添加量和悬浮液pH值。采用全功能稳定性分析仪TURBISCAN LAb研究表面活性剂十二烷基硫酸钠(SDS)添加量和悬浮液pH值对物理稳定性的影响。结果表明,当SDS质量分数太低,即颗粒表面改性不彻底时,稳定性较差;质量分数太高,容易形成表面活性剂胶束沉淀,并同时加速相变微胶囊颗粒上浮,破坏微胶囊相变悬浮液的稳定性,在选定的SDS质量分数范围内,最佳质量分数为0.2%~0.4%;pH值低于7时,溶液颗粒分散性提高不明显,并且出现加快分层现象;pH值过高时,溶液中OH-过多,压缩双电层厚度,减弱颗粒分散性,溶液稳定性提高受到抑制,在选定的pH值范围内,密胺树脂壳材的悬浮液最佳pH值为8。抑制悬浮液的分层和团聚现象对于其在传热和储热中的应用有重要意义,研究结果给制备稳定的悬浮液提供了可靠的依据。     

圆柱形等距螺旋盘管式相变蓄热装置蓄热性能研究

文章设计了一种以石蜡为相变材料的圆柱形等距螺旋盘管式相变蓄热装置,并通过实验分析了该装置的传热特性,以及传热流体入口温度、入口流量对石蜡的融化特性、相变蓄热装置的蓄热量及相变蓄热系统总传热系数的影响。分析结果表明:融化后期,石蜡的融化速率会明显加快;当传热流体入口温度一定时,随着入口流量逐渐增大,蓄热装置的最终显热蓄热量略微升高;与传热流体入口流量相比,传热流体入口温度对石蜡融化速率影响较大;相变阶段,石蜡的传热性能较强,传热流体入口温度越高,石蜡的传热性能越不稳定。     

相变材料微胶囊的国内外研究现状

相变材料微胶囊作为一种复合材料，解决了固液相变材料相变时体积变化以及泄漏问题；提供了大的传热面积改善了传热；还可防止相变材料的性质改变。相变材料微胶囊已经广泛应用于纺织品、传热流体、建筑物、军事、农业等领域。作为近年来研究热点，详细介绍了这种新型的复合材料特点、制备方法及目前研究现状。     

圆管内对流换热过程中潜热型功能流体流动阻力特性的实验研究

实验研究了由正十四烷和尿素甲醛树脂制成的相变微胶囊和水混合制成的潜热型功能流体在流过恒热流圆管进行对流换热时的流动阻力特性,获得了压降随流速的变化关系、摩擦阻力系数和表观黏度随R e的变化关系。并在同样条件下用单相水进行了对比实验。相变微胶囊的加入导致流体流动阻力较单相流体有显著增大。管路中扰动件导致单相流体的流动阻力特性在低R e条件下呈湍流特征;功能流体则呈不同规律,扰动仅导致流动阻力进一步增大,而流动阻力特性仍呈层流特征。     

基于相变微胶囊悬浮液的太阳能光伏热系统的研究进展

在太阳能光伏热系统中,光伏电池温度过高会导致太阳能发电效率下降。相变微胶囊悬浮液(MEPCMS)是一种潜热型功能性流体,将其作为冷却介质用于太阳能光伏热系统可以有效降低光伏电池温度,提高系统的能量利用率。针对相变微胶囊易泄露、导热性差等问题提出了改性方法,使其具有光热转换功能并提升了综合性能。基于性能评价指标分析了太阳能光伏热系统性能的影响因素。结果发现,流速、浓度和太阳辐照量是影响MEPCMS在太阳能光伏热系统中换热性能的关键因素。适当增加MEPCMS浓度和流速能提高工质的换热性能,在降低光伏板温度的同时增加太阳辐照量和系统热电产量,但需结合太阳辐照量大小合理匹配工质的浓度和流速。未来研究方向可集中在提升MEPCMS在太阳能光伏热系统中的换热性能、探究运行参数和太阳辐照量之间的匹配关系、优化集热器结构、利用其蓄热性解决太阳能间歇性等方面。     

Heat transfer characteristics of microencapsulated phase change material slurry in laminar flow under constant heat flux

Due to its large apparent specific heat during the phase change period, microencapsulated phase change material slurry (MPCMS) has been suggested as a medium for heat transfer. In this paper, the convective heat transfer characteristics of MPCMS flowing in a circular tube were experimentally and numerically investigated. The enhanced convective heat transfer mechanism of MPCMS, especially in the thermal fully developed range, was analyzed by using the enthalpy model. Three kinds of fluid–pure water, micro-particle slurry and MPCMS were numerically investigated. The results show that in the phase change heat transfer region the Ste number and the Mr number are the most important parameters influencing the Nusselt number fluctuation profile and the dimensionless wall temperature. Re_b, d_p and c also influence the Nusselt number profile and the dimensionless wall temperature, but they are independent of phase change process.     

Effect of dynamic load on heat transfer characteristic of a two‐phase pipe boiling flow

An experimental investigation was performed to obtain the flow and heat transfer characteristics of a single‐phase water flow and a two‐phase pipe boiling water flow under dynamic load in the present work. By analyzing the fluid resistance, effective heat, flow pattern, and heat transfer coefficient of the experimental data, the effects of dynamic load on the flow and heat transfer characteristics of single‐phase water and two‐phase boiling water flow were investigated. The results show that the dynamic load significantly influences the flow characteristic and boiling heat transfer of the two‐phase pipe flow. It will enhance the fluid resistance and heat dissipation toward the ambient environment, and reduce the heat transferred to the two‐phase fluid. The impact mixing flow caused by the dynamic load breaks the uniform and varying principle of the wall temperatures. As a result of that, the greater the dynamic load, the lower the wall inner bottom temperature and the higher the wall inner top temperature in a certain extent. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20378     

Transient convective heat transfer to water near the critical region in a vertical tube

Numerical analysis has been carried out to investigate transient forced convective heat transfer to water near the critical region in developing flow through a vertical tube. With large variations of thermophysical properties such as density, specific heat, viscosity, and thermal conductivity near the thermodynamic critical point, heat transfer in the tube is strongly coupled with fluid flow. Buoyancy force parameter has also large variation with fluid temperature and pressure in the tube. Time dependent characteristics of fluid velocity, temperature, and heat transfer coefficient with water properties are presented and analyzed. Transient Nusselt and Stanton number distributions along the tube are also compared for various pressures in the tube. Because of heat transfer from the wall transition behavior from liquid-like phase to gas-like phase of heat transfer coefficient occurs when the fluid passes through pseudocritical temperature region in the tube. Turbulent viscosity ratio also has steep variation near the pseudocritical temperature close to the wall.     

ZnO对不同流态下相变微胶囊悬浮液对流换热特性影响研究*

相变微胶囊悬浮液（MPCS）可作为热交换介质和储热流体,但其导热率较低导致其应用受到一定的限制。以水为基液使用相变微胶囊（MPCM）制备MPCS,加入氧化锌（ZnO）颗粒以提高MPCS导热率。使用旋转流变仪、差式热量扫描仪、导热仪分别测定了MPCS的黏度、相变潜热和导热系数等物理性质。设计并搭建了试验台,在内径6 mm的圆管中,使用水、MPCS以及ZnO@MPCS在层流和湍流下进行强制对流换热实验,通过对比其换热情况分析ZnO对MPCS换热特性的影响。结果表明：加入ZnO的MPCS具有良好的储热性和导热性,1%ZnO@5%MPCS导热系数较5%MPCS提高了17.9%。层流条件下MPCS的平均局部换热系数低于水,1%ZnO@5%MPCS平均局部换热系数比水高6.5%;湍流时,1%ZnO@5%MPCS在相同质量流量和功率下的平均局部换热系数相较于水提高了15.7%。     

管内流体流动管外PCM发生相变的贮能系统热性能研究

建立了分析空调贮能系统中管内流体流动管外PCM发生相变的相变的贮能器热性能的数学模型，并进行了数值计算。其中，把传热流体看作是沿轴向的—维无粘流动，对PCM相变过程的求解用显热容法。计算结果与文献中的计算结果吻合较好。所得结论对该类贮能系统的设计和性能优化有一定指导作用。     

组合相变材料储热系统的储热速率研究

建立了组合式柱内封装相变材料熔化－固化循环相变储热系统的物理模型,用有限差分法进行了数值模拟求解。结果表明,与采用单一相变材料的传统储热系统相比,在给定相变材料组合方式和传热流体进口温度条件下,传热流体流量存在最佳值;选用三种石蜡作用相变材料和水作传热流体的模拟计算结果表明,相变速率可提高１５％￣２５％左右。     

Energetic performances of a refrigerating loop using ice slurry

The consideration of environmental constraints in production, transport and distribution of cold energy resulted in reconsidering the practices of installations dimensioning in particular. Their containment led to the development of secondary refrigerants such as ice slurries to store, transport and distribute the cold energy. These heat transfer fluids should have good thermophysical properties, giving high transport capability, high heat transfer ability as well as low pressure drops. The use of ice slurries can lead to lower flow rates and smaller pumping power compared to single phase fluid. The purpose of the presented work is to study the distribution network of indirect cold systems thanks to a model allowing the evaluation of the influence of various parameters on the operating behaviour of the installation. The available domain for the use of secondary heat transfer fluid (whether in their single phase or two phase form) is determined considering the best design from an energetic point of view. Because of the essential role of the fluid distribution between the production site and consumers, we focus our study on pressure drops and pumping power due to the fluid flow in cooling loops. For each investigated case, the minimum consumption power is obtained with the two phases (solid–liquid) heat transfer fluid (ice slurry).     

Numerical Analysis of Heat Transfer and Nanofluid Flow in a Triangular Duct with Vortex Generator: Two‐Phase Model

Laminar forced convection heat transfer and nanofluids flow in an equilateral triangular channel using a delta‐winglet pair of vortex generators is numerically studied. Three nanofluids, namely; Al₂O₃, CuO, and SiO₂ nanoparticles suspended in an ethylene glycol base fluid are examined. A two‐phase mixture model is considered to simulate the governing equations of mass, momentum and energy for both phases and solved using the finite volume method (FVM). Constant and temperature dependent properties methods are assumed. The single‐phase model is considered here for comparison. The nanoparticle concentration is assumed to be 1% and 4% and Reynolds number is ranged from 100 to 800. The results show that the heat transfer enhancement by a using vortex generator and nanofluids is greater than the case of vortex generator and base fluid only, and the latest case provided higher enhancement of heat transfer compared to the case of a base fluid flowing in a plain duct. Considering the nanofluid as two separated phases is more reasonable than assuming the nanofluid as a homogeneous single phase. Temperature dependent properties model provided higher heat transfer and lower shear stress than the constant properties model.     

New challenge in advanced thermal energy transportation using functionally thermal fluids

The present review article presents the current status of some researches on thermal energy transportation using functionally thermal fluid, which is a mixture of heat transfer medium like water and other material with or without phase change like a paraffin wax as a latent heat storage material. This functionally thermal fluid offers attractive opportunities for thermal energy transportation and heat transfer enhancement of heat exchanger. This article describes classification and characteristics of functionally thermal fluids and their application. Referring to functionally thermal fluid for the usage of sensible heat, some visco-elastic fluids for flow drag reduction in a thermal energy transport system such as aqueous polymer solution and surfactant solution are mentioned. On the other hand, this article describes heat transfer and hydrodynamic characteristics of some phase change slurries like ice slurry, phase change microemulsion slurry, phase change microencapsule slurry, clathrate slurry and shape-stabilized paraffin and polyethylene pellets as functionally thermal fluids using latent heat between solid and liquid phases. Finally, it leads to the conclusion that some functionally thermal fluids are very useful for the advanced thermal energy transportation and heat exchanger systems.     

Heat transfer characteristics of liquid–solid circulating fluidized beds

An experiment was conducted to obtain heat transfer data in liquid–solid circulating fluidized beds. In the experiment, two kinds of risers were provided, their inner diameter being 24 mm and 12 mm, respectively. Tested particles were of glass and ceramics, having a diameter range from 2.10 to 4.95 mm. Water at ambient conditions was used as the fluidizing liquid. The experimental data showed a trend where the heat transfer coefficient increases gradually with increasing liquid velocity approaching that for a liquid single‐phase flow (“heat transfer enhanced region”), and finally coincides with that for a liquid single‐phase flow (“liquid single‐phase heat transfer region”). The heat transfer coefficient in the heat transfer enhanced region was found to be a function of the slip velocity between liquid and particles. Based on the experimental data, a correlation was proposed for predicting the heat transfer coefficient in the entire region from the heat transfer enhanced region to the liquid single‐phase heat transfer region, which could reproduce the experimental data with an accuracy of ±15%. The proposed correlation agreed well with existing data. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(3): 127–137, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20200