相变蓄热装置内温度场的模拟与实验研究

对螺旋盘管相变蓄热装置性能和相变材料(PCM)的传热特性开展模拟和实验研究,建立相变蓄热装置物理和数学模型,对蓄热温度场进行了数值模拟和实验测试。结果表明:自然对流换热对PCM的熔化过程影响很大,当考虑自然对流时,相变蓄热速率加快,相变分层现象明显;实验实测温度与模拟温度相近,说明所建立的模型适用于相变装置内部温度场的模拟。     

相变蓄热供热装置热性能试验研究与系统经济性分析

为分析相变蓄热装置在充热和放热过程中的热性能,设计并搭建一套相变蓄热供热装置中试实验系统,研究主要运行参数对相变蓄热装置热性能的影响;在此基础上,结合项目案例,对相变蓄热供热系统经济性进行分析。结果表明：相变材料(Phase Change Material, PCM)凝固过程中的传热主要受相变介质内部导热控制;而在其熔化过程中自然对流对传热起重要控制作用;蓄热装置充热速率快于放热速率。提高传热流体流量有助于增强PCM中的热传递,缩短充/放热时间,但蓄热装置内PCM温度分布均匀性有所降低;为降低系统能耗,提高储放热效率,优先选用小流量进行充/放热。该相变蓄热供热项目的动态投资回收期为3.55年,具有良好的经济性。研究结果可对相变蓄热供热系统的设计及应用推广提供参考依据。     

地源热泵与螺旋管式相变蓄热耦合系统的实验研究

针对地源热泵供热节能及不同地区峰谷电价政策,设计并搭建与地源热泵耦合的螺旋管式相变蓄热实验台,并进行实验研究。通过监测蓄热装置工作参数及螺旋圈内、外,螺旋圈之间相变材料的温度场、蓄热过程发生相变的过程等,分析相变材料的蓄热特点和不同工况对蓄热过程的影响。结果表明,螺旋管周边相变材料发生相变的速度最快,且可成导热与自然对流的耦合作用,自然对流对相变过程的影响最大。因此蓄热装置中的螺旋管应尽量放置在下部,可提高相变材料蓄热效率,缩短蓄热的时间,节约能源。     

制冷系统热回收装置相变材料的分析研究

以FeCl3·6H2O为制冷系统热回收装置的相变蓄能材料,建立蓄热体的物理模型及模拟简化模型,在自然对流的影响下,模拟相变材料的蓄、放热特性,找出固-液界面、温度场、速度场、液相比例、传热系数、热流密度、监测点的温度等随时间的变化规律,为制冷系统热回收装置的设计提供了理论依据。     

球形单元储热装置蓄热特性的分析与优化

为提高太阳能利用率，设计一套相变蓄热装置，研究球形相变单元蓄热影响因素，保持相变材料（PCM）总质量维持在较小浮动范围内，对比不同直径、孔隙率、层间距，探究整个装置的蓄热性能。实验与模拟结果表明：同等条件下，相变球直径越小，蓄热时间越短。而考虑到加工制造难度及成本，直径为46或50 mm的蓄热球是最优选择；孔隙率、入口温度和入口流量对整体蓄热时间影响较大；相反，改变蓄热球层间距对蓄热时间的影响较小。     

基于TRNSYS平台的PCM层储能电炉蓄热过程的数值模拟研究

为有效解决电能供需矛盾、改善储能电炉蓄热能力，研究基于TRNSYS平台的PCM层储能电炉蓄热过程的数值模拟方法。以蓄热式储能电炉物理模型为基础，在TRNSYS软件中建立储能电炉数学模型，采用PCM层分析储能电炉蓄热过程，实现建模、整理数据、求解、分析处理于一体的储能电炉蓄热数值模拟操作。实验结果表明：电炉体积为20 m³、蓄热流量为25 m³/h、蓄热温度23/13℃时电炉蓄热率最高；蓄热流量越小蓄热量越高，且蓄热流量对蓄热率影响显著；蓄热电炉上、中、下部温度总体变化趋势均为蓄热-平稳-蓄热，蓄热电炉内流体流速越大蓄热时间越短；蓄热式储能电炉内渐变螺距蓄热装置相较于恒定螺距蓄热装置的蓄热时间短；蓄热式储能电炉相变蓄热材料融化时间随密度增加而增加；当相变蓄热材料密度较小时，相变蓄热材料的平均流速大。     

管壳式蓄热装置的设计及蓄热单元三维数值模拟与优化

为弥补太阳能间歇性的缺点,设计了管壳式蓄热装置并建立了一个三维的、非稳态的、液态石蜡包含自然对流的相变蓄热装置模型,在该模型中取一个蓄热单元进行模拟研究。蓄热单元为圆柱体,内部放置石蜡,中心位置为传热管,热水通过传热管和传热管上的翅片对石蜡进行加热。对蓄热单元的蓄热过程进行了三维数值模拟,分别分析比较了有无自然对流条件,不同蓄热单元放置,以及增加内外翅片情况下蓄热单元的蓄放热性能,研究结果可为蓄能装置及集成系统的开发提供理论依据。     

太阳能储热装置的蓄热性能研究

根据能量供求关系设计太阳能储热装置,将熔盐作为相变储能材料[1-2]、导热油为传热介质,解决能量供求在时间和空间上不匹配的矛盾;通过实验研究储热装置内各监测点温度随时间的变化关系;数值分析储热装置内温度场与外界工况的关系,并与实验结果分析比较,得出了Fluent可用于预测储热装置内PCM在蓄热过程中的物性变化;在此基础上改变相变换热器的结构,强化传热,改善储热装置的蓄热性能。     

中温相变蓄热装置蓄放热性能的数值分析与实验研究

以所研制的相变温度为76℃的相变蓄热装置为研究对象,通过数值模拟和实验研究,对该相变蓄热装置的蓄、放热性能进行了模拟分析与实验验证。研究结果表明:所研究的相变温度为76℃的中温相变蓄热装置具有良好的蓄、放热性能,为在太阳能利用、工业废热利用以及暖通空调蓄热等领域的工程应用提供了可能。     

高温相变蓄热器数值模拟与实验研究

高温相变蓄热器是空间太阳能热动力发电系统的关键部件之一,相变材料(PCM)蓄热是其中的关键技术。对以LiF-CaF2为PCM和以干空气为工质的蓄热系统进行了地面实验,并分别建立了相应条件下填充纯PCM和泡沫复合相变材料(FCPCM)的蓄热单元管数学模型。经过数值计算得到结果表明,纯PCM蓄热单元管计算值与实验数据吻合得很好,表明了计算模型的有效性;此外,对填充纯PCM和FCPCM的蓄热单元管的计算结果进行了比较,结果表明,泡沫的填充强化了PCM的导热性能,提高了蓄热系统的热性能。     

Numerical investigation of the thermal mechanism of the solid-liquid phase changing process

由于相变换热储能技术可以协调能量在时间和空间尺度的分配,成为了目前研究的热点问题。本工作用焓值法分别对充填低温无机盐相变材料的二维和三维管壳式相变储能换热器模型的储/放热特性进行了模拟研究,采用Boussinesq近似研究了液相区密度变化引起的自然对流的影响。研究表明换热器的入口温度对相变换热效率影响显著;在储热过程中自然对流发挥了重要作用,换热效率与液相区的运动状态直接相关,而放热过程中的热交换主要依靠热传导完成;三维模拟的结果表明换热管出口温度与管壁的平均努赛尔数高度相关,且换热管水平放置的换热效率略低于竖直放置。     

Thermal performance of PCM thermal storage unit for a roof integrated solar heating system

The thermal performance of a phase change thermal storage unit is analysed and discussed. The storage unit is a component of a roof integrated solar heating system being developed for space heating of a home. The unit consists of several layers of phase change material (PCM) slabs with a melting temperature of 29 °C. Warm air delivered by a roof integrated collector is passed through the spaces between the PCM layers to charge the storage unit. The stored heat is utilised to heat ambient air before being admitted to a living space. The study is based on both experimental results and a theoretical two dimensional mathematical model of the PCM employed to analyse the transient thermal behaviour of the storage unit during the charge and discharge periods. The analysis takes into account the effects of sensible heat which exists when the initial temperature of the PCM is well below or above the melting point during melting or freezing. The significance of natural convection occurring inside the PCM on the heat transfer rate during melting which was previously suspected as the cause of faster melting process in one of the experiments is discussed. The results are compared with a previous analysis based on a one dimensional model which neglected the effect of sensible heat. A comparison with experimental results for a specific geometry is also made.     

A numerical investigation into the charge behaviour of a spherical phase change material particle for high temperature thermal energy storage in packed beds

基于高温相变材料,对填充床储热系统中储热单元球体的储热性能进行了模拟研究.研究了不同传热流体温度和球体直径对球体储热性能的影响规律,对导热为主的相变储热过程与导热和自然对流共同作用的相变储热过程进行了比较分析,同时还探讨了高温辐射换热的影响.结果表明,相变时间随球体直径的增大而增大,随传热流体温度的增大而减小.当考虑相变区域自然对流时,总的相变时间显著减少,和单纯导热相比,完全相变时间缩短了近16%.在导热和自然对流的基础上加上辐射传热后可以看出,辐射换热强化了球体内的传热过程,加快了相变材料的熔化速度,强化了自然对流的作用.     

Experimental study of natural convection heat transfer in a microencapsulated phase change material slurry

A new microencapsulated PCM (Phase Change Material) slurry (MEPCS) at high concentration (45% w/w) was developed based on microencapsulated Rubitherm RT6. Its heat storage and heat transfer characteristics have been experimentally investigated in order to assess its suitability for integration into a low temperature heat storage system for solar air conditioning applications. Differential scanning calorimetry tests have been conducted to evaluate the cold storage capacity and phase change temperature range. An experimental setup was built in order to quantify the natural convection heat transfer occurring from a vertical helically coiled tube immersed in the MEPCS. First, tests were carried out using water in order to obtain natural convection heat transfer correlations and then a comparison was made with the results obtained for the MEPCS. It was found that inside the phase change interval the values of the heat transfer coefficient for the MEPCS were significantly higher than for water, under identical temperature conditions.     

Eutectic compound (KNO3/NaNO3 : PCM) quasi‐encapsulated into SiC‐honeycomb for suppressing natural convection of melted PCM

The phase change eutectic compound, KNO₃/NaNO₃ (50/50 mol%) (phase change material (PCM)), which is used as the thermal energy storage material in the solar thermal power plant, was quasi‐encapsulated into the SiC‐honeycomb (SCH) for suppressing the natural convection occurring at the liquid state of PCM. The performance of the SCH as the material suppressing natural convection of PCM was investigated experimentally. PCM with three kinds of mixing ratios of SCH of 10%, 20%, and 30%, was prepared and packed in their respective stainless can with oil‐flowing pipe in the center, which is called thermal energy storage unit (TESU). Three units were linked together and stacked vertically by the connector at the inlet/outlet oil pipe. The time variation of temperature at the fixed positions inside the TESU in charging/discharging process and temperature gradient in the radial direction inside TESU when PCM was liquid state were investigated. It is concluded that the natural convection is suppressed by mixing the SCH with PCM up to around 30% in weight, because the PCM is quasi‐encapsulated into cell holes and porous structures of SCHs. And thus, the heat transfer of the PCM + 30%SCH composite is controlled mainly by its thermal conduction, which is also supported through comparison of simulation result with experimental one. And so, we conclude that SCH has a function as the quasi‐encapsulating material for suppressing the natural convection of PCM. Copyright © 2015 John Wiley & Sons, Ltd.     

Thermal and heat transfer characteristics in a latent heat storage system using lauric acid

The thermal and heat transfer characteristics of lauric acid during the melting and solidification processes were determined experimentally in a vertical double pipe energy storage system. In this study, three important subjects were addressed. The first one is temperature distributions and temporal temperature variations in the radial and axial distances in the phase change material (PCM) during phase change processes. The second one is the thermal characteristics of the lauric acid, which include total melting and total solidification times, the nature of heat transfer in melted and solidified PCM and the effect of Reynolds and Stefan numbers as inlet heat transfer fluid (HTF) conditions on the phase transition parameters. The final one is to calculate the heat transfer coefficient and the heat flow rate and also discuss the role of Reynolds and Stefan numbers on the heat transfer parameters. The experimental results proved that the PCM melts and solidifies congruently, and the melting and solidification front moved from the outer wall of the HTF pipe (HTFP) to the inner wall of the PCM container in radial distances as the melting front moved from the top to the bottom of the PCM container in axial distances. However, it was difficult to establish the solidification proceeding at the axial distances in the PCM. Though natural convection in the liquid phase played a dominant role during the melting process due to buoyancy effects, the solidification process was controlled by conduction heat transfer, and it was slowed by the conduction thermal resistance through the solidified layer. The results also indicated that the average heat transfer coefficient and the heat flow rate were affected by varying the Reynolds and Stefan numbers more during the melting process than during the solidification process due to the natural convection effect during the melting process.     

Lattice Boltzmann simulation of tree-shaped fins enhanced melting heat transfer

Abstract

A thermal lattice Boltzmann model is developed to simulate the melting process with natural convection in a cavity filled with tree-shaped solid fins, in which the velocity field and temperature field distribution functions are considered. The present model incorporates the total enthalpy and a free parameter in the equilibrium distribution function to handle conjugate heat transfer. The results indicate that natural convection of liquid phase change material (PCM) plays a significant role in the melting heat transfer of PCM. Increasing the number of branching levels leads to a more rapid melting process, and selecting appropriate bifurcation angle has more efficient heat transfer performance.     

Heat transfer enhancement in medium temperature thermal energy storage system using a multitube heat transfer array

An experimental energy storage system has been designed using an horizontal shell and tube heat exchanger incorporating a medium temperature phase change material (PCM) with a melting point of 117.7 °C. Two experimental configurations consisting of a control unit with one heat transfer tube and a multitube unit with four heat transfer tubes were studied. The thermal characteristics in the systems have been analysed using isothermal contour plots and temperature time curves. Temperature gradients along the three directions of the shell and tube systems; axial, radial and angular directions have been analysed and compared. The phase change in the multitube system was dominated by the effect of convective heat transfer compared to conductive heat transfer in the control system. The temperature gradient in the PCM during phase change was greatest in the radial direction for both the control and multitube systems. The temperature gradients recorded in the axial direction for the control and multitube systems during the change of phase were respectively 2.5 and 3.5% that of the radial direction, indicating essentially a two-dimensional heat transfer in the PCM. The onset of natural convection through the formation of multiple convective cells in the multitube system significantly altered the shape of the solid liquid interface fluid flow and indicates the requirement for an in-depth study of multitube arrangements.     

Melting of PCM in a thermal energy storage unit: Numerical investigation and effect of nanoparticle enhancement

The present paper describes the analysis of the melting process in a single vertical shell‐and‐tube latent heat thermal energy storage (LHTES), unit and it is directed at understanding the thermal performance of the system. The study is realized using a computational fluid‐dynamic (CFD) model that takes into account of the phase‐change phenomenon by means of the enthalpy method. Fluid flow is fully resolved in the liquid phase‐change material (PCM) in order to elucidate the role of natural convection. The unsteady evolution of the melting front and the velocity and temperature fields is detailed. Temperature profiles are analyzed and compared with experimental data available in the literature. Other relevant quantities are also monitored, including energy stored and heat flux exchanged between PCM and HTF. The results demonstrate that natural convection within PCM and inlet HTF temperature significantly affects the phase‐change process. Thermal enhancement through the dispersion of highly conductive nanoparticles in the base PCM is considered in the second part of the paper. Thermal behavior of the LHTES unit charged with nano‐enhanced PCM is numerically analyzed and compared with the original system configuration. Due to increase of thermal conductivity, augmented thermal performance is observed: melting time is reduced of 15% when nano‐enhanced PCM with particle volume fraction of 4% is adopted. Similar improvements of the heat transfer rate are also detected. Copyright © 2012 John Wiley & Sons, Ltd.