混合式填充床熔盐蓄热系统热性能分析

基于混合扩散中心对称(D-C)模型,考虑自然对流现象对液态相变材料(PCM)导热性能的影响,建立填充床蓄热系统的热力学模型,开发数值计算程序并通过文献实验数据对其进行验证。研究混合式填充床熔盐蓄热系统在工程规模下蓄、放热过程的循环热特性;从温度分布和填充床热装载率分布的角度评价蓄热系统的热力性能;并研究填充床结构对蓄热系统放热时间和系统容量因子的影响规律。结果表明随着总相变填充比例的增加,系统容量因子呈先增大后减小的趋势,系统放热时长则一直增加,且增幅逐渐减小;在一定的总相变填充比例下,存在最优的高、低温相变填充层体积比使系统的放热时长和容量因子达到最大值。     

相变蓄热式集热器蓄放热数值与实验分析

对一种相变蓄热式太阳集热器的蓄放热过程进行CFD软件数值模拟和实验测试。该集热器由全玻璃真空集热管、相变蓄热单元和U型换热器组成。选用相变温度在330~400 K的3种适合于太阳集热器的复合相变材料石蜡、Ba(OH)_2·8H_2O和赤藻糖醇,在蓄热式集热器中进行蓄放热性能测试。研究表明,填充Ba(OH)_2·8H_2O的蓄热式集热器在总蓄热量、材料内部温度均匀性等蓄放热方面性能优越。实验结果对蓄热式集热器的设计具有参考价值。     

球状蓄热体蓄放热性能的数值分析与实验

采用二维导热微分方程描述球状蓄热体区域,根据数值计算理论及相变储能理论,对二维球状蓄热体的蓄放热过程进行了模拟计算和实验。采用分段线性拟合的方法模拟相变材料复杂的比热-温度关系,配合编程及求解方便的热容法求解相变导热问题,得到了二维球状蓄热体的蓄放热性能曲线。通过实验验证,数值计算结果和实验结果具有较好的一致性。     

蓄热式管壳换热器传热特性研究

针对热电联产机组供热期发电负荷受供热量限制,机组调峰能力下降、电力系统弃风弃光现象严重的问题,设计了一种新型蓄热式管壳换热器。利用相变材料蓄/放热过程中温度接近恒定、释放潜热量大等优点,选取石蜡为相变材料,换热器相变区作为换热单元,采用控制变量法,针对传热流体流速、相变材料导热系数及相变层厚度等关键因素,对换热单元的蓄/放热过程进行数值模拟。结果表明:提高传热流体流速可增强换热单元蓄热能力,缩短相变材料完全熔化时间,放热过程中为保证换热器输出端热量,应适当选取传热流体流速;使用复合材料提高相变材料导热系数能够增强换热单元的换热能力,在相同传热流体流速下使换热单元平均传热系数较纯石蜡工况提升2倍以上;增加相变层厚度在放热过程中可延长传热流体出口温度维持的时间。     

太阳能热发电用相变蓄热器的数值模拟与优化

采用单级壳管式蓄热结构,运用FLUENT软件进行数值计算,对蓄、放热过程进行分析,分析了固、液界面前端位置的变化,通过对数据的曲线拟合得出熔化时间与入口温度的函数关系,讨论了蓄、放热熔化凝固时间以及管内温度的变化.通过拟合,得到的曲线显示出很好的一致性,得出最佳入口温度1025 K,流量0.05kg/s.对蓄热和放热阶段分析,得到前端位置随时间变化形状呈现锥形,管内温度的变化,显示了相变蓄热技术的优势,得出的结论对相变问题的数值模拟以及相变装置的结构设计具有重要的理论价值.     

电热型管壳式相变蓄热器的设计与性能实验

相变蓄热技术近年来在电力削峰填谷的应用中发挥了重要作用,成为供热领域的新热点。本文设计了以PTC电加热棒为发热源,水为载热介质,纳米共晶水合盐为相变蓄热材料的管壳式相变蓄热器。实验研究了蓄放热过程中装置内部水和相变材料的温度分布情况以及特定温度范围的蓄放热性能及变化规律。结果表明,以圆管正三角阵列 + 折流板为特征的管壳式换热器结构可以使蓄热器内部温度分布更加均匀;以某测温点水温75 ~ 98℃变化区间为蓄放热周期,蓄热周期的实际蓄热量为779 796 kJ,有效蓄热系数达到0.91,平均蓄热功率为94.13 kW;在放热周期,放热功率从74.2 kW随水温的下降而逐渐减小至51.8 kW,当水温降至相变温度以下时,放热功率趋于稳定。     

管壳式换热器蓄热单元数值模拟与优化

相变材料导热系数低,导致相变蓄热装置无法快速地进行热量储存和释放,文中建立了翅片管和光管式相变蓄热单元的三维计算模型,采用数值模拟方法,从蓄热速率、蓄热量以及温度场等方面比较分析了翅片管和光管结构对储热性能的影响。结果表明:在光管外壁添加翅片可以缩短相变材料完全熔化以及整个蓄/放热过程所需时间;与采用光管结构相比,采用翅片换热管时,完全熔化时间缩短32%,完全放热时间缩短14.5%。可见,在一定条件下添加翅片有助于提高蓄热体的蓄放热性能,所得结论对实际工程中相变蓄热系统的设计和优化具有一定的参考价值。     

相变蓄热同心套管传热模型和性能分析

建立了一种简便的相变蓄热同心套管传热模型,用来求解相变材料在相变过程中流体温度和相变界面随时间和向位置的变化规律,模型中考虑了相变材料导热热阻和有效传热面积随时间和位置的变化,适用于流体入口温度和流量随时间变化的情况,计算值与有关文献值吻合,验证了模型的正确性。藉此模型对文献「７」中相变贮能换热器的储、放热性能进行了模拟分析。     

相变蓄热单元蓄/放热过程的数值模拟研究

建立同心套管相变蓄热单元的二维模型,并与文献对比验证,完成网格无关性和时间步长独立性验证后,数值模拟分析模型结构和流体入口速度对蓄热特性的影响,探究导热系数和相变材料的熔点对放热特性的影响。研究结果表明:采用波节管代替光管可优化蓄热单元的蓄热特性,且3#波节管的换热性能最优,相比于光管,蓄热时间可缩短39%;其他条件不变时,增加传热流体(heat transfer fluid,HTF)的导热系数,蓄热单元的放热效率先增后降,存在最佳值;当相变材料(phase change materials,PCM)的导热系数大于HTF时,继续增加PCM的导热系数会使放热效率下降;随着PCM熔点的增加,有效放热时间先增后减,在熔点为603 K时,放热效率达到最大值0.82。     

太阳能吸热器换热管蓄热数值模拟与试验研究

对以高温共晶盐LiF—CaF2为相变材料(PCM)和以干空气为工质的相变蓄热系统，采用焓方法建立了以控制体单元为对象的单管相变蓄热模型，并对系统进行了数值分析，得到了循环工质气体出口温度、相变材料容器最高温度和平均壁温等参数的瞬态变化曲线，实验研究了吸热器换热管的蓄傲热性能，分析了工质进口温度、输入热流级工质流量对工质出口温度、PCM容器平均壁温及最高壁温的影响。计算结果和试验表明单元换热管的蓄傲热性能达到了设计要求，试验结果与数值计算吻合良好。     

Numerical and Experimental Investigation on Heat Transfer Performance of a Solar Single Storage Tank

The single-tank latent heat thermal energy storage(LHTES) of solar energy mainly consists of two modules: the first one is the phase change material(PCM) module heated by solar energy; the second is a module of heat transfer between melted PCM and the user's low-temperature water. This paper mainly focuses on the former one. To investigate the heat transfer performance of the paraffin-based solar single storage tank and find a more suitable experimental configuration, as basic research work, we established a single-tank thermal storage platform and then conducted a numerical simulation on the heat transfer process with Fluent. The result of numerical simulation shows that the test situation was basically reflected and the data agreed well with the experiment results. The numerical simulation analysis is accurate and the method is reliable. To obtain the heat transfer performance of paraffin in a single tank and strengthen heat transfer, the aspect ratio, the melting temperature of paraffin, and the heating power of the electric heater were analyzed based on simulation. The results show that the heat transfer gets more uniform when the aspect ratio is lower. This results in an increase in the liquid fraction of 61.83% to 76.47% one hour after heating when the aspect ratio of the tank reduced from 2.8 to 1.1. The higher the melting temperature of paraffin, the longer it takes for PCM to reach a stable state. And the curvature of liquid heating is greater than that of solid heating at the bottom layer. Under the constant total work, the heating power has little effect on the heat transfer performance of the paraffin. This study will provide some reference value for the optimization design of single-tank LHTES systems in the future.     

热管式吸热器单元热管传热的数值模拟分析

热管式吸热器的热性能分析对吸热器设计有着重要意义，但由于其相变过程与热管传热的耦合作用十分复杂，至今仍是很少有人深入研究的领域。本文基于焓法建立单元热管耦合传热的物理和数学模型，模拟计算了热管壁温、蓄热容器壁温、循环工质出口温度及相变材料熔化率等参数，并与基本型吸热器进行比较，验证了热管吸热器明显改善了温度分布的均匀性和相变材料的熔化率。     

Numerical and experimental investigation for heat transfer in triplex concentric tube with phase change material for thermal energy storage

This paper addresses a numerical and experimental investigation of a thermal energy storage unit involving phase change process dominated by heat conduction. The thermal energy storage unit involves a triplex concentric tube with phase change material (PCM) filling in the middle channel, with hot heat transfer fluid (HHTF) flowing outer channel during charging process and cold heat transfer fluid (CHTF) flowing inner channel during discharging process. A simple numerical method according to conversation of energy, called temperature & thermal resistance iteration method has been developed for the analysis of PCM solidification and melting in the triplex concentric tube. To test the physical validity of the numerical results, an experimental apparatus has been designed and built by which the effect of the inlet temperature and the flow rate of heat transfer fluid (HTF, including HHTF and CHTF) on the thermal energy storage has been studied. Comparison between the numerical predictions and the experimental data shows good agreement. Graphical results including fluid temperature and interface of solid and liquid phase of PCM versus time and axial position, time-wise variation of energy stored/released by the system were presented and discussed.     

Comparative study of different numerical models of packed bed thermal energy storage systems

This paper presents, compares and validates two different mathematical models of packed bed storage with PCM, more specifically the heat transfer during charge of the PCM. The first numerical model is a continuous model based on the Brinkman equation and the second numerical model treats the PCM capsules as individual particles (energy equation model). Using the Brinkman model the flow field inside the porous media and the heat transfer mechanisms present in the packed bed systems can be described. On the other hand, using the energy equation model the temperature gradient inside the PCM capsules can be analysed. Both models are validated with experimental data generated by the authors. The experimental set up consists mainly of a cylindrical storage tank with a capacity of 3.73 L full of spherically encapsulated PCM. The PCM used has a storage capacity of 175 kJ/kg between ?2–13 °C. The results from the energy equation model show a basic understanding of cold charging. Moreover, three different Nu correlations found in the literature were analysed and compared. All of them showed the same temperature profile of the PCM capsules; hence any of them could be used in future models. The comparison between both mathematical models indicated that free convection is not as important as forced convection in the studied case.     

Discharge mode for encapsulated PCMs in storage tanks

This paper is aimed at analysing the behaviour of encapsulated salt hydrates, used as latent energy storage in a heat transfer system of a domestic hot water tank. The salt is a eutectic mixture of hydrate nitrates of ammonium and magnesium, with low melting temperature, already tested for latent heat storage in domestic applications. In the discharge mode, cold water enters the tank and flows on the encapsulated melted PCM, which is cooled and solidified. In the initial condition the PCM is at its melting temperature. Suddenly its external surface is cooled to a constant temperature T₀; the duration of the solidification represents the time in which the latent heat is released to water. The discharge process of the phase change material (PCM) is analyzed analytically and its effectiveness is assessed, for constant surface temperature conditions, in three different geometrical configurations, i.e. considering the PCM encapsulated in slab, cylindrical or spherical polyethylene containers. The focus is on a model of the moving boundary within the phase-change material during the discharging mode, and the duration of the phenomenon. Results shown include transient position of the moving surface, temperature distribution, amount of solid PCM, energy released, and duration of complete solidification. The influence of the geometry and the Jacob number on the ending time of solidification is investigated. Among different geometrical configurations of the PCM, it is found that the shortest time for complete solidification is matched for small spherical capsules, with high Jacob numbers and thermal conductivity.     

Thermal performance of a solar-aided latent heat store used for space heating by heat pump

In this study, the cylindrical phase change storage tank linked to a solar powered heat pump system is investigated experimentally and theoretically. A simulation model defining the transient behaviour of the phase change unit was used. In the tank, the phase change material (PCM) is inside cylindrical tubes and the heat transfer fluid (HTF) flows parallel to it. The heat transfer problem of the model (treated as two-dimensional) was solved numerically by an enthalpy-based finite differences method and validated against experimental data. The experiments were performed from November to May in the heating seasons of 1992–1993 and 1993–1994 to measure both the mean temperature of water within the tank and the inlet and outlet water temperature of the tank. The experimentally obtained inlet water temperatures are also taken as inlet water temperature of the simulated model. Thus, theoretical temperature and stored heat energy distribution within the tank have been determined. Solar radiation and space heating loads for the heating seasons mentioned above are also presented.     

Analysis of horizontal mantle heat exchangers in solar water heating systems

The characteristics of horizontal mantle heat exchangers are investigated for application in thermosyphon solar water heaters. An experimental model of a horizontal mantle heat exchanger was used to evaluate the flow patterns in the annular passageways and the heat transfer into the inner tank. Flow visualisation was used to investigate the flow structure, and the heat transfer was measured for isothermal inner tank conditions. A numerical model of the flow and heat transfer in the annular passageway was developed and used to evaluate the heat flux distribution over the surface of the inner tank. The numerical results indicate that configurations of mantle heat exchangers used in current solar water heater applications degrade thermal stratification in the inner tank. The effects of inlet flow rate, temperature and connecting port location are quantified.     

Validation of a CFD model for the simulation of heat transfer in a tubes-in-tank PCM storage unit

A computational fluid dynamics (CFD) model was developed for the simulation of a phase change thermal energy storage process in a 100 l cylindrical tank, horizontally placed. The model is validated with experimental data obtained for the same configuration. The cold storage unit was charged using water as the heat transfer medium, flowing inside a horizontal tube bundle, and the selected phase change material (PCM) was microencapsulated slurry in 45% w/w concentration. The mathematical model is based on the three-dimensional transient Navier–Stokes equations with nonlinear temperature dependent thermo-physical properties of the PCM during the phase change range. These properties were experimentally determined using analytical methods. The governing equations were solved using the ANSYS/FLUENT commercial software package. The mathematical model is validated with experimental data for three different flow rates of the heat transfer fluid during the charging process. Bulk temperature, heat transfer rate and amount of energy stored were used as performance indicators. It was found that the PCM bulk temperatures were predicted within 5% of the experimental data. The results have also shown that the total accumulated energy was within 10% of the observed value, and thus it can be concluded that the model predicts the heat transfer inside the storage system with good accuracy.