适用于废热回收的相变蓄热装置数值模拟与实验研究

固-液相变潜热蓄热技术是一种极具前景的工业废热回收方式,通过管壳式换热器可利用相变材料(PCM)吸收工业废热加以储存再用于加热水,从而实现了工业废热的回收利用.对填充高导热多孔筛网的管壳式潜热蓄热单元(LHSU)建立了二维数学模型,并对填充与未填充筛网的蓄热容器一同进行了相变蓄热实验.实验结果表明,填充筛网能够有效改善PCM的传热性能;实验数据和计算值吻合的较好,证明了计算模型的有效性.利用计算模型,对3种PCM(石蜡P116、硬脂酸和软脂酸)蓄热系统进行了数值计算.结果表明,采用软脂酸的蓄热系统热性能最佳,能很好地满足供应生活热水的设计要求.研究结论对蓄热系统的设计和性能优化有一定的指导作用.     

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。     

圆柱形壳管式相变蓄热单元的蓄热特性研究

首先提出适合该系统的圆柱形壳管式相变材料(Phase Change Material, PCM)蓄热单元的结构形式;然后基于焓法建立了蓄热单元的数学模型,并利用文献结果对其进行了验证;以CaCl2·6H2O为相变材料,研究了蓄热单元的蓄热特性以及换热流体温度的变化规律,结果表明流体温度与流速都是影响其蓄热特性的重要因素,为季节性相变蓄能装置开发奠定了理论基础.     

高温相变蓄热的研究进展

从如下两个方面总结了高温相变蓄热的研究现状：①在高温相变材料(PCM)方面，重点介绍了高温相变材料的一些重要性能及其测量，高温相变材料的封装，高温复合相变材料及高温相变材料的应用；②在传热分析方面，主要介绍了相变过程的数值模拟和相变蓄热系统(LTES)的热力学优化。     

太阳能相变蓄热水箱性能实验研究

利用相变材料蓄热是提高太阳能系统效率的重要途径之一。为对比分析含相变材料蓄热水箱的性能,选用三水合乙酸钠,搭建了一套蓄热水箱实验系统,在初始水温为80℃、进水温度为20℃的工况下,对比分析不同进口流量下(2、6和10 L/min)相变蓄热水箱的热特性。实验结果表明:相变蓄热水箱的蓄热量较普通水箱增加了1.4%;随着流量的增加,水箱的混合数先减小后增大,火用效率逐渐降低,相变蓄热水箱的填充效率先增大后减小,且在6 L/min时达到最大值0.905。     

相变蓄热装置内部温度场的理论与实验研究

对螺旋盘管相变蓄热装置性能和相变材料 (PCM)的传热特性开展理论和试验研究,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。     

新型定形板状相变材料的蓄/放热特性

对填充了新型定形板状相变材料的蓄热槽的蓄／放热特性进行了数值计算分析和实验比较。根据数值计算，对影响蓄热槽蓄／放热特性的主要因素——相变材料的几何尺寸、相变材料的导热系数、流体流速、对流放热系数、相变材料填充率等的影响规律进行了分析研究，计算出了蓄热槽内温度分布随时间的变化；并在实验台上测试了蓄热槽初始温度、流人和流出蓄热槽流体温度、作为蓄热体的相变材料测点温度随时间的变化，计算结果与实验结果具有较好的一致性。     

组合式相变材料组分配比与储热性能研究

采用焓法对组合式相变材料(PCM)储热系统的相变过程进行了数值计算,分析了组合式相变材料中各个PCM组分质量分数的变化对系统储热性能的影响。结果表明,对于组合式相变材料储热系统,存在着最优组分配比,使得系统的储热性能达到最佳。     

基于梯度孔隙率金属泡沫的复合相变单元储热性能数值模拟北大核心CSCD

向相变材料中添加金属泡沫可以解决相变材料低导热率引起的换热效果较差等问题,提高系统的整体蓄热效率。然而,复合相变材料的传热性能受金属泡沫孔隙率分布的影响较显著,为进一步提高相变储能单元的传热性能,本工作基于低孔隙率金属泡沫-相变材料(PCM)复合储能系统,建立了一种新的梯度孔隙率金属泡沫结构,通过数值模拟方法,对蓄热单元熔化过程中的熔化率、储能速率、储能总量进行分析,系统研究了孔隙率沿加热方向负梯度分布、正梯度分布对复合相变材料熔化速度和储热性能的影响。研究结果表明,负梯度孔隙率结构可以进一步提高储能系统的储热效率,其中,孔隙率梯度为0.12(案例S-6)时增强效果最显著。在熔化周期的不同阶段,负梯度孔隙率对复合材料的传热均有不同程度增强,对于S-6,在1000 s、2000 s、2600 s时,熔化率相较于均匀孔隙率结构分别增加了0.67%、2.31%、9.90%;随着孔隙率梯度的增加,相变材料的热性能提高越显著,与均匀孔隙结构相比,改进的负梯度孔隙率结构其完全熔化时间最高可缩短7.32%,储热速率可提高8.02%。对于正梯度孔隙率结构,其对熔化速度没有显著影响,但是储热总量可提高0.49%。     

组合式高温相变蓄热器蓄热过程的数值模拟

通过对相同管径蓄热管组成的高温相变蓄热器蓄热过程进行模拟,得到了蓄热过程中相变材料(PCM)温度和液相率随时间的变化曲线以及不同时刻液相率的分布云图。针对温度曲线和云图所显示的问题,在保证蓄热量的前提下,提出了蓄热管组合式排布的设计方案,并对其蓄热性能进行了模拟。研究结果表明,采用所提出的组合式方案有效地减少了蓄热时间,降低了"死区"对蓄热器蓄热性能的影响,使PCM的液相率分布更加均匀,可为用于太阳能热发电的高温相变蓄热器的优化设计提供理论依据。     

Applied research on hot water heating wallboard with phase-change material

In this paper, the hot water heating wallboard with phase-change material (PCM) was prepared. PCM was added into the wallboard. Intermittent heating mode was used in the experiment. Heat transfer performance of the wallboards was tested and numerically simulated by ANSYS software. The results showed that the temperature and heat flow on the surface of the PCM wallboard (PCMW) were lower than that of the common wallboard without PCM in the heating process. And the decreasing in the temperature and heat flow of the PCMW was lower than that of the common wallboard. The experimental and simulated results were identical in most cases. PCMW can remain a certain heat flow on the inner surface, so it can control the fluctuation of indoor temperature, and enhanced the comfort of indoor climate. Different influence factors and optimal design of the hot water heating phase-change wallboard were analysed.     

Channel formation and visualization of melting and crystallization behaviors in direct-contact latent heat storage systems

Thermal storage systems are an essential component for increasing the share of renewable energies in residential heating and for the valorization of waste heat. A key challenge for the widespread application of thermal storage systems is their limited power-to-capacity ratio. One potential solution for this challenge is represented by direct-contact latent heat storage systems, in which a phase change material (PCM) is in direct contact with an immiscible heat transfer fluid (HTF). To demonstrate the applicability of the direct-contact concept for domestic hot water production, a PCM with a phase change temperature of 59°C is chosen. To enable cost-efficient implementation of the storage system, a eutectic mixture of two salt hydrates, magnesium chloride hexahydrate and magnesium nitrate hexahydrate, is chosen as the PCM. One key aspect for the direct-contact concept is that, during discharge, the HTF channels in the PCM do not become clogged during the solidification of the PCM. In this study, the formation and topology of the channels in direct-contact systems under an optimized flow condition are investigated via visual observation and X-ray computed tomography. The elucidation of the channel structure provides information on the melting and crystallization behaviors of the PCM, which are shown schematically.     

Feasibility study on a novel cooling technique using a phase change material in an automotive engine

The size of a cooling inventory is generally designed based on which size can endure the excessive heat load situations that occur sporadically. As a result, cooling systems are often too large for most normal driving modes. There have been numerous efforts to downsize the automotive engine cooling system using novel concepts and strategies. Efficient cooling in automobiles is beneficial in reducing harmful emissions as well as improving fuel economy. A simulation was conducted to validate the feasibility of using a novel cooling strategy that utilized the heat load averaging capabilities of a phase change material (PCM). Three prototypes were designed: full-size, down-sized, and a down-sized prototype with a heat accumulator containing the PCM inside. When the full-size of the cooling inventory was down-sized by 30%, this smaller design failed to dissipate the peak heat load and consequently led to a significant increase in the coolant temperature, around 25 °C greater than that in the full-size system. However, the peak heat load was successfully averaged out in the down-sized system with a heat accumulator. This novel cooling concept will contribute to a substantial reduction in the cooling system in terms of volume and hangover.     

The effect of using two PCMs on the thermal regulation performance of BIPV systems

Building Integrated Photovoltaics (BIPVs) is one of the most promising applications for Photovoltaics (PVs). However, when the temperature in the BIPV increases, the conversion efficiency deteriorates. A PV/PCM system using Phase Change Materials (PCM) for BIPV thermal control has been experimentally and numerically studied previously. One of the main barriers for this application is how to improve the low thermal conductivity of the PCM in order to achieve a quick thermal dissipation response with longer thermal regulation in PVs. Although the metal fins inserted inside the PCM can improve the heat transfer, the thermal regulation period declines as the volume of the PCM is substituted by the metal mass of the PV/PCM system. A modified PV/PCM system integrated with two PCMs with different phase transient temperatures for improving the heat regulation needs to be investigated. The use of combinations of PCMs, each with a set of different phase transient temperatures, is expected to enhance the thermal regulation effect of the PV/PCM system and lengthen the thermal regulation time in PVs. In this study a developed PV/PCM numerical simulation model for single PCM application has been modified to predict the thermal performance of the multi-PCMs in a triangular cell in the PV/PCM system. A series of numerical simulations tests have been carried out in static state and realistic conditions in UK. The thermal regulation of the PV/PCM system with a different range of phase transient temperature PCMs has been discussed.     

相变微胶囊功能流体相变区间的影响因素和变化趋势分析

相变微胶囊功能流体所具有的相变区间是影响其强化传热效应和工程应用价值的主要因素。采用双流体数学模型通过数值模拟发现:在层流条件下,双流体模型能够很好地模拟颗粒相体积分数、管径和R e对相变区间的影响。功能流体的相变段长度和总吸热量都随着这三个因素的增大而显著增大。同时相变段长度还取决于入口温度和边界条件等因素。以直链烷烃为相变材料的功能流体在相变段的蓄热能力相近。但在同R e下,功能流体的相变段长度和总吸热量都随着囊芯材料相变温度的升高而减小。入口温度是影响相变材料熔化速度的重要因素。相变段的长度、总温升和总吸热量与流体入口过冷度都呈线性关系。在第一类边界条件下,相变段长度与壁面过热度呈指数为负的幂指数关系,而相变段总温升和总吸热量都随壁面过热度的增大而增大。     

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。     

微重力条件下相变材料容器的二维瞬态热分析

高温相变材料是空间太阳能热动力发电系统吸热器中普遍使用的蓄热介质。基于微重力状态下的导热控制微分方程，采用焓法对相变材料容器进行了二维数值分析，对计算结果给予了讨论。     

高温固液相变蓄热容器的研究与发展

介绍了近年来国内外空间太阳能动力装置PCM蓄热容器的研制情况，分析了PCM蓄热容器的传热过程及热分析方法，概括了研究中存的主要问题，认为进一步完善传热模型，寻求强化传热新途径，提高效率和可靠性及进一步的试验研究是今后发展及研究的主要方向。     

Simulation of an integrated PCM–wallboard system

Heat transfer barriers and other practical difficulties do currently hamper the development and application of (phase change materials) PCM–wallboard systems. In this study thermal performance of randomly mixed PCM and laminated PCM–wallboard systems have been numerically evaluated and results compared. The laminated system displayed up to 50% increment in heat flux enhancement and about 18% increase in heat transfer rates. Consequently, the laminated PCM–wallboard system has greater potential for heating and cooling application in buildings than the randomly mixed system. Experimental validation and investigation into manufacturing techniques are however needed to establish the commercial viability. Copyright © 2002 John Wiley & Sons, Ltd.