On the heat removal characteristics and the analytical model of a thermal energy storage capsule using gelled Glauber's salt as the PCM

An analytical and experimental investigation was performed on a heat removal process of the thermal energy storage (TES) capsule, using gelled Glauber's salt. Transient heat flux at the capsule wall was measured for various cooling conditions. In cases where the initial temperature was lower than the saturation temperature of the phase change material (PCM), numerical analyses were performed, approximating the process by a relaxation phenomenon where crystals grew around crystal nuclei. The results of the numerical solution agreed very well with the experimental results.     

Experimental and numerical model of wall like solar heat discharge passive system

The present work raises the use of solar energy as an aid for air conditioning by means of architectural envelope parts such as walls, basically as heat discharge systems. Using a thermal balance applied to these systems, an analytic model was formulated to simulate its behavior and to consider the time variation of the environmental temperature, solar radiation, heat storage in the wall and the temperature of the room to be ventilated. The analytical results were compared against experimental data, creating an experimentally validated model that gives confidence on the accuracy and trustworthiness of the analytic proposal. Six tests were carried out in the experimental model. In four of them, the heat flux simulation was performed with electrical resistors; in the other two, solar radiation was directly employed. The results show that the thermal performance of the system can be appropriately determined and described by the analytical model, within a small margin of error. The proposed analytic model can calculate the behavior of a heat discharge system in walls by simply knowing the dimensions of the prototype and the environmental conditions.     

Latent heat thermal energy storage using cylindrical capsule: Numerical and experimental investigations

This paper is aimed at analyzing the melting behavior of paraffin wax as a phase change material (PCM) encapsulated in a cylindrical capsule, used in a latent heat thermal energy storage system with a solar water heating collector. The heat for melting of PCM in the capsule is provided by hot water surrounding it. Since it is observed experimentally that the phase change occurs in a range of temperature, the present analysis considers this range instead of constant phase change temperature and the deviation between the results of these two is presented. The numerical analysis has been carried out by using enthalpy method and the results are verified with the experimental data. The experiments have been done by visualization technique without disturbing the actual process of melting. Three distinct stages of melting process have been identified as revealed by visualization studies. Results indicate that the melting process is chiefly governed by the magnitude of the Stefan number, Ste, phase change temperature range and the capsule radius. The analysis shows that the agreement between analytical and experimental results is significantly improved when the results are obtained considering phase change temperature range and the natural convection in the liquid phase instead of considering the process to be conduction dominated only.     

Transient conjugate heat transfer in straight microchannels

Many theoretical and experimental studies have been carried out in order to study the flow and heat transfer in microchannels. In the recent years, numerical simulation has been applied to investigate the problem under a variety of conditions. However, much of the focus has been on steady-state problems and time-dependent transport has received relatively minor attention, despite its importance in practical electronic devices and systems. Employing a versatile commercial code, this paper aims to examine heat removal from straight rectangular microchannels affected by a time-dependent heat flux input. Both cosinusoidal variation and step-change application and removal of a uniform heat source are studied to determine the response time of the system. For the fluid phase, the two-dimensional momentum and energy equations are solved, considering temperature dependent properties and viscous dissipation. The effects of the amplitude of the heat flux variation, inlet velocity, and geometry, including the thickness of the heat sink, are investigated. Channels of smaller width are found to be more sensitive to the heat flux source, especially for higher input values. The velocity represents the most important parameter for channels of greatest width considered here, as it directly affects the fluid dynamics and the pressure drop when a time-dependent heat source is applied to the system.     

微通道内流动沸腾特性研究

对国内外微通道流动和换热的研究实验作了总结,阐述了影响微通道换热系数的因素,如热流密度、过热度和干度等.对去离子水在内径为0.65 mm、长为102 mm的圆形管道内流动沸腾换热进行了实验研究,得到了局部换热系数随干度的变化关系,进而根据换热系数的变化趋势讨论了饱和流动沸腾区微通道内主导的换热机制.结果表明:从换热系数随干度的变化关系很难判定主导的换热机制;将实验数据与已发表的预测关联式进行了比较,发现大多关联式都失效,说明基于常规理论的模型不再适用于微通道.     

家电商场火灾蔓延特性的实验研究

根据家电商场火灾的实际特点,对货架和堆垛两种摆放形式的模拟家电标准组件进行了全尺寸的火灾实验研究.运用实验结果分析了火灾过程中不同阶段的燃烧特点,以及温度、辐射热流和火灾载荷等参数对火灾发展和蔓延规律的影响,并将最高辐射热值拟合为实验关联式,以揭示商场内家电火灾的内在规律和预测其发展趋势.同时获取火灾过程中的基础参数.     

Granular phase change materials for thermal energy storage: Experiments and numerical simulations

The present paper reports on the utilization of granular phase change composites (GPCC) of small particle diameter (1–3 mm) in latent heat thermal energy storage (LHTES) systems. The phase changing parameters (phase change temperature, latent heat, and energy storage capacity) of GPCC have been determined using differential scanning calorimeter (DSC) and temperature-history methods. Further analysis of measurement results has been conducted to describe the evolution of latent heat with temperature during phase change in terms of liquid fraction–temperature relationships. Charging and discharging packed bed column experiments have been also carried out for different operating conditions to analyze the potential of GPCC for packed bed thermal energy storage. The present column results clearly demonstrate the dependence of temperature variation along the packed bed and the overall performance of the storage unit on the phase change characteristics of GPCC. Small and non-uniform particles diameters of GPCC and heterogeneity of the bed material complicate the phenomena of heat transfer and evolution of latent heat in the packed bed. Mathematical modeling of the packed bed that considers the GPCC and air as two separate phases with inter-phase heat transfer is presented. Comparisons between experimental and numerical results are used to evaluate the sensitivity of numerical simulations to different model parameters.     

Performance analysis on industrial refrigeration system integrated with encapsulated PCM‐based cool thermal energy storage system

Cool thermal energy storage (CTES) is an advanced energy technology that has recently attracted increasing interest for industrial refrigeration applications such as process cooling, food preservation and building air conditioning systems. An experimental investigation on the performance of an industrial refrigeration system integrated with encapsulated phase change material (PCM)‐based CTES system is carried out in the present work. In the experimental set‐up a vertical storage tank is integrated with the evaporator of the vapour compression refrigeration system. Effect of the inlet temperature of heat transfer fluid (HTF) on the temperature variation of the PCM and the HTF in the storage tank and the performance parameters namely average rate of charging, energy stored, specific energy consumption (SEC) of the chiller with and without storage system are studied in detail. The effect of porosity variation in the storage tank is also studied. A 1°C decrease in evaporator temperature results in about 3–4% increase in SEC and 1°C decrease in condensing temperature leads to 2.25–3.25% decrease in SEC. The range of HTF inlet temperature and porosity values for optimum performance is reported. Copyright © 2007 John Wiley & Sons, Ltd.     

组合相变材料柱状储热单元的储热特性实验研究

主要研究由硬脂酸,切片石蜡和月桂酸三种相变材料组合的圆柱储热单元的储热特性,测试结果表明,与单一相变材料的储热单元相比,组合相变材料的付热单元的储热速率明显提高。     

The value of thermal radiation in assessing the charge/discharge rate of high‐grade thermal energy storage using encapsulated phase change materials (PCMs)

The charge/discharge rate of a spherical phase change material (PCM) capsule was assessed in consideration of phase change phenomenon and the combined effect of thermal radiation and heat convection in the charging/discharging processes. The heat transfer model was developed based on a single PCM capsule. The equivalent heat flux was evaluated by using the thermal resistance method. In consideration of the thermal radiation, the equivalent charge/discharge rate was improved, and the temperature rising of the PCM was actually much faster in the charging/discharging processes. It was indicated that the influence of the thermal radiation became more significant for PCM capsules under a small Re number (constant air velocity) and for high‐grade thermal energy storage. The analytical results showed that the highest heat flux contributed by cold thermal radiation occupied 30% and 62% of that by heat convection for PCM capsules with radius of 10 and 40 mm, respectively. This illustrated the crucial value of thermal radiation on the charge/discharge rate of PCM capsules with a large radius. However, for smaller size PCM capsules, the equivalent heat flux was larger under the same fluid flow velocity, and it decreased more promptly with time, because the heat convection that played the dominant role in charge/discharge processes was sensitively affected by the radius of the PCM capsules. Copyright © 2016 John Wiley & Sons, Ltd.     

Experimental and numerical investigation on solar parabolic trough collector integrated with thermal energy storage unit

Solar parabolic trough collector (PTC) is the best recognized and commercial‐industrial‐scale, high temperature generation technology available today, and studies to assess its performance will add further impetus in improving these systems. The present work deals with numerical and experimental investigations to study the performance of a small‐scale solar PTC integrated with thermal energy storage system. Aperture area of PTC is 7.5 m², and capacity of thermal energy storage is 60 L. Paraffin has been used as phase change material and water as heat transfer fluid, which also acts as sensible heat storage medium. Experiments have been carried out to investigate the effect of mass flow rate on useful heat gain, thermal efficiency and energy collected/stored. A numerical model has been developed for the receiver/heat collecting element (HCE) based on one dimensional heat transfer equations to study temperature distribution, heat fluxes and thermal losses. Partial differential equations (PDE) obtained from mass and energy balance across HCE are discretized for transient conditions and solved for real time solar flux density values and other physical conditions of the present system. Convective and radiative heat transfers occurring in the HCE are also accounted in this study. Performance parameters obtained from this model are compared with experimental results, and it is found that agreement is good within 10% deviations. These deviations could be due to variations in incident solar radiation fed as input to the numerical model. System thermal efficiency is mainly influenced by heat gain and solar flux density whereas thermal loss is significantly influenced by concentrated solar radiation, receiver tube temperature and heat gained by heat transfer fluid. Copyright © 2016 John Wiley & Sons, Ltd.     

Experimental research of pool boiling heat transfer in horizontal narrow spaces

Much progress has been made in high‐performance electronic chips, the miniaturization of electronic circuits and other compact systems recently, which brings about a great demand for developing efficient heat removal techniques to accommodate these high heat fluxes. With this objective in mind, experiments were carried out on five kinds of test elements with distilled water and ethanol as working liquids. The test elements used in these experiments consisted of five parallel discs with diameters varying from 5 mm to 40 mm. The experiments were performed with the discs oriented horizontally and uniform heat fluxes applied at the bottom surfaces. The influence of narrow spacing, space size, working liquid property, and heat flux on boiling heat transfer performance in narrow spaces has been investigated. Experimental results showed that the boiling heat transfer coefficient of a narrow space was 3 to 6 times higher than that of pool boiling when the narrow space size and heat flux combine adequately, but the critical heat flux was lower than that of pool boiling. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(5): 307–315, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20017     

相变胶囊堆积形态对蓄热系统性能的影响分析

建立相变蓄热胶囊的三维有序及无序堆积模型,在此基础上分析相变蓄热胶囊有序及无序对蓄热系统特性的影响。通过对蓄热系统的蓄热量、蓄热用时、蓄热效率及系统内温度分布等关键性能指标情况分析,提出一种判定蓄热终点的新方法。结果表明:当采用有序堆积时系统的各项性能指标最优,叉排堆积时蓄热效率最低,而无序堆积时蓄热总量最少。另外,换热流体流速增加可加快蓄热过程,但此时蓄热系统蓄热效率较低。     

太阳能单罐相变蓄热系统实验测试和数值模拟研究

太阳能的高效利用是解决能源短缺和环境污染问题的主要手段之一。文章基于太阳能相变蓄热系统,建立带有辅助热源的太阳能单罐相变蓄热系统实验台,并利用该实验台对相变材料(融点为48~50℃的石蜡)进行热能存储实验。实验结果表明:加热12 min后,顶层石蜡温度达到140℃以上,并且融化完全;加载完成后,中层石蜡的最高温度为63℃;静置2 h后,储能罐中的石蜡开始放载,最后均匀混合,顶层、中层和底层石蜡的温度均达到42℃。文章在实验结果的基础上,利用Fluent软件对太阳能单罐相变蓄热系统内相变材料(石蜡和熔融盐)的换热过程进行数值模拟,模拟结果表明,在相同时间内,当采用石蜡作为相变材料时,太阳能单罐相变蓄热系统的换热效率较高。     

采用肋片强化相变蓄热器传热的模拟分析

为了增加同心套管式相变蓄热器的蓄能效果,采用环形肋片强化相变储能设备的传热,利用Fluent软件模拟了这种结构中石蜡相变的融化过程,得到了石蜡熔化过程温度场分布及熔化时间的规律,根据这些规律分析了肋片间距及厚度等参数对贮热管放热效果的影响。分析结果表明：石蜡的总融化时间随肋片间距增加而延长即传热效果变差,但是随着肋片厚度的增加而缩短,即传热效果变优,但是当或肋间距超过40mm和厚度超过2mm后,进一步增加肋片间距或者厚度对传热效果的影响变得不明显。     

严寒地区单体建筑太阳能-相变墙系统蓄热特性研究

利用太阳能对水加热并通入相变墙进行蓄热,对减少严寒地区单体建筑供热能耗有重要意义。以大庆市某单体建筑为例,结合该地区太阳能分布特点及建筑热负荷大小,对适用于该地区的太阳能-相变墙系统进行集热与储热能力计算,并采用CFD方法研究单一工况下该系统的热工变化规律及不同热水参数、换热管规格对相变墙蓄热特性的影响。结果表明：该相变墙热稳定性良好,但受自然对流影响,底部相变材料熔化较慢;管径DN25、入口流速0.3m/s、供水温度310.15K、回水温度309.15K、管间距107mm可使相变材料在4小时内完成蓄热,平均节能率为31.8%。研究结果可望为降低严寒地区建筑供热能耗提供新思路。     

Thermal storage in an air conditioning system with dual energy storage unit

在自行搭建的双蓄能实验平台上进行了制冷兼蓄热实验研究,对比了制冷兼蓄热模式和一般制冷模式,探讨了不同冷冻水流量和不同风机盘管风量对机组性能的影响.实验结果表明:蓄热对机组制冷端的影响很小,但是由于回收了大量的冷凝热,使得机组的综合能效比得到大幅提高,因此蓄热对空调节能具有较大作用.此外,在制冷兼蓄热模式下,冷冻水流量或风机盘管风量越大,机组的综合能效比越大,当风量为1033 m³/h,冷冻水流量为972 L/h时,机组综合能效比高达7.06.     

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.     

Visualization on flow patterns during condensation of R410A in a vertical rectangular channel

The visualization experiments on HFC R410A condensation in a vertical rectangular channel （14.34mm hydraulic diameter, 160mm length） were investigated. The flow patterns and heat transfer coefficients of condensation in the inlet region were presented in this paper. Better heat transfer performance can be obtained in the inlet region, and flow regime transition in other regions of the channel was also observed. Condensation experiments were carried out at different mass fluxes （ from 1.6 kg/h to 5.2 kg/h） and at saturation temperature 28~ C. It was found that the flow patterns were mainly dominated by gravity at low mass fluxes. The effects of interfacial shear stress on condensate fluctuation are significant for the film condensation at higher mass flux in vertical flow, and con- sequently, the condensation heat transfer coefficient increases with the mass flux in the experimental conditions, The drop formation and growth process of condensation were also observed at considerably low refrigerant vapor flow rate.