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.     

沸石-液态水吸附储热系统的释热特性

吸附储热是一种有着较高储热密度和较低热损失的储热方式.沸石-液态水吸附储热系统以沸石颗粒作为储热介质,具有系统简单、换热性能好和储热密度大等优点.利用Fluent建立了反应器二维轴对称对流换热模型,分析了进水流速、反应器高径比和颗粒粒径对系统释热过程出口水温的影响.研究表明,在计算条件下,该系统能够获得最大70℃的温升幅度,且进口流速越小,温升幅度越大;高径比越大,温升幅度越大,当高径比≥1.5时,温升不再随高径比的增加而增大;此外,粒径越小,反应速率和温升幅度越大,也越有利于沸石与水的充分反应.本研究有助于完善固-液吸附过程释热特性,为沸石-液态水吸附储热系统的设计和应用提供理论指导.     

光伏背板翅片散热器自然对流特性分析

聚光光伏等的快速发展都会面临散热问题,本文对翅片散热装置的散热特性作了相关分析。基于Fluent数值模拟软件,对矩形片状翅片散热器在定温加热、自然对流条件下的温度分布和速度分布进行了模拟;在温差50℃≥ΔT≥100℃、翅片高度140 mm≥H≥60 mm、翅片间距14 mm≥S≥6 mm范围内对翅片尺寸优化,得出翅片表面平均对流换热系数的变化趋势。结果表明,50~100℃内温差越大,对流换热系数越大;H取100 mm时,散热量最大,较H=80 mm时提高了32.15%;S取12 mm时散热量最大,较S=10 mm时提高了27.44%。     

柴油机排气道改进对散热量的影响

降低缸盖排气道的散热量能提高发动机热效率和排气温度,以某6缸重型柴油机为研究对象,结合流固耦合计算和一维、三维耦合计算,获得了较为准确的一维热力学模型．在此基础上设计不同表面积和直径的排气道方案,采用一维热力学仿真,计算各方案在多工况下的对流换热系数、散热量和涡前排气温度．结果表明：气侧对流换热系数基本不受气道表面积影响,但会随着气道直径的增大而迅速下降,并与直径的平方呈反比．散热量随着气道表面积减小等比例减小,也会随着直径增大而降低,且降低比例稍低于气侧对流换热系数的降低比例．涡前排气温度随着排气道散热量的降低而升高,A4方案的涡前排气温度最高,表面积减小到60%,标定工况点可以提升22.6℃．     

加装导流板的真空管热水器流动与换热的数值模拟

针对真空管与水箱连接处冷热水混流、出现随机涡流而影响换热等问题,分别建立了两种加装导流板结构模型.用FLUENT软件对两种导流结构热水器进行三维数值模拟研究,并与原结构进行比较分析,研究了真空管内的对流换热过程以及水箱内流体的分层情况.结果表明:加装导流板后,管口和水箱内的冷、热水混合换热情况得到了改善,增强了管内流体...     

全玻璃真空管太阳热水器数值模拟研究

基于FLUENT软件及TECPLOT软件,通过对均匀加热条件下的全玻璃真空管太阳热水器内流场及温度场的数值模拟,研究了热水器的对流换热与传质过程;分析了集热管倾角、真空管双面受热对热水器内流场及温度场的影响和真空管与水箱连接处、垂直等温面上的流体速度、温度分布。结果表明,在真空管与水箱连接处出现随机的涡流和真空管内出现分段的小环流,对传热传质过程不利,特别是对于带反射板的双面受热的集热器,应采取措施确保管内对流换热流动的有序性。建议加装导流板,并初步探讨了导流板的长度,确立了最佳板长模拟结构,为后续的理论研究与试验打下基础。     

偏心管翅式相变储热单元性能强化的模拟

对管翅式相变储热单元进行了二维非稳态模拟研究。在考虑自然对流与外管传热的情况下对比研究了同心管翅、偏心管翅以及翅片接触外管三种储热单元的传热特性。考虑了内管壁温度、外管材料、翅片厚度对储热性能的影响。结果表明,与采用同心管翅时相比,由于自然对流的影响,偏心管翅储热单元有效削弱了固-液界面分布不均匀现象,完全融化时间减少了29.3%,而当翅片接触金属外管时,通过翅片的传热外管温度迅速升高,增大了换热面积,完全融化时间减少了近49.3%。可见,翅片接触外管储热单元不仅削弱了自然对流引起固-液界面分布不均匀现象,而且利用了外管的传热,强化了储热换热性能。     

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。     

基于圆柱形单元的储热装置放热实验研究

蓄热水箱作为太阳能供暖系统的重要核心设备,其性能直接影响着储能系统的整体运行效率。设计一种基于圆柱形相变单元的相变储热装置,并搭建相变蓄热水箱性能测试平台,通过单一控制变量法得到储热装置放热过程的温度变化曲线。研究表明：对于空间一定的储热装置,在等质量相变材料（PCM）时,相变单元的直径对装置放热速率的影响较大;相变单元之间的间距对装置放热速率的影响较小;当增大换热流体（HTF）的入口流量及降低HTF入口温度时,能大大减少储热装置的放热时间,提高储热装置的整体性能。     

非能动余热排出系统C型管换热器数值模拟

为保证在事故工况下非能动余热排除系统有效导出余热,对其主要设备PRHR热交换器进行换热特性研究,建立了非能动余热排出系统C型管换热器的内外耦合传热分析模型,采用一维均相流模型计算管内冷凝换热与CFD程序分析水池空间的自然对流。研究进口质量流量、进口流体含气率、管倾角和水箱温度对C型管换热器换热特性的影响。结果表明:C型管换热器入口倾斜段管内始终为饱和的两相流体,在竖直段与出口倾斜段,管内流体温度逐渐下降;管内压力、流体焓值和换热系数沿管长逐渐降低;大约在冷凝70 s后,管内流体参数趋于稳定;管壁温度在入口倾斜段迅速下降,在竖直段和出口段趋于平缓。增大进口质量流量与进口流体含气率,流体温度、流体焓以及管内外换热系数增加,并且沿流动方向受两者的影响逐渐减小;若管倾斜角度增大20°,出口倾斜段管内流体温度下降约3℃;当水箱温度升高10℃,汽泡生成与脱离速度加快,水箱内部换热增强,入口倾斜段外壁温升高2℃左右,出口倾斜段外壁温大约升高0.2℃。CFD模拟结果展示出水箱内汽泡大部分聚集在C型管上部并逐渐向上流动,致使热流体向上运动,冷流体向下流动,形成自然循环。     

Comparisons of deep well and insulated shallow earth storage of solar heat

Four techniques for storing solar heat in earth are described and their costs are compared by a hypothetical example. They include heat storage by: (a) deep well; (b) deep well with a storage membrane; (c) insulated shallow earth heat exchange bed; and (d) insulated shallow earth tube exchanger. The cost comparison obtains optimal design parameters for each storage method and optimal surface area for an attached solar collector. Heat loss equations for insulated shallow earth storage are obtained by electrolytic tank models. Heat loss for deep well storage is derived by superposition of radial flow of water and diffusion of heat.     

Energy analysis and modeling of a solar assisted house heating system with a heat pump and an underground energy storage tank

An analytical model is presented and analyzed to predict the long term performance of a solar assisted house heating system with a heat pump and an underground spherical thermal energy storage tank. The system under investigation consists of a house, a heat pump, solar collectors and a storage tank. The present analytical model is based on a proper coupling of the individual energy models for the house, the heat pump, useful solar energy gain, and the transient heat transfer problem for the thermal energy storage tank. The transient heat transfer problem outside the energy storage tank is solved using a similarity transformation and Duhamel’s superposition principle. A computer code based on the present model is used to compute the performance parameters for the system under investigation. Results from the present study indicate that an operational time span of 5–7 years will be necessary before the system under investigation can attain an annually periodic operating condition. Results also indicate a decrease in the annually minimum value of the storage tank temperature with a decrease in the energy storage tank size and/or solar collector area.     

Simulated and experimental performance of a heat pipe assisted solar wall

Performance was evaluated for a passive solar space heating system utilizing heat pipes to transfer heat through an insulated wall from an absorber outside the building to a storage tank inside the building. The one-directional, thermal diode heat transfer effect of heat pipes make them ideal for passive solar applications. Gains by the heat pipe are not lost during cloud cover or periods of low irradiation. Simplified thermal resistance-based computer models were constructed to simulate the performance of direct gain, indirect gain, and integrated heat pipe passive solar systems in four different climates. The heat pipe system provided significantly higher solar fractions than the other passive options in all climates, but was particularly advantageous in cold and cloudy climates. Parametric sensitivity was evaluated for material and design features related to the collector cover, absorber plate, heat pipe, and water storage tank to determine a combination providing good thermal performance with diminishing returns for incremental parametric improvements. Important parameters included a high transmittance glazing, a high performance absorber surface and large thermal storage capacity.An experimental model of the heat pipe passive solar wall was also tested in a laboratory setting. Experimental variations included fluid fill levels, addition of insulation on the adiabatic section of the heat pipe, and fins on the outside of the condenser section. Filling the heat pipe to 120% of the volume of the evaporator section and insulating the adiabatic section achieved a system efficiency of 85%. Addition of fins on the condenser of the heat pipe did not significantly enhance overall performance.The computer model was validated by simulating the laboratory experiments and comparing experimental and simulated data. Temperatures across the system were matched by adjusting the model conductances, which resulted in good agreement with the experiment.     

A new type of phase change heat storage tank in solar energy combination system

介绍了一种新型笼屉式相变蓄热水箱,通过实验测试对比分析相变蓄热水箱与普通蓄热水箱对太阳能组合系统的太阳能保证率及系统能效比的影响。实验表明：同等水箱容积,使用相变蓄热水箱时太阳能集热系统的小时集热量为普通蓄热水箱的3.7倍,相变蓄热水箱有利于提高太阳能保证率及系统能效比。在太阳能辐照强度相似的情况下,相变蓄热水箱会使太阳能保证率平均提高72%,使系统能效比平均提高26%。同时相变蓄热水箱可减少夜间水箱上部的热损失,使水箱上部水温降减少50%。     

Analysis of heat transfer and frost layer formation on a cryogenic tank wall exposed to the humid atmospheric air

In this paper heat transfer characteristics and frost layer formation are investigated numerically on the surface of a cryogenic oxidizer tank for a liquid propulsion rocket, where a frost layer could be a significant factor in maintaining oxidizer temperature within a required range. Frost formation is modeled by considering mass diffusion of water vapor in the air into the frost layer and various heat transfer modes such as natural and forced convection, latent heat, solar radiation of short wavelength, and ambient radiation of long wavelength. Computational results are first compared with the available measurements and show favorable agreement on thickness and effective thermal conductivity of the frost layer. In the case of the cryogenic tank, a series of parametric studies is presented in order to examine the effects of important parameters such as temperature and wind speed of ambient air, air humidity, and tank wall temperature on the frost layer formation and the amount of heat transfer into the tank. It is found that the heat transfer by solar radiation is significant and also that heat transfer strongly depends on air humidity, ambient air temperature, and wind speed but not tank wall temperature.     

Discharge of a thermal storage tank using an immersed heat exchanger with an annular baffle

A number of solar domestic hot water systems and many combined space and water heating systems have heat exchangers placed directly in the storage fluid to charge and/or discharge the tank. Operation of the heat exchanger produces a buoyancy-driven flow within the storage fluid. With a view toward controlling the flow field to increase heat transfer, a cylindrical baffle is inserted in a 350 l cylindrical storage tank. The baffle creates a 40 mm annular gap adjacent to the tank wall. A 10 m-long, 0.3 m² copper coil heat exchanger is placed in the gap. The effects of the baffle on the transient heat transfer, delivered water temperature, heat exchanger effectiveness, and temperature distribution within the storage fluid are presented during discharge of initially thermally stratified and fully mixed storage tanks. The baffle increases the storage side convective heat transfer to the heat exchanger by 20%. This increase is attributed to higher storage fluid velocities across the heat exchanger.     

基于红外测温的双盘式浮顶储油罐散热损失分布规律研究

采用红外热像仪、表面温度计等对双盘式浮顶储油罐的表面温度场进行测试。结果表明:罐顶表面温度呈轴对称分布,径向温度梯度远高于周向,且距离罐中心越远,表面温度越高。油蒸汽挥发导致浮顶和罐壁间的一二次密封处散热损失明显升高,使其成为罐顶表面温度最高的区域。浮舱隔板、桁架和椽子等结构形成了热桥,使局部位置的表面温度升高,增大了罐顶的散热损失。罐壁周向表面温度梯度低于轴向,并且受油温影响较大,在罐壁保温结构的结合部位、局部保温结构破损位置的表面温度较高,散热损失较大。基于表面温度法,结合环境温度和风速测试结果,采用强迫对流换热关联式计算得到储罐不同部位的散热损失。结果表明:对于双盘式浮顶储油罐,罐顶散热损失最大,约占储罐总散热损失的67%,罐壁散热损失约占25%,罐底散热损失约占8%。     

结构参数对全玻璃真空管太阳热水器夜间热损失的影响研究

文章建立了三维非稳态的全玻璃真空管太阳热水器的数值模型,分析了夜间散热时,该热水器内的流动和传热特征,并且在夜间同一工况下,模拟研究了贮热水箱保温材料的导热系数、保温厚度,以及真空管涂层的发射率对贮热水箱温度、真空管温度和该热水器夜间热损失的影响。分析结果表明:随着散热过程的持续进行,全玻璃真空管太阳热水器内温度分层情况越来越明显,内部流体的流速越来越小,真空管内静滞区域自下往上逐渐扩大;当贮热水箱保温材料的导热系数由0.035 W/(m·℃)减小至0.020 W/(m·℃)时,该热水器的夜间热损失减少了8.5%;当贮热水箱保温厚度由50 mm增加至60 mm时,该热水器的夜间热损失减少了5.0%;当真空管涂层的发射率由0.06减小至0.05时,该热水器的夜间热损失减少了4.0%。     

A design method to determine the optimal distribution and amount of insulation for in-ground heat storage tanks

The seasonal sensible heat storage model developed by Hooper and Attwater [1] is modified to describe the thermal behaviour of the soil regime surrounding cylindrical, in-ground, heat storage tanks with optimally distributed insulation. The model assumes steady-state heat transfer, and the surrounding soil is considered to be homogeneous and isotropic. Changes in soil thermal properties due to moisture migration, whether driven by thermal or hydrostatic gradients, are assumed negligible [2]. The optimal distribution is determined using the method of Lagrange multipliers. It is shown that the marginal cost per unit of energy lost and per unit of tank surface area must be the same at all points on the surface of the tank as the condition for minimum total heat loss with a given total investment in insulation. This condition appears to apply for all axi-symmetric in-ground tank geometries. For a given volume of insulation, the incremental increase in storage efficiency with an optimal redistribution of the insulation is a function of tank geometry. The problem of determining the optimal total investment in insulation for a given marginal cost of fuel is described and a method of solution is outlined.     

Experimental investigation on a combined sensible and latent heat storage system integrated with constant/varying (solar) heat sources

The objective of the present work is to investigate experimentally the thermal behavior of a packed bed of combined sensible and latent heat thermal energy storage (TES) unit. A TES unit is designed, constructed and integrated with constant temperature bath/solar collector to study the performance of the storage unit. The TES unit contains paraffin as phase change material (PCM) filled in spherical capsules, which are packed in an insulated cylindrical storage tank. The water used as heat transfer fluid (HTF) to transfer heat from the constant temperature bath/solar collector to the TES tank also acts as sensible heat storage (SHS) material. Charging experiments are carried out at constant and varying (solar energy) inlet fluid temperatures to examine the effects of inlet fluid temperature and flow rate of HTF on the performance of the storage unit. Discharging experiments are carried out by both continuous and batchwise processes to recover the stored heat. The significance of time wise variation of HTF and PCM temperatures during charging and discharging processes is discussed in detail and the performance parameters such as instantaneous heat stored and cumulative heat stored are also studied. The performance of the present system is compared with that of the conventional SHS system. It is found from the discharging experiments that the combined storage system employing batchwise discharging of hot water from the TES tank is best suited for applications where the requirement is intermittent.