含相变微粒流体层蓄热过程的传热机理研究

对含有相变微粒并且水平放置的流体层,在底部加热和周围绝热保温条件下的蓄热传热机理进行了理论分析研究,与无相变颗粒流体的蓄热过程进行了对比数值模拟。结果表明：对较薄的蓄热流体层,含相变微粒流体的蓄热完成时间短于无相变颗粒流体,随着液层厚度的增加这种差别减小。同时对液体层内不同高度下平均温度的分布和变化过程进行了分析研究。     

水平及竖直放置套管式相变蓄热单元传热特性的数值模拟

以赤藻糖醇为相变材料,采用Fluent软件对同心套管式相变蓄热单元的熔化和凝固过程进行了三维非稳态数值研究。在考虑自然对流的前提下,对比了水平入射式、顶部入射式及底部入射式相变蓄热单元的传热特性,得到了固液界面分布图和温度云图随时间的变化特性,对比了各自的蓄放热速率。研究表明:自然对流在熔化过程中起主要作用,而在凝固过程中起的作用很小;蓄热速率从大到小排列,依次是水平入射式、顶部入射式和底部入射式,相比于底部入射式,水平入射式的总熔化时间可减少27.2%,而顶部入射式的总熔化时间仅减少3.7%;放热速率从大到小排列,依次是顶部入射式、水平入射式和底部入射式,相比于底部入射式,顶部入射式的总凝固时间可减少9.2%,而水平入射式的总凝固时间仅减少0.6%;水平入射式蓄热单元是满足蓄放热速率快这一要求的首选型式。     

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。     

三维套管式加肋相变蓄热单元的传热特性分析

采用数值模拟方法研究了不同模型的相变蓄热单元蓄/放热过程以及入口流体温度对蓄热过程的影响。结果表明：沿轴向方向,不同截面的蓄/放热时间相差很大,轴向方向的自然对流对蓄/放热的影响较小;添加分形肋片提高蓄/放热速率的效果最好;在蓄热过程中,模型4和模型5的蓄热时间相比于模型1均缩短了45.4%;在放热过程中,模型2～模型5相比于模型1的放热时间分别缩短了12.3%、13.7%、27.4%和34.2%;提高入口流体温度可以显著缩短蓄热时间,但蓄热速率的增幅逐渐减小,当入口流体温度高于358.15 K时,入口流体温度对融化时间的影响明显减小。     

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。     

梯级相变蓄热装置蓄放热性能模拟研究

文章设计了包含多种蓄热材料的梯级相变蓄热装置,建立了该装置蓄放热过程的数学模型,模拟并对比了单级、梯级相变蓄热装置在相同工况下的蓄热量、有效能利用率、液相率、传热热流密度等性能的差异。在相同工况下,与单级硬脂酸相变蓄热装置相比,梯级相变蓄热装置的蓄热量提高16.9%,有效能利用率提高10.1%,蓄热/放热平均热流密度分别提高97.6%,64.9%,蓄热/放热用时分别缩短50.5%和39.2%。梯级相变蓄热装置的蓄热用时随传热流体进口流速的增加呈现先下降后上升的趋势。梯级相变蓄热装置的蓄热用时长短受传热流体进口流速工况的影响。     

壳管式梯级相变蓄热装置内部结构参数的优化研究

梯级相变蓄热技术具有蓄放热平稳、储能密度大、易于控制、能有效克服相变材料导热系数低等优点,是最具应用前景的蓄热技术。壳管式蓄热装置是常用的蓄热装置,需要对其结构参数进行研究优化。采用焓法对壳管式梯级相变蓄热装置的相变过程进行了数值模拟,分析了梯级装置内管径和分管长对装置储热性能的影响。并以提高蓄热效率为优化目标,对参数进行了优化。对壳管式梯级相变蓄热装置的结构参数选择具有借鉴意义。     

多孔基无机复合相变材料的蓄热特性

添加多孔介质是提升PCM(相变材料)热导率、缩短其熔化时间的有效手段。本工作通过建立具有不规则分布多孔骨架的CPCM(复合相变材料)物理模型,系统研究了孔隙度、孔径、骨架形状对其蓄热特性的影响,并探讨了蓄热能力与蓄热速率之间的关系。结果表明,随着孔隙度的减小,CPCM的熔化速率增大,固液态PCM密度差引起的重力驱动力使PCM产生自然对流现象,加速了熔化过程。在相同孔隙度(0.80)下,当孔径越小时,多孔骨架表面积越大,所吸收的热通量越大,相变材料熔化越快。具有四面体形状骨架的CPCM由于具有最大比面积30.02 mm^-1,41 s内便完成熔化,相比比面积为19.93 mm^-1的二十面体快了13.5 s。孔隙度的减小虽有利于CPCM的熔化,但也会削弱其蓄热能力,本工作所确定的平衡孔隙度为0.80,可使CPCM的有效热导率达到8.07 W/(m·K)。因此,本工作可为低温蓄热材料提供理论依据和参考。     

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。     

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.     

Theoretical and experimental study of volumetric change rate during phase change process

The solid‐to‐liquid phase change material can be used to convert the thermal energy of low quality into mechanical energy. Its volumetric change rate has a great effect on the converting efficiency. The mathematical model of the volumetric change rate for the phase change processes of hexadecane within a cylindrical container was presented. A special experimental device was designed and constructed to verify the numerical model. The results at different Stefan numbers of the experiments matched well with those from the numerical simulations. The influences of Stefan number, Biot number and radius of cylinder on the volumetric change rate were studied and analyzed. The results showed that the volumetric change rate depends on the mass fraction of liquid phase and the difference between liquid and solid density of the materials. All the factors affecting the phase change rate will influence the volumetric change rate. The volumetric expansion rate is less than theoretical value under an external pressure. While a high‐pressure situation is taken into consideration, the numerical model should also be modified by adding a function calculating density varying with pressure to ensure that the model operates properly. The power output can be enhanced by reducing the total time of melting. It can also be improved when the phase change material is partly melted at a volumetric expansion rate less than 100% of the total value. Copyright © 2008 John Wiley & Sons, Ltd.     

Simulation modeling and analysis of a high temperature phase change heat storage and exchange device

相变储热因单位体积储热量大,储热和放热过程温度基本恒定等优点而成为目前研究的热点。相变过程中涉及固液两相间融化和凝固的传热问题,其储放热过程是一个复杂的非稳态相变过程。本文对高温相变储热换热装置进行换热特性研究,通过研究储热单元的换热特性,基于FLUENT软件,结合装置的设计参数和相变复合材料的物性参数,对相变储热系统储/放热过程中内部的温度分布、传热速率和储放热效率进行了数学建模及模拟分析,重点研究了不同传热流体速度对单元储/放热性能的影响规律。根据仿真结果,在相变储热装置的设计中,可选择合适的空气流速,以实现不同的散热功率及储放热时间,满足不同用户的用热需求。物理实验表明仿真结果偏差较小,可为高温相变储换热装置设计、优化等工作提供依据。     

Simulation of the daily change of soil temperature under different conditions

In this paper, by employing a mathematical model which was established to describe the physical mechanisms of heat and mass transfer in unsaturated porous media, some comparisons between the daily changes of soil temperatures under five different conditions have been conducted. The numerical simulations show that the ambient temperature, solar radiant heat flux, soil initial water content, ambient wind speed, atmospheric relative humidity, and soil porosity exert significant effects on the daily change of soil temperature. The differences between the daily changes of soil temperatures in those cases are also discussed and analyzed, which may provide some useful information for agriculture applications. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(6): 533–544, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10106     

自然对流温度场节能改造

本文通过热流理论分析和传热计算,指出了某一运行多年的自然对流温度场的不足,为了节能增效,采取了快捷的改造措施。工程竣工后试运转表明节能率可达50%,对所有自然对流温度场的改造具有普遍指导意义和重要参考借鉴作用。     

蜂窝蓄热体温度特性数学解析

基于蜂窝蓄热体气固传热精确解，研究蓄热体温度变化和切换周期设计方法，忽略沿气流流动方向的固体导热影响，建立了周期传热数学模型，并求出了气固温度分布精确解。和数值计算相比，半解析解可信，按炉内低氧稳定燃烧和蓄热体低温端不结业的要求，可进行切换周期优化设计，从而为低氧弥散燃烧设计和操控优化提供一种高效、经济、准确的解析研究方法。     

Numerical simulation of high‐temperature phase change heat storage system

In this paper, numerical results pertaining to cyclic melting and freezing of an encapsulated phase‐change material (PCM) have been reported. The cyclic nature of the present problem is relevant to latent heat thermal energy storage system used to power solar Brayton engines in space. In particular, a physical and numerical model of the single‐tube phase change heat storage system was developed. A high‐temperature eutectic mixture of LiF‐CaF₂ was used as the PCM and dry air was used as the working fluid. Numerical results were compared with available experimental data. The trends were in close agreement. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(1): 32–41, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10132     

Numerical and experimental investigation of melting of ice involving natural convection

Two‐dimensional transient melting of ice in a rectangular enclosure was numerically and experimentally investigated. Natural convection in the liquid phase due to the temperature dependency of water density was considered in the numerical model. The implicit finite difference method with fixed staggered grid approach was utilized. The SIMPLER algorithm was followed for the solution of pressure and velocity fields in the liquid phase. The prediction of the model was found to be satisfactory through preliminary experimentation. Copyright © 2002 John Wiley & Sons, Ltd.     

蜂窝陶瓷蓄热体的热应力场数值模拟

对普通正方形格孔蜂窝蓄热体和夹角圆弧过渡正方形格孔蜂窝蓄热体的热应力场进行了数值模拟计算与对比分析.结果表明,夹角圆弧过渡正方形格孔蜂窝蓄热体的低应力区域更大,应力分布也更均匀;数值模拟结果能够指导蓄热体的结构优化设计.     

半导体制冷器温度场的数值分析

半导体制冷是一种高热流密度器件,在其制冷循环过程中,有珀尔帖（Peltie）、傅立叶（Fourier）和焦耳（Joule）三种热效应在共同起作用,其中珀尔帖热效应是半导体制冷的基础。由于半导体材料塞贝克（Seeback）系数a有正负之分,而且其数值通常有所差异,因此P、N型电偶对的微观传热机理是不同的。本文着重讨论了在稳定工作状态下半导体制冷器温度场的数学模型,并对该模型进行了求解,通过数值计算得到了半导体制冷器沿电偶臂长度方向的温度分布曲线,这对进一步弄清半导体制冷器的传热机理有重要意义。     

Thermal performance investigation of a single medium temperature phase change microcapsule used for wind power absorption and heat storage system

Phase change microcapsules have a wide application in the heat storage system. The medium temperature heat storage systems such as medium temperature solar thermal plants, waste heat recovery systems and wind power absorption systems. In order to analyse the effects of configuration parameters and materials on phase change heat transfer process in a single medium temperature microcapsule, an enthalpy-transforming model was applied to trace the location of the solid-liquid interface and obtain the liquid fraction at different time in the melting process. Based on this model, the effects of particle size, the effects of wall thickness, the effects of wall materials and different medium temperature phase change materials were analyzed. The numerical results show that the larger particle size has a longer melting time, the melting time of 50 μm particle size and 250 μm particle size is 0.036 s and 2.48 s, respectively. In addition, the melting time of microcapsules with different wall thicknesses from the 1μm to 9μm is the same i.e., 0.14 s. Therefore, the wall thickness has little effect on the melting time of microcapsules. Besides, the microcapsule with the erythritol as inner material and the polystyrene as wall material has the longest melting time. Furthermore, the thermal conductivity of the wall materials is the main factor affecting the melting time. Moreover, the product of latent heat and density of phase change material is the main factor of the melting time.