A simplified model for the analysis of a phase-change material-based,thermal energy storage system

The concept of a phase-change material-based, thermal energy storage system is often used for different applications. A theoretical model to determine the thermal and fluid flow characteristics of a thermal energy storage system using a phase-change material has been developed. The model can be used to predict the energy storage behavior of different phase-change materials used with different heat transfer fluids, flow geometries, flow rates and temperatures. Results have been obtained for the case where the phase-change material is Na₂SO₄·10H₂O (Glauber's salt) and the heat transfer fluid is water. The variation of the dimensionless temperatures of the fluid and the solid, and the molten fraction of the solid during the phase change process, with dimensionless time, for different values of Biot number, Stefan number, and the flow parameter have been determined. A discussion of the results obtained and the conclusions drawn from them are also given.     

相变蓄热体的热工特性数值模拟

为了深入研究蓄热式换热器的蓄放能效果,提出了采用具有相变材料的蓄热体强化蓄放热,通过gambit软件建立了三维蓄热体相变传热过程的物理模型和数学模型,利用fluent软件模拟了具有相变材料的蓄热体与具有变截面管强化的蓄热体传热,得到了两者蓄放热过程温度场分布。根据模拟结果分析了相变材料及缩放结构对蓄热体蓄放热效果的影响,为优化设计蓄热式换热器提供了理论参考。     

相变乳状液在蛇形管中的流动和传热特性

用正十四烷、复合表面活化剂和水制成O/W型相变乳状液作为相变蓄冷及传热介质,对其作为非牛顿流体和固液两相流的流动和传热特点进行了分析,通过测试得出不同浓度乳状液在蛇形管道中的对流换热Nυ数和摩擦系数,并给出了换热和阻力的无量纲关联式。指出了对这类特殊相变蓄冷材料作进一步研究的方向。     

Spiral phase-change material for enhancing natural circulation solar water heater: An experimental study

This article studies the use of spiral phase-change material as an energy storage to improve the performance of a domestic solar water heater. The heating and cooling tests have been conducted for the vertical and horizontal position of the phase-change material in the water heater tank. The vertical position phase-change material yields better results than horizontal position. The charging energy and system thermal efficiencies of the tank are increased up to 20% and 12%, respectively, when the phase-change material is kept vertically. Also, it is observed that better heat transfer coefficient between water and phase-change material and upgraded thermal stratification during the cooling tests.     

分析半透明单晶生长传热过程的边界元方法

论文发展了一个能求解带相变运动界面非定常传热和非线性热物理特性问题的双倒易边界元方法。数值模拟了半透明单晶生长中热过程的一个例子。由于方法是纯边界积分方法,计算量与计算内存都大大减少。获得了单晶生长过程瞬态温度场分布和固液相界面形状时间推进的一些结果。     

Synthesis and characteristics of form-stable n-octadecane/expanded graphite composite phase change materials

N-octadecane/expanded graphite composite phase-change materials were prepared by absorbing liquid n-octadecane into the expanded graphite. The n-octadecane was used as the phase-change material for thermal energy storage, and the expanded graphite acted as the supporting material. Fourier transformation infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal diffusivity measurement were used to determine the chemical structure, crystalline phase, microstructure and thermal diffusivity of the composite phase-change materials, respectively. The thermal properties and thermal stability were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The DSC results indicated that the composite phase-change materials exhibited the same phase-transition characteristics as the n-octadecane and their latent heat increased with the n-octadecane content in composite phase-change materials. The SEM results showed that the n-octadecane was well absorbed in the porous network of the expanded graphite, and there was no leakage of the n-octadecane from the composites even when it was in the molten state.     

SOLIDIFICATION VOID FORMATION IN TUBES: ROLE OF LIQUID SHRINKAGE AND BUBBLE NUCLEATION

Experiments were carried out to observe the solidification sequence and void distribution for experimental liquids enclosed in Pyrex tubes under a wide range of cooling rates. A physical model based on liquid thermal shrinkage and nucleation considerations has been developed to predict the number of voids formed in terms of the heat transfer rate and thermophysical properties of the phase-change material. Agreement of the results with our experimental data is encouraging. Understanding such aspects of the solidification process is vital to development of better thermal energy storage systems for a variety of applications.     

Study on heat-storage and release characteristics of multi-cavity-structured phase-change microcapsules

Based on the solutions of apparent heat capacity method for phase-change problems, numerical simulations were performed to study the heat-storage and release processes for the multi-cavity-structured phase-change microcapsules in this paper. Moreover, the influence of the cavity structures of the phase-change microcapsules on the heat-storage and release capacity was analyzed. The results show that the rate of heat-storage and release will be accelerated by increasing the number of cavities in microcapsules, and cavity interlayer is the key factor to enhance the heat transfer.     

竖直矩形微槽内的特殊干涸与换热行为

利用宽视场体视显微镜、高速摄影仪以及CCD摄像系统对纯蒸发和沸腾换热情形下竖直矩形毛细微槽内液体的特殊干涸行为进行了观察,对液体沿微槽槽道方向的润湿高度进行了观察测量,并对液体沿微槽槽道方向的相变换热特性进行了实验研究.实验结果表明:竖直矩形毛细微槽群是一种高性能的相变换热强化表面,微槽中液体的蒸发与沸腾对干涸点高度的变化有着复杂的影响,热流密度和相变换热系数沿槽道方向的分布不均匀.     

毛细微槽内的相变传热的实验研究

本文对矩形毛细微槽竖直板的相变传热特性进行了实验研究。结果表明毛细微槽对相变换热具有很大的促进作用。当壁面过热度较小时,相变换热形式主要是三相接触线附近的蒸发换热机制。而当过热度较大时,微槽内发生剧烈的沸腾。微槽内相变换热的临界热负荷有两种产生机理:其一是当微槽长度较大时微槽内由于流动阻力而产生的液体输运临界;另一机理是当微槽长度较小时的池内沸腾临界现象,亦即由动态微液层模型决定的临界机理。实验还得到了微槽强化传热的最佳优化尺寸。     

微小空间蒸发传热的可视化实验研究

微小结构中蒸发传热过程的研究对高性能传热设备如毛细蒸发器等的设计和性能的提高 有着重要作用,因此成为近年来国内外传热界的研究热点之一。接触角在微小结构蒸发传热过程中 是一个非常重要的概念,因此,本文主要针对接触角随热负荷的变化规律进行了可视化实验研究。 实验结果表明,接触角发生了明显的滞后现象;当加热功率较大时,接触角对加热功率的反应更为敏感蒸馏水－玻璃体系的接触角大于无水乙醇－玻璃体系的接触角。     

超短激光打孔中快速相变的格子玻尔兹曼模拟

采用双重分布函数的格子玻尔兹曼模型，对单脉冲激光金属打孔过程中的快速相变传热进行研究.模型考虑了金属材料熔化后熔体的流动换热，并采用浸没移动边界方案对过程中的固液界面进行追踪.采用纯导热模型和考虑对流的换热模型计算，将结果和试验进行对比，结果表明：在激光打孔过程中熔体的流动对相变传热产生较大影响，采用考虑流动换热模型的结果与实验更接近.进而对熔化速度、熔化深度以及温度场的变化进行分析，并探讨不同激光工艺参数对相变过程的影响.模拟发现一个脉冲结束后，激光的脉宽越大，孔深越小，孔径越大，且最高温度较短脉冲激光越低.     

分析金属熔滴快速凝固过程的双倒易边界元方法

本文发展了非线性边界条件相变传热过程的轴对称双倒易边界元方法,数值模拟了金属熔滴在快速冷却条件下的快速凝固过程。分别研究了在微重力落管和落塔中及喷射成形过程中金属熔滴的快速凝固过程,得到了过冷度,再辉时间,温度变化及相变界面随时间的变化等数值结果。     

相变光存储研究的新进展

光存储朝着高密度、大容量、高数据传输速率、多功能方向发展。可擦重写相变光存储介质和技术吸引着越来越多研究者的兴趣。本文主要综述了相变光存储原理、材料性能改进和高密度相变存储技术方面的现状和新进展。     

多孔介质内轴对称流动与相变传热过程的双倒易边界元数值模拟

本文将轴对称双倒易边界元方法拓展应用于数值模拟多孔介质内轴对称的流动与相变传热过程,得到了其内非稳态温度场、压力场和速度场,及相变界面时间推进图象。     

AN EXPERIMENTAL INVESTIGATION OF COLD STORAGE IN PACKED CAPSULES WITH SOLIDIFICATION

This article investigates the thermal performance of cold storage in packed capsules with solidification. First, an experimental apparatus utilizing water capsules as phase-change material was designed and constructed. The test section has been mathematically modeled for transient response with the lump theory. Experimental data of temperature profiles for various porosities, flow rates, and different inlet coolant temperatures confirm the validity of this theory. A closed-form solution of the required time for latent heat storage has been established on the basis of the quasi-steady-state assumption of the coolant. By incorporating the experimental results and theory predictions, a correlation of internal heat transfer coefficients for cold storage in packed capsules with solidification has also been developed.     

基于可逆固体氧化物电池堆的电能高效存储

研究了固体氧化物电池堆中电能的可逆存储与产生,通过相变金属存储燃料电池模式下的热能并在电解池模式下加以利用. 系统的荷电状态(即氢燃料百分比)可显著增加开路电压,系统压力的增加有效提高了开路电压. 较高的系统压力可促进电极表面的物质扩散和输运,相应地改善电极的极化电阻. 通过有效的热能管理,系统的电能可逆存储的循环效率可高达92%.     

相变光存储研究的新进展

光存储朝着高密度、大容量、高数据传输速率、多功能方向发展。可擦重写相变光存储介质和技术吸引着越来越多研究者的兴趣。本文主要综述了相变光存储原理、材料性能改进和高密度相变存储技术方面的现状和新进展     

An enthalpy-temperature hybrid method for solving phase-change problems and its application to polymer pyrolysis and ignition

A numerical approach based on the enthalpy method is proposed for solving generalized phase-change problems. The method is applied to predict pyrolysis and ignition of polymeric combustible materials. In contrast to the traditional approach, here both enthalpy and temperature are treated as independent variables, and the conservation equations are solved simultaneously in conjunction with the constitutive equations. Also, the formulation of the constitutive equations for the phase change is not necessarily the same for all of the possible phases, but can be chosen independently according to the characteristics of the physical problem and the requirements of the numerical analysis of each respective phase. Thus with this new approach, which we refer to as the enthalpy-temperature hybrid method, the enthalpy method is applicable to the generalized phase-change problems regardless of the form of the constitutive equations. The proposed method is first applied to a one-dimensional classical freezing problem for verification. It is found that the numerical results for the temperature history and the position of the phase-change interface agree well with the analytic solution existing in the literature. The method is then applied to the numerical simulation of the pyrolysis and ignition of a composite material with a polymer as the matrix and fibreglass as the filling material. Three models of oxygen distribution in the molten layer are considered to explore the melting and oxygen effects on the polymer pyrolysis. Numerical calculation shows that high oxygen concentrations in the molten layer enhance the pyrolysis reaction, resulting in a larger amount of pyrolysate, but in lower surface temperatures of the sample. It also shows that the distribution of oxygen in the molten layer has a strong effect on the pyrolysate rate, and therefore on ignition and combustion of the polymers. Comparison with available experimental data indicates that a model of oxygen distribution in the molten layer that is limited to a thin layer near the surface best describes the ignition process for a homogeneously blended polypropylene/fibreglass composite.     

利用LNG冷能的间接冷冻法海水淡化流程比较

淡水资源缺乏已成为全球性问题,海水淡化方法的研究也日益引起重视。液化天然气在气化时有大量冷能可以利用,考虑到冷冻法海水淡化需要大量冷能,可以把LNG的冷能和冷冻法海水淡化结合起来,形成利用LNG能冷的冷冻法海水淡化流程。文中以间接冷冻法为例,介绍了LNG冷能在海水淡化中的利用;并针对系统中冷媒是否相变而提出了无相变流程和有相变流程,通过HYSYS软件进行模拟,比较分析了各自的优缺点。结果说明无相变流程设备简单、控制方便,但冷媒质量流量大;有相变流程冷媒质量流量小,但流程、设备与控制均较复杂,气相部分体积流量较大,使得气态管路直径较大,相应的换热器尺寸也会更大。