Carbon foam matrices saturated with PCM for thermal protection purposes

In the present work, numerical and experimental studies are proposed to predict and investigate the thermal characteristics of a thermal protection system consists of carbon foam matrix saturated with phase change material, PCM. Several types of carbon foam matrices with different porosities and thermal properties were introduced for the sake of a parametric study. The composite (carbon foam matrix saturated with PCM) was introduced into a cylindrical enclosure while it experiences its heat from a heat source setting on the top of the enclosure. The numerical simulation was performed using the volume averaging technique and a finite volume technique was used to discretize the heat diffusion equation while the phase change process was modeled using the enthalpy porosity method. The results are portrayed in terms of temperature and heat absorption time history and the numerical and experimental results showed good agreement. The results illustrated that the higher the porosity the more stability of the thermal performance of the matrix composite. On the other hand, the thermal conductivity of the composite matrix acts sharply to increase or decrease its heat absorption rate.     

Experimental and numerical study of heat transfer in horizontal concentric annulus containing phase change material

In the present research work, solid–liquid phase change heat transfer was studied experimentally as well as numerically for paraffin wax encapsulated in the annulus of two coaxial circular cylinders with variable heat flux. Two‐dimensional mathematical model (Navier–Stokes equation) in terms of primitive variables has been formulated to study the melting characteristics of the phase change material (PCM). The finite volume method (FVM) and semi‐implicit, that is, SIMPLE scheme are applied to discretise the governing equations and track the solid–liquid moving front. From the comparison of experimental data and numerical simulation, it is evident that the convective heat transfer is dominant in melting phase. From the computational results it is also observed that both eccentricity and the variation in the angle of inclination of the eccentricity plays an important role for the net circulation of the molten phase as well as the thermal flux at the inner surface of the annulus.     

相变热控装置内部的传热特性

针对舱内热控用相变热控装置内部的传热特点,研究装置内相变材料吸热过程的相变传热及流固耦合传热规律,建立了相应的数理模型及数值计算方法,进行了实验验证,并在此基础上分析了典型热控单元吸热特性的影响因素及作用规律。相关研究成果将为相变热控装置吸热及热控特性的有效预测,以及为装置的设计及优化提供参考。     

泡沫金属强化石蜡相变蓄热过程可视化实验

相变材料的低热导率是限制潜热蓄热广泛应用的重要原因。将相变材料石蜡真空条件下注入到泡沫金属铜内制备泡沫金属铜-石蜡复合相变材料,通过铜的高热导率及高孔隙材料的大面体比来强化相变换热过程。采用DSC示差扫描量热法对石蜡进行热物性测量获得准确的石蜡相变温度及相变潜热。以管壳式相变蓄热结构为对象,提取对称结构进行可视化设计,对比纯石蜡及泡沫金属铜-石蜡复合材料在相同运行条件下的相变过程,追踪二者熔化过程的相界面位置随时间的演化过程并布置热电偶准确测量材料内部的温度分布。结果显示加入泡沫金属后的复合材料的内部温差明显减小,温度分布均匀,蓄热热通量显著增大,有效缩短相变时间并缓解了自然对流造成的顶部过热和底部不熔化现象。     

Thermal conductivity improvement of phase change materials/graphite foam composites

Effective thermal conductivity of composites of graphite foam infiltrated with phase change materials (PCM) was investigated numerically and experimentally. Graphite foam, as a highly-conductive, highly-porous structure, is an excellent candidate for infiltrating PCM into its pores and forming thermal energy storage composites with improved effective thermal conductivity. For numerical simulation, the graphite structure was modeled as a three-dimensional body-centered cube arrangement of uniform spherical pores, saturated with PCM thus forming a cubic representative elementary volume (REV). Thermal analysis of the developed REV was conducted for unidirectional heat transfer and the total heat flux was determined, which leads to the effective thermal conductivity evaluation. For experimental verification, a sample of graphite foam was infiltrated with PCM. The effective thermal conductivity was evaluated using the direct method of measuring temperature within the sample under fixed heat flux in unidirectional heat transfer. The results indicate a noticeable improvement in the effective thermal conductivity of composites compared to the PCM. Our numerical and experimental results are in agreement and are also consistent with reported experimental results on graphite foam. Moreover, the role of natural convection within the pores is found to be negligible.     

圆柱系统中石蜡/纳米银复合相变材料热特性分析

相变材料由于具有相变潜热,被应用于各领域的热管理。锂离子动力电池作为一种新能源,近年来广泛应用于电动汽车,相变冷却作为一种有效的被动冷却方式,能够有效减缓锂电池的热聚集。为将相变材料应用于减缓锂电池热失控,本工作建立了石蜡/纳米银复合相变材料(CPCM)的圆柱系统,使用相变模型及流体体积(VOF)模型研究了相变材料的融化过程,得到了初始时期空气/石蜡气液交界面的变化以及石蜡的液相分布,与实验结果具有很好的一致性。在此基础上分析了相变过程的吸热及储热情况。同时,针对不同质量分数的石蜡/纳米银复合相变材料进行模拟,结果表明,添加0.5wt%~2wt%的纳米银颗粒能够改善石蜡的导热性能,但潜热会有所降低。相变结束后,材料吸收的热量将转化为显热,底面传热减小,主要是通过壁面传热。另外分析了融化过程中液相的流动情况,相变材料液态层增厚,Nu数下降并趋于稳定,增加纳米银浓度也会降低Nu数。     

Characterization of the Thermal Response of Munitions in Storage Exposed to Real and Arbitrary Weather Conditions

The objective of this investigation is to characterize the thermal response of stored munitions exposed to real weather conditions in order to validate current test procedures as well as develop the ability to provide an estimate of the thermal exposure over the lifecycle of a munition. Currently, the thermal history of munitions in storage cannot be determined without continuous monitoring of individual items and there is no method to provide a detailed estimate of the exposure by analyzing existing data when continuous data was not collected. This work describes the experimental and initial numerical investigations of instrumented munitions in storage to characterize their thermal response. The numerical model has the capability to validate broad trends observed from the experimental data. For this investigation, data is being collected through field experiments of inert munitions instrumented with thermocouples that are collocated near a weather station. Data from field experiments are used to develop and validate the numerical model. The numerical model will save time and resources in future investigations of the thermal exposure of munitions in storage as well as advance the understanding of the implications of selecting a long term storage environment. At a fundamental research level, this work contributes to the advances in combined heat transfer by natural convection and radiation inside a 3D enclosure combined with external forced convection and radiation. The numerical model includes several features including heat transfer to the enclosure by solar radiation, conduction, and forced convection, heat transfer between the enclosure container walls and the projectiles by radiation, conduction, and natural convection, heat transfer within the projectile by conduction, distributed thermal energy storage in projectiles, and transient temperature boundary condition at the enclosure walls.     

相变微胶囊悬浮液圆管内换热特性模拟

相变微胶囊悬浮液在相变过程中具有较大的相变潜热,可以减小温度的变化程度,且比单相流体具有更高的对流换热系数,成为广泛关注的新型热工流体。本文针对相变微胶囊悬浮液在等热流边界条件下的管内层流,根据差示扫描量热法所得到的相变温度范围,采用矩形等效比热容模型,进行了数值模拟分析,并结合以溴代十六烷为相变材料的相变微胶囊悬浮液的实验数据,将数值模拟结果与实验结果对比并进行误差分析。又对在不同质量分数、不同热流密度条件下的对流换热进行研究,分析了不同参数对对流换热强度的影响。并通过拟合得到了相变微胶囊悬浮液圆管内对流换热关联式。然后改变管径、流速条件重新模拟验证该关联式的通用性,其结果表明模拟结果与预测公式高度吻合,该关联式的通用性较好。     

MIG焊接V型熔池特性的数理模型建立

通过对MIG坡口焊接工艺过程的分析将三维焊接物理过程简化为二维数理模型,建立移动高斯热源模型用于描述电弧与工件的热作用,建立熔池内的CFD模型用以描述熔池内熔化／凝固过程的流动传热传质现象,对典型V型坡口焊接过程进行实验和数值模拟研究,通过与实验得到的工件截面金相图对比分析验证了数理模型的准确性,最后对数理模型中高斯热源参数及熔池流动的驱动力机理模型进行了进一步讨论。结果表明通过合理的模型参数选择二维数理模型可以准确地分析MIG坡口焊过程熔池的演变过程。     

纳米复合相变蓄热材料的制备与特性

相变蓄热材料（phase change materials,PCMs）是相变蓄热技术研究的基础。针对普通相变蓄热材料热导率低的缺点,采用纳米技术改善石蜡的相变传热性能,从而提高其热导率及热扩散系数。通过纳米颗粒-石蜡复合材料熔化过程测试和纳米颗粒沉降过程观察,确定铜纳米颗粒和Hitenol BC-10分别作为实验用纳米颗粒和分散剂,在制备稳定的纳米铜颗粒-石蜡复合相变材料的基础上,对其热物性进行了实验研究。结果表明纳米铜颗粒的添加使得石蜡热导率增幅最大,实验测得固态纳米铜-石蜡热导率提高7.9%,液态提高3.8%,而固、液态热扩散系数则分别提高了20.6%和16%。     

石膏基相变储能构件的数值模拟分析

以石蜡为相变储能材料,石膏为基础材料,制备不同配比的石膏基-石蜡相变储能构件并利用现有热工测试方法进行了热工性能测试。将测试数据代入经验证过的相变模型模拟了北京地区被动式建筑中相变构件的应用效果,并对结果进行了优化处理。结果表明：随着石蜡含量的增大,相变构件的热导率随之减小;制备的四种配比中PCM质量分数为33%的相变构件在北京地区被动式建筑中应用效果最好,在过渡季中较传统建筑可节能10%;增大构件的热导率与提高墙体表面对流传热系数均可提高室内舒适度与节能效果,且对流传热系数的影响程度更大。     

TPMS多孔铝-石蜡复合相变材料蓄热过程数值模拟及实验

传统相变材料受限于自身热导率小,其相变蓄热效率难以提升,通过在相变材料中添加具有高热导率的金属多孔结构是强化传热的重要手段之一。本文建立了三周期极小曲面（triply periodic minimal surface,TPMS）多孔铝-石蜡复合相变材料的三维、瞬态包含自然对流的相变蓄热模型,利用数值仿真结合实验的方法研究了TPMS多孔铝-石蜡复合相变材料在蓄热过程中的固液相界面演变规律、实时温度变化、热传输特性以及蓄热性能。结果表明,在纯石蜡中添加primitive杆状（primitive sheet,PS）、primitive壳状（primitive network,PN）两种TPMS多孔铝结构后,石蜡相变温度范围内出现明显的相变温度平台,PS-石蜡、PN-石蜡复合相变材料的相变起始时间较纯石蜡分别减少了74.1%与91.4%,竖直方向上的最大温度梯度由纯石蜡的1605.7℃/m分别下降至PS-石蜡、PN-石蜡复合相变材料的840℃/m、943.8℃/m,蓄热速率较纯石蜡分别提高3.10倍、4.69倍。最后,通过选区激光熔化（selective laser melting,SLM）技...     

基于多孔支撑体的形状稳定复合相变储能材料的研究进展

固-液相变材料（PCMs）是热能储存（TES）技术发展的关键因素,然而一些固有的问题如泄漏和热导率低等严重制约了相变材料的性能。因此,选择合适的方法构建形状稳定的复合相变材料（FSCPCMs）,并有效地提高其热导率是实现相变材料实用化的重要前提。多孔载体封装相变材料为构建具有高储能密度和优异热传输性能的定形复合相变材料提供了一条有效的途径。本文对不同FSCPCMs的制备、结构热学性能、应用等方面进行了综述,详细总结和讨论了孔径和几何形状、表面改性、作用力、组成等因素对FSCPCMs相变行为的影响。重点介绍了具有高热导率、高负载率和高潜热的新型多孔复合相变材料的设计和应用。最后,基于理论、数值和实验方法,展望了FSCPCMs在约束结构中的相变和多尺度传热方面未来的研究方向及其在能源转换方面的商业化应用。     

Polymeric phase change composites for thermal energy storage

This article describes a group of thermal energy storage (TES) composites that combine TES and structural functionality. The composites are encapsulations of low melt temperature phase change materials (PCM) such as paraffin waxes in polymer matrices. Room temperature cured bisphenol‐A epoxy and styrene–ethylene–butylene–styrene (SEBS) polymers are chosen as matrix materials because of their excellent chemical and mechanical properties. The polymeric network structure in the composite encapsulates the PCMs, which transform from the solid to the liquid phase. The PCMs provide the energy storage function via the solid–liquid latent heat effect. The resulting composite exhibits dry‐phase transition in the sense that fluid motion of the PCM, when in the liquid phase, is inhibited by the structure of the polymer matrix. The polymer matrix is formulated to provide structural functionality. The latent heat, thermal conductivity and contact conductance, and structural moduli of composites having various PCM‐to‐matrix volume fractions are measured. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1240–1251, 2004     

Heat transfer and friction factor of Therminol liquid phase heat transfer fluid in a ribbed tube

Experiments and simulations on flow and heat transfer behavior of Therminol-55 liquid phase heat transfer fluid have been conducted in a ribbed tube with the outer diameter and inner diameter 25.0 and 20.0 mm,pitch and rib height of 4.5 and 1.0 mm.respectively.Experimental results show that the heat transfer and thermal performance of Therminol-55 liquid phase heat transfer fluid in the ribbed tube are considerably improved compared to those of the smooth tube.The Nusselt number increase with the increase of Reynolds number.The increase in heat transfer rate of the ribbed tube has a mean value of 2.24 times.Also,the pressure drop results reveal that the average friction factor of the ribbed tube is in a range of 2.4 and 2.8 times over the smooth tube.Numerical simulations of three-dimensional flow behavior of Therminol-55 liquid phase heat transfer fluid are carried out using three different turbulence models in the ribbed tube.The numerical results show that the heat transfer of ribbed tube is improved because vortices are generated behind ribs,which produce some disruptions to fluid flow and enhance heat transfer compared with smooth tube.The numerical results prove that the ribbed tube can improve heat transfer and fluid flow performances of Therminol liquid phase heat transfer fluid.     

开孔泡沫铝内石蜡融化相变过程的可视化实验研究

采用光学显微与红外热成像技术对开孔泡沫铝内石蜡的融化相变过程进行了可视化实验,更加直观地考察泡沫铝强化石蜡传热性能,并重点分析泡沫金属微观孔隙结构下石蜡融化界面演化、融化液相流体流动及温度分布特征。研究结果表明：相变材料中填充泡沫金属可有效降低导热热阻、强化相变传热;与纯石蜡系统相比,石蜡/泡沫铝复合材料系统温度分布更加均匀,具有更好的热响应性能。与纯石蜡系统存在明显的融化前沿界面相比,石蜡/泡沫铝复合系统中融化区与非融化区交错分布,融化界面相对模糊。此外,实验还观测到泡沫铝内石蜡融化液相中存在大量片絮状悬浮物现象。     

硬脂酸/膨润土纳米复合相变储热材料的制备、结构与性能

引　言相变储热技术可用于解决热能供给和需求失配的矛盾 ,在太阳能利用、电力的“移峰填谷”、废热和余热的回收利用以及工业与民用建筑采暖与空调的节能等领域具有广阔的应用前景[1] .其技术关键就是高性能相变储热材料的应用 .近年来 ,国内外学者不断致力于储热材料的研究工     

Numerical and experimental investigations on thermal debinding of polymeric binder of powder injection molding compact

A mathematical model is established to describe the thermal debinding process of polymeric binder from a powder injection molding compact. The model takes into account of the thermal degradation of liquid polymer into liquid volatile fragment, the evaporation of liquid volatile fragment, the capillary driven liquid phase transport, the binary diffusion in solution, the convection and diffusion of gas phases, and the heat transfer in a porous medium. The proposed model is solved numerically based on a finite volume method and validated with experimental data. Based on the numerical results, the binder removal, the pressure buildup, the binder distribution, the mass transfers, and the removal mechanisms during thermal debinding are studied.     

Free Heat and Mass Transfer in a Porous Enclosure with Side Vents

Free heat and mass transfer during drying in a porous enclosure with free vents has been investigated numerically. Enclosed moist air interacts with the surrounding air through freely vented ports situated on both sides perpendicular to the heated wall. Air, heat, and moisture transport structures are visualized respectively by streamlines, heat lines, and mass lines. Effects of thermal Rayleigh number, Darcy number, vent location, and enclosure inclination on the convective heat/moisture transfer rate and volume flow rate across this enclosure are discussed. For each case, partially enclosed fluid flow undergoes different phases, increasing with buoyancy ratio; that is, heat transfer–driven flow, heat- and moisture-aided flow, and moisture transfer–dominated flow. Numerical results demonstrate that the convective heat and moisture transport patterns and transport rates greatly depend on thermal Rayleigh number, properties of porous medium, and enclosure inclination. Practices for enhancing heat and moisture transfer have been suggested for drying processes.     

采用泡沫复合相变材料的电子元件热控制单元数值仿真

将相变材料(PCM)应用于移动电子设备热控制单元(TCU)是一种极为理想的被动式热设计方案,但因PCM导热率低,TCU热性能较差,需强化PCM的导热以提高TCU的热性能。对填充泡沫复合相变材料(FCPCM)的TCU建立了二维数学模型,并对TCU的热性能进行了计算分析。其中,FCPCM的传热模型考虑了空穴的影响;PCM相变过程采用等效热容法求解。对填充FCPCM的TCU设计和加肋设计进行了比较分析,此外,还分析了泡沫孔隙率、热源功率以及泡沫骨架材料对TCU热性能的影响。结果表明,填充铝制FCPCM极大提高了TCU的热性能,可以很好地满足电子元件热控制设计要求。