Simulation of the Hygrothermal Behavior of a Building Envelope Based on Phase Change Materials and a Bio-Based Concrete

Phase Change Materials(PCMs)have high thermal inertia,and hemp concrete(HC),a bio-based concrete,has strong hygroscopic behavior.In previous studies,PCM has been extensively combined with many materials,however,most of these studies focused on thermal properties while neglecting hygroscopic aspects.In this study,the two materials have been combined into a building envelope and the related hygrothermal properties have been studied.In particular,numerical studies have been performed to investigate the temperature and relative humidity behavior inside the HC,and the effect of adding PCM on the hygrothermal behavior of the HC.The results show that there is a high coupling between temperature and relative humidity inside the HC,since the relative humidity changes on the second and third days are different,with values of 8%and 4%,respectively.Also,the variation of relative humidity with temperature indicates the dominant influence of temperature on relative humidity variation.With the presence of PCM,the temperature variation inside the HC is damped due to the high thermal inertia of the PCM,which also leads to suppression of moisture evaporation and thus damping of relative humidity variation.On the second and third days,the temperature changes at the central position are reduced by 4.6%and 5.1%,compared to the quarter position.For the relative humidity change,the reductions are 5.3%and 5.4%on the second and third days,respectively.Therefore,PCM,with high thermal inertia,acts as a temperature damper and has the potential to increase the moisture buffering capacity inside the HC.This makes it possible for such a combined envelope to have both thermal and hygric inertia.

Numerical Simulation Considering Non-Equilibrium Phase Change Volatile Oil Reservoir: A Case Study of Wenchang 8-3 Oil Reservoir

As a typical volatile oil reservoir, the actual production characteristics of Wenchang 8-3 reservoir are inconsistent with the results of traditional phase equilibrium experiments. The conventional isothermal instantaneous phase equilibrium theory cannot meet the production performance or numerical simulation analysis requirements of this type of reservoir. The thermodynamic properties of volatile oil reservoirs are like those of condensate gas reservoirs. As the formation pressure drops below the dew point pressure during the mining process, the balance between the liquid phase and the gas phase is not completed instantaneously. Based on the non-equilibrium phase recovery treatment method of the condensate gas reservoir, the phase behavior change curve of the A4h well of Wenchang 8-3 oil reservoir recovered from the saturation pressure to three different pressures is analyzed. The accuracy of the numerical simulation results with or without non-equilibrium phase transition is compared. The results show that the non-equilibrium phase change has a great impact on the production performance of volatile oil reservoirs;the numerical simulation results considering the non-equilibrium phase transition are in good agreement with the actual production performance of a single well and can better reflect the actual situation of this type of reservoir. Therefore, considering the effects of non-equilibrium phase transitions has important guiding significance for the dynamic analysis of volatile oil reservoirs, numerical simulation, and the formulation of development management strategies.

Melting Characteristics of a Phase Change Material Mixed with Nano Particles of Cobalt Oxide Bounded in a Trapezoidal Structure

A novel trapezoidal design for storage of heat energy through melting of phase-change material(PCM)is investigated.Latent heat thermal energy storage system(LHTES)is a promising option to diminish mis-match between energy consumption and supply.For this purpose,Paraffin:Rubitherm-35(RT35)material is successively melted in aluminum structure which is heated from one side and the other sides are kept adiabatic.Melting of PCM is observed experimentally and melt fronts are photographed for various time lengths.The fluid-solid module in COMSOL Multiphysics 5.4 has been utilized.The transient heat conduction with enthalpy function is hired.Simulations are carried out for enhancement of thermal conductivity through addition of nano-entities of cobalt oxide Co3O4.Themelting time is notably reduced with inclusion of nano-entities to enhance thermal conductivity.The time spans for melt start and total melt in case of pure PCM are 375 and 4500(s)respectively whereas for the nano mix case,these are 150 and 3000 s.Thus 33%shorter time length is noticed for charging of the PCM trapezoidal matrix with nano entities of Co3O4 aremixed.The results fromsimulation and lab observations depict similar patterns and are in quite close comparison.

Antarcticite, a Phase Change Material for ThermalEnergy Storage: Experiments and Simulation

Antarcticite, a mineral with composition CaCl2·6H2O and structure P321, is an ideal phase change material (PCM) due to the high energy storage density and good thermal conductivity. However, the existence of supercooling and incongruent melting would weaken its thermal properties and then hinder its application. In this paper, based on the cooling curve method and DSC measurement, we experimentally selected the minor SrCl2·6H2O as the nucleator and carboxyl methyl cellulose as the thickening agent, which could significantly reduce supercooling and partly restrain the incongruent melting. Moreover, we incorporated Antarcticite as PCM into building envelopes in four different cases, the simulation of the heat transfer processes showed that the temperature fluctuation could be reduced to about 2℃ in the best case.

Contact size scaling of a W-contact phase-change memory cell based on numerical simulation

In the design of phase-change memory（PCM）,it is important to perform numerical simulations to predict the performances of different device structures.This work presents a numerical simulation using a coupled system including Poisson＇s equation,the current continuity equation,the thermal conductivity equation,and phase-change dynamics to simulate the thermal and electric characteristics of phase-change memory.This method discriminates the common numerical simulation of PCM cells,from which it applies Possion＇s equation and current continuity equations instead of the Laplace equation to depict the electric characteristics of PCM cells,which is more adoptable for the semiconductor characteristics of phase-change materials.The results show that the simulation agrees with the measurement,and the scalability of PCM is predicted.

Thermal performance of a 3D printed lattice-structure heat sink packaging phase change material

Thermal storage technology is becoming more and more significant with the increase of high-power equipment in space applications.In this paper,3 D printing technology and Phase Change Material(PCM)were combined into a Thermal Energy Storage(TES)system,which could fulfill the requirements of light weight and high thermal conductivity.A 3 D-printed lattice-structure TES plate with N-tetradecane as the PCM and aluminum alloy as the thermal conductivity enhancer was manufactured,and experimentally tested in a thermal vacuum chamber.In addition,a simplified simulation model of the lattice cell was established to clearly analyze the heat transfer process of the TES plate.The effects of initial temperature distribution and heat load gradient on the thermal storage performances were investigated experimentally and theoretically.The equivalent thermal conductivity of the 3 D-printed lattice-structure TES plate turns out to be 13 times of the pure PCM thanks to the aluminum skeleton.The heat transfer enhancement appears at the end of the phase change stage due to the sudden mixture of the PCM with different temperature.The simulation results agree well with the experimental data.The equivalent thermal conductivity obtained by the phase change simulations are a little higher than those of the experiments,which is mainly caused by the initial uneven temperature distribution in the tests.Additionally,the effects of non-uniform heat load and the presence of the PCM in the TES plate are studied.This work successfully validates the feasibility and effectiveness of 3 D printing technology and TES technology for the temperature control in space applications.

Evaluation of Thermal Effect of PCM Wallboards by Coupling Simplified Phase Change Model with Design Tool

A simplified PCM wallboard model is coupled to an existing design-oriented model of multi-zone buildings. Using a reference model and a basic simulation configuration, the accuracy of the resulting PCM wallboard-building thermal design tool is evaluated. A new performance indicator, called PCM utilization factor, is then proposed in order to estimate the thermal efficiency of using PCM wallboards in buildings. Using this PCM Utilization factor and a degrees-hours indicator, the ability of the PCM wallboard-building thermal design tool to evaluate the effect of PCM wallboards on heating loads and summer thermal comfort in the early design phase of a project is examined in two real case studies: a family house project and an existing office building. The user-friendliness of this design tool, and the short calculation times it leads to when performing a year-long simulation using a standard office computer, make it a well-adapted tool for sensibility studies or multi-criterion optimization for buildings that contain PCM wallboards.

The Effects of Thickness and Location of PCMon the Building’s Passive Temperature-Control–A Numerical Study

Building energy consumption and building carbon emissions both account for more than 20%of their total national values in China.Building employing phase change material(PCM)for passive temperature control shows a promising prospect in meeting the comfort demand and reducing energy consumption simultaneously.However,there is a lack of more detailed research on the interaction between the location and thickness of PCM and indoor natural convection,as well as indoor temperature distribution.In this study,the numerical model of a passive temperature-controlled building integrating the developed PCM module is established with the help of ANSYS.In which,the actual weather condition of Beijing city is set as the boundary conditions and the indoor natural convection is simulated with the consideration of radiation model.The effects of PCM’s thickness and location on the internal temperature field are analyzed and discussed.The results show that the room could maintain within the human comfort temperature range with the longest ratio of 94.10%and the shortest ratio of 51.04%as integrating PCM.In comparison,the value is only 26.70%without PCM.The room’s maximum temperature fluctuation can also be improved;it could be lowered by 64.4%compared to the normal condition.When the quantity of PCM is sufficient,further increasing the PCM amount results in a temperature fluctuation reduction of less than 0.1°C and does not increase the comfort time.Placing PCM on the wall induces an apparent variation in indoor temperature along the vertical direction.Conversely,placing PCM on the roof can lead to a heat transfer rate difference of up to seven times.The optimal placement of PCM depends on the difference between the environmental and phase change temperatures.If the difference is positive,placing PCM on the roof is more effective;conversely,the opposite holds.According to the results over the entire cycle,PCM application on vertical walls yields better performance.The significant difference in natural convection caused by the same thickness of PCM but different application positions,coupled with the influence of air movement on the melting and solidification of PCM,further impacts indoor temperature fluctuations and comfort.This study can provide guidance for the application location and thickness of PCM,especially for scenarios where temperature regulation is required at a specific time.

Review on thermal performance of phase change energy storage building envelope

Improving the thermal performance of building envelope is an important way to save building energy consumption. The phase change energy storage building envelope is helpful to effective use of re-newable energy, reducing building operational energy consumption, increasing building thermal com-fort, and reducing environment pollution and greenhouse gas emission. This paper presents the con-cept of ideal energy-saving building envelope, which is used to guide the building envelope material selection and thermal performance design. This paper reviews some available researches on phase change building material and phase change energy storage building envelope. At last, this paper pre-sents some current problems needed further research.

A Numerical Study of the Irradiated Ignition of Obliquely Oriented Thermally-Deformable Polymer

This study numerically investigates the influence of molten matter dynamics on the gasification and subsequent ignitability of an inclined thermoplastic specimen subjected to localized irradiation heat flux normal to the surface. A thermoplastic material is modeled as a phase change material with predefined solidification and melting temperatures, respectively, and the gasification process is modeled by the Arrhenius law of molten matter. Gas phase kinetics is not considered for simplicity purposes;instead, the onset of ignition of polymer is estimated on the basis of the critical mass flux concept. According to the numerical results, as the inclination angle becomes steeper (toward the vertical angle), the estimated ignition delay becomes shorter, showing ignition is promoted, whereas it is turned to be difficult to occur when inclination angles are above the vertical angle (>90?) having a longer delay time for the onset of gasification. With careful observation, the thermal interaction between the hot molten matter and unmelted (cold) solid is found to play an important role in gasification. The formation of a bulge due to resolidification to suppress the dripping downstream could be the source to promote ignition. By contrast, the hot molten matter is enforced to detach from the unmelted solid and “freely fall-off” to prohibit ignition for inclination angles beyond 90?. This supports the notion that high-enthalpy caused by the external heating is simply lost because of dripping, and there is less chance of catching fire there.

Numerical Assessment on Fin Design Parameters Employed for Augmentation of Natural Convection and Fluid Flow in a Horizontal Latent Heat Thermal Energy Storage Unit

The present work focus on the thermal performance of a horizontal concentric heat exchanger, which is numerically investigated to evaluate the heat transfer enhancement process by adding fins with different configurations. As a part of this investigation, the melting process is simulated from the onset of phase change to the offset involving physics of natural convection in PCM fluid pool. The investigation is carried out by ANSYS Fluent code, which is an efficient numerical analysis tool for investigating fluid flow and convective heat transfer phenomena during PCM melting process. The attention is mainly focused on the extension of contact area between the PCM body and cylindrical capsule to enhance heat transfer rates to PCM bodies during the melting process by employing longitudinal fins in the enclosed capsule. Two commercial PCMs: RT50 and C58, are introduced in a 2D cylindrical pipe with their thermo-physical properties as input for modelling. The selected modelling approach is validated against experimental result with respect to the total enthalpy changes that qualify our model to run in the proceeding calculation. It is ensured that an isothermal boundary condition (373 K) is applied to the inner pipe throughout the series of simulation cases and the corresponding Rayleigh number (Ra) ranges from 104 - 105 and Prandtl number (Pr) 0.05 - 0.07. Finally, parametric study is carried out to evaluate the effect of length, thickness and number of longitudinal fins on the thermal performance of PCM-LHTES (Latent Heat Thermal Energy Storage) system associated with the physics of natural convection process during PCM melting.

A Modified Kelvin Model for Thermal Performance Simulation of High Mechanical Property Open-Cell Metal Foams

This paper proposes a modified Kelvin model for high mechanical property open-cell metal foams and investigates its application in thermal simulations. The thermal conductivity is simulated based on the steady state method and the results are consistent with experimental values. The melting process of phase change materials (PCMs) in Kelvin model and its modified model is numerically investigated under a temperature constant heat resource. By detecting the temperature variations, it shows that the metal foam greatly improves the heat transfer in energy storage systems. Besides, the comparison of the melting process in two foam models indicates that the systems based on high mechanical property metal foams have a shorter melting time. The melting process of paraffin in modified Kelvin metal foam models with three different porosities (65%, 70% and 75%) are numerically analyzed and compared.

石蜡与低熔点合金双级联相变材料强化板翅式散热器换热性能的数值模拟

相变散热技术可以通过相变材料的物态变化,有效吸收或释放热量,从而实现被动式热管理。本文建立了一种板翅式相变散热器的结构模型,研究级联相变技术的相变传热过程及优化传热性能的措施,重点分析了相变材料(PCM)的组合方式、翅片结构以及不同的PCM体积比工况对相变散热器传热性能的影响。研究结果表明,改变PCM的组合方式对相变材料传热过程有显著影响,对于采用双PCM的级联相变散热器,PCM1和PCM2呈相变温度递减排列具备更优的热管理性能;当翅片数为16时,具备更低的工作温度,并且相比于四翅片结构的相变散热单元,两种级联PCM组合的相变散热单元的凝固时间分别缩短了25.3%和22.5%。因此,当翅片数为16及翅片厚度为0.5 mm时,热管理性能较优;采用石蜡和低熔点合金材料,当PCM体积比为1∶2时,两个临界温度下的热管理时间分别延长了17.2%和15%,温升速率也随着低熔点合金所占体积增多而逐渐减小,综合考虑热管理时间和温升速率,可知PCM体积比为1∶2时级联相变散热器的热管理性能较优。

相变储能材料在建筑围护结构领域的应用及研究进展

相变材料(PCM)具有结构稳定性好、高储能密度、可控相变温度、较大的相变潜热以及出色的储热能力,被用作建筑物中有效的潜在储能元件,以改善由温室气体排放引起的严重环境问题,可有效地减缓燃料和电能消耗,同时在建筑围护结构中保持舒适的环境,最大限度地减少温度波动。简述相变材料的分类及其选取,相变材料的封装工艺,重点介绍了相变储能材料在围护结构领域的研究与应用,指出了影响相变围护结构效率的因素,对其未来的研究方向进行了展望。

不同温度条件下相变石蜡力学性能分析

对3种不同相变温度的石蜡材料(PCM-28、PCM-18和PCM-10)在不同温度下的基本力学性能参数进行了系统的压缩实验,计算出对应的弹性模量和泊松比。结果表明:PCM-18和PCM-10的抗压强度随温度的降低而有所提高,但PCM-28的抗压强度随温度的降低反而呈现出下降趋势。在单轴压缩条件下PCM-28和PCM-18的应力-应变曲线主要经历了压实阶段、弹性变形阶段、裂纹扩展阶段和裂纹后破坏阶段4个明显阶段,最终表现为脆性破坏。PCM-10在经历压实阶段和弹性变形阶段后进入塑性阶段,曲线发展近似水平,且整个过程无明显裂纹出现。

不同振动条件下水平冷表面上液滴冻结过程数值模拟

液滴冻结作为结霜初期霜层生长的初始开端,对后续霜层生长发挥着关键作用。为研究液滴在振动条件下的冻结规律,通过对水平冷表面添加正弦周期性振动条件进行数值模拟分析,并搭建试验平台进行验证。研究结果发现:振动条件改变了固液相界面形态,缩短了液滴完全冻结时间,其中改变振动频率和振动幅值均会加快液滴冻结;液滴冻结前期,受凝固放热和固相成长影响,测点温度变化呈现先慢后快再趋于平缓的趋势,测点温度平均变化速率与振动频率和振动幅值成正比;液滴冻结过程中,相界面波动程度主要受振动幅值和液滴大小影响,体积恒定时,振动幅值越大,相界面波动程度越明显,相同振动条件下,液滴体积越小,相界面波动程度越明显。

三套管式加肋相变蓄热单元的强化传热特性

相变蓄热技术在利用工业余热、太阳能等解决热能与用户之间的供需不平衡问题方面起着重要的作用,然而受相变材料自身物性的限制,蓄热熔化时仍存在蓄热速度低、熔化不均匀等问题。本文以三套管式相变蓄热单元为基本结构,利用FLUENT软件模拟研究了在考虑自然对流情况下添加纵向肋片的不同参数(肋片数量、长度、厚度、偏心距及肋片排布方式)对三套管式相变蓄热单元蓄热性能的影响。结果表明,三套管式蓄热单元内外双壁面共同加热,与普通套管式蓄热单元相比,换热面积增大,相变材料全部熔化需要的时间缩短;加装纵向直肋片的三套管蓄热单元的熔化速度进一步加快,相同数量肋片下,肋片的长度和厚度是影响蓄热单元蓄热量与平均蓄热速率的主要因素,肋片长度占相变材料区域径向长度的50%时,蓄热单元的熔化速度加快幅度明显且蓄热量与平均蓄热速率较高。通过优化设计,发现内管向下偏移距离e对相变材料的熔化过程作用显著,偏心距离较短时相变材料完全熔化所需时间最少,相比光滑管蓄热单元,相变材料的熔化速率提升了48.5%,偏移距离过长则会延长蓄热时间;通过加密蓄热单元底部外层肋片的排布,蓄热单元的熔化进程有不同程度的加快,总体蓄热速率较原肋片结构有所提高。

松铺覆盖下相变黏土施工防冻控温数值模拟

堤坝及路基等工程冬季负温下施工,土料会因短时冻结或冻融而引起压实性能降低,并诱发冻胀、融沉、开裂等病害。常规覆盖保温措施工艺复杂,难以实现负温下连续施工,易使工期延长。相变黏土为改善土料防冻性能、延长冬季施工时间提供了可能,但相变材料(PCM)掺量过高影响土料力学性能。为此,本文提出了土料掺混PCM与松铺覆盖联合的防冻控温措施,利用原有施工中上层土料松铺工序,尽可能减少PCM掺量,减小施工干扰,同时解决负温施工暴露时段土料的表层防冻问题。首先,建立了考虑上层土料松铺的施工相变传热有限元数值模型,并验证了模型的有效性;然后,定量分析了不同松铺厚度、PCM掺量对相变黏土冬季施工控温效果的影响,提出了相变黏土有效施工时长的估计方法;最后,给出了防冻控温措施建议,采用2%PCM掺量和9.6 cm松铺覆盖层厚度,即可满足现场施工控温的要求。本研究为解决寒区土料冬季施工过程中的防冻控温难题提供了新的技术途径。

不同压缩比螺旋翅片对相变储能罐蓄热的影响机制

在传统螺旋翅片基础上提出3种不同压缩比的螺旋翅片结构。首先数值模拟不同压缩比的螺旋翅片储能罐相变材料的熔化过程;其次,讨论4个储能罐中温度场和固液界面的演变情况;再次,分析液相分数、平均温度和平均努塞尔数的变化趋势;最后,对比不同压缩比下相变材料的总蓄热量、熔化时间和平均蓄热速率。结果表明:设计压缩比合适的螺旋翅片可有效促进储能装置相变材料的熔化,减轻下部温度垂直分层现象,增强相变材料的自然对流,提升相变储能罐的蓄热性能;随着压缩比的增加,蓄热速率呈先增加后减小的趋势,而完全熔化时间则先减少后增加;与无压缩螺旋翅片的储能罐相比,当压缩比为3时,石蜡完全熔化时间缩短27.27%,平均蓄热速率提升33.33%,但当压缩比增加到7时,平均蓄热速率反而下降25%,完全熔化时间增加34.67%。

基于TRNSYS的主动式建筑相变蓄冷空调系统模拟

以夏热冬冷地区的办公建筑为例,利用瞬态系统仿真程序(transient system simulation program, TRNSYS)构建主动式相变蓄能地板模块,建立了主动式相变蓄能地板空调系统和常规风机盘管加新风空调系统的仿真模型,对其节能性和经济性进行模拟计算.以典型日和供冷季晚间蓄冷运行工况为条件,研究冷冻水温对主动式相变蓄能地板的蓄释能特性、室温波动与热泵制冷系数的影响.结果表明:夜间相变蓄冷工况下,在满足每平方米冷负荷为74.78 W情况下,9℃为最佳供水水温;采取晚间低电价时段间歇运行蓄冷热泵的方案,能够有效提高热泵运行时的制冷系数,并降低运行费用;主动式相变蓄能地板空调系统供冷季的能耗相比于常规风机盘管加新风空调系统减少30.5%,运行费用减少44.24%,夏季制冷综合能效比达到了2.38.