Optimization Design of Thermal Conduction Enhanced PCM Plates for Simulating the Infrared Signature of Steel Plate

Phase change material(PCM) can be used to prepare the infrared false targets for realizing all-weather passive infrared decoy,but its low thermal conductivity is a great blockage to the simulation of the infrared signature of thick metal plates.For that reason,a method of simulating the infrared signature of thick steel plates by thermal conduction enhanced PCM,including the aluminum fins,is proposed.A physical and mathematic model is set up,and the infrared signature simulation of thick steel plate is investigated numerically.The effects of the distribution density and thickness of fins and the thickness of PCM plate on the simulation results are discussed,and the reasonable construction parameters of PCM plates used to simulate the steel plates of different thickness are obtained.

Thermal Conductance of Cu and Carbon Nanotube Interface Enhanced by a Graphene Layer

Thermal conduetances between Cu and graphene covered carbon nanotubes （gCNTs） are calculated by molecular dynamics simulations. The results show that the thermal conductance is about ten times larger than that of Cu- CNT interface. The enhanced thermal conductance is due to the larger contact area introduced by the graphene layer and the stronger thermal transfer ability of the Cu-gCNT interface. From the linear increasing thermal conductance with the increasing total contact area, an effective contact area of such an interface can be defined.

Thermal Property Enhancement of a Novel Shape-Stabilized Sodium Acetate Trihydrate-Acetamide/Expanded Graphite-Based Composite Phase Change Material

Phase change materials(PCMs)are a kind of highly efficient thermal storage materials which have a bright application prospect in many fields such as energy conservation in buildings,waste heat recovery,battery thermal management and so on.Especially inorganic hydrated salt PCMs have received increasing attention from researchers due to their advantages of being inexpensive and non-flammable.However,inorganic hydrated salt PCMs are still limited by the aspects of inappropriate phase change temperature,liquid phase leakage,large supercooling and severe phase separation in the application process.In this work,sodium acetate trihydrate was selected as the basic inorganic PCM,and a novel shape-stabilized composite phase change material(CPCM)with good thermal properties was prepared by adding various functional additives.At first,the sodium acetate trihydrate-acetamide binary mixture was prepared and the melting point was adjusted using acetamide.Then the binary mixture was incorporated into expanded graphite to synthesize a novel shape-stabilized CPCM.The thermophysical properties of the resultant shape-stabilized CPCM were systematically investigated.The microscopic morphology and chemical structure of the obtained shape-stabilized CPCM were characterized and analyzed.The experiment results pointed out that acetamide could effectively lower the melting point of sodium acetate trihydrate.The obtained shape-stabilized CPCM modified with additional 18%(mass fraction)acetamide and 12%(mass fraction)expanded graphite exhibited good shape stability and thermophysical characteristics:a low supercooling degree of 1.75℃and an appropriate melting temperature of 40.77℃were obtained;the latent heat of 151.64 kJ/kg and thermal conductivity of 1.411 W/(m·K)were also satisfactory.Moreover,after 50accelerated melting-freezing cycles,the obtained shape-stabilized CPCM represented good thermal reliability.

Characterization of Thermo-Physical Properties of Cement-Based Blocks of Varied Sand Types Using Cost-Effective Enhancement Approach

The dominant property of building envelope fabric which contributes significantly to minimize electricity utilization in building is the thermo-physical properties. There is inadequate literature on representative practical data of thermo-physical properties of the dominant building envelope components in Ghana. This study aims to use cost-effective approach to characterize the thermo-physical properties of only cement-based mortar and concrete blocks used in Ghana for building components specifically wall design. Mixed methods research design was employed to achieving the aim. A questionnaire survey was used among sampled building fabric components manufacturers to pick representative data on thermos-physical properties of their mortar and concrete blocks. Also, an experimental procedure employing a transient technique with a TCi Thermal Analyser was used to determine the thermo-physical properties of selected mortar and concrete blocks from Ghana in addition to designed parametric mortar and concrete blocks with varied ratios obtained from the survey were undertaken at University of Nottingham. From the study, a trend of decreasing thermal conductivity and thermal effusivity with corresponding decreasing sand content was observed with all the different sand types. The thermal conductivities of both mortar and concrete parametric blocks meet the range of expected standard values outlined in Chattered Institute of Building Services Engineers (CIBSE) Guide A. The major limitation of the work is the dimension of the sample size;which is not inconsistent with standard block size due to the experimental setup used. It is expected that, the characterization of the predominant cement-based building fabrics components will contribute to improved building performance analysis with significant savings in electricity utilization for space cooling.

Effect of temperature-dependent rock thermal conductivity and specific heat capacity on heat recovery in an enhanced geothermal system

The modeling of heat recovery from an enhanced geothermal system(EGS)requires rock thermal parameters as inputs such as thermal conductivity and specific heat capacity.These parameters may encounter significant variations due to the reduction of rock temperature during heat recovery.In the present study,we investigate the effect of temperature-dependent thermal conductivity and specific heat capacity on the thermal performance of EGS reservoirs.Equations describing the relationships between thermal conductivity/specific heat capacity and temperature from previous experimental studies were incorporated in a field-scale single-fracture EGS model.The modeling results indicate that the increase of thermal conductivity caused by temperature reduction accelerates thermal conduction from rock formations to fracture fluid,and thus improves thermal performance.The decrease of specific heat capacity due to temperature reduction,on the contrary,impairs the thermal performance but the impact is smaller than that of the increase of thermal conductivity.Due to the opposite effects of thermal conductivity increase and specific heat capacity decrease,the overall effect of temperature-dependent thermal parameters is relatively small.Assuming constant thermal parameters measured at room temperature appears to be able to provide acceptable predictions of EGS thermal performance.

Wet-resilient graphene aerogel for thermal conductivity enhancement in polymer nanocomposites

Three-dimensional(3 D)graphene-based aerogels have significant potential for adsorption,sensors,and thermal management applications.However,their practical applications are limited by their disorganized structure and ultra-low resilience after compression.Some methods can realize a well-aligned structure,however,they involve high costs and complex technology.Herein,a 3 D graphene hybrid aerogel with an anisotropic open-cell and well-oriented structure is realized by unidirectional freeze casting,which combines the‘soft’(e.g.graphene oxide,Tween-80)and‘hard’(e.g.graphene assembly)components to realize full recovery after flattening.A graphene aerogel annealed at a moderate temperature(200℃)can possess superhydrophilicity and outstanding wet-resilience properties,including after being pressed under40 MPa.Furthermore,the graphene aerogel annealed at a high temperature of 1500℃exhibits excellent thermal conductivity enhancement efficiency in polydimethylsiloxane(PDMS).The resultant nanocomposites clearly demonstrate anisotropic thermal conductivity and promising applications as thermal interface materials.This strategy offers new insights into the design and fabrication of 3 D multifunctional graphene aerogels.

芒硝基相变储能材料的研究进展

芒硝(Na_(2)SO_(4)·10H_(2)O)作为一种原料易得、储能密度大的无机水合盐类相变储能材料(Phase Change Materials,PCMs),拥有相变温度适中、储能高等优点,然而,过冷度大、相分层严重、易泄露和导热率低等问题限制了芒硝基PCMs的大规模应用。通过总结关于过冷和相分离问题的改性方法,归纳比较了各类封装的原理及效果,并概述了通过导热强化来提高芒硝基PCMs储放热效率的研究进展。最后,对该材料的实际应用现状进行阐述,并展望了其发展方向,为开发高稳定性、高效封装、高导热率芒硝基PCMs提供参考依据。

The regulation mechanism and heat transfer enhancement of composite mixed paraffin and copper foam phase change materials

Phase change materials(PCMs)have remarkable energy storage capacity and promising applications in the field of thermal control of electronic products.The problem of thermal property improvement and heat transfer of PCMs in metal-foam heatsinks is an important task for thermal management of electronic components.Mixed paraffin samples were prepared by mixing appropriate proportions of paraffin(mass)at various temperatures.Differential scanning calorimetry analysis revealed that the maximum enthalpy of 206.3 J/g is obtained by mixing 20%of 17°C liquid paraffin and 80%of 29℃ solid paraffin.Heating and cooling cycling tests revealed that mixed paraffin exhibits excellent thermal stability and that the regulation method marginally affects thermal stability.Moreover,composites were prepared by embedding PCM into a copper foam by melt impregnation.The thermal conductivity of the composites increased to 4.35 W/(m K),corresponding to 20 times its original value.In addition,density functional theory and experimental results were in good agreement,indicating that the regulation method is practical and effective.

MA/SBR/CNT定型复合相变材料的制备及其性能研究

以肉豆蔻酸(MA)为储能基元材料、丁苯橡胶(SBR)为储能载体、碳纳米管(CNT)为导热增强材料,通过熔融共混法制备了MA/SBR/CNT低温复合定型相变材料。采用SEM、FT-IR、XRD、DSC等测试表征手段对复合相变材料的微观结构、物化稳定性和热性能进行了分析。结果表明,SBR能很好地包裹MA,维持复合材料结构稳定且保证液相材料相变过程中不发生泄漏。CNT的加入可以有效地提高复合相变材料的热导率,当添加质量分数为7%的CNT时,复合相变材料热导率可达到0.4 W/(m·K),其导热性能大大提高;在该添加比例下,复合材料熔化、凝固相变温度分别为53.03℃、51.49℃,相变焓变分别为156.53 kJ/kg、152.35 kJ/kg。

Research on the Heat-Conduction Enhancement for Liquid with Nano-Particle Suspensions

By measuring the effective thermal conductivity, taking photos of the distribution of the nano-particles in the fluids and photos that reflect the interfacial phenomena between nano-particles and fluids, we try to explain the possible mechanism for heat conductive enhancement.

Numerical Study on Heat Transfer Characteristics of Heated/Cooled Rods Having a Composite Board in between: Effect of Thermal Vias

By placing a sample between a heated and a cooled rod, a thermal conductivity of the sample can be evaluated easily with the assumption of a one-dimensional heat flow. However, a three-dimensional constriction/spreading heat flow may occur inside the rods when the sample is a composite having different thermal conductivities. In order to investigate the thermal resistance due to the constriction/spreading heat flow, the three-dimensional numerical analyses were conducted on the heat transfer characteristics of the rods. In the present analyses, a polymer-based composite board having thermal vias was sandwiched between the rods. From the numerical results, it was confirmed that the constriction/spreading resistance of the rods was strongly affected by the thermal conductivity of the rods as well as the number and size of the thermal vias. A simple equation was also proposed to evaluate the constriction/spreading resistance of the rods. Fairly good agreements were obtained between the numerical results and the calculated ones by the simple equation. Moreover, the discussion was also made on an effective thermal conductivity of the composite board evaluated with the heated and the cooled rod.

双螺旋盘管内膨胀石墨对相变材料的传热强化机制与蓄放热性能

本工作提出了一种双螺旋盘管与膨胀石墨强化传热的相变蓄热器结构,通过建立双螺旋管相变蓄热器模型,模拟了双螺旋管的螺距、材料热物性对蓄放热性能的影响规律。探讨了限制空间下相变材料自然对流与材料导热增强在传热强化过程中的控制作用。结果表明,在加工螺距限制条件下,减小螺距及增加螺旋数可以提升蓄热器放热性能。膨胀石墨可以提升相变材料热导率,同时会导致材料黏度增大,因此添加膨胀石墨能增强蓄热器的导热换热,但抑制自然对流换热。通过对比分析发现,在限制空间下材料的导热增强带来的传热强化程度能够弥补由于材料自然对流不足导致的传热损失。因此,本工作揭示了相变蓄热器传热强化的控制步骤在于材料热导率强化。基于实验和仿真结合的方式,本工作设计了一款用于加热生活热水的蓄热器结构,验证了所设计的蓄热器在宽进口温度、进口流量等操作工况下具有优异的放热性能。该蓄热器与电加热器相结合,能够在电热功率小于2200 W的条件下实现热水加热功率超3500 W,突破了家用电热设备的功率限制。

SiC对能源桩混凝土传热与力学性能的影响

能源桩系统作为一种建筑节能技术,通过开发清洁能源可有效缓解能源危机。以添加SiC粉末制备的传热强化型混凝土为研究对象,综合能源桩强化换热模型分析、室内传热与力学性能试验和中心热源热成像结果可知,在相同环境温度下,随着SiC含量的增加,SiC混凝土导热系数呈线性增加。在力学性能方面,当SiC质量分数达到10%时,混凝土的抗压强度和抗拉强度较素混凝土分别增加6.3%和8.3%,当SiC质量分数达到15%时,混凝土的抗压强度增长速率有所放缓,抗拉强度有所下降。中心热源热成像结果也进一步验证SiC试样自内向外的传热效率显著高于普通混凝土,与SiC含量为0%的试样相比,SiC含量为10%的试样的中心温度降低39.2%。由此可见,当SiC粉末添加量达到10%时,强化型混凝土的导热效率显著增加且抗压强度提升最为明显,这为提高能源桩的热效益提供一种可靠的参考方法。

相变储能材料开发与封装技术研究进展

相变储能技术具有储热密度大、温度恒定等优点,大规模商业化潜力巨大。相变材料的开发对相变储能技术的应用至关重要,为解决相变材料传热性能差、易泄露等问题,围绕相变材料的传热和储热强化、封装开展了广泛研究。对比了热储能技术的特点,概述了潜热储能相变材料的分类与性质;并从换热面积增大以及热导率、熔化潜热和比热容提高等方面,讨论传热和储热强化研究进展;论述了相变材料的封装,对今后相变储能的发展方向进行了展望。

3D打印技术在高导热复合材料中的应用研究

分析了3D打印技术在制备导热复合材料上的发展过程,介绍了不同种类的3D打印技术制备的高导热复合材料,包括碳纤维型、石墨烯型、碳纳米管型、氮化硼型、氮化铝型等,概述了不同类型的成型过程并归纳和总结了其导热性能。最后,对3D打印技术制备导热复合材料进行了总结与展望。

A Numerical Study on the Heat Transfer Enhancement Behavior of Water-Microparticles Suspension

The heat transfer enhancement characteristics of water with polystyrene particles are examined in the present numerical study. The numerical study is conducted in the hydrodynamically fully developed turbulent flows within a circular duct with the wall boundary condition of a constant heat flux. The thermal conductivity of the turbulent flow obtained by the Reynolds analogy is 1000 times as much as the thermal conductivity of water. On the contrary, the enhancement of thermal conductivity caused by water-microparticles suspension is relatively low. Slight enhancements of the local Nusselt number are obtained in the numerical calculations of Newtonian turbulent flows with the micro-convection effects, thus showing large deviations from the experimental data. The numerical results in non-Newtonian flows are in agreement with the experimental data. Thus, the main cause for the enhancement of the heat transfer of the suspension might be not due to the micro-convection effects but to the non-Newtonian

高效节能低过冷高导热相变蓄冷材料

传统冰蓄冷的蓄冷系统其制冷温度通常需要达到-10℃左右,若使用相变温度高于冰的材料作为相变蓄冷材料,可提升制冷系统的蒸发温度,从而提升蓄冷系统整体能效。四丁基溴化铵(TBAB)水溶液是一种可替代冰的相变蓄冷材料,相变温度分布在5~15℃范围内,可与双工况及单工况空气源热泵机组联用,与常规冰蓄冷相比可大大提高蓄冷整体能效。但TBAB水溶液相变过程具有过冷度大,导热性较低等问题。将提出一种基于有机-无机纳米掺杂工艺,采用分散改性导热鳞片石墨与花状纳米氧化铝作为改性材料的低过冷高导热相变蓄冷材料制备方法。蓄冷材料可与空气源热泵联用,相变温度区间为6~12℃,相变焓为157.3 J/g,过冷度不大于2℃,导热率与未改性前相比提升70%以上。

Comparisons of Heat Transfer Enhancement of an Internal Blade Tip with Metal or Insulating Pins

Cooling methods are needed for turbine blade tips to ensure a long durability and safe operation.A common way to cool a tip is to use serpentine passages with 180-deg turn under the blade tip-cap taking advantage of the threedimensional turning effect and impingement like flow.Improved internal convective cooling is therefore required to increase the blade tip lifetime.In the present study,augmented heat transfer of an internal blade tip with pin-fin arrays has been investigated numerically using a conjugate heat transfer method.The computational domain includes the fluid region and the solid pins as well as the tip regions.Turbulent convective heat transfer between the fluid and pins,and heat conduction within pins and tip are simultaneously computed.The main objective of the present study is to observe the effect of the pin material on heat transfer enhancement of the pin-finned tips.It is found that due to the combination of turning,impingement and pin-fin crossflow,the heat transfer coefficient of a pin-finned tip is a factor of 2.9 higher than that of a smooth tip at the cost of an increased pressure drop by less than 10%.The usage of metal pins can reduce the tip temperature effectively and thereby remove the heat load from the tip.Also,it is found that the tip heat transfer is enhanced even by using insulating pins having low thermal conductivity at low Reynolds numbers.The comparisons of overall performances are also included.

煤基新材料——煤基石墨烯的制备及石墨烯在导热领域应用研究进展

以石墨烯等为代表的具有优良物性和高附加值的新型材料得到越来越广泛的研究和应用,煤作为一种低价的高碳资源可通过不同的物理或化学方法制备成石墨烯及其化合物,使煤炭达到清洁高效利用,形成高附加值的煤基新材料。本文首先对石墨烯的制备方法进行综述进而引出煤基石墨烯的制备方法。煤基石墨烯是以煤炭为原料转化而成的石墨烯,通过高温热处理后,采用常规石墨烯的制备方法获取煤基石墨烯。随后对石墨烯具有超高的导热性能机理进行分析。随着电子工业的快速发展,越来越多的高性能电子产品面临越来越大的散热困境。石墨烯的导热系数是目前已知材料中最高的,远超常规金属。利用石墨烯具有超高的横向导热机理,其单质或化合物可作为电子元器件理想的散热材料,逐渐取代传统金属导热材料,应用前景广阔。再分别对少数层石墨烯、石墨烯的复合材料、氧化石墨烯的复合材料在导热领域方面的研究进展及性能表现进行综述。其中石墨烯的复合材料的导热能力更强,应用范围更广。借助外力使石墨烯纳米片取向与传热方向对齐,可极大提高石墨烯复合材料导热能力。为进一步扩大石墨烯的应用范围、增强石墨烯平面外导热能力,可将石墨烯与碳纳米管结合。最后对煤基石墨烯的制备及石墨烯在导热领域的研究方向及发展前景进行了展望。

碳纤维/石蜡/膨胀石墨复合相变材料的制备及强化传热研究

以石蜡为相变材料,膨胀石墨为载体,碳纤维为强化传热介质,利用膨胀石墨对石蜡良好的吸附性及碳纤维高的导热性,制备了碳纤维/石蜡/膨胀石墨复合相变材料。采用扫描电镜、差示扫描量热仪、温度巡检仪对制备的碳纤维/石蜡/膨胀石墨复合相变材料热性能进行了测试和表征。实验结果表明,添加了碳纤维的石蜡/膨胀石墨复合相变材料的相变潜热随石蜡含量的降低而减小,随碳纤维含量的增加复合相变材料的相变温度略有降低,相变提前发生,随碳纤维含量的增加复合相变材料的导热性能大幅度提高。