RT6/纳米铝相变蓄冷材料的热物性研究

石蜡类相变材料RT6相变潜热高,相变温度低,化学性质稳定,但其具有导热系数低,蓄能时间长的缺点,固采用纳米粒子强化换热。通过粒度观测法选取分散稳定性较好的纳米Al粒子制备纳米相变材料,实验分析强化后的相变过程、纳米流体石蜡/纳米铝的导热性和相变特性。实验结果表明,复合材料稳定性好,相变温度基本不变为4～8℃,相变焓值略有下降,但导热系数明显提高,可以作为蓄冷系统的新型蓄冷材料广泛应用。     

Hydrophilic expanded graphite–magnesium nitrate hexahydrate composite phase change materials: Understanding the effect of hydrophilic modification on thermophysical properties

Expanded graphite (EG) has shown excellent performances in compression resilience, thermal conductivity, and adsorption ability. EG can adsorb liquid phase change materials (PCMs) mainly because of capillary action; however, EG is hydrophobic, which makes it less compatible with hydrated salts. Herein, hydrophilic EG (HEG) was prepared with Triton X‐100 (TX‐100) as surface modifier. The HEG–magnesium nitrate hexahydrate (HEG‐MNH) composite as a PCM was investigated for thermal energy storage (TES) to understand the effect of hydrophilic modification on thermophysical properties. The powder‐state HEG is added into MNH to prepare HEG‐MNH composite PCM, which contains 1.71 wt% of TX‐100, 7.29 wt% of EG, and 91.00 wt% of MNH by control variable method. The melting point and latent heat of HEG‐MNH composite PCM were 89.05°C and 137.28 J/g, respectively. The endothermic enthalpy change of HEG‐MNH composite PCM only decreased by 0.90%, along with the exothermic values of HEG‐MNH composite that increased by 3.80% after 100 cycles. The thermal conductivity is higher 5.17 times than that of the pure MNH. Our work suggests that the HEG‐MNH composite PCM has a great potential to be used as a PCM for TES.     

Effects of variable thermophysical properties on mixed convection slip flow over a wedge

This paper reveals the characteristics of mixed convection slip flow of an electrically conducting fluid over a wedge subject to temperature dependent viscosity and thermal conductivity variations. The system of dimensionless nonsimilar governing equations has been solved by an implicit finite difference method. We also use stream‐function formulation to reduce the governing equations into a convenient form, which are valid for small and large time regimes. These are solved employing the perturbation method for small time and the asymptotic method for large time. Numerical solutions yield a good agreement with the series solutions. Because of the increase in the mixed convection parameter, the peak of the velocity profile increases whereas the maximum temperature decreases. In contrast, the local skin‐friction coefficient and local Nusselt number are found to increase with the mixed convection parameter. For higher values of the velocity slip and temperature jump conditions, the local skin‐friction coefficient and the local Nusselt number are found to increase. The viscosity parameter enhances the local skin friction and the local Nusselt number. But the converse characteristic is observed for the thermal conductivity parameter. The results could be used in microelectromechanical systems, fabrication, melting of polymers, polishing of artificial heart valves, etc.     

地源热泵岩土热物性测试简介与分析

地源热泵系统作为利用可再生能源的暖通空调技术,具有节能、环保等优点,在世界范围内被广泛使用。土壤作为地源热泵系统的冷热源,对整个系统有着至关重要的影响。不同建筑负荷特性要求系统对土壤的取放热量不同,二者的不平衡会使土壤的温度发生变化,影响整个系统的运行。对特定建筑地源热泵系统土壤的热物性测试是设计地埋管系统的重要依据。本文对热物性测试的理论依据进行了简单介绍,并对具体事例进行了分析计算,得出岩土体的导热系数等具体热物性参数,为地源热泵系统的精确设计提供了依据。     

Thermal properties of a low-melting-point nitrate molten salt system

利用硝酸盐熔盐熔点低、比热容高、热分解温度高的特性,制备一种新型硝酸盐熔盐[在Ca（NO₃）₂：KNO₃为0.47：0.53的熔盐体系中添加0.1%~15%的新型添加剂],并对该熔盐的热力学特性及使用成本进行了测试分析。结果表明,新型熔盐的熔点为120.1℃,熔化潜热为76.37 J/g,分解温度为588℃,平均比热容为1.598 J/（g·K）,相比较于传统的Solar salt和Hitec熔盐热力学性能具有较大提升。采用测量范围内比热容的积分平均值来代替整个温度范围内熔盐的比热容,通过计算得出该体系熔盐的显热蓄热成本为108元/（kW·h）。此外,对新型熔盐的热扩散系数以及导热系数进行的测试分析,进一步证明了该熔盐在太阳能光热发电中作为蓄热传热介质具有巨大的潜在应用价值。     

Effect of thermophysical properties models on the predicting of the convective heat transfer coefficient for low concentration nanofluid

The term of nanofluid refers to a solid–liquid mixture with a continuous phase which is a nanometer sized nanoparticle dispersed in conventional base fluids. In order to study the heat transfer behavior of the nanofluids, precise values of thermal and physical properties such as specific heat, viscosity and thermal conductivity of the nanofluids are required. There are a few well-known correlations for predicting the thermal and physical properties of nanofluids which are often cited by researchers to calculate the convective heat transfer behaviors of the nanofluids. Each researcher has used different models of the thermophysical properties in their works. This article aims to summarize the various models for predicting the thermophysical properties of nanofluids which have been commonly cited by a number of researchers and use them to calculate the experimental convective heat transfer coefficient of the nanofluid flowing in a double-tube counter flow heat exchanger. The effects of these models on the predicted value of the convective heat transfer of nanofluid with low nanoparticle concentration are discussed in detail.     

The influences of thermophysical properties of porous media on superadiabatic combustion with reciprocating flow

The influences of thermophysical properties of porous media on superadiabatic combustion with reciprocating flow is numerically studied in order to improve the understanding of the complex heat transfer and optimum design of the combustor. The heat transfer performance of a porous media combustor strongly depends on the thermophysical properties of the porous material. In order to explore how the material properties influence reciprocating superadiabatic combustion of premixed gases in porous media (short for RSCP), a two‐dimensional mathematical model of a simplified RSCP combustor is developed based on the hypothesis of local thermal non‐equilibrium between the solid and the gas phases by solving separate energy equations for these two phases. The porous media is assumed to emit, absorb, and isotropically scatter radiation. The finite‐volume method is used for computing radiation heat transfer processes. The flow and temperature fields are calculated by solving the mass, moment, gas and solid energy, and species conservation equations with a finite difference/control volume approach. Since the mass fraction conservation equations are stiff, an operator splitting method is used to solve them. The results show that the volumetric convective heat transfer coefficient and extinction coefficient of the porous media obviously affect the temperature distributions of the combustion chamber and burning speed of the gases, but thermal conductivity does not have an obvious effect. It indicates that convective heat transfer and heat radiation are the dominating ways of heat transfer, while heat conduction is a little less important. The specific heat of the porous media also has a remarkable impact on temperature distribution of gases and heat release rate. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(5): 336–350, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20120     

水平多管式换热器内部相变石蜡储热特性实验研究

设计并搭建了水平多管式相变储热系统,以水为传热流体(HTF)、石蜡为相变材料(PCM),通过实验对储热系统的具体蓄热特性和不同操作条件下HTF与PCM之间的传热特性进行定量分析,评估了HTF体积流量和进口温度对紧凑型低温相变储热系统功率输入、吸热完成时间以及储存能量的影响。该系统主要由一个聚碳酸酯壳和水平定向的多管换热器以及石蜡组成,其中石蜡相变温度约为41℃。结果表明：随着HTF进口温度或体积流量的增加,吸热完成时间减少,平均吸热功率增大,且增加速率都随着进口温度的增大而变小;HTF体积流量分别为4.5,6.0和7.5 L/min时,吸热过程耗时300.7,252.9和226.7 min;在58,64和70℃的进口温度下,吸热完成时间分别为270.1,226.7和204.9 min;提高HTF进口温度,会导致换热结束时石蜡温度与HTF出口温度出现越来越靠近的趋势,而在提高HTF体积流量时,却呈现相反的趋势。     

Binary eutectics of acetamide with inorganic nitrates: thermophysical properties relevant for heat storage

Binary mixtures of acetamide with the following nitrate salts were studied: potassium nitrate, sodium nitrate, ammonium nitrate and calcium nitrate Eutectic compositions, melting points and enthalpies of melting were defined from DSC thermograms. It was shown that the eutectics may exist in two crystalline forms, differing in melting point and enthalpy of fusion. Specific heat as a function of temperature was also measured by DSC. Latent heat storage density in these eutectics of 160–270 MJ/m³, strongly suggests their consideration as phase change materials (PCMs) in solar heat storage systems.     

Evaluation of the underground soil thermal storage properties in Libya

Experimental investigation was conducted of temperature distribution through the underground soil of Tripoli (Capital of Libya). The aim of the experiment is to monitor the temperature variation of the underground soil under a depth of 4 m and around the year, in order to know the thermal capacity ability of the soil to be used as a seasonal thermal storage. The measurements covered two types of systems: the first one is dry soil and the second is dry soil covered by a glass sheet. The measurements indicate that, at a depth of 4 m, the average temperatures for the dry and dry-glass covered systems are 21, 46 °C, with maximum temperatures of 21.5 and 47 °C during December and January, and the minimum temperatures occurred in May and June, are reached values of 19, 44 °C, respectively. The temperatures for the two systems were almost constant through the year and fluctuating with a monthly period of 2π/12. Results show that, the underground thermal capacity can be used as a source of heating and cooling of buildings leading to reduce the energy consumption in this application. Furthermore, for industrial and domestic heating processes, one can utilize the dry-glass covered system to cover a significant part of the heating load. Anyhow, the experimental study may not applicable everywhere, so an analytical presentation for the system will be necessary to save money and efforts. The first step to put the analytical model in reality is to get the thermal properties of the underground soil, and this is the aim of the present study.The paper described the followed procedure during theoretical-heat transfer approach. The thermal properties were presented as a function of the ground depth, furthermore, the paper presented the measured temperatures of the two systems for Tripoli underground soil.     

Investigations of the thermophysical properties of monocrystalline tungsten

Monocrystalline tungsten and its alloys are of interest in the design and manufacture of high temperature facilities operating at high (up to 2500 °C) temperatures in vacuum under mechanical loads. This paper demonstrates advantages of using monocrystalline tungsten as a structural material in high-tech industry in comparison with polycrystalline materials. The results of comparative studies of the thermophysical properties of polycrystalline and monocrystalline tungsten 4% tantalum alloy are presented. It is shown that monocrystalline tungsten based materials are more suitable for use under high temperatures and high mechanical stresses than polycrystalline. Use of monocrystalline tungsten as a structural material makes it possible to prolong the service life of modern technical facilities.     

二甲醚热物理特性的比较研究

当二甲醚(DME)运用于柴油机时,准确了解二甲醚的热物理特性,将有助于对二甲醚的喷射和燃烧过程开展数值模拟.对二甲醚的饱和蒸汽压、液体粘度、弹性模量、汽化潜热、表面张力、热传导和扩散系数等热物理参数与温度或压力的关系式进行了比较和分析,可为数值模拟提供精确的参考依据.     

Characteristics of medium deep borehole thermal energy storage

Seasonal energy storage is an important component to cope with the challenges resulting from fluctuating renewable energy sources and the corresponding mismatch of energy demand and supply. The storage of heat via medium deep borehole heat exchangers is a new approach in the field of Borehole Thermal Energy Storage. In contrast to conventional borehole storages, fewer, but deeper borehole heat exchangers tap into the subsurface, which serves as the storage medium. As a result, the thermal impact on shallow aquifers is strongly reduced mitigating negative effects on the drinking water quality. Furthermore, less surface area is required. However, there are no operational experiences, as the concept has not been put into practice so far. In this study, more than 250 different numerical storage models are compared. The influence of the characteristic design parameters on the storage system's behaviour and performance is analysed by variation of parameters like borefield layout, fluid inlet temperatures and properties of the reservoir rocks. The results indicate that especially larger systems have a high potential for efficient seasonal heat storage. Several GWh of thermal energy can be stored during summertime and extracted during the heating period with a high recovery rate of up to 83%. Medium deep borehole heat exchanger arrays are suitable thermal storages for fluctuating renewable energy sources and waste heat from industrial processes. Copyright © 2016 John Wiley & Sons, Ltd.     

两种深层岩土热物性测试方法的比较

通过对现场热物性测试和现场冷热量测试两种测试方法的对比,说明了各自在深层岩土热物性测试的优缺点。结合实际工程,对两种方法在测试中出现的问题进行分析,指出了现场热物性测试是一种适用范围更广的测试方法。     

三元混合碳酸熔盐热物性实验研究

为满足超临界二氧化碳太阳能光热发电系统高温传热储热的要求,对3种不同比例的高温混合碳酸熔盐的热物性进行研究.实验结果表明,1号、2号、3号混合碳酸熔盐的熔点分别为388.7、337.9、391.3℃;初晶点分别为454.9、639.7、535.4℃;1号、3号混合碳酸熔盐的分解温度分别为916.2、920.4℃.通过分...     

Comparative study on adsorbent characteristics for adsorption thermal energy storage system

The adsorption performance of the thermal energy storage (TES) system changes depending on the material properties of the adsorbent itself, but the change of the hardware structure can also substantially change the adsorption characteristics. In this study, a laboratory‐scale adsorption‐based TES system was constructed, and the adsorption performance of three adsorbents was evaluated in the same system to compare the adsorption performance between adsorbents. The adsorption characteristics of silica gel, zeolite 13X, and 4A, which are the most preferred adsorbents in the physical adsorption‐based TES system, were selected for evaluation. Experiments with each adsorbent were performed, including heat recovery to evaluate the heat transfer effect and the amount of heat recoverable in the actual TES system. Experimental results have identified several key characteristics of the adsorption and performance of each adsorbent in the TES system, as well as operating parameters that determine the influence of adsorption performance on the TES system. The actual energy storage density of the adsorbent is affected not only by the enthalpy of adsorption of the material itself but also by other factors. These factors include the difference in thermal conductivity that causes a difference in temperature distribution and the magnitude of mass transfer resistance due to the shape of the adsorbent particle and the actual TES system reactor structure. If the reaction heat generated during the adsorption reaction cannot be effectively released, the adsorption performance is significantly lowered due to the increased temperature of the reactor inside. This phenomenon was commonly observed in adsorbents examined in the present study. The uptake amount, X [g/g], was increased by allowing the inside of the reactor to be maintained at a lower temperature through heat recovery. In case of silica gel, the temperature rise during adsorption reaction is not high due to the difference of isotherm characteristics compared with zeolites, but it is possible to absorb more amount of adsorbate and to recover heat for a longer time. The energy storage density is affected by the temperature increase effect and the uptake amount of adsorbate during the adsorption reaction. The experimental results show that the energy storage density of zeolite 13X is 15% and 28.7% higher than that of silica gel and 4A, respectively, and the temperature rise due to heat generation during adsorption reaction is also high, which is advantageous in adsorption TES system performance.     

流体热物性测试用全自动高精度恒温槽的研制

为解决高精度流体热物性测试中的恒温环境问题 ,研制了一套高精度的恒温槽。恒温槽的温度控制系统采用准二维模糊控制器和 PID控制相结合的方法。同时 ,利用 C ++语言 ,在 Windows环境中实现了恒温槽温度控制和测量的全自动化。实验结果表明 :恒温槽的可控温度为室温至 2 5 0℃ ,波动度优于± 3 m K/1 5min,达到国外同类产品的水平 ,完全能够满足高精度流体热物性测试的需要     

Evidence of the illegitimacy of the additive approach to the determination of the thermophysical properties of coal-water fuel with glycerol

For the fuel ignition, the thermal conductivity and heat capacity are the key properties that determine the pre-ignition behavior of the drop of the fuel. The classic monophase fuels, such as natural gas, liquid propellants, or solid one-component fuels, have been investigated for a long time; and their thermophysical properties are well known in most of the cases. Composite fuels, which have recently attracted the attention of the researchers, have complex contents. In many cases, composite fuel is a mixture of solid and liquid components in the form of a slurry. Coal-water fuel and its derivatives with different additives are examples of such type fuels. For those fuels, the thermophysical properties are usually unknown. Nowadays, researchers use simple additivity theory for the calculation of the thermophysical properties of complex fuels for the first approach. Authors of this research believe that the simple additivity approach is not correct and can lead to the wrong results in the case of the numerical research of the ignition and burning processes of such a fuel. In the present research, the thermophysical properties of coal-water fuel with glycerol additives were experimentally obtained. It was found that the coefficient of thermal conductivity increases with temperature and varies in the range of 0.45 to 0.53 W/(m·K). The heat capacity of the fuel also increases with the temperature and varies from 4.7 to 5.5 kJ/kg·K. The higher the glycerol content, the lower the thermal conductivity and heat capacity of the composite fuel in the investigated temperature range. The results confirm the failure of the approach of the additivity law usage. Neither, thermal conductivity coefficient or heat capacity of the coal-water fuel with the addition of up to 20% glycerol complies with the additivity law. Differences between real values of the thermophysical properties and calculated ones are more than 30% to 50%. Empirical expressions for calculation of the thermophysical properties of coal-water fuel with the addition of up to 20% glycerol are presented.     

Thermal properties and crystallization kinetics of pentaglycerine/graphene nanoplatelets composite phase change material for thermal energy storage

Solid-solid phase change materials (SSPCMs) used in thermal energy storage (TES) system attract much attention in recent days. Here, graphene nanoplatelets (GnPs) were introduced into pentaglycerine (PG) with mass ratios of 1 wt%, 2 wt%, and 4 wt% to obtain PG/GnPs PCMs. The structure and thermal property of PG/GnPs PCMs were characterized by SEM, XPS, FT-IR, POM, DSC, thermal conductivity tester, and heat transfer performance test system. The effect of GnPs on the crystallization kinetic of PG was investigated by isoconversional method. The results indicated that PG and GnPs were uniformly mixed together by physical reaction. GnPs reduced the subcooling and enhanced the thermal conductivity of the PG/GnPs. The heat transfer rate of PG/GnPs was improved during to the high thermal conductivity. Crystallization kinetic results presented that the activation energy increases with the GnP content. In summary, GnPs improved the thermal behaviors of PG.     

Preparation and thermophysical properties of nanoparticle-in-paraffin emulsion as phase change material

In this study, phase change material (PCM) embedded by nanoparticles was prepared by emulsifying alumina (Al₂O₃) nanoparticles in paraffin (n-octadecane) by means of a non-ionic surfactant. The formulated nanoparticle-in-paraffin emulsions contain the nanoparticles of 5 wt.% and 10 wt.%, respectively; their effective thermophysical properties, such as latent heat of fusion, density, dynamic viscosity, and thermal conductivity, were investigated experimentally. The experimentally measured density of the emulsions agrees excellently with that predicted based on the mixture theory. The measured thermal conductivity and dynamic viscosity for the nanoparticle-in-paraffin emulsions formulated show a nonlinear increase with the mass fraction of the nanoparticles compared with that for the pure paraffin, depending on the temperature.