The Measurement of Thermal Conductivities of Silica and Carbon Black Powders at Different Pressures by Thermal Conductivity Probe

This investigation was done to study the gas filled powder insulation and thermal conductivity probe for themeasurement of thermal conductivity of powders.The mathematical analysis showed that the heat capacity ofthe probe itself and the thermal resistance between the probe and powder must be considered.The authorsdeveloped a slender probe and measured the effective thermal conductivity of silica and carbon black powdersunder a variety of conditions.     

Simultaneous measurement of thermal properties by thermal probe using stochastic approximation method

A novel thermal probe method is proposed for the simultaneous measurement of the thermal properties by the Monte Carlo stochastic approximation method. In this method, thermal capacity of probe and thermal contact resistance between probe and sample are considered. An experimental system is set up with the method to validate the measurement accuracy of the method. The thermal properties of several liquid samples as well as solid samples are measured. The results show that: (1) the thermal conductivity and the volumetric heat capacity can be measured with an error of less than 1.2% and 3% respectively, therefore, the measurement accuracy by the method is much higher than the conventional method and (2) the thermal contact resistance has a great effect on thermal conductivity for solid sample, while little influence on thermal conductivity for liquid sample and volumetric heat capacity.     

Methods for measuring the effective thermal conductivity of metal hydride beds: A review

The effective thermal conductivity of metal hydride beds is a crucial parameter for metal hydride reactor design. In this review, methods and principles for the measurement of the effective thermal conductivity of metal hydride beds are discussed, including steady-state techniques (the radial heat flow, comparative cut bar, guarded heat flow meter, and guarded hot plate methods) and transient techniques (the hot-wire, thermal probe, transient plane source, and laser flash methods). Reports of effective thermal conductivity measurements for characterizing metal hydride beds have been reviewed, including the measurement methods, material composition, measurement results, temperature, and gas pressure. The advantages, disadvantages, and applications of each measurement method have been presented, and an assessment regarding different techniques of measurement has been conducted. Laser flash and transient plane source are found to be the most frequently used methods, and have been increasingly applied in recent years. Finally, a brief discussion of recommended future development of effective thermal conductivity measurement is presented.     

Measuring method of three thermophysical parameters of solids by a thermal probe with instantaneous point contact: Measuring principle

A new method for instantaneous measurement of three thermophysical parameters of solids in situ is proposed. The measurement principle is based on a transient heat conduction model of a thermal probe point contacting a testing body. The measurement of temperature response has been made using the probe of a sheathed K‐type thermocouple in this experiment. Ratios of both thermal conductivity and thermal effusivity between the probe and the testing body are determined from curve‐fitting with the theoretical response to the measured one. As a result, it is shown that the measurement is reproducible and the accuracies of measured thermophysical parameters are good enough to apply this method to many kinds of solids. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(3): 191–201, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10084     

直埋闭式地源热泵回填土性能研究

回填土的性能对直埋式地源热泵的设计有一定的影响．对直埋闭式地源热泵用回填土的性能参数中的密度、含水率、饱和度以及导热系数进行了数学定义，并指出影响导热系数的主要因素是密度和含水率。用平板探针原理的室内试验的方法研究了各种物质组成情况、时间以及不同温度对导热系数的影响，并对试验的结果进行了详细的分析，得出了随各种影响因素而变化的曲线。试验结果表明，膨润土不适合单独用于回填材料，需与水泥配合并推荐使用非饱和态。导热系数随水灰比的减小而增加。掺人大颗粒的骨料是提高导热系数的有效途径，砂含量的增加使导热系数往往呈线性增长。当水灰比为0．45且砂的置换率为80％时回填材料具有较满意的导热性能，值得推荐使用。     

Finite difference modeling of heat distribution in multilayer soils with time-spatial hydrothermal properties

In this study, the heat distribution throughout the profile of unsaturated multilayered soil is determined using finite difference method while its thermal diffusivity varies with time and depth. First, the input parameters such as water content, dry density and sand content of the soil profile are provided. These data are coupled with the theoretical approaches to estimate thermal properties of soil such as thermal conductivity and thermal diffusivity of multilayered soil. Second, finite difference method is used to model heat distributions in soil profile taking into account the initial and boundary conditions. A continuity of heat flux between each layer is performed as a condition in the numerical model. A comparison of estimated temperature within time throughout the profile with the thermal probe measurements shows a satisfactory capacity of the numerical model. Finally, different cases of nonhomogeneous and homogeneous soil show that thermal response of homogeneous and nonhomogeneous soils are almost similar at average value of thermal diffusivity where hydrothermal characteristics of each soil layer (such as water content, dry density, and soil texture) are required to calculate this average value.     

Recent Studies on Fluid Flow and Heat Transfer in Thermal Microdevices

Control and measurement of fluid flow and heat transfer in microdevices is of great importance to the development and application of MEMS and Bio-MEMS such as thermal inkjet printer heads, microchemical reactors, and PCR. Thus, the detailed flow behavior, in particular the two-phase flow in microdevices, has attracted much attention in recent years. Several types of thermal micropumps have been developed, although there is still room for further development. Various techniques for measuring the temperature, which are applicable to microscale devices, have also been proposed. As the cooling problem in microdevices becomes increasingly significant, a prospective view on integrated heat and mass transfer is quite necessary. Thus, in this work, some issues and future prospects for fluid dynamics and heat transfer of thermal microdevices are presented and discussed, in terms of thermocapillary pump, temperature measurement in microdevices, and flow near an evaporating meniscus.     

北方地区岩土导热系数及换热量的测试研究

岩土导热系数是地源热泵地埋管换热器的重要设计参数;测井单位深度换热量是地埋管换热器系统的设计依据。掌握工程区域岩土的热物性及换热性能,是保证地源热泵系统高效、稳定运行的关键。文章建立了现场测试岩土导热系数及换热量的方法,并结合沈阳浑南高新技术产业开发区某地源热泵工程,测试分析了岩土导热系数和测井单位深度换热量。结果表明,该区域的岩土具有较好的导热能力,适合采用地埋管地源热泵系统;在特殊地理条件下设计地源热泵系统方案前,应对拟建区域的地质条件进行全面勘探,以优选工程区域,为岩土热响应测试结果的可靠性提供保障。     

A practical method for computing the thermal properties of a Ground Heat Exchanger

The aim of this paper is to show a practical way of estimating the thermal ground properties, namely the ground thermal conductivity, and in particular the thermal diffusivity and the volumetric heat capacity in a reliable manner, for sizing Ground Heat Exchangers (GHEs). A well-known thermal model, proposed by Blackwell in 1954, is applied and is validated both in the heating mode and in the cooling mode, using a GHE as a probe. The value of the thermal conductivity can be easily determined by the model but the procedure also requires knowledge of the ground specific heat capacity and density, which are normally deduced from the (non-accurate) geological data of the site.In addition to the above, the thermal model is also solved analytically –based on the actual parameters used in the experiment–leading to the computation of the ground thermal diffusivity, the volumetric heat capacity and the thermal resistance of the GHE. The possible errors and drawbacks of the whole method are then discussed and finally a complete set of guidelines is provided to the field Engineer for estimating the ground thermal properties from a single test, rendering the use of the geological data of the side unnecessary.     

New challenge in advanced thermal energy transportation using functionally thermal fluids

The present review article presents the current status of some researches on thermal energy transportation using functionally thermal fluid, which is a mixture of heat transfer medium like water and other material with or without phase change like a paraffin wax as a latent heat storage material. This functionally thermal fluid offers attractive opportunities for thermal energy transportation and heat transfer enhancement of heat exchanger. This article describes classification and characteristics of functionally thermal fluids and their application. Referring to functionally thermal fluid for the usage of sensible heat, some visco-elastic fluids for flow drag reduction in a thermal energy transport system such as aqueous polymer solution and surfactant solution are mentioned. On the other hand, this article describes heat transfer and hydrodynamic characteristics of some phase change slurries like ice slurry, phase change microemulsion slurry, phase change microencapsule slurry, clathrate slurry and shape-stabilized paraffin and polyethylene pellets as functionally thermal fluids using latent heat between solid and liquid phases. Finally, it leads to the conclusion that some functionally thermal fluids are very useful for the advanced thermal energy transportation and heat exchanger systems.     

Monitoring of transient 3D temperature distribution and thermal stress in pressure elements based on the wall temperature measurement

Abstract

Two methods for monitoring the thermal stresses in pressure components of thermal power plants are presented. In the first method, the transient temperature distribution in the pressure component is determined by measuring the transient wall temperature at several points located on the outer insulated surface of the component. The transient temperature distribution in the pressure component, including the temperature of the inner surface is determined from the solution of the inverse heat conduction problem (IHCP). In the first method, there is no need to know the temperature of the fluid and the heat transfer coefficient. In the second method, thermal stresses in a pressure component with a complicated shape are computed using the finite element method (FEM) based on experimentally estimated fluid temperature and known heat transfer coefficient. A new thermometer with good dynamic properties has been developed and applied in practice, providing a much more accurate measurement of the temperature of the flowing fluid in comparison with standard thermometers. The heat transfer coefficient on the inner surface of a pressure element can be determined from the empirical relationships available in the literature. A numerical-experimental method of determination of the transient heat transfer coefficient based on the solution of the 3D-inverse heat conduction problem has also been proposed. The heat transfer coefficient on the internal surface of a pressure element is determined based on an experimentally determined local transient temperature distribution on the external surface of the element or the basis of wall temperature measurement at six points located near the internal surface if fluid temperature changes are fast. Examples of determining thermal and pressure stresses in the thick-walled horizontal superheater header and the horizontal header of the steam cooler in a power boiler with the use of real measurement data are presented.     

家用太阳热水器热性能试验方法及标准探讨

对家用太阳热水器热性能的评价方法进行了分析,国标中采用“平均日效率”反映热水器的热性能有一定局限性,“平均热损系数”定义的物理含义不清晰,数学表达式的推导也值得商榷。国标中提出的测定水箱平均水温的方法对不同种类的热水器及不同种类的水箱会带来不易控制的误差。     

Heat transfer design in adsorption refrigeration systems for efficient use of low-grade thermal energy

Adsorption refrigeration and heat pump systems have been considered as important means for the efficient use of low-grade thermal energy of 60–150 °C. Sorption systems are merely thermodynamic systems based on heat exchangers, and therefore a good design to optimize heat and mass transfer with reaction or sorption processes is very important, for which the notable technique is the use of expanded graphite to improve both heat and mass transfer in the chemisorption beds. Studies have also shown the need to enhance the heat transfer in adsorption bed by matching with the efficient heat transfer of thermal fluids. Heat pipes and good thermal loop design coupled with adsorption beds could yield higher thermal performance of a sorption system. A novel design with passive evaporation, known as rising film evaporation coupled with a gravity heat pipe was introduced for high cooling output. It has also been shown that the performance of traditional heat and mass recovery in the sorption systems is limited, and novel arrangement of thermal fluid and refrigerant may improve the performance of sorption systems. Based upon the above researches, various sorption systems have been developed, and high performances have been reached.     

Performance of a building integrated photovoltaic/thermal (BIPVT) solar collector

The idea of combining photovoltaic and solar thermal collectors (PVT collectors) to provide electrical and heat energy is an area that has, until recently, received only limited attention. Although PVTs are not as prevalent as solar thermal systems, the integration of photovoltaic and solar thermal collectors into the walls or roofing structure of a building could provide greater opportunity for the use of renewable solar energy technologies. In this study, the design of a novel building integrated photovoltaic/thermal (BIPVT) solar collector was theoretically analysed through the use of a modified Hottel-Whillier model and was validated with experimental data from testing on a prototype BIPVT collector.The results showed that key design parameters such as the fin efficiency, the thermal conductivity between the PV cells and their supporting structure, and the lamination method had a significant influence on both the electrical and thermal efficiency of the BIPVT. Furthermore, it was shown that the BIPVT could be made of lower cost materials, such as pre-coated colour steel, without significant decreases in efficiency.Finally, it was shown that by integrating the BIPVT into the building rather than onto the building could result in a lower cost system. This was illustrated by the finding that insulating the rear of the BIPVT may be unnecessary when it is integrated into a roof above an enclosed air filled attic, as this air space acts as a passive insulating barrier.     

Assessment of Thermal Properties via Nanosecond Thermoreflectance Method

Nanosecond time-domain thermoreflectance (ns-TDTR) is an all optical method of determining independently a variety of thermal parameters of both homogeneous and layered materials. Despite its relative experimental simplicity, the sensitivity of the temperature decay (measured by the transient reflectivity signal) to the relevant thermal properties has yet to be fully characterized. In principle, it is possible to simultaneously extract multiple thermal parameters from a single measurement. In practice, however, changes to several of these parameters may result in experimentally indistinguishable variations to the transient reflectivity signal. In this work, we focus on investigating thermal properties of bulk material and the contact resistance between the thin-film coating that is needed for the ns-TDTR method and the bulk substrate. To extract multiple properties from one temperature decay trace, we divide the data into temporal sub-regions known to be influenced to different degrees by each individual thermal parameter and iteratively fit with a 1-D heat conduction model to independently determine the contact resistance and cross-plane thermal conductivity.     

A novel method used to study growth of ash deposition and in situ measurement of effective thermal conductivity of ash deposit

Ash deposition always brings boilers some trouble due to fouling or slagging. In this paper, a completely controlled system was developed to study the growth of ash deposit. A novel sampling probe was designed to online measure the heat flux through ash deposit. Additionally, the thickness of ash deposit can be obtained by an online figure collecting system. The results of this research showed that as the thickness of ash deposit increased, the heat flux decreased. It was also found that at the initial stage of ash deposition when the thickness of ash deposit is approximately 1 mm, the heat flux through ash deposit had a sharp reduction. An effective method was attempted to situ measure the effective thermal conductivity of the ash deposit in the simulated combustion flue gas. It was found that temperature of the ash deposit layer had no obvious effect on its value. It was concluded that the structure of ash deposit had no obvious change in a short deposition time of 30 min with varied surface temperatures of the probe head between 400 °C and 600 °C.     

Preparation and thermal property of a tetradecane-octanoic acid eutectic phase change material

采用低共熔法研制了一种相变温度在0~3℃的二元有机相变蓄冷材料,该材料由十四烷和正辛酸按一定比例混合组成。首先通过理论计算预测二元最低共熔混合物的比例,确定其理论最低共熔点温度以及潜热值,然后围绕共晶点配制了5种不同比例的混合物。通过差示热量扫描仪、步冷曲线、Hot disk热常数分析仪测量其热物性,并利用高低温交变箱进行循环稳定性实验。当十四烷和正辛酸的摩尔质量比为51：49时,有最低共熔点温度为1.0℃,相变潜热为191.8 J/g,热导率为0.379 W/（m·K）。对其进行100次充放冷实验,循环后相变温度为0.9℃,相变潜热为191.5 J/g,相变蓄冷时间缩短了24.3%,热稳定性良好。实验结果表明,十四烷-正辛酸有机复合相变材料在低温储能中有可观的应用价值。     

Experimental investigation of a new thermal energy storage system using thermo‐sensitive magnetic fluid and porous medium

In this paper, a novel thermal energy storage (TES) system based on a thermo‐sensitive magnetic fluid (MF) in a porous medium is proposed to store low‐temperature thermal energy. In order to have a better understanding about the fluid flow and heat‐transfer mechanism in the TES system, four different configurations, using ferrofluid as the basic fluid and either copper foam or porous carbon with different porosity (90 and 100 PPI, respectively) as the packed bed, are investigated experimentally. Furthermore, two thermal performance parameters are evaluated during the heat charging cycle, which are thermal storage velocity and thermal storage capacity of the materials under a range of magnetic field strength. It is shown that heat conduction is the primary heat‐transfer mechanism in copper foam TES system, while magnetic thermal convection of the magnetic fluid is the dominating heat‐transfer mechanism in the porous carbon TES. In practical applications in small‐scale systems, the 90‐PPI copper foam should be selected among the four porous materials because of its cost efficiency, while porous carbon should be used in industrial scale systems because of its sensitivity to magnetic field and cost efficiency.     

地源热泵地下岩土热物性的测定方法

提出了可用于现场测量计算地下岩土综合热物性参数的方法。垂直埋设在地下的换热器与周围岩土换热过程可以近似地看作半无限大介质中常热流边界条件下的非稳态导热过程来处理。这种方法克服了其他常用研究模型对钻孔中埋管的具体位置、上升管及下降管之间的距离、换热器材料和回填材料的物性参数的要求,相应地消除了上述各个参数所带来的误差。通过测量地埋换热器的循环水流量、进出水口温度,加热器的加热功率等数据确定地下岩土综合的热物性参数。     

用于现场测量深层岩土导热系数的简化方法

为便于工程上实现应用现场测量确定实际介质的物性，采用一种简化的传热分析方法确定深层岩土导热系数。该方法不需要测量钻孔中埋管的具体位置、上升管和下降管之间的距离以及埋管和回填材料的特性等参数，可消除上述参数测量带来的误差。通过现场测量地下埋管回路的加热热流、回路循环水流量以及回路出入口水温度随时间的变化，利用简化分析和最优化估计方法，确定了某工地地下岩土的导热系数，检验证实了该方法的实用性和可靠性。