An application of thermal microscopes to the measurement of thermal effusivity of films

The application of thermal microscopes to the measurement of local thermal properties has drawn considerable scientific interest. We report on the application of a thermal microscope to the measurement of thermal effusivity for films comprising alumina deposited on a substrate, which were fabricated by an electrophoretic deposition method. The measured data was analyzed to consider the undulations on the sample surface The thermal effusivity of these samples was approximately 1×10³ Js^?0.5m^?2K^?1; this value is smaller than that for dense alumina because the alumina grain makes contact with a point. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20227     

Polycrystalline silicon films prepared by improved pulsed rapid thermal annealing

An improved pulsed rapid thermal annealing (PRTA) has been used for the solid-phase crystallization (SPC) of a-Si films prepared by PECVD. The SPC can be completed with time-temperature budgets such as 10 cycles of 60-s 550°C thermal bias/1-s 850°C thermal pulse. The microstructure and surface morphology of the crystallized films are investigated by X-ray diffraction (XRD). The results indicate that this PRTA is a suitable post-crystallization technique for fabricating large-area poly-Si films on low-cost substrate.     

Determination of thermal parameters of poor conductors by transient techniques

The authors compare the transient hot wire method and the parallel wire method of determining thermal conductivity, using the second to find the thermal diffusivity of two materials. The wires are sandwiched between two samples of the material to be investigated. The influence of pressure is also studied in order to identify the measurement conditions that can be easily achieved. A method is chosen to evaluate thermal parameters and to determine the field of data to be used in relation to sample dimensions.     

Measurement of the local convection coefficient by pulsed photothermal radiometry

Pulsed photothermal radiometry is used for the determination of the convection coefficient. The theoretical analysis of the transient wall temperature after a brief excitation is based on the use of the zeroorder temporal moment. Different solutions to determine the convection coefficient from experimental thermograms are proposed and tested for insulating walls as well as conducting ones. Experimental results are compared to those given by a dissipation fluxmeter in order to validate the pulsed method.     

The HEATSEP method for the synthesis of thermal systems: An application to the S-Graz cycle

In the last decades component synthesis has become a critical issue in the research field about new highly integrated energy conversion systems. Several heuristic methodologies following experience-based guidelines have been proposed to simplify the problem of synthesis optimization. This paper describes an application of the HEATSEP method, which consists in the isolation of all the heat transfer processes of an energy system in an undefined “black-box”. Then, synthesis optimization can be split in two subproblems, the first about the synthesis/design optimization of the basic plant configuration (which is made up of all the components but heat transfer devices) and the other about the synthesis of the heat exchanger network inside the black-box. The chosen test case is the design optimization of the basic plant configuration of an S-Graz cycle based power plant, as it is suitable to show the potentialities of the method.     

Improvement of the pulsed photothermal technique for the measurement of the convective heat transfer coefficient

<正>The present study concerns the measurement of the convective heat transfer coefficient on the solid-fluid interface by the pulsed photothermal method.This non-intrusive technique is apphed for the measurement of the local heat transfer coefficients in cooling of a rectangular slab that simulates an electronic component.The heat transfer coefficient is deduced from the evolution of the transient temperature induced by a sudden deposit of a luminous energy on the front face of the slab.In order to draw up the heat transfer cartography by a non-destructive tool, the infrared thermography has been used.Two inverse techniques for the identification of the heat transfer coefficient are presented here.The first one is based on the assumption that heat transfer coefficient remains constant during the pulsed experiment,and the second one considered it variable in space and time.The temporal and spatial evolutions are expressed as a constant heat transfer coefficient(h_0)multiplied by a function of time and space f(x,t).The function f is deduced from the resolution of the conjugated convection-conduction problem,by a control volume technique for the case of thermally thick sample.The results are given for different air velocities and deflection angles of the flow.     

Numerical and experimental assessment of thermal performance of vertical energy piles: An application

A district space heating and cooling system using geothermal energy from bearing piles was designed in Shanghai and will be installed in two years before 2010. This paper describes the pile-foundation heat exchangers applied in an energy pile system for an actual architectural complex in Shanghai, 30% of whose cooling/heating load was designed to be provided by a ground-source heat pump (GSHP) system using the energy piles. In situ performance tests of heat transfer are carried out to figure out the most efficient type of energy pile and to specify the design of energy pile system. Numerical investigation is also performed to confirm the test results and to demonstrate the medium temperature variations along the pipes. The averaged heat resistance and heat injection rate of different types of energy piles are calculated from the test and numerical results. The effect of pile type, medium flow rate and inlet temperature on thermal performance is separately discussed. From the viewpoint of energy efficiency and adjustability, the W-shaped underground heat exchanger with moderate medium flow rate is finally adopted for the energy pile system.     

Effective thermal conductivity of compressed woods

Compression is one solution to improve the strength of softwoods. The effective thermal conductivities of compressed Japanese cedars (cryptomeria japonica), which were compressed in the radial direction of the wood, were measured. Both the effective thermal conductivities in the tangential and fiber directions increase proportionally to the density increment due to the compression. However, the thermal conductivity in the radial direction (compression direction) increases slightly with the density increment. Numerical computations were conducted to explain the characteristics of thermal conductivity in the radial direction by using a microscopic heat conduction model for the compressed wood. The numerical results were compared with the measured values. And the physical mechanism of the heat conduction in the compressed woods is discussed.     

An analytical model for silicon MIS/IL solar cells to optimize cell parameters through thermal annealing

A model has been developed which simulates the effect of cell parameters in order to optimize them by controlling the fabrication conditions, namely, annealing time and annealing temperature. Calculation of the efficiency as a function of surface states density D_it, positive fixed oxide charge density Q_f and mobile charge density Q_m, that depend on anealing conditions are carried out. A compromise between D_it and Q_m for different anealing temperatures for high performance cells has been investigated.     

SOLAR CONTROL: An integrated approach to solar control techniques

In the framework of the CE JOULE III Programme, further to the SCL activities in PASCOOL Project, the project SOLAR CONTROL has been supported. The project participants, co-ordinated by Conphoebus, include also Universities of Sevilla and Ljubljana, NOA, TNO, BRE, VTT, Fraunhofer and GENEC, as well as leading manufacturers such as SOMFY and Pilkington.The project is aimed at undertaking investigations on the performance of shading devices in order to overcome the existing lack of support towards industry and final user, thus improving the dissemination of knowledge and the effectiveness of the application of these techniques.The final outputs of this project are:

• ll Proposal of Prenormative Testing Standards for shading devices;

• ll Upgrade of the existing simulation models by integration of new validated algorithms;

• ll Design and implementation of a smart solar control assembly;

• ll Design Guidelines & Shading Component Handbook.

In the light of the above objectives, the main ongoing activities concern:

• - characterisation of the most interesting shading devices (external, interpane and internal, isotropic and non isotropic) for the assessment of the overall performance (thermal, daylighting, ventilation)

• - development, validation and integration in existing simulation tools of new algorithms of the shading devices behaviour in respect to solar radiation, visible radiation and ventilation

• - development of new smart solar control systems for integration into the building envelope, with special attention on their impact on energy consumption and improvement of the indoor comfort as well

• - preparation of Design Guidelines and a Shading Component Handbook for European architects and building designer.

     

Effective thermal conductivity of wet particle assemblies

The effective thermal conductivity of mono- and poly-dispersed random assemblies of spherical particles and irregular crystals, both dry and partially or fully saturated by wetting and non-wetting liquids, has been determined computationally by numerical solution of the Fourier’s law on 3-D reconstructed media and experimentally by the transient hot wire method. The effect of spatial distribution and volume fractions of the vapour, liquid, and solid phases on effective thermal conductivity was systematically investigated. A power-law correlation for estimating the effective conductivity, valid over a wide range of phase volume fractions and relative conductivities of components, has been proposed.     

Effect of thermal annealing on the structural and optical properties of nanostructured zinc oxide thin films prepared by pulsed laser ablation

Zinc oxide (ZnO) films are prepared by pulsed laser ablation, on an optically flat quartz substrate for different deposition time. The influence of annealing temperature, on the structural and optical properties of ZnO films is investigated systematically using X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectra, UV-vis spectroscopy and photoluminescence spectroscopy (PL). The XRD pattern shows that the as-deposited films are amorphous and the annealed films are polycrystalline. The average size of the crystalline grains varies from 9 to 26 nm in the films. The SEM and AFM images reveal uniform distribution of grains in the films and the grains are in the nanoscale dimension. Raman spectra suggest the hexagonal wurtzite phase for the ZnO films. The UV-visible spectra show an increase in transmittance with annealing temperature. The observation of very intense PL emission from the films annealed at 773 K, suggest the suitability of these films for applications as light emitters in the visible region. The ability to produce the stochiometric ZnO thin films with reproducible structural, morphological and optical characteristics should be useful as a suitable window material for practical industrial solar cell and display devices.     

Using midday surface temperature to estimate cooling degree-days from NOAA-AVHRR thermal infrared data: An application for Athens, Greece

Cooling degree-days (CDD) are a practical method for assessing the effect ambient air temperature has on the energy performance of buildings. In this study, the relationship between midday land surface temperatures derived from NOAA-AVHRR data and mean daily air temperature observations recorded at standard meteorological stations is defined and statistically validated. The relationship is further used for the calculation of CDD. The benefit of this approach is the direct application of daily satellite data for the definition of CDD in urban areas at a spatial resolution of 1.1 km.     

Effective thermal conductivity of highly porous two-phase systems

A theoretical expression for predicting effective thermal conductivity (ETC) of highly porous two-phase systems has been developed. The porous system has been assumed to contain particles of irregular shape, being dispersed randomly within the continuous medium. The concept of averaging the temperature field within different phases has been used. Resistor model has been applied to determine ETC of two-phase porous systems, as a porous medium is neither composed of slabs parallel nor perpendicular to the heat flux. It is proposed to use slabs inclined at an angle θ with the heat flux lines. An effort is made to correlate angle of inclination θ in terms of the ratio of thermal conductivity of the constituent phases and the physical porosity. Best-fitted expression so obtained for θ is used in the derived model and found that the predicted values of ETC are quite close to the experimental results.     

太阳能蓄热分层水箱数值模拟优化分析

蓄热水箱能够存储和调配能量。将蓄热水箱应用到太阳能热水系统中,可以弥补太阳能的不稳定性和不连续性,有效地提高太阳能热水系统的热利用率。文章基于小型太阳能热水系统,建立蓄热水箱物理模型,应用Fluent软件模拟分析了各个工况下蓄热水箱的温度分层情况,从而寻求较优的温度分层。分析结果表明:当热水入口质量流量小于2.8 kg/s时,蓄热水箱的温度分层比较明显;当热水入口质量流量大于2.8 kg/s时,随着热水入口质量流量逐渐增大,蓄热水箱温度分层越来越不明显;热水入口温度与水箱初始温度对于蓄热水箱温度分层影响不大;当热水入口质量流量为2.8 kg/s时,存在最佳热水入口直径(9 mm),此时蓄热水箱冷、热水不发生混合,蓄热水箱的热利用率较高。     

变频器在火电厂给粉系统中的应用

1　概述火电厂大型煤粉锅炉的煤粉由磨煤机磨制。制粉系统分直吹式和储仓式二类。储仓式系统一般采用钢球式磨煤机 ,送粉采用乏气送粉或热风送粉。采用储仓式系统的锅炉 ,燃煤量应随负荷及工况变化而改变 ,而改变燃煤量的手段是通过改变给粉机的转速来实现的。采用直吹式系统的锅炉 ,当负荷及工况发生变化时 ,是通过直接改变给煤机的给煤量 ,即调节给煤机转速来改变锅炉的燃煤量。以往的煤粉锅炉一般采用直流调速、调压调速或滑差电机等调整手段 ,其中以滑差电机进行调速较为普遍 ,此种方法存在设备复杂、操作繁琐、运行可靠性差、调速精度…     

非对称树状二分岔结构的有效导热系数

采用热电模拟的方法研究了非对称树状二分岔网络结构的有效导热系数,给出了此类结构有效导热系数的解析表达式。结果表明,此类结构的有效导热系数大小与结构本身的非对称率α,分岔处直径的幂律指数p以及分岔级数m有关,并分析了各参数对整个结构有效导热系数的具体影响。结果发现,当幂律指数p=2、非对称率α=1即为对称分岔时,整个结构的有效导热系数取得最大值,且等于组成该结构的材料本身的导热系数。     

Effective thermal conductivity analysis of xonotlite-aerogel composite insulation material

A 3-dimensional unit cell model is developed for analyzing effective thermal conductivity of xonotlite-aerogel composite insulation material based on its microstructure features. Effective thermal conductivity comparisons between xonotlite-type calcium silicate and aerogel as well as xonotlite-aerogel composite insulation material are presented. It is shown that the density of xonotlite-type calcium silicate is the key factor affecting the effective thermal conductivity of xonotlite-aerogel composite insulation material, and the density of aerogel has little influence. The effective thermal conductivity can be lowered greatly by composite of the two materials at an elevated temperature.     

Determination of synchronous machine parameters using networksynthesis techniques

This paper deals with the problem of the determination of synchronous machine parameters starting from two-port information. Network synthesis techniques are used to show that no unique solution can be found for models containing more than one damper winding. Only a limited number of parameters can be determined in a unique way from two-port information