Thermal conductivities study on silica aerogel and its composite insulation materials

This paper presents a theoretical and experimental study on thermal conductivities of silica aerogel, xonotlite-type calcium silicate and xonotlite–aerogel composite insulation material. The transmittance spectra of silica aerogel and xonotlite-type calcium silicate samples are obtained through FTIR measurements. The corresponding extinction coefficient spectra of the three materials are then obtained by applying Beer’s law. The thermal conductivities of aerogel, xonotlite-type calcium silicate, and xonotlite–aerogel composite insulation material are measured from 300 to 970 K and from 0.045 Pa to atmospheric pressure with the transient hot-strip (THS) method. The thermal conductivity models developed for coupled heat transfer of gas and solid based on the unit cell method are compared with the experimental measurement results. It is shown that the effective thermal conductivity models matches well with the experimental data. The specific spectral extinction coefficients of xonotlite-type calcium are larger than 10 m² kg^?1, and the specific spectral extinction coefficients of aerogel are larger than 7 m² kg^?1 over the whole measured spectra. 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.     

含二氧化硅气凝胶和相变材料三层玻璃窗热性能分析

为了研究含二氧化硅气凝胶和相变材料三层玻璃窗对严寒地区建筑能耗的影响,建立了相变材料层与其他透明壁层结合发生的传热数值模型。分析了含二氧化硅气凝胶和相变材料三层玻璃窗在不同二氧化硅气凝胶厚度、导热系数和不同保温材料下的动态热调节性能,得到了含二氧化硅气凝胶和相变材料三层玻璃窗内表面热流密度和液相率随时间的变化规律。结果表明:随着二氧化硅气凝胶厚度增加,总传热量降低和液相率增加,当二氧化硅气凝胶厚度为20～30 mm时,可以实现有效的利用太阳能;随着二氧化硅气凝胶导热系数增加,总传热量升高和液相率降低;当二氧化硅气凝胶的导热系数从0.022降低到0.014 W/(m·K)时,最大液相率从0.83增加到1.00。二氧化硅作为保温层比相变材料作为保温层具有更好的保温隔热作用。     

Monolithic silica aerogel in superinsulating glazings

Silica aerogel is an open-pored porous transparent material with optical and thermal properties that makes the material very interesting as an insulation material in windows. A number of different aerogels have been investigated for their optical and thermal performance. High thermal resistance of aerogel was found for all the investigated samples and the samples showed very high solar as well as light transmittance. However, all the investigated aerogel samples showed a tendency to scatter the transmitted light, resulting in a reduced optical quality when the aerogels are integrated in glazings. This phenomenon is considered as being the main obstacle to incorporating the material in clear glazings, but a significant improvement of the optical quality of aerogel has been observed during the last five years. A number of prototypical evacuated 500×500×28 mm³ aerogel double glazed units employing a new edge seal technique were manufactured and characterized for their optical and thermal properties. As expected, the same scattering of light was found in the aerogel glazings as in the aerogel samples, but excellent thermal performance was found, indicating a glazing type that, from a thermal point of view, is without competition in heating dominated climates.     

Thermal conductivity of a clay-based aerogel

Aerogel materials exhibit superior thermal insulation characteristics due largely to their highly porous internal structure. A recently developed class of montmorillonite clay-based aerogels provides the attractive thermal properties of traditional aerogel materials using constituents that are chemically benign and abundantly available. Results are compared for aerogels made from clay alone and those with polyvinyl alcohol introduced during processing. Results demonstrate that as well as strength advantages, the addition of the polymer also leads to a reduction in thermal conductivity. Experimental thermal conductivity data as well as a model to describe the mechanisms involved in impeding thermal transport are presented.     

Infrared radiative heat transfer in highly transparent silica aerogel

The radiative heat transfer in silica aerogel is determined by measuring the spectral infrared extinction (absorption) E(Λ) between 2.5 and 14 μm. From E(Λ) we calculate a temperature-dependent Rosseland mean for extinction. Using a suitable diffusion model for the infrared radiation the temperature-dependent radiative conductivity λ_r(T) is determined. For T = 280 K, a mean temperature within the insulation, a value as low as λ_r 0.002 W/(m·K) results. Between T = 250 K and 450 Kλ_r increases roughly ∝ T^α, with α = 5.6.     

Bulk and surface light scattering from transparent silica aerogel

Elastic light scattering has been used to study structural properties of different transparent aerogels, which may be used as filling materials in super-windows. With a goniometer having an angular resolution better than 0.6° and a He-Ne laser as the light sourcewe investigated the angular distribution of scattered intensity from transparent silica aerogels and one xerogel. The densities ranged between 0.11 and 0.60 g cm^-3. An exponential correlation function for the density fluctuations of a random porous medium has been utilised to analyse the large-angle scattering, which is dominated by bulk scattering, for different polarisation of the incident light. The determination of correlation lengths in the nanometre range was possible, because the absolute scattering intensities were determined. For relative angular dependence measurements, this range would have been accessible only to small angle X-ray scattering (SAXS). The resulting mean pore sizes between 8 nm and 50 nm and specific surface areas between 500 and 700 m²/g agree well with nitrogen-porosimetry data from the literature.The data compare quite well with correlation lengths calculated from specular transmittance data from an ordinary spectrophotometer. This method, which is not sensitive to the angular distribution of superposed forward scattering with large correlation lengths, has also been applied to a series of base-catalysed TMOS aerogels with different catalystconcentrations.The forward scattering peak of the signal may be attributed to correlation lengths in the micrometre range. Experimental results for aerogel surfaces with evaporated aluminium indicate that this might be due to the surface properties. A quantitative analysis, however, is not possible yet.     

二氧化硅气凝胶制备条件对热导率的影响

二氧化硅(SiO2)气凝胶由于其内部的多孔网状结构与气体填充,热导率极低,可应用于多种保温隔热材料。本文通过实验研究了二氧化硅气凝胶的热学性能,并分析了不同水解时间、蒸馏水体积、反应物体积比、pH值对其热学物性参数的影响规律。结果表明,在所研究的范围内,随着水解时间的增加,二氧化硅气凝胶热导率减小;随着反应物体积比、蒸馏水体积和pH值的增加,二氧化硅气凝胶热导率先减小后变大。     

Thermal characteristics of a solar tank with aerogel surface insulation

The transient thermal characteristics of a solar tank (a shallow solar-pond water heater) with a silica aerogel surface insulation have been examined both experimentally and theoretically. It is found that the temperature drop of the water within the solar tank after sunset can be appreciably suppressed by introducing the aerogel surface insulation system and that the proposed theoretical model can predict sunny-day hourly variations in the hot-water temperature within the solar tank with acceptable accuracy.     

Radiative properties and heat transfer characteristics of fiber-loaded silica aerogel composites for thermal insulation

The radiative properties and heat transfer in fiber-loaded silica aerogel composites were investigated using modified anomalous diffraction theory in a combined heat conduction and radiation model. The randomly parameterized 2-D fiber distribution was generated to simulate a very realistic material structure. The finite volume method was then used to solve a two flux radiation model and the steady-state energy equation to calculate the effective thermal conductivity of the composite. The numerical results provide theoretic guidelines for material designs with optimum parameters, such as the inclination angle, diameter and length-to-diameter ratio of the fibers. The results show that the fiber extinction coefficient increases as the fiber length-to-diameter ratio is reduced or the fiber inclination angle is increased. The effective thermal conductivity of the fiber-loaded aerogel can be reduced by reducing the fiber length-to-diameter ratio and the inclination angle and by moderately increasing the fiber volume fraction. The 4–6 μm diameter silicon fibers are optimum for high-temperature thermal insulation.     

Heat loss coefficients for box-type solar cookers

The top and overall heat loss coefficients for the entire feasible operating range of box-type solar cookers are evaluated experimentally and presented in a graphical form as a function of the difference between mean plate temperature and ambient temperature with wind velocity and number of glass covers as parameters.

The range of plate temperatures considered is from 50°C to 180°C. While the wind velocity is varied from 0 to 3.33 m/s, and the number of glass covers considered are from 1 to 4.

Based on these experimental results, a correlation for the determination of top loss coefficient in terms of optical properties of cooker, the spacing between glass cover and absorber plate, wind velocity and number of glass covers, is derived and presented in the paper.     

A method of measuring windmill torque loss coefficients

The aim of this work is to investigate a method of measuring windmill torque loss coefficients of a windmill power system. We discussed the mathematical windmill model and defined the windmill torque loss coefficients. An estimating technique based on Kalman filtering was developed for decision of the value of these coefficients. We manifested the valdity of this method by the estimated results from both the simulated data and wind-tunnel experimental data.     

Microwave-assisted synthesis of silica aerogel supported pt nanoparticles for self-humidifying proton exchange membrane fuel cell

In this study, the mesoporous silica aerogel supported Pt nanoparticle (SAP) was synthesized by the simple microwave-assisted method within 90 s and characterized by WXRD and BET measurements. SAP was then used as a filler to prepare the self-humidifying Nafion^®-based composite membrane (N/SAP). The dispersion of the catalyst in N/SAP as well as the water uptake and proton conductivity of N/SAP were investigated. Compared to that of the recast Nafion^® membrane (RN), the water uptake and the proton conductivity of N/SAP was improved for about 38% and 109%, respectively. In addition, the power density of the PEMFC single cell fabricated with N/SAP at 50 °C was 1104 mW cm⁻² and 913 mW cm⁻² measured under the humidified condition and dry condition, respectively, which was approximately 91% and 5.5 times higher than that with RN membrane, respectively.     

Influence of silica aerogel on the properties of polyethylene oxide-based nanocomposite polymer electrolytes for lithium battery

In this study, a series of nanocomposite polymer electrolytes (NCPEs) with high conductivity and lithium ion transference number, PEO/LiClO₄/SAP, were prepared from high molecular weight polyethylene oxide (PEO), LiClO₄ and low content of homemade silica aerogel powder (SAP), which had higher surface area and pore volume than the conventional silica particle. From the SEM images it was found that the SAP nanoparticles were well dispersed in the PEO polymer electrolyte matrix. The characterization and interactions in the CPEs were studied by DSC, XRD, FT-IR and ⁷Li NMR analysis. The ac impedance results showed that the ionic conductivity of the CPE was significantly improved by the addition of the as-prepared SAP. The maximum ambient ionic conductivity obtained from the CPE with EO/Li = 6 and 2 wt.% of SAP (O6A2) was about threefold higher than that of the corresponding polymer electrolyte without SAP (O6). In addition, the lithium ion transference number (t⁺) of O6A2 at 70 °C was as high as 0.67, which was also three times higher than that of O6 and has not been previously reported for the PEO–LiX-based polymer electrolytes.     

Determination of loss coefficients for high-temperature flow devices: An entropy-based approach

Thermodynamic analysis of a high-temperature confined turbulent gas-jet is presented in this paper. The numerical model is two dimensional, steady, and includes the effect of gravity in the governing equations. Computations are carried out with a commercial CFD code and the local exergy losses are determined as post processed quantities. The analysis takes into account the second law effects of viscous dissipation, heat conduction and convection, and radiative heat transfer. The study is extended by conducting a parametric investigation to determine the effects of Reynolds number, inlet fluid temperature, optical thickness, and Planck number on the exergy loss coefficient, which is defined as the total exergy destroyed per unit mechanical energy input. The results show that exergy loss trough radiation entropy production is higher than that due to heat conduction and convection when the inlet gas temperature is high. It has also been found that in contrast to the conventional head loss coefficient, the exergy loss coefficient increases with inlet gas temperature, optical thickness, and Planck number.     

叶片表面粗糙度对透平叶栅气动性能影响的试验研究

燃气轮机的透平前温在不断提高,采用热障涂层技术是保证透平叶片能够在高温下安全工作的重要手段.但是热障涂层的喷涂会改变叶片表面的粗糙度,粗糙度的变化对透平的气动性能会有多大的影响是必须关注的问题.本文试验研究了叶片表面粗糙度对透平气动性能的影响.试验结果表明,在喷涂过程中必须控制叶片表面粗糙度,否则会显著增加损失.     

Solar heat gain factors and heat loss coefficients for passive heating concepts

The concept of solar heat gain factor has been introduced for calculating the net energy gain of passive heating elements and other components of a building as a result of incident solar radiation. For passive heating concepts (namely, the direct gain, mass wall, water wall, Trombe wall, and solarium), exact analytical expressions have been obtained for the solar heat gain factors and the corresponding overall heat loss coefficients. These will allow a building designer to calculate immediately the overall heat gain/loss in a building. Numerical calculations have been done for typical values of solar radiation and ambient temperature of typical climatic conditions in India. The method has been compared with the other methods reported in the literature so far. A good comparison is found between the earlier methods and the method of using solar gain factors and the corresponding heat transfer values.     

碱金属热电直接发电装置的热辐射损失

寄生热辐射损失，特别是BASE管外表面的热辐射是影响碱金属热电转换器(AMTEC)高效运行的主要因素之一。为了量化BASE管外表面的热辐射对碱金属热电转换器热电转换效率的影响程度，文章用一简化模型，计算了采用不同层数遮热屏的碱金属热电转换器中的BASE管外表面的净热辐射量。结果表明，恰当设计AMTEC装置的结构，并在BASE管外加数层遮热屏，可使BASE管外单位电极表面的净热辐射损失在其工作温度范围内控制在1W／cm^2以下。     

Dimensioning, thermal analysis and experimental heat loss coefficients of an adsorptive solar icemaker

The dimensioning, a thermal parameters analysis and the experimental heat loss coefficients of an adsorptive solar refrigerator prototype used for ice production are presented. The solar icemaker operates in an intermittent cycle, i.e. without recovering heat. It uses the activated carbon–methanol pair whose basic components are an adsorber coupled to a static solar collector, a condenser and an evaporator. Some innovations were considered, especially those brought about by French researchers, in which the adsorber was always box-shaped with extended surfaces, and air condensers were used. For the present system, the adsorber is bi-facially irradiated and covered with transparent insulation material (TIM), the geometric configuration of the main components is multi-tubular, and a water condenser is used. TIM polycarbonate covers are used on the top and bottom of the adsorber. The components of the prototype were dimensioned after the results from numerical simulations using meteorological data valid for the hottest six months in João Pessoa (7°8′S, 34°50′WG), whose climate is typically hot and humid. The machine was designed to produce up to 10 kg of ice/day per square meter of solar collection surface. Analyses of the thermal parameters influence on the ice production as well as parameters for dimensioning each component of the machine are presented. The overall heat loss coefficient by the top and the bottom of the adsorber–solar collector component are experimentally evaluated. The tests were performed using an incandescent lamp panel disposed on a 1 m² surface, totalizing a thermal power of 3600 W. The results show a good efficiency of the TIM covers, achieving overall heat loss coefficient values between 0.54 and 1.90 W m⁻² K⁻¹.