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.     

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.     

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.     

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.     

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.     

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.     

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.     

Thermal accommodation coefficients between polyatomic gas molecules and soot in laser-induced incandescence experiments

Time-resolved laser-induced incandescence demands detailed knowledge of the thermal accommodation coefficient, but to date little is understood about the gas/surface scattering physics underlying this parameter in LII experiments. This paper presents a molecular dynamics (MD) simulation that models polyatomic molecules as rigid rotors and soot as crystalline graphite. A Monte Carlo integration over incident gas molecular speeds and surface atomic vibrational phases yields simulated thermal accommodation coefficients that match the experimentally-measured values. The MD simulation is then extended to assess how α changes with gas and soot temperature, and finally to define a Cercignani–Lampis–Lord scattering kernel that will form a boundary condition in future Monte Carlo simulations of heat and mass transfer between soot aggregates and the surrounding gas.     

Thermal characterization of phase change materials based on linear low-density polyethylene,paraffin wax and expanded graphite

Thermal characterization of Phase Change Materials (PCMs) based on linear low-density polyethylene (LLDPE), paraffin wax (W) and expanded graphite (EG) is reported in this paper. Investigated PCMs showed high potential for application in energy storage systems.The latent heat, L_m, sensible heat Q_sens, and the ability of the prepared PCMs to store and release thermal energy were investigated using specific home-made equipment based on the transient guarded hot plane method (TGHPT). The sensible heat of PCM containing 40 wt.% of paraffin wax was investigated in the temperature range 25–35 °C, they exhibited a drop in Q_sens from 31 to 24 J/g depending on the concentration of EG. A similar decrease in sensible heat with increased loading of EG was observed for PCMs containing 50 wt.% of EG.The storage and release of thermal energy during phase change which is associated with the latent heat of the materials were investigated within the temperature range 20–50 °C. PCMs containing 40 wt.% of paraffin wax exhibited latent heat of 36 J/g, whereas the latent heat of PCMs containing 50 wt.% of paraffin wax was 49 J/g. The addition of EG decreased the time needed to melt and solidify PCMs due to increase in thermal conductivity of PCMs with increase in EG content. This behavior was confirmed by the thermal conductivity measurements, where thermal conductivity increased from 0.252 for sample without EG to 1.329 W/m × °C for PCM containing 15 wt.% of EG.The reproducibility of storage and release of thermal energy by PCMs was demonstrated by subjecting them to repeated heating and cooling cycles (over 150 cycles).     

Thermal efficiency of evaporative heat loss for open and closed cycle systems

The authors present an analytical expression for the thermal efficiency of evaporative heat loss for open and closed cycle systems in terms of system design and climatic parameters. The derived analytical expression can be used for optimum design of evaporative cooling (open cycle) and distillation system (closed cycle). The theoretical results have also been validated experimentally for open cycle system.     

Modification of carbon aerogel supports for PEMFC catalysts

Nitrogen enriched carbon aerogels and Co-based non-noble metal catalysts supported on carbon aerogels have been synthesized and tested using XPS, HRTEM, XRD and RDE techniques. XPS spectra of unmodified carbon aerogels indicated a presence of two oxygen O(1s) groups and five carbon C(1s) groups in deconvoluted spectra. XPS spectra of chemically modified samples indicated nitrogen N(1s) introduced in the carbon aerogel structure by acidic (HNO₃) or basic (NH₄OH) chemical treatment.Synthesis of aerogel supported Co catalysts performed by using Co-methoxy-tetra-phenylporfirin as a macrocyclic compound incorporated into the aerogel structure, and sintered at 700 and 900 °C in N₂, revealed the presence of Co-metal nano-particles with 20 nm diameter. HRTEM and diffraction patterns show a β-Co FCC structure with many {111}<110> micro-twins in the Co nano-particles. The electrochemical properties of the synthesized catalysts in O₂-free and O₂-saturated sulfuric and perchloric acid solutions, evaluated by a rotating disc electrode (RDE) technique, demonstrated catalytic activity in hydrogen oxidation and oxygen reduction reactions.     

建筑用太阳能光热相变储能瓦片设计

为了解决太阳能不连续、不稳定的问题,在传统的太阳能光热系统中,创新性地加入了相变储能模块,并利用相变温度为58℃的相变材料,持续地向用户提供热水。在此太阳能光热系统中,太阳能光热被转化为显热和潜热并储存起来,这样能够显著提高太阳能的综合利用率。研究结果表明:串联方式下,相变材料的蓄热时间较短,并且当循环水入口流量为0.3 L/min时,相变材料的蓄热时间最短,为134 min;并联方式下,夜间相变储能瓦片的热水生成量较大,并且当循环水出口流量为0.5 L/min时,热水的生成量为9.6 L。