Comparison of measured and predicted performance of different heat storage systems

This paper presents a comparison between the measured and predicted performance of different sensible heat storage units. Also, a comparison between the performance of water- and air-based heat storage systems has been conducted. In the air-based systems, natural soil available at Mu'tah site, Jordan, was used as a sensible heat storage material.An experimental set-up was designed for three different storage systems, namely a water storage system, an unstratified air storage system, and a stratified air-based system. The performance of the storage units, i.e. the heat accumulated by the storage material (soil in air storage systems, and water in water storage system) was calculated.A computer program was developed to determine the temperature distribution of the air-based storage systems. The numerical model uses finite difference techniques to solve the governing equations for both the storage material and the circulating fluid.The results showed that the computer model is in good agreement with the experimental results. Also, results showed that the stratified tank performed better than the unstratified one and the water system is superior in storage energy, in spite of its harmful corrosion effects.     

Numerical modeling and simulation of a diffusion-controlled liquid–solid phase change in polycrystalline solids

A fully implicit two-dimensional moving-mesh finite element simulation model was developed to study the influence of grain boundaries in polycrystalline solids on diffusion-controlled liquid–solid transition during transient liquid phase (TLP) bonding. The new model, which was developed without the non-trivial symmetry assumption in existing numerical models for the process, was found to conserve solute and its calculated solutions were unconditionally stable and in good agreement with experimental results. Contrary to the assumption that increased grain boundary diffusion coefficient would significantly accelerate the rate of liquid–solid interface migration, numerical calculations and experimental verification showed that enhanced intergranular diffusivity had a minimal effect on the time required to achieve complete diffusion-induced solidification in cast superalloys. The results indicate that reducing the number of grain boundaries in structural alloys through directional solidification casting techniques did not constitute a disincentive to efficient application of TLP bonding to this class of materials.     

Measurement of Thermoelectric Properties of Single Semiconductor Nanowires

We have measured the thermopower and the thermal conductivity of individual silicon and indium arsenide nanowires (NWs). In this study, we evaluate a self-heating method to determine the thermal conductivity λ. Experimental validation of this method was performed on highly n-doped Si NWs with diameters ranging from 20 nm to 80 nm. The Si NWs exhibited electrical resistivity of $\rho = (8\pm4)\, \hbox{m}\Upomega\,\hbox{cm}$ ρ = ( 8 ± 4 ) m Ω cm at room temperature and Seebeck coefficient of ?(250 ± 100) μV/K. The thermal conductivity of Si NWs measured using the proposed method is very similar to previously reported values; e.g., for Si NWs with 50 nm diameter, λ = 23 W/(m K) was obtained. Using the same method, we investigated InAs NWs with diameter of 100 nm and resistivities of $\rho = (25\pm5)\, \hbox{m}\Upomega\,\hbox{cm}$ ρ = ( 25 ± 5 ) m Ω cm at room temperature. Thermal conductivity of λ = 1.8 W/(m K) was obtained, which is about 20 to 30 times smaller than in bulk InAs. We analyzed the accuracy of the self-heating method by means of analytical and numerical solution of the one-dimensional (1-D) heat diffusion equation taking various loss channels into account. For our NWs suspended from the substrate with low-impedance contacts the relative error can be estimated to be ≤25%.     

Synthesis and Evaluation of Semiconductive Polymer Compositions

A semiconductive polymer composition was prepared in which micronized particulate conductive filler, such as copper or graphite, is dispersed in a polymeric component that is of an essentially amorphous thermoplastic resin (such as polymethylmethacrylate). These molding powders of polymethylmethacrylate were prepared by suspension polymerization of methylmethacrylate monomer in presence of these conductors. The prepared molding powder contains 4.8% and 40.44% by volume (with respect to polymethylmethacrylate (PMMA)) of copper and graphite, respectively.

The effect of blending copper and graphite at different vol % to the prepared molding powder on the specific resistance of the product was studied.

It was concluded that the specific resistance of molding powder was decreased from 2.1 × 10¹¹ to 3.05 × 10⁶ ohm.cm at 70% volume copper and from 2.3 × 10⁴ to 2 × 10² ohm cm at 80% volume graphite.     

Para-xylene Maximization—Part VI: Shape-Selective Platinum-Promoted Methylation of Toluene

Abstract

The need for increased production of para-xylene, which is the primary material for producing the polyester fibers, activated this research. Although the alkylation reaction is acid catalyzed, we found that a Pt promoter activates this reaction by virtue of the presence of a vacant d-orbital in the Pt atom. In this work, a series of catalysts containing 0.1, 0.2, or 0.3% Pt in H-ZSM-5 zeolite was tested for alkylating toluene with methanol, aiming to produce the xylenes and maximizing para-xylene production in a temperature range of 300°C–500°C in the presence of hydrogen flow in a continuous-flow fixed-bed reactor. The catalysts were characterized by temperature programmed desorption (TPD) of ammonia for acid sites distribution analysis and platinum dispersion in the catalysts by hydrogen chemisorption. Moreover, the diffusion resistance extent in the current catalysts during the alkylation reaction has been evaluated via estimation of the Thiele modulus, Φ _L . The selectivity for para-xylene production was found to increase systematically with increasing the Pt content in the catalysts, whereas the unloaded zeolite did not follow this order. The Φ _L values calculated were accordingly found to increase also with increasing Pt content in the catalysts. Although para-xylene was the highest on the 0.3% Pt/H-ZSM-5 catalyst, the heavy undesired trimethylbenzenes were the lowest to be formed on this catalyst.     

TEM analysis of diffusion brazement microstructure in a Ni<Subscript>3</Subscript>Al-based intermetallic alloy

Transmission electron microscopy study of brazed joint microstructure in a Ni₃Al-based intermetallic alloy IC 6 was performed. A continuously distributed microconstituents consisting of γ solid-solution, M₃B₂-type and M₂₃B₆-type borides, which were likely formed by eutectic-type reaction(s) from residual liquated insert during cooling from brazing temperature, were observed along the joint centreline. Consideration of possible incipient melting due to eutectic-type transformation reaction of these phases is pertinent to development of post braze heat treatment for modifying the brazement microstructure.     

Design of a solar water heating system for Alexandria, Egypt

The purpose of this work is two-fold. First, to introduce a comparison between steady state and dynamic test methods for two different collectors. Second, to design a solar water heating system to satisfy both hot water and space heating demands for a multi-family house in Alexandria, Egypt.     

Book review

This Letter communicates the discovery of an exceptionally large, double‐ringed crater in the eastern part of the Great Sahara, North Africa. The crater is centred at 24.40° N 24.58° E, straddling the boarder between Egypt and Libya. It is the 15th and largest impact crater identified in the Sahara. Landsat Enhanced Thematic Mapper Plus (ETM+) images as well as Radarsat‐1 data reveal a discontinuous outer rim, 31 km in diameter, and a group of prominences forming an inner ring. The Nubian sandstone surface in which the crater was formed has undergone severe erosion. Thus, the crater morphology was affected by both aeolian and fluvial processes. Courses of a major river and smaller streams, now dry, have eroded much of the crater's outer rim as revealed by Shuttle Radar Topography Mission (SRTM) data. The probable impact that created the crater, named Kebira, meaning large in Arabic, is possibly the source of the silica glass fragments that abound on the desert surface between giant linear dunes of the Great Sand Sea in southwestern Egypt.     

Studies on Epdm/Nr Blends. I. Dielectric and Mechanical Properties

Ethylene-propylene-diene terpolymer rubberlnatural rubber blends (EPDM/NR) in different proportions (100 : 0, 75 : 25, 50 : 50, 25 : 75, and 0 : 100) reinforced with 20 phr semireinforcing furnace carbon black have been studied. The permittivity ?' and dielectric loss ?″ for these samples were determined in the frequency range 10²–10⁷ Hz. Also, their physico-mechanical properties were investigated.

The EPDM/NR blend (75 : 25) possessed the best mechanical properties and stability against aging. Thus, it was chosen to explore the effect of adding carbon black in different quantities up to 50 phr on its electrical and mechanical properties. Three different types of carbon black. namely high-abrasion furnace black, fast-extrusion furnace black, and medium thermal black, were used. At a certain concentration of carbon black, referred to as the threshold percolation concentration, an abrupt increase in ?′ and ?″, which depends on its type, was detected. This could be attributed to some sort of interaction between the blend and the carbon black at higher concentrations.     

Incorporation of density relaxation in the analysis of residual stresses in molded parts

Thermo-rheologically/piezo-rheologically simple viscoelastic constitutive equations are adopted for the material behavior of a generic polystyrene, in both the deviatoric and dilatational domains, in order to investigate the effect of density relaxation on the development of the thermal residual stresses in a thin injectionmolded strip. A preliminary study is undertaken to assess the ability of the proposed dilatational viscoelastic constitutive equations to capture some of the density-relaxation behavior such as the isobaric volume relaxation following a sudden quench from above the glass-transition temperature and upon constant rates of cooling at different temperatures and pressures. In this preliminary study, different combinations of relaxation functions and shift factors are investigated. An appropriate combination is selected and used for the residual-stress analysis. The numerical simulation of the development of the stresses in a one-dimensional cavity qualitatively predicts the correct stress profile across the thickness of the molded part, as well as the dependency of this profile on some of the material properties and molding conditions. In general, the investigation presented in this paper suggests that density relaxation plays an important role in the development of residual stresses in molded parts.