Effect of front air attack angles on heat transfer coefficient of the cross‐flow of four flat tube

Heat transfer coefficients are studied experimentally for airflow over a four‐flat‐tube configuration with different inlet attack angles. The range of Re numbers was from 1668 to 3782. The heat fluxes are 13.2, 38.5, and 99.8 W/m². In this paper, 45 samples of a flat tube with inlet airflow angles (30°, 45°, and 90°) are set up to investigate its thermal performance. The design of experimental approach was used to conduct the heat transfer parametric study by designing 45 experiments, analyzing the empirical model, and optimizing heat transfer. The response was modeled using a general factorial design model based on experimental data. From analysis of variance (ANOVA), we obtained significant coefficients by performing the 5% test of level of significance. The results indicated that both the Nu and Re numbers increase for all heat flux cases and these decrease with increasing the air attack. Also shown are that the Colburn j‐factor decreases with increase of air velocity. It was found that the ANOVA results are appropriate to the collected experimental data, as the R² and R²‐adjusted statistics are found to be 92.61% and 90.97%, respectively, for Nu number. In a similar manner, for Colburn j‐factor, the R² and R²‐adjusted statistic are 96.97% and 96.29%, respectively.     

Constructal design of multiscale elliptic tubes in crossflow

The optimal configuration of two‐scale elliptic tubes in crossflow is found on the basis of the constructal design. The larger tubes are installed inside a domain of fixed length and height. In the same domain, smaller tubes are inserted between the larger tubes in the entrance region at the mid leading edge to leading edge distance of the larger tubes. The spacing between the larger tubes, the semiminor axis of the larger tubes, the major axis of the smaller tubes, and the semiminor axis of the smaller tubes are varied inside the domain freely to find the optimal configuration. There are two optimal configurations: one without the smaller tubes and the other with the presence of the smaller tubes. Both the larger and the smaller tubes are heated at a constant surface temperature. The flow is induced by a fixed pressure difference. The equations for steady, laminar, two‐dimensional, and incompressible flow are solved by finite volume method. In the absence of the smaller tubes, the range of Bejan number (dimensionless pressure drop) is ${10}^3<\mathit{Be}<{10}^5$, and in the presence of the smaller tubes, Bejan number is Be = 10⁵. The range of the dimensionless larger tubes semiminor axis is 0.1 ≤ B ≤ 0.4. Air is used to cool the row of the tubes with Prandtl number equal to 0.7. The results show that for different semiminor axes of the larger tubes, the heat transfer rate is enhanced when the smaller tubes are placed between the larger tubes.     

Thermal evaporation and condensation synthesis of metallic Zn layered polyhedral microparticles

Metallic zinc layered polyhedral microparticles have been fabricated by thermal evaporation and condensation technique using zinc as precursor at 750 °C for 120 min and NH₃ as a carrier gas. The zinc polyhedral microparticles with oblate spherical shape are observed to be 2-9 μm in diameter along major axes and 1-7 μm in thickness along minor axes. The structural, compositional and morphological characterizations were performed by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). A vapour-solid (VS) mechanism based growth model has been proposed for the formation of Zn microparticles. Room temperature photoluminescence (PL) emission spectrum of the product exhibited a strong emission band at 369 nm attributed to the radiative recombination of electrons in the s, p conduction band near Fermi surface and the holes in the d bands generated by the optical excitation.     

Solvo-solid preparation of Zn3N2 hollow structures; their PL yellow emission and hydrogen absorption characteristics

Zinc nitride (Zn₃N₂) hollow structures with 10-25 μm size have been prepared by solvo-solid approach using aqueous ammonia treated Zn precursor at reaction temperature of 600 °C for reaction time of 240 min under ammonia gas flow. The structural, compositional and morphological characterizations of the as-obtained product were performed by XRD, EDS and SEM. Room temperature photoluminescence (PL) spectrum of zinc nitride hollow structures (ZNHSs) exhibited UV emission band at 384 nm and a defect related yellow emission band at 605 nm. The first ever studies on hydrogen absorption characteristics of ZNHSs performed at 373 K showed an absorption capacity of 1.29 wt.%. Growth mechanism proposed for the formation of ZNHSs is also discussed briefly.     

Large-scale synthesis of highly pure Cd metal hexagonal nanosheets

Highly pure and large-scale single crystalline cadmium (Cd) metal hexagonal nanosheets with 10-12 nm thickness were obtained by catalyst-free thermal decomposition of cadmium oxide (CdO) powder under nitrogen gas at 1000 °C. The as-prepared product was characterized by X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy, transmission electron microscopy and selected area electron diffraction. Room temperature photoluminescence (PL) spectrum for Cd nanosheets exhibited three very distinct emission bands at 360 nm, 402 nm and 426 nm. UV emission band at 360 nm is attributed to radiative recombination of electrons in the s, p conduction band near the Fermi surface and the holes in the d bands generated by xenon light excitation whereas visible emission bands at 402 nm and 426 nm may be due to surface oxidation effects or induced defects in the nanosheets. The growth mechanism for the formation of Cd metal hexagonal nanosheets is also proposed and discussed briefly.     

High Efficiency Neutron Detector for Low Neutron Flux Measurement

A neutron flux detector composed of a bank of nine He³ detectors is developed. It is employed successfully to record neutron flux from a pulsed source as low as 5 × 10² neutrons per m², which is about 0.5 nrem per shot. It may find applications in recording low neutron flux at large distances from the source or in detecting leakage from the radiation shield, and hence may help to estimate health hazards.     

VLS and VS effect on ferromagnetic behaviour of SnO2 nanobelts

Fabrication of SnO₂ nanobelts has been carried out by employing vapour liquid solid (VLS) and vapour solid (VS) mechanism. Nanobelts obtained by VLS mechanism were fabricated at relatively low temperature using Fe powders as a catalyst by means of chemical vapour deposition (CVD) technique. Direct thermal evaporation of SnO₂ nanobelts was carried out at 1350°C in the atmosphere of Argon gas via VS mechanism. In both cases, ramp rate was adjusted to 10°C/min. The structural, compositional and morphological characterisations of synthesised SnO₂ nanobelts were conducted by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X?ray spectroscopy (EDS). Magnetic behaviour of the nanobelts was studied by vibrating sample magnetometer (VSM). Cathodoluminescence (CL) spectra were studied using Renishaw Raman spectroscopy system. The growth of nanobelts is found to be homogeneous and dense. The nanobelts formed by VLS mechanism show ferromagnetic behaviour along with some other exciting results whereas the nanobelts formed by VS mechanism show a diamagnetic behaviour. The observed room temperature ferromagnetism in SnO₂ nanobelts is superior to other oxides.     

Prospects,challenges, and latest developments in lithium–air batteries

Metal–air batteries are being envisioned as a clean and high energy fuel for the modern automotive industry. The lithium–air battery has been found most promising among the various practically applicable metal–air systems, that is, Al–air, Li–air, Mg–air, Fe–air, and Zn–air. The theoretical specific energy of the Li–air battery is ~12 kWh/kg, excluding the oxygen mass. This is comparable with the energy density of gasoline, which is ~13 kWh/kg. It has been hypothesized that the Li–air battery could supply an energy ~1.7 kWh/kg after losses from over potentials to run a vehicle ~300 miles on a single charge. During the first decade of this century, a fair amount of research has been conducted on Li–air battery system. Yet, Li–air batteries could not make an industrial breakthrough, and are still in the laboratory phase since their birth. In this article, we technically evaluated the recent developments, and the inferences have been analyzed from the practical/commercial point of view. The study concludes that low discharge rate, lower number of cycles, oxidation of lithium anode, discharge products at the cathode, and side reactions inside the battery are the key limiting factors in the slow progress of Li–air batteries on an industrial scale. The ongoing researches to overcome these hurdles have also been discussed. This analysis will help the reader to understand the current standing of the lithium–air battery technology. Copyright © 2014 John Wiley & Sons, Ltd.     

Zinc—air alkaline batteries — A review

The basic principles involved in the operation of an alkaline zinc—air system are considered. Fully developed primary and mechanically rechargeable cells and their applications are reviewed. Various obstacles pertaining to the development of an electrically rechargeable zinc—air alkaline battery and possible means of overcoming them to some degree are summarized.     

Airborne Laser Terrain Mapping for Expediting Highway Projects: Evaluation of Accuracy and Cost

A typical highway project generally takes 5 years or more from planning phase to construction stage, particularly in wooded and difficult terrain using traditional topographic terrain mapping methods. This paper presents an application of airborne laser terrain mapping technology for a 9?km (5.9?mi.) long highway project in a difficult densely wooded terrain with steep slopes and ravines. Elevation data accuracy, efficiency, and cost effectiveness were compared with the traditional aerial photogrammetry and ground based total station survey methods. The elevations of centerline and 15 different cross sections were compared with groundtruthing data from the total station survey. Using appropriate flight mission parameters, the airborne laser technology permits elevation accuracy of 0.13?m (5?in). There are less operating constraints which adversely affect the productivity of traditional methods, such as cloud and vegetation cover, time of day, and intrusion into private properties. It is recommended to combine the low-altitude airborne laser technology with centerline staking by total station survey and aerial photography. The recommended combined approach saves 33% of the budget and 35% of time.