Convective Heat Transfer and Pressure Drop over Elliptical and Flattened Tube

A numerical study on the effect of the effect of elliptical and flattened tube bundle geometry on the convective heat transfer and pressure drop is presented in this article. The analysis has been carried out to evaluate the performance of these bundle geometries in the design of a compact and effective single phase shell and tube heat exchanger. The temperature, velocity, and pressure drop profiles are obtained from solving the mass, momentum, and energy conservation equations. The comparison is made for inline and staggered bundle with different pitch to diameter ratio and inlet velocity for elliptical and flattened tubes. The pitch to diameter ratio is varied from 1.25 to 2.5 for Reynolds number ranging from 200 to 2000 which is in the laminar flow region. The heat transfer coefficient over the staggered and inline tube bundle decrease with an increase in pitch. The same kind of variation is also observed for the pressure drop in the case of both elliptical and flattened tube bundle. The study shows that the transverse pitch with respect to cross flow affects more than the longitudinal pitch.     

A unified model for multi-turn closed-loop pulsating heat pipe

Numerical modeling of  the multi-turn closed-loop pulsating heat pipe (CLPHP) in the bottom, horizontal, and top heat mode is presented in this paper, with water as working fluid. Modeling is carried out for 2-mm ID CLPHP having 5, 16, and 32 turns at different orientations for 10 different cases. Momentum and heat transfer variations with time are investigated by numerically solving the one-dimensional governing equations for vapor bubble and liquid plugs. Instead of considering all the vapor bubble at saturation temperature, vapor bubbles are allowed to remain in super-heated condition. Film thickness is found using a correlation. Two-phase heat transfer coefficient is calculated by considering conduction through the thin film at liquid–vapor interface. Liquid plug merging and splitting result in continuous variation in the number of liquid plugs and vapor bubble with time, which is also considered in the code. During the merging of liquid plugs, a time step-adaptive scheme is implemented and this minimum time step was found to be 10⁻⁷ s. Model results are compared with the experimental results from literature for heat transfer and the maximum variation in heat transfer for all these cases is below ±39%.     

Technical advances and experimental devices for enteroscopy

Enteroscopy remains the procedure in the gastrointestinal tract that is most inaccessible to endoscopy, and technical limitations severely impair the ability to advance and examine the small bowel reliably or completely. Push-type enteroscopy not only suffers limitations owing to looping in the stomach, but is intrinsically associated with increasing loss of transmission of force to the tip and consequent failure of advancement. Development is this area has been slow, partly owing to the limited clinical need compared with upper gastrointestinal endoscopy, and colonoscopy and consequent financial limitations imposed on further development. Practical and useful technical advances have been made, especially in push-type as well as sonde-type enteroscopy. These are reviewed briefly and are covered in other articles elsewhere in this issue.     

IP-networked multimedia conferencing

This article looks at video conferencing, specifically the VideoTalks system developed by AT&T Labs. This system was designed to alleviate some of the problems of both user experience and audiovisual signal quality. It allows talks and presentations generated in one auditorium or conference room to be received in other conference rooms, multicast to desktop computers, and received at home or on the road. VideoTalks is a prototypical system that can be retrofitted to provide for a variety of Internet services, such as remote learning, narrowcasting to niche audiences, video telephone, video conferencing, low cost video-on-demand, etc. An effort was also made to make the system function without a full-time operator     

BeSleep: Blockchain-enabled distributed sleeping strategies of small base stations in ultra dense networks

Small cell networks can fulfill the increasing demandfor the high data rate of wireless applications. Energy efficiency is an important design parameter of the ultra dense small cell network (UDSCN). The sleeping strategy of small base stations (s-BSs) is used to enhance the network's energy efficiency. An efficient sleeping strategy of s-BSs is required while preserving users' quality of service (QoS). The idle s-BSs can be switched to sleep mode. This paper proposes a blockchain-enabled solution for the sleeping strategy of s-BSs. Here, a blockchain-enabled small cell network is created between the s-BSs. The network is decentralized, which eliminates the workload of the macro base station (MBS). The proposed network architecture is enabled as a decentralized network through blockchain. The blockchain provides distributed control over the s-BS operations through a smart contract. Here, smart contracts act as distributed self organizing network features to handle self-transactions among small cells for switching off s-BSs in the network. All the software logic required to perform s-BS operations is written in a smart contract using Ethereum. The proposed solution improves energy efficiency and enables the ultra dense small cell network to be decentralized.     

Room-temperature ferromagnetic organic magnets derived from fluoro-graphite via facile halide exchange

Room-temperature ferromagnetic organic magnet is developed via a facile halide exchange process using fluoro-graphite (FG) as a starting material. Structural and chemical analysis reveals that heterogeneous C–F bond cleavage in the defect sites of FG is essentially related to the formation of ferromagnetic hydroxyl-graphene (HG). Pyrolysis of FG in a polar solvent with iodine ions induces a formation of metastable C–I bond and subsequent replacement of the I sites with the hydroxyl groups. Specifically, defective sites can be formed in FG due to nucleophile attack where the OH groups can be easily generated. As a result, the FG can be transformed into the HG with a network of sp²-conjugated carbon motifs in a sp³-based graphene matrix. Hence, the paramagnetic centers in the π-electron system of FG can be transformed to the long-range ordered ferromagnetic centers in the sp²-conjugated system of HG. Electron paramagnetic resonance data demonstrate the weak ferromagnetic property of HG stable at room temperature.     

Electrochemical performance of diamond thin-film electrodes from different commercial sources

The electrochemical properties of two commercial (Condias, Sumitomo) boron-doped diamond thin-film electrodes were compared with those of two types of boron-doped diamond thin film deposited in our laboratory (microcrystalline, nanocrystalline). Scanning electron microscopy and Raman spectroscopy were used to characterize the electrode morphology and microstructure, respectively. Cyclic voltammetry was used to study the electrochemical response, with five different redox systems serving as probes (Fe(CN)(6)(3)(-)(/4)(-), Ru(NH(3))(6)(3+/)(2+), IrCl(6)(2)(-)(/3)(-), 4-methylcatechol, Fe(3+/2+)). The response for the different systems was quite reproducibile from electrode type to type and from film to film for electrodes of the same type. For all five redox systems, the forward reaction peak current varied linearly with the scan rate(1/2) (nu), indicative of electrode reaction kinetics controlled by mass transport (semi-infinite linear diffusion) of the reactant. Apparent heterogeneous electron-transfer rate constants, k degrees (app), for all five redox systems were determined from deltaE(p)-nu experimental data, according to the method described by Nicholson (Nicholson, R. S. Anal. Chem. 1965, 37, 1351.). The rate constants were also verified through digital simulation (DigiSim 3.03) of the voltammetric i-E curves at different scan rates. Good fits between the experimental and simulated voltammograms were found for scan rates up to 50 V/s. k degrees (app) values of 0.05-0.5 cm/s were observed for Fe(CN)(6)(3)(-)(/4)(-), Ru(NH(3))(6)(3+/2+), and IrCl(6)(2)(-)(/3)(-) without any extensive electrode pretreatment (e.g., polishing). Lower k degrees (app) values of 10(-)(4)-10(-)(6) cm/s were found for 4-methylcatechol and Fe(3+/2+). The voltammetric responses for Fe(CN)(6)(3)(-)(/4)(-) and Ru(NH(3))(6)(3+/2+) were also examined at all four electrode types at two different solution pH (1.90, 7.35). Since the hydrogen-terminated diamond surfaces contain few, if any, ionizable carbon-oxygen functionalities (e.g., carboxylic acid, pK(a) approximately 4.5), the deltaE(p), i(p)(ox), and i(p)(red) values for the two systems were, for the most part, unaffected by the solution pH. This is in contrast to the typical behavior of oxygenated, sp(2) carbon electrodes, such as glassy carbon.     

Boron-doped diamond microelectrodes for use in capillary electrophoresis with electrochemical detection

The fabrication and characterization of boron-doped diamond microelectrodes for use in electrochemical detection coupled with capillary electrophoresis (CE-EC) is discussed. The microelectrodes were prepared by coating thin films of polycrystalline diamond on electrochemically sharpened platinum wires (76-, 25-, and 10-microm diameter), using microwave-assisted chemical vapor deposition (CVD). The diamond-coated wires were attached to copper wires (current collectors), and several methods were explored to insulate the cylindrical portion of the electrode: nail polish, epoxy, polyimide, and polypropylene coatings. The microelectrodes were characterized by scanning electron microscopy, Raman spectroscopy, and cyclic voltammetry. They exhibited low and stable background currents and sigmoidally shaped voltammetric curves for Ru(NH3)6(3+/2+) and Fe(CN)6(3-/4-) at low scan rates. The microelectrodes formed with the large diameter Pt and sealed in polypropylene pipet tips were employed for end-column detection in CE. Evaluation of the CE-EC system and the electrode performance were accomplished using a 10 mM phosphate buffer, pH 6.0, run buffer, and a 30-cm-long fused-silica capillary (75-microm i.d.) with dopamine, catechol, and ascorbic acid serving as test analytes. The background current (approximately 100 pA) and noise (approximately 3 pA) were measured at different detection potentials and found to be very stable with time. Reproducible separation (elution time) and detection (peak current or area) of dopamine, catechol, and ascorbic acid were observed with response precisions of 4.1% or less. Calibration curves constructed from the peak area were linear over 4 orders of magnitude, up to a concentration between 0.1 and 1 mM. Mass limits of detection for dopamine and catechol were 1.7 and 2.6 fmol, respectively (S/N = 3). The separation efficiency was approximately 33,000, 56,000, and 98,000 plates/m for dopamine, catechol, and ascorbic acid, respectively. In addition, the separation and detection of 1- and 2-naphthol in 160 mM borate buffer, pH 9.2, was investigated. Separation of these two analytes was achieved with efficiencies of 118,000 and 126,000 plates/m, respectively.     

Heat transport and stagnation‐point flow of magnetized nanoliquid with variable thermal conductivity,Brownian moment,and thermophoresis aspects

The improvement of heat transport is a very important phenomenon in nuclear reactors, solar collectors, heat exchangers, and coolers, which can be achieved by choosing the nanofluid as the functional fluid. Nanofluids improve thermophysical properties; as a result, they have made great progress in engineering, biomedical, and industrial applications. Therefore, a numerical study has been proposed to analyze the flow and heat transport of nanoliquids over an extendable surface near a stagnation point with variable thermal conductivity under the influence of the magnetic field, due to their importance in the engineering field. Nanoliquid attributes explain the Brownian motion and the diffusion of thermophoresis. The effects of the chemical reaction and the uniform internal heat source/heat sink are also considered. The Nachtsheim‐Swigert shooting procedure based on the Runge‐Kutta scheme is used for numerical calculation. The impact of effective parameters on velocity, temperature, and volume fraction of the nanoparticles is shown in the graphs and reported in detail. The surface criteria are also estimated with respect to the shear stress and the rate of heat and mass transfer. The aspects of the Brownian moment and Lorentz force are positively correlated to the thermal field of the nanoliquid. Also, the variable thermal conductivity aspect favors the growth of the thermal boundary layer.