CFD modeling of hydrogen dispersion under cryogenic release conditions

The use of hydrogen as a fuel should always be accompanied by a safety assessment concerning the case of an accidental release. To evaluate the potential hazards in a spill accident both experiments and simulations are performed. In the present work, the CFD code, ADREA-HF, is used to simulate the liquefied hydrogen (LH2) spill experiments (test 5, 6, 7) conducted by the Health Safety Laboratory (HSL). Two horizontal releases, the one along the ground and the other one at a distance above the ground, and one vertical release are examined with spill rate 60 lt/min. The main focus of this study is on the presence of humidity in the atmosphere and its effect on the vapor dispersion. When humidity is present is cooled, condenses and freezes due to the low prevailing temperature (∼20 K near the release), and releases heat. In addition, during the release hydrogen droplets are formed due to mechanical and flashing break up, and water droplets and ice crystals due to humidity phase change. Therefore, two models are tested: the hydrodynamic equilibrium model, which assumes that the phases are in thermodynamic and kinematic equilibrium and the non hydrodynamic equilibrium model (slip model), which assumed that the phases are in thermodynamic equilibrium but they can obtain different velocities. The fluctuating wind direction was also taken into account, since it greatly affects the hydrogen dispersion. The computational results are compared with the experimental measurements, and it is concluded that humidity along with the slip effect influences the buoyancy of the cloud to a great extent. The best simulation case (humidity and slip effect) is consistent with the experiment for all three tests for the majority of the sensors.     

Methacrylated Calix[6]arenes as Low Shrinking Additives for Dental Restorative Composites

p‐tert‐Butylcalix[6]arene was acylated with different carboxylic acid anhydrides to improve their solubility in dimethacrylate cross‐linkers. An esterification of p‐tert‐butylcalix[6]arene 1 with octanoic anhydride followed by methacrylation of the residual hydroxy groups with methacrylic anhydride resulted mainly in the formation of a pentaoctanoatemonomethacrylate 2a . In the radical polymerization of methyl methacrylate the inhibition effect of residual hydroxy groups in 2a was evaluated. Dimethacrylate containing composites based on a monomer matrix of cross‐linking dimethacrylates and 0–30 wt.% of 2a were prepared. The addition of 2a resulted in a significant decrease of the polymerization shrinkage, whereas the modulus of elasticity of the visible light cured composites was not affected.

     

Hydrogen absorption/desorption kinetics of magnesium nano-nickel composites synthesized by dry particle coating technique

Dry particle coating technique was utilized for coating nano-nickel on magnesium particles. The objectives of this study were to evaluate the effects of the nanocatalyst (Ni) on the hydrogen absorption and desorption kinetics of the Mg-based composite (Mg–Ni). Hydrogen absorption curves plotted as a function of time showed that composites processed for longer periods of time exhibited significantly higher hydrogen absorption rates. With increased coating time, the catalyst was more evenly distributed over the Mg surface, resulting in the formation of a composite with increased hydrogen capacity and kinetics. Meanwhile, no significant morphology change in the composites was observed. A change in the hydrogen absorption rate as a function of time was observed. This change in rate implies a change in the rate limiting mechanism from chemical absorption on the Mg particle to diffusion into the Mg particle. TGA results showed that lower coating speeds resulted in lower initial desorption temperatures as well as steeper desorption rates. Higher heating rates resulted in faster reaction rates and therefore reduced hydrogen absorption time and increased hydrogen storage capacity.     

Manipulating surface states in topological insulator nanoribbons

Topological insulators display unique properties, such as the quantum spin Hall effect, because time-reversal symmetry allows charges and spins to propagate along the edge or surface of the topological insulator without scattering. However, the direct manipulation of these edge/surface states is difficult because they are significantly outnumbered by bulk carriers. Here, we report experimental evidence for the modulation of these surface states by using a gate voltage to control quantum oscillations in Bi(2)Te(3) nanoribbons. Surface conduction can be significantly enhanced by the gate voltage, with the mobility and Fermi velocity reaching values as high as ~5,800?cm(2)?V(-1)?s(-1) and ~3.7?×?10(5)?m?s(-1), respectively, with up to ~51% of the total conductance being due to the surface states. We also report the first observation of h/2e periodic oscillations, suggesting the presence of time-reversed paths with the same relative zero phase at the interference point. The high surface conduction and ability to manipulate the surface states demonstrated here could lead to new applications in nanoelectronics and spintronics.     

Teachers’ Perceptions οn the Use of Botanic Gardens as a Means of Environmental Education in Schools and the Enhancement of School Student Benefits from Botanic Garden Visits

The potential of Botanic Gardens (BGs) to contribute to environmental education is well documented. To what degree this potential is met concerning school students’ environmental education is to be considered. In Greece, Environmental Education Centres (EECs) develop Environmental Education Programmes (EEPs) for schools to implement. A questionnaire survey addressing teachers employed at the EECs in Greece investigated the use of BGs as a means of environmental education and identified their most important features in supporting school EEPs. Findings indicated that the majority of EEC teachers (90.6%) have not developed EEPs involving the use of BGs, although BGs were ranked as most suitable amongst other green spaces for conducting such programmes, particularly for primary school students. Teachers identified the need for BGs to be designed accordingly and provide the necessary infrastructure to be educationally effective that included facilitating teaching (e.g. open spaces, areas to congregate, easy access paths and indoor facilities) and plant-orientated elements as well as plant management displays. A strategy by BGs to promote their role in environmental education and a multidisciplinary design approach would maximise student benefit.     

Electrochemical oxidation of ammonia (NH4/NH3) on thermally and electrochemically prepared IrO2 electrodes

The electrochemical oxidation of ammonia (NH₄⁺/NH₃) in sodium perchlorate was investigated on IrO₂ electrodes prepared by two techniques: the thermal decomposition of H₂IrCl₆ precursor and the anodic oxidation of metallic iridium. The electrochemical behaviour of Ir(IV)/Ir(III) surface redox couple differs between the electrodes indicating that on the anodic iridium oxide film (AIROF) both, the surface and the interior of the electrode are electrochemically active whereas on the thermally decomposed iridium oxide films (TDIROF), mainly the electrode surface participates in the electrochemical processes.On both electrodes, ammonia is oxidized in the potential region of Ir(V)/Ir(IV) surface redox couple activity, thus, may involve Ir(V). During ammonia oxidation, TDIROF is deactivated, probably by adsorbed products of ammonia oxidation. To regenerate TDIROF, it is necessary to polarize the electrode in the hydrogen evolution region. On the contrary, AIROF seems not to be blocked during ammonia oxidation indicating its fast regeneration during the potential scan. The difference between both electrodes results from the difference in the activity of the iridium oxide surface redox couples.     

Anomaly‐based intrusion detection of jamming attacks,local versus collaborative detection

We present intrusion detection algorithms to detect physical layer jamming attacks in wireless networks. We compare the performance of local algorithms on the basis of the signal‐to‐interference‐plus‐noise ratio (SINR) executing independently at several monitors, with a collaborative detection algorithm that fuses the outputs provided by these algorithms. The local algorithms fall into two categories: simple threshold that raise an alarm if the output of the SINR‐based metrics we consider deviates from a predefined detection threshold and cumulative sum (cusum) algorithms that raise an alarm if the aggregated output exceeds the predefined threshold. For collaborative detection, we use the Dempster–Shafer theory of evidence algorithm. We collect SINR traces from a real IEEE 802.11 network, and with the use of a new evaluation method, we evaluate both the local and the Dempster–Shafer algorithms in terms of the detection probability, false alarm rate, and their robustness to different detection threshold values, under different attack intensities. The evaluation shows that the cusums achieve higher performance than the simple threshold algorithms under all attack intensities. The Dempster–Shafer algorithm when combined with the simple algorithms, it can increase their performance by more than 80%, but for the cusum algorithms it does not substantially improve their already high performance.Copyright © 2013 John Wiley & Sons, Ltd.     

Performance Evaluation of Lazy Deletion Methods in R-trees

Motivated by the way R-trees are implemented in commercial databases systems, in this paper we examine several deletion techniques for R-trees. In particular, in commercial systems R-tree entries are mapped onto relational tables, which implement their own concurrency protocols on top of existing table-level concurrency mechanisms. In analogy, the actual industrial implementations of B-trees do not apply the well-known merging procedure from textbooks in case of node underflows, but rather they apply the free-at-empty technique. This way, space is sacrificed for the benefit of faster deletions and less locking operations, whereas the search performance practically remains unaffected. In this context, we examine the efficiency of modifications to the original R-tree deletion algorithm, which relax certain constraints of this algorithm and perform a controlled reorganization procedure according to a specified criterion. We present the modified algorithms and experimental results about the impact of these modifications on the tree quality, the execution time for the deletion operation and the processing time of search queries, considering several parameters. The experimental results indicate that the modified algorithms improve the efficiency of the deletion operation, while they do not affect the quality of the R-tree and its performance with respect to search operations.     

Nickel and nickel-phosphorous matrix composite electrocoatings

Nickel and nickel-phosphorous matrix composite coatings reinforced by TiO₂, SiC and WC particles were produced under direct and pulse current conditions from an additive-free Watts' type bath. The influence of the variable electrolysis parameters (type of current, frequency of current pulses and current density) and the reinforcing particles properties (type, size and concentration in the bath) on the surface morphology and the structure of the deposits was examined. It is demonstrated that the embedding of ceramic particles modifies in various ways the nickel electrocrystallisation process. On the other hand, Ni-P amorphous matrix is not affected by the occlusion of the particles. Overall, the imposition of pulse current conditions leads to composite coatings with increased embedded percentage and more homogenous distribution of particles in the matrix than coatings produced under direct current regime.