Buoyancy Induced Heat Transfer Flow Inside a Tilted Square Enclosure Filled with Nanofluids in the Presence of Oriented Magnetic Field

This paper analyzes heat transfer and fluid flow of natural convection in an inclined square enclosure filled with different types of nanofluids having various shapes of nanoparticles in the presence of oriented magnetic field. The Galerkin weighted residual finite element method has been employed to solve the governing non-dimensional partial differential equations. In the numerical simulations, water, ethylene glycol, and engine oil containing copper, alumina, titanium dioxide nanoparticles are considered. The effects of model parameters such as Rayleigh number, Hartmann number, nanoparticles volume fraction, magnetic field inclination angle, geometry inclination angle on the fluid flow and heat transfer are investigated. The results indicate that increment of the Rayleigh number and nanoparticle volume fraction increase the heat transfer rate in a significant way, whereas, increment of the Hartmann number decreases the overall heat transfer rate. It is also observed that a blade shape nanoparticle gives higher heat transfer rate compared to other shapes of nanoparticles. The critical geometry inclination angle at which the maximum heat transfer rate is achieved depends on the nanoparticle volume fraction as well as on the magnetic field orientation. These results are new and have direct applications in solar thermal collectors and thermal insulator of buildings.     

A review on hydrogen generation from the hydrolysis of sodium borohydride

Hydrogen is a promising alternative energy source to conventional fuels, including fossil fuel. Thus, several methods were reported for the generation of hydrogen. This review provided a comprehensive bibliometric analysis of the publications that focus on the hydrolysis or hydrolytic dehydrogenation of sodium borohydride (NaBH₄). Surveying articles in the literature showed a promising future for this technology, although some challenges lie ahead. The process can be reversible via the regeneration of the reaction by-product (NaBO₂?2H₂O or NaBO₂?4H₂O). The key parameters affecting the hydrolysis reaction of aqueous NaBH₄ were also summarized. The analysis of the publications indicated that hydrogen production techniques need further investigations to be competitive and for renaissance of the current applications. This review also presented concerns behind the commercialization of the generation of hydrogen gas using the hydrolysis of NaBH₄. Several materials have been reported for hydrogen generation, but thus far, no single material can simultaneously meet all the required criteria for mobile applications. Researchers and political decision-makers should manage the progress and open new channels for commercialization purposes. The hydrolysis of NaBH₄ is promising for several applications, including material science, environmental fields, and energy-based applications.     

Influence of Sn content on the electrocatalytic activity of NiSn alloy nanoparticles-incorporated carbon nanofibers toward methanol oxidation

Improvement of the electrocatalytic activity of nickel toward methanol oxidation can be conducted by exploiting the synergetic influence of a co-catalyst and/or utilizing a proper support. In this study, utilizing tin as a co-catalyst and supporting on carbon nanofibers are proposed to enhance methanol oxidation in the alkaline media. Typically, NiSn nanoparticles alloy-incorporated carbon nanofibers could be prepared by calcination of electrospun nanofibers composed of poly (vinyl alcohol), nickel acetate tetrahydrate and tin chloride under argon atmosphere at a high temperature. The influence of the co-catalyst content and the calcination temperature on the morphology, composition and electrocatalytic activity of the proposed nanofibers was investigated. Smooth electrospun nanofibers can be prepared regardless the tin chloride content up to 35 wt%, and the calcination process did not distinctly affect the nanofibrous morphology. Mostly, Ni₃Sn and Ni₃Sn₂ nanoparticles-incorporated amorphous carbon nanofibers were obtained at all the utilized calcination temperatures (700, 850 and 1000 °C) and examined SnCl₂ contents. However, at 10 wt% SnCl₂ content and 850 °C calcination temperature, single metallic compound (Ni₃Sn₂)-incorporated carbon nanofibers were synthesized. Electrochemical measurements indicated that the electrocatalytic activity depends strongly on the tin content as well as the calcination temperature. The nanofibers obtained from electrospun solution containing 10 wt% SnCl₂ and calcined at 850 °C showed very good performance compared to the other formulations. Typically, the corresponding onset potential of the methanol oxidation reaction using these nanofibers catalyst is 315 mV (vs. Ag/AgCl) while it was 405 mV for the nanofibers obtained from electrospun solution containing 0, 5, 15, 25 and 35 wt% SnCl₂. Moreover, the best nanofibers reveal the highest current density. Kinetic study indicated that the corresponding activation energy is 15.6 kJ/mol.     

A study on optimization of acid sites concentration versus pore dimensions in modified solid acid catalysts for biodiesel production

The acid sites concentration of silica sulfuric acid was optimized against pore dimensions depending on the intensity of silanol groups as a key for binding modifier molecules. The texturally modified rice husk silica, of determined intensity of silanol groups, was functionalized with different loadings of SO₃H groups to produce solid acid catalysts with different concentrations of acid sites and different sizes of the pore systems. The catalysts were employed in trans-esterification of waste-cocking oil. The obtained optimum catalytic activity was attributed to the proper compensation between acid sites intensity and pore dimensions. The estimated TOF for methyl ester production was found to decrease with the increase of SO₃H loading on the catalyst surface. The role of compatibility of the reactants phase with the catalyst nature could be validated by the observed much higher activity of sulfonated lignin in the studied esterification reaction than the modified RHS-acid catalysts under study.     

Molecular identification and pectate lyase production by Bacillus strains involved in cocoa fermentation

We have previously reported the implication of Bacillus in the production of pectinolytic enzymes during cocoa fermentation. The objective of this work was to identify the Bacillus strains isolated from cocoa fermentation and study their ability to produce pectate lyase (PL) in various growth conditions. Ninety-eight strains were analyzed by Amplified Ribosomal DNA Restriction Analysis (ARDRA). Four different banding patterns were obtained leading to the clustering of the bacterial isolates into 4 distinct ARDRA groups. A subset of representative isolates for each group was identified by 16S rRNA gene partial sequencing. Six species were identified: Bacillus subtilis, Bacillus pumilus, Bacillus sphaericus, Bacillus cereus, Bacillus thuringiensis, together with Bacillus fusiformis which was isolated for the first time from cocoa fermentation. The best PL producers, yielding at least 9 U/mg of bacterial dry weight, belonged to B. fusiformis, B. subtilis, and B. pumilus species while those belonging to B. sphaericus, B. cereus and B. thuringiensis generally showed a low level of activity. Two kinds of PL were produced, as revealed by isoelectrofocusing: one with a pI of 9.8 produced by B. subtilis and B. fusiformis, the other with a pI of 10.5 was produced by B. pumilus. Strains yielded about 2 fold more PL in a pectic compound medium than in glucose medium and maximum enzyme production occurred in the late stationary bacterial growth phase. Together all these results indicate that PL production in the bacilli studied is modulated by the growth phase and by the carbon source present in the medium.     

Enabling seamless multimedia wireless access through QoS‐based bandwidth adaptation

Effective support of real‐time multimedia applications in wireless access networks, viz. cellular networks and wireless LANs, requires a dynamic bandwidth adaptation framework where the bandwidth of an ongoing call is continuously monitored and adjusted. Since bandwidth is a scarce resource in wireless networking, it needs to be carefully allocated amidst competing connections with different Quality of Service (QoS) requirements. In this paper, we propose a new framework called QoS‐adaptive multimedia wireless access (QoS‐AMWA) for supporting heterogeneous traffic with different QoS requirements in wireless cellular networks. The QoS‐AMWA framework combines the following components: (i) a threshold‐based bandwidth allocation policy that gives priority to handoff calls over new calls and prioritizes between different classes of handoff calls by assigning a threshold to each class, (ii) an efficient threshold‐type connection admission control algorithm, and (iii) a bandwidth adaptation algorithm that dynamically adjusts the bandwidth of an ongoing multimedia call to minimize the number of calls receiving lower bandwidth than the requested. The framework can be modeled as a multi‐dimensional Markov chain, and therefore, a product‐form solution is provided. The QoS metrics—new call blocking probability (NCBP), handoff call dropping probability (HCDB), and degradation probability (DP)—are derived. The analytical results are supported by simulation and show that this work improves the service quality by minimizing the handoff call dropping probability and maintaining the bandwidth utilization efficiently. Copyright © 2006 John Wiley & Sons, Ltd.     

Data-driven safe gain-scheduling control

Data-based safe gain-scheduling controllers are presented for discrete-time linear parameter-varying systems (LPV) with polytopic models. First, $\lambda$-contractivity conditions are provided under which the safety and stability of the LPV systems are unified through Minkowski functions of the safe sets. Then, a data-based representation of the closed-loop LPV system is provided, which requires less restrictive data richness conditions than identifying the system dynamics. This sample-efficient closed-loop data-based representation is leveraged to design data-driven gain-scheduling controllers that guarantee $\lambda$-contractivity and, thus, invariance of the safe sets. It is also shown that the problem of designing a data-driven gain-scheduling controller for a polyhedral (ellipsoidal) safe set amounts to a linear program (a semi-definite program). The motivation behind direct learning of a safe controller is that identifying an LPV system requires satisfying the persistence of the excitation (PE) condition. It is shown in this paper, however, that directly learning a safe controller and bypassing the system identification can be achieved without satisfying the PE condition. This data-richness reduction is of vital importance, especially for LPV systems that are open-loop unstable, and collecting rich samples to satisfy the PE condition can jeopardize their safety. A simulation example is provided to show the effectiveness of the presented approach.