Homogeneous Schemes Research Articles

The Advents of Ubiquitous Computing in the Development of Smart Cities—A Review on the Internet of Things (IoT)

By leveraging ubiquitous computing and the Internet of Things (IoT), smart cities gain potential to provide a wider range of services. Different homogeneous and heterogeneous networking schemes and applications have been proposed in the literature to date. In these networking schemes, human and computer are connected for social, economic, physiological, and technological growth. However, there is a dearth of recent literature that incorporates recently proposed and operating techniques and technologies capable of enhancing the productivity of human and machine in IoT technologies. The role of this research is to investigate the protocols, followed by the advance frameworks for IoT, the characteristics and services that are being governed using IoT for establishing information-rich smart cities. To this end, likewise, physical layer, media access control, networking and applications protocols, and encapsulation standards of IoT for smart cities applications are critically reviewed. Certain open issues are discussed based on the literature collected that would improve the autonomous behavior, process control, device handling, and the QoS in smart environments.

Numerical investigation of premixed hydrogen/air combustion at lean to ultra-lean conditions and catalytic approach to enhance stability

Premixed combustion of hydrogen/air over a platinum (Pt) catalyst is numerically investigated in a planar channel burner with the aim of stabilising the flame at lean to ultra-lean conditions. A steady laminar species transport model is examined in conjunction with elementary heterogeneous and homogeneous chemical reaction schemes and validated against experimental results. A stability map is obtained in a non-catalytic burner for the equivalence ratios (φ) of 0.15–0.20, which serves as the basis for the catalytic flame analysis. Over the Reynolds numbers (Re) investigated in the non-catalytic burner, no flame is observed for φ ≤ 0.16, and flame extinction occurs at Re < 571 and Re < 381 for φ = 0.18 and 0.20, respectively. Moreover, a significant amount of unburned H2 exits the burner in all cases. With the Pt catalyst coated on the walls, complete H2 combustion is attained for 0.10 ≤ φ ≤ 0.20 where the contribution of gas phase (homogeneous) reaction increases with Re. Furthermore, radiation on the wall and at the inlet affects the combustion kinetics and flame temperature. Finally, NOx emission is investigated under the same conditions and found to increase with equivalence ratio but has a negligible effect with the inflow Reynolds number.

Applicability of conventional protocols for benchmarking of unidirectional and bidirectional multi-electron homogeneous molecular catalysts beyond the pure kinetic regime

Accurate and rational benchmarking of molecular catalysts is essential to achieve high selectivity and efficiency towards electrochemical reactions associated with key energy and environmental challenges. Homogeneous catalytic schemes commonly involve intricate multi-electron, multi-step processes, the behaviour of which can depart significantly from that of the paradigmatic first-order one-electron catalytic mechanism (EC’ scheme). Nevertheless, currently, the quantitative analysis of such complex processes is mostly based on methods developed for the EC’ case. Since no general theoretical framework has been proposed for multi-electron multi-step processes, the degree of applicability of such protocols is unknown. This is investigated in this work by modelling the voltammetry of unidirectional and bidirectional two-electron catalysts (EEC’-type mechanisms). It is demonstrated that so-called pure kinetic (PK) conditions could be difficult to attain in the case of multi-electron catalysts so that the widely-used PK-based protocols of benchmarking can yield inaccurate results. In order to overcome this issue, novel suitable tools are presented.

Fuel-rich hetero-/homogeneous combustion of C3H8/O2/N2 mixtures over rhodium

The catalytic (heterogeneous) and gas-phase (homogeneous) combustion of C3H8/O2/N2 mixtures over rhodium was investigated experimentally and numerically at 5 bar and at fuel-rich equivalence ratios φ = 2.0-3.5 relevant to propane Catalytic Partial Oxidation (CPO). In situ spatially-resolved Raman measurements of major gas-phase species concentrations and Planar Laser Induced Fluorescence (PLIF) of formaldehyde were applied in an optically accessible channel-flow reactor to monitor the catalytic and gas-phase processes, respectively, while accompanying 2D simulations were carried out with detailed hetero-/homogeneous chemical reaction mechanisms. Due to the high gas-phase reactivity of propane, homogeneous chemistry could not be ignored over most of the reactor's oxidation zone length (upstream zone where the deficient reactant oxygen is not fully consumed). The presence of gas-phase chemistry deteriorated the otherwise high catalytic syngas (H2 and CO) selectivities over the oxidation zone. Raman measurements of major gas-phase species concentrations over the restricted oxidation zone length without appreciable gas-phase chemistry showed that the catalytic reaction mechanism slightly underpredicted (overpredicted) the H2 (CO) formation. The same behavior was also attested over the remaining length of the oxidation zone where combined catalytic and gas-phase chemistry was present. The production of considerable amounts of H2 at the highest investigated equivalence ratio of 3.5 accelerated the onset of homogeneous ignition and the formation of strong flames. The discrepancies between measured and predicted homogeneous ignition distances were less than 6.8% in all cases, illustrating the validity of the employed hetero-/homogeneous kinetic schemes. Contrary to past methane CPO studies, the contribution of gas-phase chemistry and the formation of strong flames in propane CPO was detrimental to syngas production.

An analytical and experimental comparison of maximal lottery schemes

Maximal lottery ( ML ) schemes constitute an interesting class of randomized voting rules that were proposed by Peter Fishburn in 1984 and have been repeatedly recommended for practical use. However, the subtle differences between different ML schemes are often overlooked. Two canonical subsets of ML schemes are schemes (which only depend on unweighted majority comparisons) and schemes (which only depend on weighted majority comparisons). We prove that schemes are the only homogeneous ML schemes that satisfy SD -efficiency and SD -participation, but are also among the most manipulable ML schemes. While all ML schemes are manipulable and even violate monotonicity, they are never manipulable when a Condorcet winner exists and satisfy a relative notion of monotonicity. We also evaluate the frequency of manipulable preference profiles and the degree of randomization of ML schemes via extensive computer simulations. In summary, ML schemes are rarely manipulable and often do not randomize at all, especially for few alternatives. The average degree of randomization of schemes is consistently lower than that of schemes.

Modelling of a soft tunable lens actuated by an annular dielectric elastomer actuator with homogeneous and inhomogeneous prestretches

Unlike the conventional tunable lens which contains cumbersome elements to fulfill the focus adjustment, soft liquid lens shows distinct advantages in compactness and flexibility. This paper presents a computational model of a dielectric elastomer (DE)-based, liquid-coupled soft tunable lens. The vital component of the lens system is a DE film which is divided into lens part and actuation part. The actuation part is annular which is coated with compliant electrodes while the lens part is circular and uncoated. A fixed volume of glycerol is injected into the lens to act as a transparent medium. An applied voltage will trigger the transmission of liquid between the two parts and eventually change the focal length. Governing equations for the two parts are established and the shooting method with two initial guesses is developed to solve the boundary value problems. The simulation results agree well with the experimental data. Several parametric analyses are carried out based on the developed model. It is found that the film thickness, initial volume of the liquid, the prestretch, and areas of the two parts all have significant effects on the focusing ability of the tunable lens. Both the homogeneous and inhomogeneous prestretched schemes are considered. An inhomogeneous prestretch on the annular actuation part is beneficial for improving the focusing ability of the tunable lens.

Reversible Multiple Items Authentication Scheme for WSNs

In this article, a reversible authentication scheme for wireless sensor networks (WSNs) is proposed aimed at multiple items authentication and good imperception of transmitted data. Firstly, the WSNs data stream is divided into some authentication groups, and each authentication group is composed of a generator group and a carrier group. Then, the cyclical redundancy check (CRC) code of the generator group is generated as the authentication information, which is composed of the CRC codes of all collected items in the generator group. In the carrier group, first, to decrease the visual perception introduced by information embedding, the beginning parameter T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> and the end parameter T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> are used to select the fluctuation region of the prediction-error histogram (PEH) where large prediction errors occur. Then, using the prediction-error-based histogram shifting method, the authentication information is reversibly embedded into the fluctuation region. Experimental results and analysis demonstrate that, compared with up-to-date homogeneous schemes, the proposed scheme achieved better performance on invisibility, false positive rate, type of authenticated items, and attraction to attackers.

ОДНОРОДНЫЕ РАЗНОСТНЫЕ СХЕМЫ ДЛЯ СОПРЯЖЕННЫХ ЗАДАЧ ГИДРОДИНАМИКИ И УПРУГОСТИ

ОДНОРОДНЫЕ РАЗНОСТНЫЕ СХЕМЫ ДЛЯ СОПРЯЖЕННЫХ ЗАДАЧ ГИДРОДИНАМИКИ И УПРУГОСТИ

Homogeneous Catalysis of Electrochemical Reactions: The Steady-State and Nonsteady-State Statuses of Intermediates

In the context of modern energy challenges, there is an increasing need to decipher the mechanism of complex, multistep catalytic processes, as a basis of their optimization and improvement. Cyclic voltammetry (CV) is one of the most popular electrochemical techniques in this purpose. Mechanistic complexities often trigger a quest for simplification in the treatment of data, such as the application of the steady-state approximation to intermediates. The validity of such assumptions actually needs justification. This is the object of the present work, which examines the question for five homogeneous catalytic reaction schemes of practical interest, which can also serve as tutorial examples for the analysis of further schemes. The analysis is simplified by the consideration of pure kinetic conditions and constancy of substrate concentration. These conditions can be achieved, in practice, by appropriate manipulation of the scan rate and concentrations. The current–potential responses are consequently S-shape...

Однородные разностные схемы для решения сопряжённых задач гидродинамики и упругости

Однородные разностные схемы для решения сопряжённых задач гидродинамики и упругости

Homogeneous ignition during fuel-rich H2/O2/N2 combustion in platinum-coated channels at elevated pressures

The hetero-/homogeneous combustion of fuel-rich H2/O2/N2 mixtures (equivalence ratios φ = 2.5-6.5) was investigated experimentally and numerically in a platinum-coated channel at pressures p = 1-14bar. One-dimensional Raman measurements of major gas-phase species concentrations over the catalyst boundary layer assessed the heterogeneous combustion processes, while planar laser induced fluorescence (LIF) of OH at pressures below ∼5bar and of hot-O2 at pressures above ∼5bar (wherein OH-LIF was not applicable) determined the onset of homogeneous ignition. Simulations were carried out using a 2-D code with detailed hetero-/homogeneous chemical reaction schemes and transport. Both Raman measurements and numerical simulations attested a transport-limited catalytic conversion of the deficient O2 reactant over the gas-phase induction zones. The agreement between measured and predicted homogeneous ignition distances was better than 12%, thus establishing the aptness of the employed hetero-/homogeneous chemical reaction mechanisms. Analytical homogeneous ignition criteria revealed that the catalytic reaction pathway introduced a scaling factor 1/p to the homogeneous ignition distances. This outcome, in conjunction with the intricate pressure dependence of the gaseous ignition chemistry of hydrogen, yielded shorter homogeneous ignition distances at 14bar compared to 1bar. The practical implication for gas turbine burners utilizing the catalytic-rich/gaseous-lean combustion concept was that the high operating pressures of such systems promoted the onset of homogeneous ignition within the catalytic module. Sensitivity analysis has finally identified the key catalytic and gaseous reactions affecting homogeneous ignition.

On Fine-Grained Runtime Power Budgeting for Networks-on-Chip Systems

Power budgeting is an essential aspect of networks-on-chip (NoC) to meet the power constraint for on-chip communications while assuring the best possible overall system performance. For simplicity and ease of implementation, existing NoC power budgeting schemes treat all the individual routers uniformly when allocating power to them. However, such homogeneous power budgeting schemes ignore the fact that the workloads of different NoC routers may vary significantly, and thus may provide excess power to routers with low workloads, whereas insufficient power to those with high workloads. In this paper, we formulate the NoC power budgeting problem in order to optimize the network performance over a power budget through per-router frequency scaling. We take into account of heterogeneous workloads across different routers as imposed by variations in traffic. Correspondingly, we propose a fine-grained solution using an agile algorithm with low time complexity. Frequency of each router is set individually according to its contribution to the average network latency while meeting the power budget. Experimental results have confirmed that with fairly low runtime and hardware overhead, the proposed scheme can help save up to <inline-formula> <tex-math notation="LaTeX">$50$</tex-math></inline-formula> percent application execution time when compared with the latest proposed methods.

Homogeneous and Heterogeneous Combustion in Hydrogen-Fueled Catalytic Microreactors

The hetero-/homogeneous combustion and interaction of hydrogen-fueled catalytic microreactors were investigated numerically. A two-dimensional CFD (computational fluid dynamics) model was developed, using elementary homogeneous and heterogeneous chemical reaction schemes, surface radiation heat transfer, heat conduction in the solid wall, and external heat losses. Computations were carried out to study the effects of the wall thermal conductivity, equivalence ratio, microreactor dimension, and inlet velocity on combustion characteristics, flame stability, and hetero-/homogeneous interaction. Despite the micro-scale, large transverse gradients in species mass fractions and temperature exist in the fluid and large axial gradients in temperature may exist in the walls. Wall thermal conductivity is crucial in determining the flame stability, as the walls transfer heat upstream for ignition of the cold incoming reactants but at the same time are responsible for heat losses. Combustible mixtures with compositions away from the stoichiometric point decrease the homogeneous chemistry contribution and the operating temperature. The microreactor dimension and inlet velocity have a strong effect on homogeneous flame stability. Smaller microreactors result in extinction because of the inhibition of homogeneous combustion induced by heterogeneous reaction; larger microreactors result in blowout due to the reduction of the heterogeneous contribution. Hetero-/homogeneous interaction maps were constructed in terms of microreactor dimension and inlet velocity.

Homogeneous and Heterogeneous Combustion in Hydrogen-Fueled Catalytic Microreactors

The hetero-/homogeneous combustion and interaction of hydrogen-fueled catalytic microreactors were investigated numerically. A two-dimensional CFD (computational fluid dynamics) model was developed, using elementary homogeneous and heterogeneous chemical reaction schemes, surface radiation heat transfer, heat conduction in the solid wall, and external heat losses. Computations were carried out to study the effects of the wall thermal conductivity, equivalence ratio, microreactor dimension, and inlet velocity on combustion characteristics, flame stability, and hetero-/homogeneous interaction. Despite the micro-scale, large transverse gradients in species mass fractions and temperature exist in the fluid and large axial gradients in temperature may exist in the walls. Wall thermal conductivity is crucial in determining the flame stability, as the walls transfer heat upstream for ignition of the cold incoming reactants but at the same time are responsible for heat losses. Combustible mixtures with compositions away from the stoichiometric point decrease the homogeneous chemistry contribution and the operating temperature. The microreactor dimension and inlet velocity have a strong effect on homogeneous flame stability. Smaller microreactors result in extinction because of the inhibition of homogeneous combustion induced by heterogeneous reaction; larger microreactors result in blowout due to the reduction of the heterogeneous contribution. Hetero-/homogeneous interaction maps were constructed in terms of microreactor dimension and inlet velocity.

Detecting and Preventing the Formation of Photosensitizer‐Catalyst Colloids in Homogeneous Light‐Driven Water Oxidation

Combining cationic ruthenium photosensitizers (RuPS) with anionic polyoxometalate water oxidation catalysts (POM-WOCs) is the standard approach for light-driven POM-based water oxidation. Here, we show that colloid formation by electrostatic aggregation of a molecular photosensitizer {e.g., [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine)} and a POM-WOC {e.g., [Co4(H2O)2(α-PW9O34)2]10–} significantly affects catalytically relevant system parameters. A facile, quantitative procedure for colloid detection using syringe filtration and UV/Vis spectroscopy is presented, and we illustrate that photosensitizer-POM colloid formation is a general phenomenon under typical WOC conditions and is observed for a range of photosensitizers and POMs. It is further demonstrated that for some systems an increase in the ionic strength of the solution prevents colloid formation. Significant changes in the electronic interactions between RuPS and POM-WOC under colloidal and homogeneous conditions are reported, thus highlighting the need for fast and reliable colloid identification. In addition, the study raises awareness about colloid formation in homogeneous solar-energy conversion schemes driven by two or more ionic species.

Unequal diffusivities case of homogeneous–heterogeneous reactions within viscoelastic fluid flow in the presence of induced magnetic-field and nonlinear thermal radiation

This article presents the effects of nonlinear thermal radiation and induced magnetic field on viscoelastic fluid flow toward a stagnation point. It is assumed that there exists a kind of chemical reaction between chemical species A and B. The diffusion coefficients of the two chemical species in the viscoelastic fluid flow are unequal. Since chemical species B is a catalyst at the horizontal surface, hence homogeneous and heterogeneous schemes are of the isothermal cubic autocatalytic reaction and first order reaction respectively. The transformed governing equations are solved numerically using Runge–Kutta integration scheme along with Newton’s method. Good agreement is obtained between present and published numerical results for a limiting case. The influence of some pertinent parameters on skin friction coefficient, local heat transfer rate, together with velocity, induced magnetic field, temperature, and concentration profiles is illustrated graphically and discussed. Based on all of these assumptions, results indicate that the effects of induced magnetic and viscoelastic parameters on velocity, transverse velocity and velocity of induced magnetic field are almost the same but opposite in nature. The strength of heterogeneous reaction parameter is very helpful to reduce the concentration of bulk fluid and increase the concentration of catalyst at the surface.

Hetero-/homogeneous combustion of hydrogen/air mixtures over platinum: Fuel-lean versus fuel-rich combustion modes

The catalytic combustion of hydrogen/air mixtures in platinum-coated channels was investigated by means of two-dimensional numerical simulations with detailed heterogeneous (catalytic) and homogeneous (gas-phase) chemical reaction schemes, detailed species transport, and heat transfer mechanisms in the solid wall. Three different combustion modes relevant to power generation were analyzed and their ranges of applicability were delineated. The investigated modes included the fuel-lean catalytically stabilized thermal combustion (CST), the inverse catalytically stabilized thermal combustion (i-CST), and the fuel-rich catalytic/gaseous-lean combustion. In the fuel-lean CST approach, the less-than-unity Lewis number of hydrogen led to superadiabatic surface temperatures. Even though the presence of homogeneous combustion moderated the wall temperature superadiabaticity, reactor and catalyst thermal stability considerations limited the CST mode to equivalence ratios less than 0.2. In the i-CST mode, the maximum attained wall temperatures strongly depended on the location of homogeneous ignition, as it directly affected the magnitude of the wall temperature superadiabaticity induced by heat recirculation inside the solid walls. The i-CST had advantages over CST in terms of reactor thermal management and extended the range of applicable equivalence ratios to 0.3. In the fuel-rich catalytic combustion mode, the larger-than-unity Lewis number of the deficient oxygen reactant and the cooling of the catalytic channel by the resulting bypass air led to surface temperatures below the adiabatic equilibrium value, even for those cases where homogeneous combustion was established in the channel. This mode facilitated safe reactor operation for overall equivalence ratios up to 0.5 and inlet temperatures up to 800 K, an operational range of particular interest for gas turbines.

Homogeneous combustion of fuel-lean syngas mixtures over platinum at elevated pressures and preheats

The gaseous oxidation of H2/CO/CO2/O2/N2 mixtures was investigated experimentally and numerically in a platinum-coated channel at fuel-lean stoichiometries (equivalence ratios φ⩽0.30), H2:CO molar ratios 0.47–4.54, pressures 2–14bar, and reactant preheats up to 736K. Two-dimensional laser induced fluorescence of the OH radical monitored the homogeneous (gaseous) combustion, while 1-D Raman spectroscopy assessed the heterogeneous (catalytic) conversion of H2 and CO. Numerical simulations, which were carried out with a 2-D elliptic code and detailed hetero-/homogeneous reaction schemes, reproduced the measured onset of homogeneous ignition, the ensuing flame shapes, and the mass-transport-limited catalytic conversion of H2 and CO. Additional simulations in practical tubular channels with 1mm diameter have shown that gaseous oxidation was suppressed at atmospheric pressure due to the intrinsic slow gas-phase ignition kinetics in conjunction with the competition from the catalytic pathway for H2 and CO consumption. At pressures p>4bar, homogeneous combustion was largely controlled by flame propagation characteristics due to the near-wall confinement of the established flames. The decrease in laminar mass burning rates at p>4bar led to a push of the gaseous combustion zone close to the channel wall, to leakage of H2 and CO through the flame and, finally, to subsequent catalytic conversion of the leaked fuel components. Radical heterogeneous reactions promoted mildly the onset of homogeneous ignition at p⩾2bar due to the net desorptive flux of OH over the gaseous induction zone. The catalytically produced H2O had a strong kinetic impact on homogeneous combustion by inhibiting the gaseous oxidation of both H2 and CO at high H2:CO ratios and by promoting CO gaseous oxidation at low H2:CO ratios. The catalytically produced CO2 always inhibited kinetically the gaseous combustion of H2 and CO, although its effect was much weaker compared to that of H2O.

Hetero-/homogeneous combustion of ethane/air mixtures over platinum at pressures up to 14 bar

The hetero-/homogeneous combustion of fuel-lean ethane/air mixtures over platinum was investigated experimentally and numerically at pressures of 1–14bar, equivalence ratios of 0.1–0.5, and surface temperatures ranging from 700 to 1300K. Experiments were carried out in an optically accessible channel-flow reactor and included in situ 1-D Raman measurements of major gas phase species concentrations across the channel boundary layer for determining the catalytic reactivity, and planar laser induced fluorescence (LIF) of the OH radical for assessing homogeneous ignition. Numerical simulations were performed with a 2-D CFD code with detailed hetero-/homogeneous C2 kinetic mechanisms and transport. An appropriately amended heterogeneous reaction scheme has been proposed, which captured the increase of ethane catalytic reactivity with rising pressure. This scheme, when coupled to a gas-phase reaction mechanism, reproduced the combustion processes over the reactor extent whereby both heterogeneous and homogeneous reactions were significant and moreover, provided good agreement to the measured homogeneous ignition locations. The validated hetero-/homogeneous kinetic schemes were suitable for modeling the catalytic combustion of ethane at elevated pressures and temperatures relevant to either microreactors or large-scale gas turbine reactors in power generation systems. It was further shown that the pressure dependence of the ethane catalytic reactivity was substantially stronger compared to that of methane, at temperatures up to 1000K. Implications for high-pressure catalytic combustion of natural gas were finally drawn.

The Classes of the Quasihomogeneous Hilbert Schemes of Points on the Plane

Abstract: In this paper we give a formula for the classes (in the Grothendieck ring of complex quasi-projective varieties) of irreducible components of -quasi-homogeneous Hilbert schemes of points on the plane. We find a new simple geometric interpretation of the -Catalan numbers. Finally, we investigate a connection between -quasi-homogeneous Hilbert schemes and homogeneous nested Hilbert schemes. Key words and phrases: Hilbert scheme, torus action, -Catalan numbers.