A microscopic model for catalyst surfaces—II: Supported catalysts

A new “pebbly surface” model of supported metal catalysts is presented which can explain observed multiplicity and oscillatory behaviour of catalyst particles. This model combines the dynamic behavior of the individual metal crystallites on the surface of the support with diffusion and heat transfer processes in the porous particle. The bifurcation of oscillations is analyzed in terms of relaxation oscillations as well as by Hopf's theorem and it is shown how the bifurcation phenomena depend on the system parameters. The parameter study may be used to suggest operating conditions and catalyst design parameters such that sintering effects may be reduced. Qualitative comparisons with data for carbon monoxide oxidation at various temperatures show very good agreement; however, this present simple form of the pebbly surface model seems unable to predict the long period oscillations observed in hydrogen oxidation.     

Periodic pressure fluctuations in fluidized beds

In shallow gas fluidized beds harmonic oscillations of the pressure around its equilibrium value can be observed. Three aspects of these vibrations have been analysed: the frequency, the critical bed height and the damping. The frequency decreases with the inverse of the square root of the bed height for values below the critical height.For bed heights larger than the critical height the fluctuations cease to be harmonic, the bed breaks up and voids are formed leading to the formation of bubbles or slugs. The critical bed height can be calculated from the frequency and the wave velocity. The maximum value of the critical bed height is a few hundred particle diameters, thus most beds will fluidize heterogeneously. Damping of the oscillations is governed by the ratio of the fluid- to solids-density; the lower this values the higher the damping. The damping is liquid fluidized beds is such that oscillations are prevented.     

Self-organization phenomena at semiconductor electrodes

Anodically dissolving semiconductor electrodes such as Si, Ge, GaAs, InP, or GaP exhibit a number of self-organization phenomena such as current oscillations in time and/or in space; some phenomena of this kind are also found during the anodic formation of porous metal oxides. Current oscillations in space are expressed in correlated pore growth and other effects like pore diameter oscillation; this will be introduced and discussed in some detail. Some less well-known effects like self-induced growth mode transitions or pore density oscillations are also included. The paper endeavors to sort through the various self-organization phenomena observed so far and to look for underlying principles that transcend semiconductor-specific dissolution chemistry. Intrinsic time and length scales provide one such principle and this will be discussed with emphasis on the so-called current burst model originally developed for current oscillations in time.     

基于谱熵的自适应广义S变换时频滤波器设计

将信息熵引入到广义S变换时频滤波中,提出一种基于谱熵的自适应广义s变换时频滤波器设计方法。计算信号广义s变换的时频谱,将时频谱各时间点的频率区间划分成若干个小区间,计算各区间时频谱熵,以时频谱熵最大区间的时频谱的平均值作为阅值,以此来衡量信号的杂乱程度。若该时间点上的时频谱值大于闻值,则将该点时频谱值清零。采用广义s逆变换进行重构。该方法可自适应地调节时频滤波因子,有效滤除干扰信息。     

Impedance study of formic acid oxidation on platinum electrodes

The electrooxidation of formic acid on polycrystalline platinum electrodes in sulphuric acid is studied by a dynamic impedance method. Impedance spectra in the frequency range 0.7 Hz to 20 kHz, assembled from ac voltammograms, are interpreted in terms of a mechanism with three adsorbed species, although one of these may be free sites. The spectra show evidence for incipient oscillations in two potential regions, and observed oscillations were related to these spectra. The zeroes of the interfacial impedance were directly extracted from suitable equivalent circuits in distinct potential regions where oscillations may be triggered. The observation of only real zeroes of the interfacial impedance eliminates the possibility of pure potentiostatic oscillations arising from chemical reasons alone, and therefore potential is an essential variable.     

Interface Oscillations in Boiling

Interface oscillations during boiling are studied experimentally and theoretically. The output-signal spectra of light and acoustic sensors indicate that the liquid-pressure and interface oscillations are interrelated. A model is suggested for the oscillations of a bubble growing on a heating surface, and the existence of an attractor is deduced for the first time from the solution of model equations. The data obtained enable one to determine the ranges of interface oscillation amplitude and frequency and can be used in the modeling of boiling.     

Catalytic Performance of Ce1−x Ni x O2 Catalysts for Propane Oxidative Steam Reforming

The model, proposed by Peskov et al. (Catal Today 105:223, 2005) to describe kinetic oscillations observed in the N₂O + H₂ reaction on Ir(110), is used to illustrate the effect of periodic perturbation of the reactant pressures on oscillations. The attention is focused on the conversion of natural period-1 oscillations to period-1 oscillations with the imposed frequency. For this conversion, simultaneous perturbation of both reactant pressures is found to be much more efficient compared to perturbation of one of the pressures.     

Time-scale transformations of discrete time processes

Abstract.  This paper investigates the effects of temporal aggregation when the aggregation frequency is variable and possibly stochastic. The results that we report include, as a particular case, the well-known results on fixed-interval aggregation, such as when monthly data are aggregated into quarters. A variable aggregation frequency implies that the aggregated process will exhibit time-varying parameters and non-spherical disturbances, even when these characteristics are absent from the original model. Consequently, we develop methods for specification and estimation of the aggregate models and show with an example how these methods perform in practice.     

Autoregression method for determining the decrement and frequency of the natural oscillations of a gas in a combustion chamber

Conclusions A new method for determining the decrement and frequency of the natural oscillations of the gas from the intrachamber noise for stable combustion regimes has been developed. The accuracy of estimates of the decrement and frequency of oscillations obtained by the given method has been analyzed, and rules for processing of realizations of random oscillations of the gas have been determined which provide the required accuracy of the estimates of the decrement in the range 0.01–0.4. The proposed approach for the construction of an algorithm for the processing of experimental data reduces the problem to a linear one and allows the application of mini- and micro-computers to its solution. The method possesses low sensitivity to the presence of additive noise in the recorded signal and provides the required accuracy for the determination of small values of the decrement. For the choice of a filter which ensures a signal/noise ratio of 16 or greater, application of the method is not required if simulators are employed in the calibration of the analytic procedure. The proposed method can be implemented in all branches of technology where the mechanism of generation of processes to be analyzed can be represented as the response of a linear, oscillatory system to a wide-band random input. Moscow. Translated from Fizika Gorenia i Vzryva, Vol. 23, No. 6, pp. 33–40, November–December, 1987.     

Measurement of polymer viscoelastic response during an impact experiment

Instrumented impact data on polymers typically show high frequency oscillations that decay in amplitude during the impact event. By undertaking a complete impact analysis including effects of inertial loading, external damping (friction), and internal damping (viscoelastic relaxations), we have been able to predict all the details of the high frequency oscillations. The most consistent analysis of impact data on Delrin® 100 polyacetal is one that ignores friction and attributes the decay of the high frequency oscillations to the effect of viscoelastic relaxations in the polymer. The impact analysis therefore allows us to extract viscoelastic relaxation information from Instrumented impact data. It is further suggested that this direct observation of viscoelastic relaxations during impact testing should make it possible to make a quantitative assessment of the role that viscoelastic relaxations play in influencing the impact toughness of polymers.     

Rapid method for determination of the thermostability of refractory products

A method for estimating the thermostability of refractories by the change in the frequency of natural oscillations of a specimen after a single thermal shock is presented. The method has been tested in estimating the thermostability of silicon carbide plates on silica binder produced by the Volzhskii Abrasive Plant. At the same time, a direct dependence of the apparent density on the frequency of natural oscillations of a specimen is established, the method is quicker and less laborious than the conventional one. Translated from Ogneupory, No. 4, pp. 18 – 19, April, 1994.     

Strategies for damping the oscillations of the alternating direction implicit method of simulation of diffusion-limited chronoamperometry at disk electrodes

Five methods are described for reducing or eliminating the error oscillations resulting from the application of the Peaceman-Rachford alternating direction implicit algorithm to disk electrode simulation problems involving an initial discontinuity, such as results from a potential jump at an electrode. The methods are: (a) the straight-forward application of the Peaceman-Rachford ADI algorithm, using sufficiently small time intervals so that the oscillations are damped at times for which accurate current values are needed; (b) using the first-order alternating direction implicit algorithm by Douglas and Rachford; (c) subdivision of the first simulation time interval into subintervals; (d) beginning the simulation with a few steps of the Douglas-Rachford algorithm followed by the Peaceman-Rachford method; and (e) presetting a reasonable approximation to the concentration profile at the end of the first simulation step and simulating from there on.Methods (d) and (e) are found clearly to be the most efficient at damping the oscillations, but method (b) also eliminates oscillations and leads to reasonable computation times, about one third of those needed for a sparse matrix solution of a two-dimensional system.     

Methods for root cause diagnosis of plant‐wide oscillations

Plant‐wide oscillations are common in many industrial processes. They may impact the overall process performance and reduce profitability. It is important to detect and diagnose such oscillations. This paper reviews advances in diagnosis of plant‐wide oscillations. The main focus of this study is on identifying possible root causes of oscillations using two techniques, one based on data analysis in the temporal and spectral domains and the other based on process connectivity analysis. The process data‐based analysis provides an effective way to capture the difference between the root cause variable and the secondary propagated oscillating variables. It is shown that process topology‐based methods are capable of finding oscillation propagation pathways and, thus, help in determining the root cause. This paper discusses and compares five such methods—spectral envelope, adjacency matrix, Granger causality, transfer entropy, and Bayesian network inference methods— by application to an industrial benchmark dataset. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2019–2034, 2014     

Effect of frequency and oscillation amplitude on moldability of alumina-foam-polystyrene bodies

Conclusions Molding alumina-foam polystyrene bodies at low vibration frequencies (1000 oscillations per minute) is not recommended. The high-frequency vibration (6000 oscillations per minute) with maximum amplitudes is of no great advantage over a frequency of 3000 oscillations per minute in relation to any reduction in the viscosity of the bodies. At a frequency of 3000 oscillations per minute and an amplitude of 1.0 mm the products have the maximum strength. The optimum moisture content of these bodies, facilitating immediate deshuttering of the freshly molded products is 22% using Soviet alumina, and 19% using Hungarian alumina.Translated from Ogneupory, No. 3, pp. 55–58, March, 1973.     

Radiation-absorption,chemical reaction,Hall and ion slip impacts on magnetohydrodynamic free convective flow over semi-infinite moving absorbent surface

The investigation of radiation-absorption,chemical reaction,Hall and ion-slip impacts on unsteady MHD free convective laminar flow of an incompressible viscous,electrically conducting and heat generation/absorbing fluid enclosed with a semi-infinite porous plate within a rotating frame has been premeditated.The plate is assumed to be moving with a constant velocity in the direction of fluid movement.A uniform transverse magnetic field is applied at right angles to the porous surface,which is absorbing the fluid with a suction velocity changing with time.The non-dimensional governing equations for present inves-tigation are solved analytically making use of two term harmonic and non-harmonic functions.The graphical results of velocity,temperature and concentration distributions on the analytical solutions are displayed and discussed with reference to pertinent parameters.It is found that the velocity profiles decreased with an increasing in Hartmann number,rotation parameter,the Schmidt number,heat source parameter,while it increased due to an increase in permeability parameter,radiation-absorption param-eter,Hall and ion slip parameters.However,the temperature profile is an increasing function of radiation-absorption parameter,whereas an increase in chemical reaction parameter,the Schmidt num-ber Sc or frequency of oscillations decrease the temperature profile on cooling.Also,it is found that the concentration profile is decreased with an escalating in the Schmidt number or the chemical reaction parameter.     

螺旋管内单相液体紊流脉动流动传热

以水为工质对螺旋管内单相液体充分发展紊流脉动传热特性进行了实验研究 ,发现了紊流脉动传热系数与流量脉动相位间的反相关系 ,引进了两个新的准则数 :脉动频率数W0 和脉动振幅率Ap,对紊流脉动传热规律进行了分析讨论 ,并根据紊流脉动传热参数的瞬态变化规律 ,对脉动过程中的管内二次流特征和变化作了详细讨论和分析 .     

On specific properties of electrochemical immittance close to discontinuity points

Unusual electrochemical immittance spectra (eis) recorded at Cu rotating disc electrode in CuSO₄+H₂SO₄, solution and analogous to spectra at states close to oscillations reported by other authors, were interpreted as representative for non-minimum phase (nmp) and minimum phase (mp) systems. The electrode was stable under voltage control (potentiostatic conditions, pc) but its stability under current control (galvanostatic conditions, gc) changed at Hopf bifurcation corresponding to zero admittance (infinite impedance).CNLS fitting procedure was applied using 3rd order rational function of complex frequency and from its parameters the electrical equivalent circuit and zero-pole representations of electrode impedance were calculated. All zeros were real negative numbers (stability under pc); two, out of three, poles were complex conjugate numbers, which in case of dataset II (stability under gc, mp behavior) had negative real parts and for dataset I (unstable under gc, nmp behavior)—positive real parts.Transition from nmp- to mp-type of immittance resulting from short anodic polarization was attributed to local activation of the passive electrode corresponding to the appearance of parallel conductance. This was demonstrated by numerical simulation showing transformation of the mp-type immittance to the nmp-type one as a result of increasing parallel conductance.