CO-activator model for reconstructing Pt(100) surfaces: local microstructures and chemical turbulence

We present the results of the modelling of CO adsorption and catalytic CO oxidation on inhomogeneous Pt(100) surfaces which contain structurally different areas. These areas are formed during the CO-induced transition from a reconstructed phase with hexagonal geometry of the overlayer to a bulk-like (1x1) phase with square atomic arrangement. In the present approach, the surface transition is explained in terms of nonequilibrium bistable behavior. The bistable region is characterized by a coexistence of the hexagonal and (1x1) phases and is terminated in a critical bifurcation point which is located at (T_c ~680 K, p_CO ~10 Torr). Due to increasing fluctuations, the behavior at high temperatures and pressures in the vicinity of this cusp point should be qualitatively different from the hysteresis-type behavior which is typically observed in the experiments under ultrahigh vacuum conditions. On the inhomogeneous surface, we find a regime of nonuniform oscillations characterized by random standing waves of adsorbate concentrations. The resulting spatial deformations of wave fronts allow to gain deeper insight into the nature of irregular oscillations on Pt(100) surface.Comment: 11 pages, 11 figures, to appear in Physical Review

Experimental investigation of oscillatory heat release mechanisms and stability margin analysis in lean -premixed combustion

Lean-premixed combustion has become an acceptable means of achieving ultra-low NOx emissions from land-based gas turbines. Further reduction may be possible through the use of hydrogen augmented or syngas fuels. However, advanced combustor designs developed to utilize these technologies often encounter thermoacoustic instabilities that may significantly hamper engine performance and shorten component life-cycles. These dynamics, although not fully understood, occur through a complex interaction between variations in heat release rate and acoustic properties of the system, and can be exacerbated by variable fuel properties in natural gas and syngas applications.;Theoretical models of thermoacoustic instabilities have attempted to describe the coupling process through reduced-order models that represent mechanisms suspected of contributing to variations in the heat release rate such as variations in fuel/air mixing, fluctuations of heat release through vortex shedding and periodic changes in the flame structure. These reduced-order models have demonstrated only a modest ability at predicting instabilities even in relatively simple systems. This may be due to the inherent complexity from interacting processes, the use of over-simplifying assumptions and the lack of experimental verification.;In this study a simple conical flame, used to reduce the number of contributing mechanisms, is utilized to experimentally evaluate the relationship between the heat release rate and variations in the flame surface area. Results indicated that while area perturbations can adequately describe the magnitude of heat release fluctuations, the area perturbations are not a direct indicator of the phase of heat release needed for closed-loop stability analysis.;Time-resolved particle image velocimetry was used to quantify the near-field acoustics and the dilatation rate field in the pre- and post-flame regions of the flow. Measurements indicated that multi-dimensional acoustics dominate the pre-combustion flow field with radial and axial acoustic velocities of similar magnitudes. Variations in the flame structure potentially due to alternating regions of positive and negative flame stretch were also observed and may result in variations in the flame speed. As it is common to assume constant flame speed and one-dimensional acoustics, the experimental identification of these altered mechanisms may help to resolve discrepancies compared to a number of published reduced-order models

The mechanism for the electrooxidation of procarbazine pharmaceutical preparation in alkaline media and its mathematical description

The mechanism for the electrooxidation of procarbazine in alkaline media has been proposed. The process is realized completely on the electrode surface and is adsorption-controlled. The oscillatory behavior in this case is more probable, than for neutral media and may be caused by influences of electrochemical oxidation and salt dissolution from the electrode surface

On the design and simulation of an airlift loop bioreactor with microbubble generation by fluidic oscillation

Microbubble generation by a novel fluidic oscillator driven approach is analyzed, with a view to identifying the key design elements and their differences from standard approaches to airlift loop bioreactor design. The microbubble generation mechanism has been shown to achieve high mass transfer rates by the decrease of the bubble diameter, by hydrodynamic stabilization that avoids coalescence increasing the bubble diameter, and by longer residence times offsetting slower convection. The fluidic oscillator approach also decreases the friction losses in pipe networks and in nozzles/diffusers due to boundary layer disruption, so there is actually an energetic consumption savings in using this approach over steady flow. These dual advantages make the microbubble generation approach a promising component of a novel airlift loop bioreactor whose design is presented here. The equipment, control system for flow and temperature, and the optimization of the nozzle bank for the gas distribution system are presented. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V All rights reserved

High frequency periodic forcing of the oscillatory catalytic CO oxidation on Pt(110)

Resonant periodic forcing is applied to catalytic CO oxidation on platinum (110) in the oscillatory regime. The external parameters are chosen such that the unperturbed system spontaneously develops chemical turbulence. By periodically modulating the CO partial pressure, changes in the spatiotemporal behaviour of the system can be induced: the turbulent behaviour is suppressed and frequency locked patterns with sub-harmonic entrainment develop. A novel gas-driving compressor has been implemented to perform the experimental work

Information coding with frequency of oscillations in Belousov-Zhabotinsky encapsulated disks

Information processing with an excitable chemical medium, like the Belousov-Zhabotinsky (BZ) reaction, is typically based on information coding in the presence or absence of excitation pulses. Here we present a new concept of Boolean coding that can be applied to an oscillatory medium. A medium represents the logical TRUE state if a selected region oscillates with a high frequency. If the frequency fails below a specified value, it represents the logical FALSE state. We consider a medium composed of disks encapsulating an oscillatory mixture of reagents, as related to our recent experiments with lipid-coated BZ droplets. We demonstrate that by using specific geometrical arrangements of disks containing the oscillatory medium one can perform logical operations on variables coded in oscillation frequency. Realizations of a chemical signal diode and of a single-bit memory with oscillatory disks are also discussed

Phase reduction approach to synchronization of spatiotemporal rhythms in reaction-diffusion systems

Reaction-diffusion systems can describe a wide class of rhythmic spatiotemporal patterns observed in chemical and biological systems, such as circulating pulses on a ring, oscillating spots, target waves, and rotating spirals. These rhythmic dynamics can be considered limit cycles of reaction-diffusion systems. However, the conventional phase-reduction theory, which provides a simple unified framework for analyzing synchronization properties of limit-cycle oscillators subjected to weak forcing, has mostly been restricted to low-dimensional dynamical systems. Here, we develop a phase-reduction theory for stable limit-cycle solutions of infinite-dimensional reaction-diffusion systems. By generalizing the notion of isochrons to functional space, the phase sensitivity function - a fundamental quantity for phase reduction - is derived. For illustration, several rhythmic dynamics of the FitzHugh-Nagumo model of excitable media are considered. Nontrivial phase response properties and synchronization dynamics are revealed, reflecting their complex spatiotemporal organization. Our theory will provide a general basis for the analysis and control of spatiotemporal rhythms in various reaction-diffusion systems.Comment: 19 pages, 6 figures, see the journal for a full versio