Oscillator self-sustained

Similar phenomena are observed for m-xylene combustion. As we already shown [23] there are two types of oscillation self-sustained and damped ones. As the system is not stable under certain conditions the oxidation of m-xylene may show oscillations. Oscillation was observed only in high activity state. Preliminary experiments showed that oscillation can be initialized by (i) step change in m-xylene concentration at low conversion, or (ii) by step change in the temperature at fiill conversion, so the phenomenon is similar to one observed for NO decomposition. 

K. Fichthom, E. Gulari, R. Ziff". Self-sustained oscillations in a heterogeneous catalytic reaction A Monte Carlo simulation. Chem Eng Sci 44 1403-1411, 1989. 

In its applications, science was encountering gradually-increasing difficulties in view of the impossibility of explaining numerous oscillatory phenomena, particularly those connected with the so-called self-sustained oscillations (first, the oscillating arcs and gaseous discharges and still later, the electron tube oscillators). 

The oxidation of CO on Pt is one of the best studied catalytic systems. It proceeds via the reaction of chemisorbed CO and O. Despite its complexities, which include island formation, surface reconstruction and self-sustained oscillations, the reaction is a textbook example of a Langmuir-Hinshelwood mechanism the kinetics of which can be described qualitatively by a LHHW rate expression. This is shown in Figure 2.39 for the unpromoted Pt( 111) surface.112 For low Pco/po2 ratios the rate is first order in CO and negative order in 02, for high pco/po2 ratios the rate becomes negative order in CO and positive order in 02. Thus for low Pcc/po2 ratios the Pt(l 11) surface is covered predominantly by O, at high pco/po2 ratios the Pt surface is predominantly covered by CO. 

Figure 8.31. Induction of self-sustained rate and catalyst potential, or work function, oscillations by NEMCA during CO oxidation on Pt. Inlet composition Pco 0.47 kPa, Po2=16 kPa, T=297°C.33 Reprinted with permission from Academic Press.

Figure 8.33. Effect of applied constant current on the frequency of the self-sustained rate and Uwr oscillations during CO oxidation on Pt conditions as on Fig. 8.32 Filled circles on the frequency vs current diagram are oscillatory states of this figure open circles include states shown on Fig. 8.31.33 Reprinted with permission from Academic Press.

D. Durox, T. Schuller, and S. Gandel. Self-sustained oscillations of a premixed impinging jet flame on a plate. Proc. Combust. Inst., 29 69-75, 2002. 

Self-sustained Oscillations. Under certain conditions, isothermal limit cycles in gaseous concentrations over catalysts are observed. These are probably caused by interaction of steps on the surface. Sometimes heat and mass transfer effects intervene, leading to temperature oscillations also. Since this subject has recently been reviewed (42, 43) only a few recent papers will be mentioned here. 

Even though the bifurcation behavior exhibits a Z-shaped curve, it is more complicated due to the existence of the HB. For example, upon ignition, the system is expected to oscillate because no locally stable stationary solutions are found (an oscillatory ignition). Time-dependent simulations confirm the existence of self-sustained oscillations [7, 12]. The envelope of the oscillations (amplitude of H2 mole fraction) is shown in circles (a so-called continuation in periodic orbits). 

As the surface temperature decreases slightly below the HB temperature, self-sustained harmonic oscillations of small amplitude appear. The oscillations increase sharply in amplitude, as the surface temperature decreases (see... 

The local stability in the neighborhood of the second set of turning points is simply deduced because no new HB point is found the intermediate branch is locally unstable, whereas the partially ignited branch and fully ignited branch are locally stable. The temperature range for self-sustained oscillations is larger at this higher pressure. 

As the pressure increases further, a second HB point (HB2) appears at the extinction point E and shifts toward the other HB HBi) point. An example is shown for 4 atm in Fig. 26.1c. Ignition Ii is no longer oscillatory, because the stationary partially ignited branch becomes locally stable in the vicinity of /i. Time-dependent simulations indicate that the two HB points are supercritical, i.e., self-sustained oscillations die and emerge at these points with zero amplitude. In this case, the first extinction Ei defines again the actual extinction of the system. 

To explain the role of transport, simulations have been also performed in an isothermal PSR. Oscillatory instabilities were again found [8]. These facts indicate that oscillations are radical induced. However, without the heat of reactions, no self-sustained oscillations are found for these conditions. The heat of reactions is a prerequisite at these conditions to pull the HB point outside the multiplic-... 

Figure 26.3 The mole fraction of H2 just above the surface (a) and the wall heat flux (6) as functions of the dimensionless time, 2at, for 10% H2-air mixture at a surface temperature of 1100 K. Self-sustained oscillations and stationary solutions are represented by solid and dashed curves, respectively. The pressure is 4 atm and the strain is a = 200 s ...

Bui, P.-A., D. G. Vlachos, and P. R. Westmoreland. 1997. Self-sustained oscillations in distributed flames modeled with detailed chemistry. Eastern States Section, Chemical and Physical Processes in Combustion Proceedings. Pittsburgh, PA The Combustion Institute. 337-40. 

However, there is an apparently important difference between SCN and non-SCN rhythms in vitro-. SCN explants remain rhythmic while other regions of the brain and peripheral tissues damp after a minimum of two, but a maximum of about 15 cycles (Fig. 1). The robustness and persistence of the rhythmicity depends on the tissue and does vary in individual cultures. It has been assumed from results like these that the SCN is a self-sustained pacemaker while peripheral clocks are damped oscillators, but it cannot be excluded that in isolated peripheral tissues, I erl may damp despite the persistent rhythmicity of other clock components or that the cultures lack some factor continuously present in vivo that is required for sustained rhythmicity of non-SCN tissue. 

Neurons of the mammalian suprachiasmatic nucleus (SCN) contain cell-autonomous, self-sustained oscillators, which are able to maintain circadian periodicity even when isolated in vitro or when the animal is placed under... 

Kosbash If you trawl the literature, how many studies have taken a tissue, dissociated it into single cells, prevented the cells from having any kind of contact with each other (including signalling molecules), and then seen longterm self-sustained oscillations Very few, I suspect. 

These studies demonstrate the general mechanism of synchronization of biochemical systems, which I expect to be operative in even more complex systems, such as the mitochondrial respiration or the periodic activity of the slime mold Dictyostelium discoideum. As shown in a number of laboratories under suitable conditions mitochondrial respiration can break into self-sustained oscillations of ATP and ADP, NADH, cytochromes, and oxygen uptake as well as various ion transport and proton transport functions. It is important to note that mitochondrial respiration and oxidative phosphorylation under conditions of oscillations is open for the source, namely, oxygen, as well as with respect to a number of sink reactions producing water, carbon dioxide, and heat. 

Bittger, I., Pettinger, B., Schedel-Niedrig, T. et al. (2001) Self Sustained Oscillations Over Copper in the Catalytic Oxidation of Methanol, Elsevier, Amsterdam. 

Werner, H., Herein, D., Schulz, G. et al. (1997) Catalysis Letters, 49, 109-119. Slin ko, M.M. and Jaeger, N.I. (1994) Physicochemical Basis for the Appearance of Self-Sustained Oscillations in Heterogeneous Catalytic Systems, Elsevier. 

In the resonance region the system oscillates with the external frequency X and with an increased amplitude (entrainment region). Far away from resonance, the internal free oscillations are present. This behaviour is completely absent in systems, where no self-sustained oscillations can exist. A typical example of such a system is a nonlinear conservative system. The resonance diagram has been drawn in Figure 2 for both, the small and the large ampli tude oscillation. 

Before we can start to develop a model we also have to decide how to interpret the behavior observed in Fig. 2.1. The variations in insulin and glucose concentrations could be generated by a damped oscillatory system that was continuously excited by external perturbations (e.g. through interaction with the pulsatile release of other hormones). However, the variations could also represent a disturbed self-sustained oscillation, or they could be an example of deterministic chaos. Here, it is important to realize that, with a sampling period of 10 min over the considered periods of 20-24 h, the number of data points are insufficient for any statistical analysis to distinguish between the possible modes. We need to make a choice and, in the present case, our choice is to consider the insulin-glucose regulation to operate... 

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