Fluctuations Of Reaction Rate Research Articles

Modelling the response of a turbulent jet flame to acoustic forcing in a linearized framework using an active flame approach

This study performs a linear mean field analysis of a turbulent reacting methane-air jet flame, with the goal of predicting the response of the reacting flow to upstream acoustic actuation. Unlike previous studies, this work develops and applies an active flame approach by taking the heat release oscillations of the flame resulting from the acoustic fluctuations into account. For an active flame approach in the linear mean field analysis, a linearized combustion model is necessary. Linearizing Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS) combustion models leads to closure problems, making their application in this context troublesome, whereas Reynolds-averaged Navier Stokes (RANS) combustion models prove to circumvent this problem making them suitable candidates for this purpose. The RANS combustion models are linearized around the temporal mean state variables of the turbulent jet flame, which is obtained by LES. An a priori analysis shows that a linearized RANS–Eddy Break Up (EBU) model is the best suited among all investigated combustion models for the investigated set-up and reproduces with high accuracy the fluctuations in reaction rate obtained in the LES. Furthermore, the linearized governing equations of the flow including the linearized EBU model for the reaction rate are solved for incoming acoustic perturbations. The response modes show that the reaction rate oscillations are caused by Kelvin–Helmholtz vortex rings, which perturb the jet flame. The results are in good agreement with the LES simulations in terms of the mode shapes of both reaction rate and velocity fluctuations. This study represents a basis for linear mean field analysis of turbulent flames and monolithic modelling approaches for thermoacoustic instabilities in gas turbine combustors.

Product molecule numbers and reaction rate fluctuations in elementary reactions

In many chemical reactions, reaction rate fluctuations are inevitable. Whenever chemical reactions occur, reaction rates vary due to their dependence on the number of reaction events or products. Accordingly, understanding the impact of rate fluctuations on the product number counting statistics is of the utmost importance when developing a quantitative explanation of chemical reactions. In this work, we examine the relationship between the reaction rate and product number fluctuations. Product number counting statistics uncover stochastic properties of the product number; the latter directly manipulates the reaction rate. Specifically, we find that the product number displays super-Poisson characteristics as the reaction rate increases with the product number. On the other hand, when the product number shows sub-Poisson behavior, a decrease in the reaction rate follows. Furthermore, our analysis, dealing with reaction rate fluctuations, allows for quantifying the deviations of an elementary reaction process from a renewal process.

Three-dimensional direct numerical simulation of turbulent channel flow catalytic combustion of hydrogen over platinum

The turbulent catalytic combustion of a fuel-lean hydrogen/air mixture (equivalence ratio ϕ=0.24) was investigated by means of three-dimensional direct numerical simulation (DNS) in a platinum-coated plane channel with a prescribed wall temperature of 960K and an incoming Reynolds number, based on the channel height, of 5700. Heat transfer from the hot catalytic walls laminarized the flow, as manifested by the progressive suppression of the high vorticity components of the flow aligned parallel to the channel walls at increasing streamwise distances. The impact of turbulence suppression on the mass transfer towards or away from the catalytic wall was subsequently assessed. Far upstream where high turbulence fluctuations persisted, the instantaneous local transverse gradient of the limiting hydrogen reactant (a quantity proportional to the catalytic reaction rate) as well as the instantaneous hydrogen concentration at the wall exhibited strong fluctuations by up to 300%, a result of finite-rate chemistry induced by the high inrush events towards the catalytic walls. Fourier analysis of the reaction rate fluctuations yielded peak frequencies of less than 1kHz, values comparable to the thermal response frequencies of typical materials in commercial catalytic geometries. This has direct implications on the thermal stress of the reactor walls as well as on the decoupling between flow and solid thermal modeling currently used in practical catalytic reactors. Far downstream, the dampening of turbulence resulted in weaker hydrogen concentration fluctuations with nearly symmetric distributions. Finally, computed transverse turbulent species fluxes indicated inherent weaknesses of near-wall turbulence models in describing turbulent transport of species with disparate molecular diffusivities.

Direct numerical simulation of an isothermal reacting turbulent wall-jet

In the present investigation, Direct Numerical Simulation (DNS) is used to study a binary irreversible and isothermal reaction in a plane turbulent wall-jet. The flow is compressible and a single-step global reaction between an oxidizer and a fuel species is solved. The inlet based Reynolds, Schmidt, and Mach numbers of the wall-jet are Re = 2000, Sc = 0.72, and M = 0.5, respectively, and a constant coflow velocity is applied above the jet. At the inlet, fuel and oxidizer enter the domain separately in a non-premixed manner. The turbulent structures of the velocity field show the common streaky patterns near the wall, while a somewhat patchy or spotty pattern is observed for the scalars and the reaction rate fluctuations in the near-wall region. The reaction mainly occurs in the upper shear layer in thin highly convoluted reaction zones, but it also takes place close to the wall. Analysis of turbulence and reaction statistics confirms the observations in the instantaneous snapshots, regarding the intermittent character of the reaction rate near the wall. A detailed study of the probability density functions of the reacting scalars and comparison to that of the passive scalar throughout the domain reveals the significance of the reaction influence as well as the wall effects on the scalar distributions. The higher order moments of both the velocities and the scalar concentrations are analyzed and show a satisfactory agreement with experiments. The simulations show that the reaction can both enhance and reduce the dissipation of fuel scalar, since there are two competing effects; on the one hand, the reaction causes sharper scalar gradients and thus a higher dissipation rate, on the other hand, the reaction consumes the fuel scalar thereby reducing the scalar dissipation.

Improving accuracy by leaps and unbounds

In the same way that arrangements of digital logic gates form the building blocks of electronic computers, networks of biochemical reactions constitute the computing hardware of cells. These networks carry out the processes of life, from cell growth and death to responses to environmental cues such as nutrients and toxins. Biological receptors—the cell’s “particle detectors”—measure the concentrations of external and internal signaling molecules, performing a generic and essential task of the cell’s computing machinery. Readouts from the receptors are used to regulate decisionmaking circuits within the cell that control the expression of genes and proteins, and cellular motion, growth and division. The energy scale for biomolecular interactions is typically a few times kBT, so it is natural to ask: How reliably can these measurements be carried out in the face of inherent fluctuations? Do biological sensors reach the detection limits set by the laws of physics? Thierry Mora and Ned Wingreen at Princeton University in the US report theoretical results in Physical Review Letters[1] that add to recent efforts exploring these questions. Specifically, they suggest cells may be able to measure changes in time in the concentration of signaling molecules twice as accurately as previous theoretical bounds. The ability to perform increasingly quantitative and noninvasive experiments on biological systems places us in an exciting era in which to look at cellular phenomena from the standpoint of statistical physics. We know of many examples where signal processing occurs with exceptional accuracy, such as the ability of rod photoreceptor cells to count single photons [2], the reproducibility of concentration profiles of morphogens (signaling molecules governing the pattern of tissue development in the embryo) and the accuracy with which they can be read out by cells [3], and the sensitivity of the bacterial flagellar motor to changes in concentration of an internal signal [4, 5]. In these examples, the reliability of the output is limited by the inherent random nature of the input signal—photon shot noise or diffusive counting noise—thereby approaching a physical lower limit. The requirement of nearly perfect detection and processing of the input to generate the macroscopic output poses constraints on the underlying strategies, resulting in the adoption of common signal processing approaches in these biological systems [3, 6]. Roughly thirty years ago, Berg and Purcell derived statistical limits of cell sensing in their classic article on the physics of chemoreception [7]. Their work focused on chemotaxis, the behavioral response of singlecelled organisms that helps them move toward favorable chemicals and away from harmful ones. (Chemotaxis is involved in the motile response of immune cells hunting down bacterial predators, wound healing, the spread of cancerous cells, sperm navigation, and the preferential growth of neurons.) This canonical sensory response is best studied in the well-controlled and experimentally accessible setting offered by two model organisms. The eukaryotic social slime mold Dictystelium discoidium (or Dicty) and the prokaryotic bacterium Escherichia coli employ very different methods of chemotactic sensing and response and have emerged as “hydrogen atoms” in the study of cellular sensing. A central result from Berg and Purcell’s work was that the accuracy with which cells can detect the concentration of signaling molecules is fundamentally limited due to the random arrival of diffusing signaling molecules at their targets. A hypothetical “perfect” device, able to count molecules exactly, is limited by this diffusive noise only. In actuality, biological sensors “count” the number of diffusing signaling molecules by registering chemical binding and unbinding interactions (Fig.1). Thermal fluctuations in the free energy levels of the bound and unbound receptor-signal system lead to effective binding/unbinding reaction rate fluctuations and result in additional counting error. This means that in addition

Real-time Enzyme Dynamics Illustrated with Fluorescence Spectroscopy of p-Hydroxybenzoate Hydroxylase

We have used the flavoenzyme p-hydroxybenzoate hydroxylase (PHBH) to illustrate that a strongly fluorescent donor label can communicate with the flavin via single-pair Förster resonance energy transfer (spFRET). The accessible Cys-116 of PHBH was labeled with two different fluorescent maleimides with full preservation of enzymatic activity. One of these labels shows overlap between its fluorescence spectrum and the absorption spectrum of the FAD prosthetic group in the oxidized state, while the other fluorescent probe does not have this spectral overlap. The spectral overlap strongly diminished when the flavin becomes reduced during catalysis. The donor fluorescence properties can then be used as a sensitive antenna for the flavin redox state. Time-resolved fluorescence experiments on ensembles of labeled PHBH molecules were carried out in the absence and presence of enzymatic turnover. Distinct changes in fluorescence decays of spFRET-active PHBH can be observed when the enzyme is performing catalysis using both substrates p-hydroxybenzoate and NADPH. Single-molecule fluorescence correlation spectroscopy on spFRET-active PHBH showed the presence of a relaxation process (relaxation time of 23 micros) that is related to catalysis. In addition, in both labeled PHBH preparations the number of enzyme molecules reversibly increased during enzymatic turnover indicating that the dimer-monomer equilibrium is affected.

Assessment of closure schemes in second-order conditional moment closure against DNS with extinction and ignition

The performance of second-order conditional moment closure (CMC) depends on models to evaluate conditional variances and covariances of temperature and species mass fractions. In this paper the closure schemes based on the steady laminar flamelet model (SLFM) are validated against direct numerical simulation (DNS) involving extinction and ignition. Scaling is performed to reproduce proper absolute magnitudes, irrespective of the origin of mismatch between local flamelet structures and scalar dissipation rates. DNS based on the pseudospectral method is carried out to study hydrogen–air combustion with a detailed kinetic mechanism, in homogeneous, isotropic, and decaying turbulent media. Lewis numbers are set equal to unity to avoid complication of differential diffusion. The SLFM-based closures for correlations among fluctuations of reaction rate, scalar dissipation rate, and species mass fractions show good comparison with DNS. The variance parameter in lognormal PDF and the constants in the dissipation term have been estimated from DNS results. Comparison is made for the resulting conditional profiles from DNS, first-order CMC, and second-order CMC with correction to the most critical reaction step according to sensitivity analysis. Overall good agreement ensures validity of the SLFM-based closures for modeling conditional variances and covariances in second-order CMC.

Intermittent Single-Molecule Interfacial Electron Transfer Dynamics

We report on single-molecule studies of photosensitized interfacial electron transfer (ET) processes in Coumarin 343 (C343)-TiO(2) nanoparticles (NP) and Cresyl Violet (CV(+))-TiO(2) NP systems, using time-correlated single-photon counting coupled with scanning confocal fluorescence microscopy. Fluorescence intensity trajectories of individual dye molecules adsorbed on a semiconductor NP surface showed fluorescence fluctuations and blinking, with time constants distributed from milliseconds to seconds. The fluorescence fluctuation dynamics were found to be inhomogeneous from molecule to molecule and from time to time, showing significant static and dynamic disorders in the interfacial ET reaction dynamics. We attribute fluorescence fluctuations to the interfacial ET reaction rate fluctuations, associating redox reactivity intermittency with the fluctuations of molecule-TiO(2) electronic and Franck-Condon coupling. Intermittent interfacial ET dynamics of individual molecules could be characteristic of a surface chemical reaction strongly involved with and regulated by molecule-surface interactions. The intermittent interfacial reaction dynamics that likely occur among single molecules in other interfacial and surface chemical processes can typically be observed by single-molecule studies but not by conventional ensemble-averaged experiments.

Specific Features of Kinetics of Stochastically Gated, Diffusion-Controlled Reactions

The kinetics of diffusion-controlled, stochastically gated biochemical reactions is analyzed within the markovian approximation for stochastic fluctuations of reaction rate. With the use of methods developed in the theory of magnetic field effects on chemical reactions, several general expressions for reaction rate and transient kinetics of geminate and bulk reactions are derived. In particular, it is shown that gating strongly manifests itself not only in steady-state reaction rates but also in the long time tail of kinetics. Specific features of gated reactions in the presence of attractive potential, resulting in the long-lived intermediate state (cage), are discussed. Two simple markovian models of gating are considered which allow significant simplification of the general expressions obtained. Within these models simple analytical formulas for reaction rate and reaction kinetics are derived and analyzed in detail.

Control of adenine nucleotide metabolism and glycolysis in vertebrate skeletal muscle during exercise.

The turnover of adenosine triphosphate (ATP) in vertebrate skeletal muscle can increase more than a hundredfold during high-intensity exercise, while the content of ATP in muscle may remain virtually unchanged. This requires that the rates of ATP hydrolysis and ATP synthesis are exactly balanced despite large fluctuations in reaction rates. ATP is regenerated initially at the expense of phosphocreatine (PCr) and then mainly through glycolysis from muscle glycogen. The increased ATP turnover in contracting muscle will cause an increase in the contents of adenosine diphosphate (ADP), adenosine monophosphate (AMP) and inorganic phosphate (P(i)), metabolites that are substrates and activators of regulatory enzymes such as glycogen phosphorylase and phosphofructokinase. An intracellular metabolic feedback mechanism is thus activated by muscle contraction. How muscle metabolism is integrated in the intact body under physiological conditions is not fully understood. Common frogs are suitable experimental animals for the study of this problem because they can readily be induced to change from rest to high-intensity exercise, in the form of swimming. The changes in metabolites and effectors in gastrocnemius muscle were followed during exercise, post-exercise recovery and repeated exercise. The results suggest that glycolytic flux in muscle is modulated by signals from outside the muscle and that fructose 2,6-bisphosphate is a key signal in this process.

Advances in Numerical Simulation of Pulsating Combustion at ENEL

Abstract This paper concerns the simulation of unsteady combustion processes. It gathers up-to-date results of simulation activities, part of a long-term research program between the Italian National Institute for Electric Power (ENEL) and the University of Padova. The software used is ‘Fluent’1; the hardware a Vax 8530 and a Cray X-MP/14. The paper is divided in two major sections: one dealing with combustion instabilities, the second with pulse combustion. In the first section, of the several known forms of combustion instabilities, our attention is directed toward the investigation of one caused by fluctuations in turbulent mixing rate. Assuming that the combustion rate is controlled by mixing of the chemical species (and not by chemical kinetics), we show that the fluctuations of velocity which accompany acoustic waves produce fluctuations in turbulence and then fluctuations in reaction rate. We also show that mean flow plays a key role in determining the sign of these fluctuations and therefore the o...

The statistical nature of the acetycholine potential and its molecular components.

1. When a steady dose of acetylcholine (ACh) is applied to an end-plate, the resulting depolarization is accompanied by a significant increase in voltage noise.2. The characteristic properties of this ACh noise (amplitude and time course) are examined under various experimental conditions. The voltage noise is analysed on the assumption that it arises from statistical fluctuations in reaction rate, and in the frequency of the elementary current pulses (;shot effects') produced by the action of ACh molecules.3. The elementary ACh current pulse (amplitude approximately 10(-11) A), arises from a conductance change of the order of 10(-10) Omega(-1) which lasts for approximately 1 ms (at 20 degrees C), and produces a minute depolarization, of the order of 0.3 muV. It is associated with a net charge transfer of nearly 10(-14) C, equivalent to approximately 5 x 10(4) univalent ions.4. At low temperature, and during chronic denervation, the duration of the elementary current pulse increases, and the elementary voltage change becomes correspondingly larger.5. Curare has little or no effect on the characteristics of the elementary event.6. A comparative study of ACh and carbachol actions shows that carbachol produces considerably briefer, and therefore less effective, current pulses than ACh.

Chemical effects on sound propagation

A theoretical study of coupling between chemical kinetics and sound propagation is presented. Examination of equations governing propagation of a travelling acoustic wave in a chemically reacting, stagnant, gaseous medium shows that chemical effects are, in general, of two kinds; viz., those due to changes in mean conditions resulting from chemical reaction and those due to fluctuations in reaction rate resulting from acoustic waves. These coupled effects are found to depend on activation energy, enthalpy of reaction, and ratio of chemical to acoustic time, and have, in particular, different qualitative features for exothermic and endothermic reactions. Several cases have been investigated. When chemical time is long as compared with acoustic time, the main effects are due to fluctuations in the reaction rate. If reaction rates increase with rising temperatures, the acoustic waves are amplified for exothermic reactions and attenuated for endothermic reactions. The sound propagation speed is only slightly reduced from its isentropic frozen value within the region of validity of this quasisteady analysis. When the reaction rate is constant, the chemical effects are due to changes in the mean conditions. Contrary to what is predicted for the quasisteady case, the acoustic waves are here attenuated for exothermic reactions and amplified for endothermic reactions. Furthermore, effects on propagation speeds become pronounced. When both kinds of chemical effects are considered, the results depend strongly on activation energy. For the case of small activation energy, where the reaction rate is weakly dependent on temperature, the qualitative effects are similar to those observed for constant reaction rate. For large activation energy, however, effects due to fluctuations in reaction rate become pronounced, leading to amplification for exothermic reactions and attenuation for endothermic reactions. In all cases, chemical effects become more significant at higher reaction rates.