Fluctuations Of Species Concentrations Research Articles

On the individual importance of temperature and concentration fluctuations in the turbulence-radiation interaction in pool fires

This paper presents a study of resolved-scale turbulence-radiation interaction (TRI) effects in large-scale methanol and ethanol pool fires. A broad investigation of the magnitude of the phenomenon when assuming the participating medium as gray or as non-gray is first conducted, followed by an analysis of the individual importance of turbulent fluctuations of temperature and species concentrations in the prediction of the mean radiation field. For these purposes, transient data generated by large eddy simulation are compared to results of independent radiative transfer calculations initialized with mean temperature and medium composition fields. A new methodology is proposed to isolate the influence of each fluctuating scalar on the overall TRI. In all test cases, turbulence-radiation interaction was found to increase the region of radiation loss, leading to differences between 60% and 80% in the radiant fraction of the flame compared to solutions neglecting turbulent fluctuations. When treating the media as non-gray, TRI effects were globally more significant, even though in some parts of the domain simulations employing the gray assumption yielded larger deviations in the mean radiative heat source due to TRI. By isolating the contributions of fluctuations of temperature and fluctuations of species concentrations, the latter resulted in mean radiation fields very similar to the ones obtained by neglecting all scalar fluctuations, while the former, although being closer to the solution considering full TRI effects, still showed differences as high as 60% relative to that solution. These findings indicate that temperature fluctuations are more important to the turbulence-radiation interaction phenomenon, but fluctuations in medium composition need to be taken into account in order to obtain reliable predictions of the mean radiative heat transfer.

An analysis of fluctuations in supercooled TIP4P/2005 water

Large-scale, long-time molecular dynamics simulations are used to investigate fluctuations in the TIP4P/2005 water model in the supercooled region (240-190 K). Particular attention is focused in the vicinity of a previously reported liquid-liquid critical point [J. L. F. Abascal and C. Vega, J. Chem. Phys. 133, 234502 (2010)]. Water is viewed as an equimolar binary mixture with "species" defined based on a local tetrahedral order parameter. A Bhatia-Thornton fluctuation analysis is used to show that species concentration fluctuations couple to density fluctuations and completely account for the anomalous increase in the structure factor at small wave number observed under supercooled conditions. Although we find that both concentration and density fluctuations increase with decreasing temperature along the proposed critical isochore, we cannot confirm the existence of a liquid-liquid critical point. Our simulations suggest that the parameters previously reported are not a true liquid-liquid critical point and we find no evidence of two-phase coexistence in its vicinity. It is shown that very long simulations (on the order of 8 μs for 500 molecules) are necessary to obtain well converged density distributions for deeply supercooled water and this is especially important if one is seeking direct evidence of a two-phase region.

A study of the accuracy of moment-closure approximations for stochastic chemical kinetics

Moment-closure approximations have in recent years become a popular means to estimate the mean concentrations and the variances and covariances of the concentration fluctuations of species involved in stochastic chemical reactions, such as those inside cells. The typical assumption behind these methods is that all cumulants of the probability distribution function solution of the chemical master equation which are higher than a certain order are negligibly small and hence can be set to zero. These approximations are ad hoc and hence the reliability of the predictions of these class of methods is presently unclear. In this article, we study the accuracy of the two moment approximation (2MA) (third and higher order cumulants are zero) and of the three moment approximation (3MA) (fourth and higher order cumulants are zero) for chemical systems which are monostable and composed of unimolecular and bimolecular reactions. We use the system-size expansion, a systematic method of solving the chemical master equation for monostable reaction systems, to calculate in the limit of large reaction volumes, the first- and second-order corrections to the mean concentration prediction of the rate equations and the first-order correction to the variance and covariance predictions of the linear-noise approximation. We also compute these corrections using the 2MA and the 3MA. Comparison of the latter results with those of the system-size expansion shows that: (i) the 2MA accurately captures the first-order correction to the rate equations but its first-order correction to the linear-noise approximation exhibits the wrong dependence on the rate constants. (ii) the 3MA accurately captures the first- and second-order corrections to the rate equation predictions and the first-order correction to the linear-noise approximation. Hence while both the 2MA and the 3MA are more accurate than the rate equations, only the 3MA is more accurate than the linear-noise approximation across all of parameter space. The analytical results are numerically validated for dimerization and enzyme-catalyzed reactions.

Scalar Fluctuation Modeling for High-Speed Aeropropulsive Flows

A Reynolds-averaged Navier-Stokes-based scalar-variance model is described that extends a previous low-speed nonreacting jet model to more generalized high-speed compressible reacting flows. The model is cast in a k-e turbulence model framework. Transport equations for energy variance and its dissipation rate are solved to predict temperature fluctuations and provide a thermal time scale for use in calculating a variable turbulent Prandtl number. For multispecies problems, mixture-fraction variance and dissipation rate equations are solved that predict species concentration fluctuations and provide a species mixing time scale for use in calculating a variable turbulent Schmidt number. The formulation accounts for compressibility and near-wall damping effects. A series of high-speed flow simulations are presented for both nonreacting and reacting configurations and the predictions are compared to available measured data and companion LES calculations. Results demonstrate the models' capabilities over a range of conditions and suggest that the proposed formulation will provide improved predictions in practical high-speed aeropropulsive configurations of interest, such as scramjet combustors, where turbulent Prandtl and Schmidt numbers vary substantially.

Noise, clutter, and determinism in infrared spectral signatures under varying meteorological conditions

Infrared remote detection of chemical and biological agents in a complex environment depends on the ability to recognize threat signatures and differentiate them from the signatures of innocuous materials. In this paper, we addressed the methods of producing the constraint spectra needed to ensure reliable operation in a meteorologically changing environment. We collected arrays of background spectra of ground, woods, and low-angle sky on an irregular basis over a period of a year. Based on the hypothesis that the concentration fluctuations of species in the sensor's field of view can be exploited to form signatures, the standard deviations of the array (the result is characteristic of all fluctuations) and the difference array (the result is characteristic of sensor fluctuations) were computed. Subtracting these two spectra and filtering the result produced a spectrum, which is a measure of the IR fluctuations in the scene. The resulting set of scene spectra were processed into aberrant noise, and deterministic groups by numerical filtering and statistical methods.

Measurement and Modelling of NOxFormation in a Gas Fired Furnace

ABSTRACT This paper reports results of NOx measurements carried out on a gas fired furnace using radial fuel injection. The data consists of a complete mapping of NOx concentration in the furnace under variable swirl conditions. Swirl produced locally higher NOx concentrations but little overall increase. Measured stack NOx variations with burner input conditions indicate that, for this system, operating with a fuel-rich mixture is more effective in suppressing NOx formation than with excess air operation. The data is needed for testing and developing theoretical models. Theoretical predictions of the overall NOx formation are made using a well-stirred reactor model based on the Zeldovich mechanism with account for the species concentration fluctuations. The model predictions compare satisfactorily with the measurements

Turbulence effects on chemical reactions in smog chamber flows

Turbulent mixing effects on the reaction rate of a non-premixed flow are presented for a moderately slow second-order irreversible chemical reaction. The turbulent mixing process leads to inhomogeneities in the concentration of the reactants. Chemical reactions are normally highly non-linear and large errors can result from using average concentrations in the computation of mean reaction rates. A brief review of the literature on this problem and its application areas is made with particular emphasis placed on near isothermal flows where the fluctuation in the reaction rate constant can be neglected. The reaction between mixing air jets containing dilute nitric oxide, NO, and ozone, O 3, is studied in a large Turbulent Smog Chamber (TSC) under conditions of high Reynolds number, three-dimensional flow. The measured reactant concentrations, obtained with good time and space resolution, are used to investigate the effects of the reactant species concentration fluctuations on the mean reactant species concentration field through their contribution to the mean reaction rate. Two flow geometries have been studied: two opposed jets issuing into a large chamber and a plume-like source issuing into a turbulent background flow. A three-dimensional finite-difference computation has been carried out for the flow in the chamber using the k- ε- g model of turbulence and mixing. The reacting species concentration field is calculated by considering the transport of a ‘perturbation variable’ equal to the mean of the difference between the species concentration and its corresponding fast chemistry value. A closure for the mean chemical reaction rate based on this quantity is presented and its experimental validation discussed.