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  • 1
    Electronic Resource
    Electronic Resource
    Binary scalar mixing and reaction in homogeneous turbulence: some linear eddy model results (1993)
    s.l. : American Chemical Society
    Energy & fuels 7 (1993), S. 827-834 
    Details
    ISSN: 1520-5029
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Energy, Environment Protection, Nuclear Power Engineering , Process Engineering, Biotechnology, Nutrition Technology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ef00042a019
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  • 2
    Electronic Resource
    Electronic Resource
    Direct numerical simulation of a laminar vortex ring (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 8 (1996), S. 2400-2414 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Results are presented of direct numerical simulations (DNS) of a viscous, laminar ring. The effects of different generator configurations and velocity programs on the formation and post-formation characteristics of the ring are studied. It is shown that during the formation phase of the ring, total circulation and impulse in the flowfield are approximately the same for the "nozzle'' and "orifice'' generators. It is also found that throughout this period the slug flow model under-predicts the total circulation in the flow. During the formation phase, the simulation results for the time evolution of total circulation and location of the vortex spiral center are in agreement with the experimental findings of Didden [J. Appl. Mech. Phys. (ZAMP) 30, 101 (1979)]. The results of the flow visualization studies show that during the post-formation phase a vortex bubble is formed. As the bubble propels itself forward a wake is formed in the rear of the bubble. The impulse and vorticity from the bubble are continuously shed into this wake. It is found that the total value of the circulation in the flow varies as (t1*)−0.33 which is consistent with Maxworthy's [J. Fluid Mech. 81, 465 (1977)] prediction of the decay of circulation for a vortex ring. The transport of a passive Shvab-Zeldovich scalar variable is used to study the mixing and to obtain the maximum product formation in a chemical reaction of the type A+B→Products in a vortex ring. It is found that as the bubble containing the fuel propels itself forward, the outside oxidizer flow is entrained into it and reacts to form a product. Some of this product then is de-entrained into the wake of the bubble. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869041
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  • 3
    Electronic Resource
    Electronic Resource
    Interaction of a vortex ring with a diffusion flame (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 14 (2002), S. 668-681 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The interaction between a laminar vortex ring and a flat diffusion flame is investigated via direct numerical simulations. The diffusion flame is generated by a "spark" and is implemented as initial condition for the simulations. The chemistry is modeled by an Arrhenius, single step, irreversible reaction. A heat-releasing flame sheet is also considered in addition to the finite rate flames. The vortex ring is generated by an axisymmetric discharge of fuel-inert species mixture that enters a quiescent medium with the same composition and temperature. Two stages are identified during the finite rate diffusion flame–vortex ring interaction. The first stage corresponds to the head-on collision between the flame and the vortex ring, and lasts until the flame is quenched near the centerline. The unsteady effects are dominant and examination of the terms in the temperature transport equation reveals that, depending on the relative strengths of the vortex ring and the flame, convection and/or diffusion terms are responsible for the local flame extinction. The extinction patterns obtained from the flame–vortex ring interaction during this stage, are compared with the results from unsteady counterflow diffusion flame simulations. The second stage corresponds to the passage of the ring through the flame and its interaction with the flame from the oxidizer side. During this stage, the vortex ring loses its strength and, in addition to the unsteady effects, curvature effects can also become important. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1429248
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  • 4
    Electronic Resource
    Electronic Resource
    Flame–vortex interaction in a reacting vortex ring (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 10 (1998), S. 189-205 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Direct numerical simulations are used to study the flame–vortex interaction in a laminar reacting vortex ring. The chemical reaction occurs by a one-step, Arrhenius-type reaction that mimics the combustion of typical hydrocarbon and air. The ring is generated by an axisymmetric jet that is impulsed to emit a cold fuel through a nozzle. The fuel enters a quiescent ambient at a much higher temperature. By adjusting the ratio of the ambient and fuel temperatures, the ignition either occurs during the formation or post-formation phase of the ring. When ignition occurs during the formation phase of the ring, the bulk of combustion is by a flame at the front of the vortex bubble. When ignition is delayed until after the formation phase, most of the reaction occurs inside the vortex ring. It is found that premixing the fuel and the oxidizer enhances the amount of product formation. The heat released from the reaction significantly affects production, redistribution, and diffusion of the vorticity throughout the field. The results of the simulations also reveal that the heat of reaction affects the strain rate fields differently depending on when the ignition of the ring occurs. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869560
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  • 5
    Electronic Resource
    Electronic Resource
    Characteristics of chemically reacting compressible homogeneous turbulence (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 12 (2000), S. 1189-1209 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Direct numerical simulations (DNS) are conducted to study the turbulence-chemical reaction interactions in homogeneous decaying compressible fluid flows. The reaction is of a single-step irreversible Arrhenius type. The results indicate that the heat of reaction has a noticeable influence on the solenoidal and the dilatational turbulent motions. The effect of reaction on the solenoidal velocity field is primarily due to variation of the molecular diffusivity coefficients with temperature and appears at small scales. However, the dilatational motions are affected more than the solenoidal motions and are intensified at all length scales. The decay rate of the turbulent kinetic energy is dependent on the molecular dissipation and the pressure-dilation correlation. In isothermal reacting cases, the net contribution of the pressure-dilatation is small and the turbulent energy decays continuously due to viscous dissipation. In the exothermic reacting cases, the pressure-dilatation tends to increase the turbulent kinetic energy when the reaction is significant. Analysis of the flow structure indicates that the flow is dominated by strain in the reaction zones. Also, consistent with previous studies, the scalar gradient tends to align with the most compressive strain eigenvector and the vorticity vector tends to align with the intermediate strain eigenvector. The heat of reaction weakens this preferential alignment, primarily due to variation in molecular transport coefficients. The spatial and the compositional structure of the flame are also affected by the modification of the turbulence and the molecular coefficients. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870370
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  • 6
    Electronic Resource
    Electronic Resource
    The structure and the small-scale intermittency of passive scalars in homogeneous turbulence (1995)
    Springer
    Journal of scientific computing 10 (1995), S. 151-180 
    Details
    ISSN: 1573-7691
    Keywords: Scalar intermittency ; homogeneous turbulence ; direct numerical simulation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science
    Notes: Abstract Results generated by direct numerical simulations (DNS) are used to study the structure and the small-scale intermittency of a passive scalar contaminant in a homogeneous turbulent shear flow. Simulations are conducted of flows with and without a constant mean scalar gradient. In all cases, the probability density functions (PDFs) of the scalars adopt an approximate gaussian distribution at the final stages of mixing. In the presence of the mean gradient, the scalar fields yield a nearly identical asymptotic state independent of initial conditions. In these cases, the gradient of the fluctuating scalar field shows preferred directions of orientation with respect to the strain eigenvectors; and the mean transverse velocity conditioned on the scalar is linear. These fields also portray increased flatness and skewness of the scalar-difference field as the separation distance becomes small. Larger than gaussian tails are observed in the PDF of both the velocity- and the scalar-derivatives, and the intermittency of the scalar derivative is shown to be more pronounced in the presence of the mean scalar gradient. Conditional averages of the angle between the scalar gradient and the strain eigenvectors suggest that the scalar field may be viewed as a random gaussian background field superimposed with sporadic scalar structures which are responsible for intermittency. With this view, a Langevin transport equation is proposed for the mapping of the scalar derivative PDF from a gaussian reference field. This is done in the context of the “two-fluid” model of She (1990). With this model, the PDF of the scalar dissipation is produced and the results are compared with DNS data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02087964
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  • 7
    Electronic Resource
    Electronic Resource
    Reactant conversion in homogeneous turbulence: Mathematical modeling, computational validations, and practical applications (1992)
    Springer
    Theoretical and computational fluid dynamics 4 (1992), S. 79-93 
    Details
    ISSN: 1432-2250
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Closed form analytical expressions are obtained for predicting the limiting rate of mean reactant conversion in homogeneous turbulent flows under the influence of a binary reaction of the type F+rO→(1+r) Product. These relations are obtained by means of a single-point Probability Density Function (PDF) method based on the Amplitude Mapping Closure (Kraichnan, 1989; Chen et al., 1989; Pope, 1991). It is demonstrated that with this model, the maximum rate of the mean reactants' decay can be conveniently expressed in terms of definite integrals of the parabolic cylinder functions. For the cases with complete initial segregation, it is shown that the results agree very closely with those predicted by employing a beta density of the first kind for an appropriately defined Shvab-Zeldovich scalar variable. With this assumption, the final results can also be expressed in terms of closed form analytical expressions which are based on the incomplete beta functions. With both models, the dependence of the results on the stoichiometric coefficient and the equivalence ratio can be expressed in an explicit manner. For a stoichiometric mixture the analytical results simplify significantly. In the mapping closure these results are expressed in terms of simple trigonometric functions. For the beta density model they are in the form of gamma functions. In all the cases considered, the results are shown to agree well with data generated by Direct Numerical Simulations (DNS). Due to the simplicity of these expressions and because of nice mathematical features of the parabolic cylinder and the incomplete beta functions, these models are recommended for estimating the limiting rate of mean reactant conversion in homogeneous reacting flows. These results also provide a valuable tool in assessing the extent of validity of turbulence closures for the modeling of unpremixed reacting flows. Some discussions are provided on the extension of the models for teating more complicated reacting systems, including realistic kinetics schemes and multiscalar mixing with finite rate chemical reactions in more complex configurations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00418237
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  • 8
    Electronic Resource
    Electronic Resource
    Comparative assessment of closures for turbulent reacting flows (1993)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 39 (1993), S. 899-903 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690390519
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