Effect of Damkohler number on the reactive zone structure in a shear layer☆
Abstract
Numerical simulation is used to study an unstable, two-dimensional, two-stream, spatially developing, confined, reacting shear layer. The physical model is based on a low-heat-release, temperature-independent chemical reaction and is used to investigate the effect of the flow field on the burning rate at high Reynolds number and moderate Damkohler numbers. The unsteady mass, mometum, energy, and species conservation equations are integrated using a grid-free, Lagrangian field method in which gradients of the primary variables are transported along particle trajectories to preserve the numerical accuracy as strong strains arise in the flow. Results reveal a strong similarity between the distributions of product concentration and vorticity at a wide range of Damkohler numbers. This similarity is due to the entrainment field associated with vorticity amalgamation into large-vortex eddies. At low Damkohler numbers, products form in distributed zones within the large eddies, following the mixing of entrained reactants. At high Damkohler numbers, products form within thin zones around the outer edges of the eddies and then get entrained into the their cores. Between the large eddies, the strain field reduces product concentration by thinning the braids and reducing the time allowed for mixing. The effect of chemical kinetics rates on product formation is strongest around a Damkohler number of 1, and reaches its limit around Damkohler number of 20. Product formation is strongly governed by the entrainment field, and is thus weakly dependent on the Reynolds number. The reactants' ratio across the layer affects the rate of product formation, through the mechanism of asymmetric entrainment, by enforcing stronger potential of chemical activity.
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Effects of Damköhler number on vortex-flame interaction
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Oscillating heat release associated with large-scale vortex structures in a dump combustor was investigated experimentally. The objectives were to understand vortex–heat release interactions that may lead to combustion instability and to characterize spatial patterns in fluctuating heat release for future control purposes. The inlet jet, which consisted of a turbulent premixed ethylene–air flow, was acoustically forced to control shedding of periodic vortices. The reacting flowfield was characterized using a phase-locked schlieren technique, while the corresponding heat release pattern was investigated using CH* chemiluminescence. Phase-resolved data taken at various flow conditions were compared to identify important physical parameters. In the context of vortex-driven combustion instability, the results showed that the Damköhler number was a critical parameter for vortex–heat release interaction. When the Damköhler number was relatively high, the leading edge of the vortices coincided with the high point in the local heat release cycle and the trailing edge with the low point. However, a clear shift in the heat release pattern was observed at Damköhler numbers lower than four. Under these conditions, the high point in local heat release was observed either in the vortex core or at the trailing edge of the vortex. Accordingly, the effect of the Damköhler number must be taken into consideration when designing an active control strategy based on secondary fuel injection.
On the effects of the inlet boundary condition on the mixing and burning in reacting shear flows
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Applications of direct numerical simulation to premixed turbulent combustion
1995, Progress in Energy and Combustion ScienceThis paper describes recent progress in the field of Direct Numerical Simulations (DNS) for premixed turbulent combustion. A classification of Computational Fluid Dynamics techniques used in reacting flows is first proposed. Numerical methods and problems specific to DNS of combustion like grid resolution or boundary conditions treatment are discussed. The presentation of DNS results is done in the framework of flamelet modeling. The domain of validity of the flamelet assumptions is studied with simulations of flame vortex interactions. The local structure of flamelets in premixed turbulent combustion is investigated using two and three-dimensional DNS with simple chemistry models. Effects of complex chemistry are considered through recent two-dimensional DNS performed with realistic chemical schemes. Flame surface densities, flame strain and curvature statistics obtained from various three dimensional simulations are given and implications for modeling are presented. DNS is also exploited to deal with some specific problems like turbulent flame ignition or flame-wall interactions. This review is concluded with a discussion of current limitations and a list of future challenges.
Experimental identification of mixing regimes in the analysis of turbulent diffusion flames
1994, Combustion and FlameA statistical criterion for the classification of gaseous mixing regimes is presented. It is based on the evaluation of two quantities: the mixing layer thickness and the separation distance between two neighboring interface segments, which are preliminarily defined in a specific section along with other quantities (stretch ratio and stretch rate) relevant to the mixing analysis. A unique experimental method for a lagrangian measure of the aforementioned quantities as a function of the residence time is described. Measurements use a two-dimensional laser light scattering technique in a two-dimensional, transitional, isothermal flow which can be considered a prototypical condition of mixing. The probability density function of the stretch rate as well as average values of stretch ratio, mixing layer thickness and interface separation distance are also reported. The last two quantities are used in the determination of a single parameter named “saturation factor” (Csat), that allows the identification of three regimes:
(a) “Isolated mixing layer regime” (Csat = 1) in which both the mixing and oxidative reactions occur in the neighborhood of the interface. This is a well recognized regime, to which many studies on the characterization of one-dimensional time-dependent stretched diffusion flames refer.
(b) “Interacting mixing layer regime” (10−2 < Csat < 1) in which the diffusion still occurs in the whole flow field, but more sophisticated models than those relative to the previous regime are needed for its characterization.
(c) “Saturated mixing layer regime” (Csat < 10−2) in which the mixing process is no longer related to the intermaterial surface in its wholeness, but only to that part for which it is still far from other segments of the interface.
A physical simulation model for turbulent mixing layer combustion
1992, Symposium (International) on CombustionA previously proposed discrete vortex model is applied to reveal the underlying physics of diffusion combustion in vortical flow fields generated by pairing vortices. The model accounts for the finite size of each localized vorticity region as a circulation monopole plus a vortex quadrupole, and describes a self-rotating elliptic vortex deformable in a stretching flow due to the presence of other vortices and flame. The coalescence of pairing vortices can be recognized as a resonance phenomenon which takes place, through interaction between a pair of quadrupoles, when the magnitude of the stretching rate in the vorticity regions exceeds that of the self-rotating angular velocity. The Shvab-Zeldovich formulation is used for combustion modeling. Results of simulation performed for a temporally evolving mixing layer are in fairly good agreement with existing numerical calculation results.
The effect of thermal expansion is incorporated in the form of volume source at the flame front, so that changes from constant density case attribute to the expanding flow from the flame. The burning rate is reduced since the flow opposes the diffusive fuel and oxidizer fluxes toward the flame. The distance of separation between flame segments is enlarged, resulting in an rapid increase in the flame length and mixing layer thickness. This has another important significance in the light of the present model because the effect of vortex quadrupole decays faster than that of circulation monopole away from each vortex core and the preservation of localized vorticity regions through the repeated processes of vortex pairing and coalescence is inhibited. In addition to the commonly mentioned increased viscosity due to temperature rises, this mechanism is proposed to provide a physical explanation why turbulent free shear layer with combustion assumes a simpler structure in a shorter time.
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This work was supported by the Air Force Office of Scientific Research Grant 84-0356, The National Science Foundation Grant CBT-8709465 and the Department of Energy Grant DE-FG04-87AL44875.