Numerical and experimental investigation of vortical flow-flame interaction

A massively parallel coupled Eulerian-Lagrangian low Mach number reacting flow code is developed and used to study the structure and dynamics of a forced planar buoyant jet flame in two dimensions. The numerical construction uses a finite difference scheme with adaptive mesh refinement for solving the scalar conservation equations, and the vortex method for the momentum equations, with the necessary coupling terms. The numerical model construction is presented, along with computational issues regarding the parallel implementation. An experimental acoustically forced planar jet burner apparatus is also developed and used to study the velocity and scalar fields in this flow, and to provide useful data for validation of the computed jet. Burner design and laser diagnostic details are discussed, along with the measured laboratory jet flame dynamics. The computed reacting jet flow is also presented, with focus on both large-scale outer buoyant structures and the lifted flame stabilization dynamics. A triple flame structure is observed at the flame base in the computed flow, as is theoretically expected, but was not observable with present diagnostic techniques in the laboratory flame. Computed and experimental results are compared, along with implications for model improvements

Vida Verde. Barcelona

International audienceWe demonstrate, on a scramjet combustion problem, a constrained probabilistic learning approach that augments physics-based datasets with realizations that adhere to underlying constraints and scatter. The constraints are captured and delineated through diffusion maps, while the scatter is captured and sampled through a projected stochastic differential equation. The objective function and constraints of the optimization problem are then efficiently framed as non-parametric conditional expectations. Different spatial resolutions of a large-eddy simulation filter are used to explore the robustness of the model to the training dataset and to gain insight into the significance of spatial resolution on optimal design

Premixed flame response to unsteady strain-rate and curvature

The interaction of a premixed stoichiometric methane-air flame with a counter-rotating vortex-pair is studied using a skeletal C{sub 1} chemical description of the reaction process. The focus is on the modification to flame structure and dynamics due to unsteady strain-rate and curvature. The detailed description of flame structure and dynamics in response to unsteady flow is necessary to establish relevant extinction criteria in unsteady multi-dimensional flow, which, based on recent experimental evidence, may be significantly different from those of steady one-dimensional counterflow stagnation flames. Present results suggest that the increasing unsteady tangential strain-rate causes modification of flame structure that leads to reduced reaction rates of key chain-branching reactions which are active on the products side of the flame. This causes a reduction in the concentrations of active radicals, such as H, OH, and O, which are necessary for the breakdown of hydrocarbons on the reactants side of the flame

Planar laser-induced fluorescence imaging of flame heat release rate

Local heat release rate represents one of the most interesting experimental observables in the study of unsteady reacting flows. The direct measure of burning or heat release rate as a field variable is not possible. Numerous experimental investigations have relied on inferring this type of information as well as flame front topology from indirect measures which are presumed to be correlated. A recent study has brought into question many of the commonly used flame front marker and burning rate diagnostics. This same study found that the concentration of formyl radical offers the best possibility for measuring flame burning rate. However, primarily due to low concentrations, the fluorescence signal level from formyl is too weak to employ this diagnostic for single-pulse measurements of turbulent reacting flows. In this paper the authors describe and demonstrate a new fluorescence-based reaction front imaging diagnostic suitable for single-shot applications. The measurement is based on taking the pixel-by-pixel product of OH and CH{sub 2}O planar laser-induced fluorescence images to yield an image closely related to a reaction rate. The spectroscopic and collisional processes affecting the measured signals are discussed and the foundation of the diagnostic, as based on laminar and unsteady flame calculations, is presented. The authors report the results of applying this diagnostic to the study of a laminar premixed flame subject to an interaction with an isolated line-vortex pair

CSP Analysis of a Transient Flame/Vortex Interaction: Time Scales an Manifolds

The interaction of a two-dimensional counter-rotating vortex-pair with a premixed methane-air flame is analyzed with the Computational Singular Perturbation (CSP) method. It is shown that, as the fastest chemical time scales become exhausted, the solution is attracted towards a manifold, whose dimension decreases as the number of exhausted time scales increases. A necessary condition for a chemical time scale to become exhausted is that it must be much faster than the locally prevailing diffusion and convection time scales. Downstream of the flame, the hot products are in a regime of near-equilibrium, characterized by a large number of exhausted fast chemical time scales and the development of a low dimensional manifold, where the dynamics are locally controlled by slow transport processes and slow kinetics. In the flame region, where intense chemical and transport activity takes place, the number of exhausted chemical time scales is relatively small. The manifold has a large dimension and the driving time scale is set by chemical kinetics. In the cold flow region, where mostly reactants are present, the flow regime can be described as frozen, as the active chemical time scales are much slower than the diffusion and convection time scales; the driving scale set by diffusion. The algebraic relations among the elementary rates, which describe the manifold, are discussed along with a classification of the unknowns in three classes: i) CSP radicals; ii) trace; and, iii) major species. It is established that the optimal CSP radicals must be: i) strongly affected by the exhausted fast chemical time scales; and, ii) significant participants in the algebraic relations describing the manifold. The identification of CSP radicals, trace and major species, is a prerequisite for simplification or reduction of chemical kinetic mechanisms

Effect of equivalence ratio on premixed flame response to unsteady strain-rate and curvature

The detailed dynamical response of flames in turbulent reacting flow involves a complex interaction between unsteady flow structures and flame chemistry. Two essential features of this interaction are the unsteady strain-rate and curvature disturbances to the reaction zone. In this work, the authors focus on a single flow length/time scale feature in two dimensions (2D), and its effect on a premixed flame for a range of mixture conditions. In particular, they study the interaction of a premixed freely propagating methane-air flame with a 2D counter-rotating vortex pair in an unbounded domain. In earlier work, they studied this flow using C{sub 1} kinetics, at stoichiometric conditions. Notable observations include the shift of the reaction zone into the products on the vortex-pair centerline, leading to depletion of H, O and OH, and the consequent general drop in reaction rates on the centerline flame segment. Curvature-induced focusing/defocusing effects were observed at the positively curved flame cusp, leading to modifications in internal transport fluxes of various species and radicals in the flame, and associated effects on H production and fuel consumption rates. These results were extended to more detailed kinetics, using other C{sub 1} and C{sub 2} mechanisms, which demonstrated the effect of choice of chemical mechanism on the observed transient flame response. The present study focuses on the dependence of the transient flame response on reactants mixture equivalence ratio. Two reactants mixture conditions are studied: case 1 is a stoichiometric conditions - equivalence ratio {Phi} = 1.0, and case 2 is rich at {Phi} = 1.2. In both cases, the reactants are 20% N{sub 2}-diluted

Triple flame structure and dynamics at the stabilization point of a lifted jet diffusion flame

A coupled Lagrangian-Eulerian low-Mach-number numerical scheme is developed, using the vortex method for the momentum equations, and a finite difference approach with adaptive mesh refinement for the scalar conservation equations. The scheme is used to study the structure and dynamics of a forced lifted buoyant planar jet flame. Outer buoyant structures, driven by baroclinic vorticity generation, are observed. The flame base is found to stabilize in a region where flow velocities are sufficiently small to allow its existence. A triple flame is observed at the flame base, a result of premixing of fuel and oxidizer upstream of the ignition point. The structure and dynamics of the triple flame, and its modulation by jet vortex structures, are studied. The spatial extent of the triple flame is small, such that it fits wholly within the rounded flame base temperature field. The dilatation rate field outlines the edge of the hot fluid at the flame base. Neither the temperature field nor the dilatation rate field seem appropriate for experimental measurement of the triple flame in this flow

The spectral characterisation of reduced order models in chemical kinetic systems

The size and complexity of multi-scale problems such as those arising in chemicalkinetics mechanisms has stimulated the search for methods that reduce the numberof species and chemical reactions but retain a desired degree of accuracy. The time-scale characterisation of the multi-scale problem can be carried out on the basis oflocal information such as the Jacobian matrix of the model problem and its relatedeigen-system evaluated at one pointPof the system trajectory. While the originalproblem is usually described by ordinary differential equations (ODEs), the reducedorder model is described by a reduced number of ODEs and a number of algebraicequations (AEs), that might express one or more physical conservation laws (mass,momentum, energy), or the fact that the long-term dynamics evolves within a so-calledSlow Invariant Manifold (SIM). To fully exploit the benefits offered by a reduced ordermodel, it is required that the time scale characterisation of then-dimensional reducedorder model returns an answer consistent and coherent with the time-scale characteri-sation of theN-dimensional original model. This manuscript discusses a procedure forobtaining the time-scale characterisation of the reduced order model in a manner thatis consistent with that of the original problem. While a standard time scale characteri-sation of the (original)N-dimensional original model can be carried out by evaluatingthe eigen-system of the (N×N) Jacobian matrix of the vector field that defines thesystem dynamics, the time-scale characterisation of then-dimensional reduced ordermodel (withn<N) can be carried out by evaluating the eigen-system of a (n×n)con-strainedJacobian matrix,JC, of the reduced vector field that accounts for the role ofthe constraints.info:eu-repo/semantics/publishe

A CSP-Based Skeletal Mechanism Generation Procedure: Auto-Ignition and Premixed Laminar flames in n-Heptane/Air Mixtures

We use a procedure based on the decomposition into fast and slow dynamical components offered by the Computational Singular Perturbation (CSP) method to generate automatically skeletal kinetic mechanisms for the simplification of the kinetics of n-heptane oxidation. The detailed mechanism of the n-heptane oxidation here considered has been proposed by Curran et al. [H.J. Curran, P. Gaffuri, W.J. Pitz, and C.K. Westbrook, n-Heptane, detailed mechanism, Version 2, www-cms.llnl.gov/combustion/combustion2.html, 2002] and involves 561 species and 2538 reactions. This work achieved three main goals. First, we carried out a thorough error analysis aimed at verifying which of the two algorithmic options involving or not the scaling of the CSP indices is the most suited for achieving accurate skeletal mechanisms. The comparative analysis showed that although both options produce valid mechanisms, scaling the indices seems to offer a smoother dependence of the accuracy with respect to the number of species retained in the mechanism, and for this reason seems to be the most preferable. Second, by using the scaled index option, we generated two series of accurate skeletal mechanisms for n-heptane oxidation, one series valid for both a wide range of initial temperatures and equivalence ratios, and the other only for the high temperatures regime (and for different equivalence ratios). Finally, we verified that it is possible to use skeletal mechanisms generated with respect to auto-ignition phenomena for computing premixed laminar flames with high accuracy, especially with respect to macroscopic parameters such as laminar flame speed, equilibrium temperature, and velocity, temperature and major species fields across the flame. This test showed that for a premixed laminar flame it is not essential to include the low temperature kinetics of n-heptane. This allowed us to obtain a satisfactory approximation of the flame structure with a rather small mechanism, which includes only 66 species out of the original 561. These findings empirically demonstrate that the reduction can be performed in a simple configuration, like the homogeneous auto-ignition considered in this paper and the resulting reduced mechanism applied successfully to a more complex configuration such as a premixed or counterflow flame, or, even, a fully multidimensional CFD reactive flow simulation. It is noteworthy to stress, in closing, that the 66-species mechanism seems a good and affordable candidate to tackle the direct simulation of n-heptane combustion