Luis Cifuentes

Local flow topologies and scalar structures in a turbulent premixed flame

A three-dimensional direct numerical simulation of a propagating turbulent premixed flame is performed using one-step Arrhenius model chemistry. The interaction of the flame thermochemical processes with the local geometries of the scalar field and flow topologies is studied. Four regions (“fresh reactants,” “preheating,” “burning,” and “hot products”), characterized by their reaction rate and mass fraction values, are examined. Thermochemical processes in the “preheating” and “burning” regions smooth out highly contorted iso-scalar surfaces, present in the “fresh reactants,” and annihilate large curvatures. Positive volumetric dilatation rates, −P = ∇ · u, display maxima for elliptic concave and minima for convex scalar micro-structures. Constant average tangential strain rates, aT, with large fluctuations, occur throughout the flow domain, whereas normal strain rates, aN, follow the trends of volumetric dilatation rates. Focal topologies, present in the “fresh reactants,” tend to disappear in favor of n...

The Physics of Scalar Gradients in Turbulent Premixed Combustion and Its Relevance to Modeling

ABSTRACT Equations for the absolute value of the scalar gradient and for the infinitesimal distance, between two adjacent iso-scalar non-material surfaces are obtained. ‘Effective’ strain rate normal to the iso-surfaces, which includes flow and physicochemically ‘added’ contributions, is shown to be an essential variable, causing either enhancement or destruction of scalar gradients and reduction or growth of distances between surfaces. Two DNS datasets for turbulent premixed flames, one simulating a statistically planar propagating front in an inflow-outflow configuration and the other computing a jet of a methane-air mixture, surrounded by a coflow of hot products, have been examined. DNS are used to estimate the relative importance of different processes determining the gradients fate. The flow normal strain rate apparently scales with the inverse of the Kolmogorov time microscale. Using as characteristic time, , and length, , where and are the laminar flame thickness and propagating velocity, a dimensionless equation for the time rate of change of depends on five dimensionless parameters, among them the Karlovitz number, ; the contribution of every term in the rate equation depends on the magnitude of compared to the other dimensionless groups. The chemically ‘added’ normal strain rate dominates the time evolution of in the burning and a good share of the preheat regions of the statistically planar flame, whereas ‘added’ and flow normal strain rates are comparable in the turbulent jet flame. Large values of some of these dimensionless parameters hint at the likely importance of accounting for Reynolds and/or Damhöhler numbers dependencies in future work. A consistent definition of an average premixed turbulent flame thickness is presented and its computed values are compared to a previous proposal. Some suggestions to model the molecular mixing term in the context of the scalar PDF transport methodology are discussed. It is hypothesized that the characteristic mixing time should be proportional to the inverse of the ‘effective’ normal strain rate.

Vorticity budgets in premixed combusting turbulent flows at different Lewis numbers

A direct numerical simulations database of statistically planar turbulent premixed flames using a simple Arrhenius type irreversible chemistry for different values of global Lewis numbers, Le, (0.34, 0.60, 0.80, 1.00, 1.20) has been examined to analyze the effects of Le on vorticity transport within the flame. To meet this objective, a general enstrophy conservation equation has been considered, which distinctly describes contributions from vortex-stretching, destruction by volumetric dilatation rates, baroclinic and viscous force torques, viscous transport, and dissipation. The average statistical behavior of the various contributions conditioned upon the value of the reaction progress variable, c, has been analyzed in the preheat and reacting regions of the flame. The mean values of enstrophy monotonically decays with c from fresh reactants toward hot products for Le equal to 0.8, 1.0, and 1.2; vortex-stretching and viscous dissipation are the leading contributors, while the remaining contributions are ...

Analysis of flame curvature evolution in a turbulent premixed bluff body burner

The physical mechanisms responsible for flame curvature evolution of a methane-air premixed flame attached to a bluff-body burner have been investigated using a high-fidelity flame-resolved three-dimensional simulation database. The contributions to the mean curvature generation due to the fluid flow motion and to a combination of flow and flame propagation induced strain rates have been analyzed in detail and dominant contributions in different zones (reactants, ame and products) of the flame have been identified. The effect of fluid flow on the mean curvature evolution is important on the unburned gas side, whereas the ame propagation dominates the mean curvature evolution in the reaction region and towards the hot products. The statistical contributions of the mean curvature transport equation have been analysed in terms of the iso-scalar surface geometry, characterized by the mean and Gauss curvatures. This information has subsequently been used to provide physical insights into the dominant mechanisms of curvature evolution for different flame topologies. This project has received funding from the European Unions Horizon 2020 research and innovation program under grant agreement No 706672 - ITPF.

Influence of the Lewis Number on Effective Strain Rates in Weakly Turbulent Premixed Combustion

ABSTRACT The influence of the global Lewis number, Le, on the statistical behavior of the “effective” normal and tangential strain rates have been analyzed based on three-dimensional direct numerical simulation data of freely propagating statistically planar turbulent premixed flames with Le = 0.34, 0.60, 0.80, 1.00, and 1.20. The volumetric dilatation rate is found to be mostly positive and its magnitude increases with decreasing Le. The flow normal strain rate predominantly assumes positive values and thus tends to pull adjacent iso-scalar surfaces apart, which reduces scalar gradients. By contrast, the “added” normal strain rate due to derivatives of the displacement speed normal to iso-surfaces has the propensity to push them closer together, and therefore increase the magnitude of scalar gradients. The balance between flow and added normal strain rates along with the advective transport determines whether scalar gradients are enhanced or destroyed. Iso-surface elementary area stretching by the fluid flow increases with decreasing Lewis number, and the added tangential strain rate exhibits predominantly negative values and is determined by the correlation between displacement speed components and flame curvature. It has been found that turbulent flames with small values of Lewis number exhibit flame thinning and high values of the flame surface area and these tendencies strengthen with decreasing Lewis number. This behavior has been explained in detail in terms of the statistical behaviors of effective normal and tangential strain rates.