Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session H26: Reacting Flows VI: Premixed |
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Chair: Guillaume Blanquart, California Institute of Technology Room: 321 |
Monday, November 25, 2013 10:30AM - 10:43AM |
H26.00001: High Karlovitz {\it n}-Alkane Premixed Flame DNS: Effects of Turbulence on the Flame Structure Bruno Savard, Brock Bobbitt, Guillaume Blanquart The effects of turbulence on the structure of a statistically flat, slightly lean ($\phi=0.9$), {\it n}-heptane/air premixed flame is investigated using three dimensional direct numerical simulations at a Karlovitz number close to 100. Two simulations are performed: one with unity Lewis numbers and one with non-unity Lewis numbers. The first simulation is used to investigate deviations away from the laminar flamelet structure as eddies penetrate the preheat and reaction zones. The second is to analyze the relative importance of molecular {\it vs} turbulent mixing and their effects on species transport. The conditional mean profiles of the species mass fraction {\it vs} temperature from both simulations are evaluated to assess the influence of turbulence on scalar transport. As expected, turbulent mixing and molecular mixing are of comparable magnitude and, as a result, the structure of the flame is altered. Using a method developed in a previous work, the effective Lewis numbers of the different species are identified. These effective Lewis numbers are closer to unity than their laminar value, showing the effect of turbulent mixing. Interestingly, with this change in Lewis numbers, the structure of the turbulent flame compares very favorably with that of a laminar flame. [Preview Abstract] |
Monday, November 25, 2013 10:43AM - 10:56AM |
H26.00002: High Karlovitz \textit{n}-Alkane Premixed Flame DNS: Effects of the Flame on Turbulence Characteristics Brock Bobbitt, Bruno Savard, Guillaume Blanquart The study of premixed turbulent combustion requires understanding turbulence and chemistry independently as well as their effects upon one another. This coupling alters their inherent characteristics in a complex fashion. Unfortunately, the transformation of the turbulence across the flame is not well understood and it is common to assume homogeneous, isotropic turbulence before and after the flame. To this end, direct numerical simulations were performed of homogeneous isotropic turbulence interacting with a premixed flame. These were done at a Karlovitz number of approximately 100 using both tabulated and detailed \textit{n}-heptane air chemistry. The integral length scale was four times the laminar flame thickness allowing study of both large and small scale turbulence. The transformation of these turbulent scales across the flame was investigated throughout and behind the flame. A model for the transfer function was developed by applying a generalized expansion to the continuity, momentum, vorticity, temperature, and species transport equations. From this, equations are derived which describe to leading order the transformation of turbulent velocity and length scales across the flame. [Preview Abstract] |
Monday, November 25, 2013 10:56AM - 11:09AM |
H26.00003: A simulation of a bluff-body stabilized turbulent premixed flame using LES-PDF Jeonglae Kim, Stephen Pope A turbulent premixed flame stabilized by a triangular cylinder as a flame-holder is simulated. The computational condition matches the Volvo experiments (Sjunnesson~{\em et al.}~1992). Propane is premixed at a fuel lean condition of $\phi = 0.65$. For this reactive simulation, LES-PDF formulation is used, similar to Yang~{\em et al.} (2012). The evolution of Lagrangian particles is simulated by solving stochastic differential equations modeling transport of the composition PDF. Mixing is modeled by the modified IEM model (Viswanathan~{\em et al.}~2011). Chemical reactions are calculated by ISAT and for the good load balancing, PURAN distribution of ISAT tables is applied (Hiremath~{\em et al.}~2012). To calculate resolved density, the two-way coupling (Popov \& Pope~2013) is applied, solving a transport equation of resolved specific volume to reduce statistical noise. A baseline calculation shows a good agreement with the experimental measurements in turbulence statistics, temperature, and minor species mass fractions. Chemical reaction does not significantly contribute to the overall computational cost, in contrast to non-premixed flame simulations (Hiremath~{\em et al.}~2013), presumably due to the restricted manifold of the purely premixed flame in the composition space. [Preview Abstract] |
Monday, November 25, 2013 11:09AM - 11:22AM |
H26.00004: Impact of Chemistry Models on Flame-Vortex Interaction Simon Lapointe, Brock Bobbitt, Guillaume Blanquart In premixed turbulent combustion, accurate modeling of the fuel chemistry for numerical simulations is a critical component of capturing the relevant physics. Various chemical models are currently being used including detailed chemistry, tabulated chemistry, one-step chemistry, and rate-controlled constrained-equilibrium. However, the differences between these models and their impacts on the fluid dynamics are not well understood. Towards that end, the interaction between a laminar premixed hydrogen flame and a two-dimensional vortex was studied through Direct Numerical Simulations using each of these different chemistry models. In these simulations, the flame thickness, flame speed, viscosity, diffusivity, conductivity, density ratio, and vortex characteristics were held constant providing comparison of the effects of each chemical model alone. A convergence study was performed for each model assessing the numerical requirements of domain size, grid spacing and time step to completely resolve both the fluid dynamics and the chemistry. The converged results from each model were compared by considering the evolution of the flame structure and characteristics of the vortex. [Preview Abstract] |
Monday, November 25, 2013 11:22AM - 11:35AM |
H26.00005: Folds and pockets in the propagation of premixed turbulent flames Navin Fogla, Moshe Matalon, Francesco Creta We examine the propagation of premixed flames in two-dimensional turbulent flows within the context of a hydrodynamic model that treats the flame as a surface of density discontinuity using a hybrid Navier-Stokes/interface capturing technique. Employing an improved interface capturing technique, which allows the flame front to attain multivalued configurations and form pockets of unburned gas before being consumed, broadens the range of applicability of our results to include the corrugated flamelet regime ($u'/S_L > 1$) of turbulent combustion. Three regimes are identified, depending on the mixture composition, thermal expansion coefficient and turbulence intensity: a regime where, on the average, the flame brush remains planar and unaffected by the Darrieus-Landau (DL) instability, a regime where the DL effects, responsible for frequent intrusions of the flame front into the burned gas region, have a marked influence on the flame brush that remains resilient to turbulence, and a highly turbulent regime where the influences of the DL instability progressively decrease and play limited to no role on the flame propagation. Particular attention is given in this presentation to the effects of folding/pocket formation on the flame structure and dynamics. [Preview Abstract] |
Monday, November 25, 2013 11:35AM - 11:48AM |
H26.00006: Turbulent combustion modeling using explicit convolution of 1-D laminar flame S. Mukhopadhyay, R.J.M. Bastiaans, J.A. van Oijen, L.P.H. de Goey Increasing computational power is enabling highly resolved Large Eddy Simulation (LES) of turbulent reacting flows. However resolving chemical scales in a practical combustor even with tabulated chemistry methods, still remains unaffordable and requires a model. DNS of a premixed slot flame is performed and \textit{a priori} analysis indicates that laminar flame filtered at suitable scale can represent the chemical state in a turbulent reacting flow. But to represent all the chemical states, multiple filter widths will be required. This work explores a new modeling approach, Filtered Flamelet Generated Manifold (FFGM) based on explicit convolution of 1-D laminar flame solutions with spatial filter kernel of varying widths. To test the validity of the model \textit{a posteriori} analysis, using tabulated chemistry constructed by convoluting a premixed laminar flame with top hat kernel of multiple widths is performed for the DNS configuration. The results indicate good performance of the model compared to DNS at a fraction of computational cost. [Preview Abstract] |
Monday, November 25, 2013 11:48AM - 12:01PM |
H26.00007: Numerical forcing of an M-flame: linear analysis Mathieu Blanchard, Peter J. Schmid, Denis Sipp, Thierry Schuller, Sebastien Candel Direct numerical simulations of a high Mach number (Ma=0.1), low Reynolds number (Re = 1000), premixed, lean M-flame have been studied with the goal of characterizing and quantifying the response of this generic flame to acoustic modulations. This response is essential to a description of thermo-acoustic instabilities. The flame is submitted to energy disturbances introduced in the injection tube of reactants using random binary signals. The unit impulse response of the flow variables in the burnt gases is computed. It features disturbances of acoustic and hydrodynamic nature. The short time response of this function is controlled by acoustic disturbances, while large hydrodynamic perturbations dominate the long time response of the unit impulse function. The mechanisms controlling the short and long time responses of the flame are examined. A sensitivity analysis is then conducted for selected characteristic frequencies and the structures of the linear optimal forcing are determined. [Preview Abstract] |
Monday, November 25, 2013 12:01PM - 12:14PM |
H26.00008: Flame Thickness and Conditional Scalar Dissipation Rate in a Premixed Temporal Turbulent Reacting Jet Swetaprovo Chaudhuri, Hemanth Kolla, Evatt Hawkes, Jacqueline Chen, Chung Law The flame structure corresponding to a lean H$_{2}$/air premixed flame in intense sheared turbulence in the thin reaction zones regime is quantified from flame thickness and conditional scalar dissipation rate statistics obtained from recent direct numerical simulation data of premixed temporally-evolving turbulent slot jet flames. The local alignment between the progress variable iso-surface normals and the most compressive principal strain rate is observed to increase traversing from the unburnt reactants to the burnt products. Such preferential alignment coupled with sub-unity Le associated with lean H$_{2}$/air mixtures results in increasing normalized mean conditional scalar dissipation rate and a resultant decrease of the normalized mean flame thickness. On average, these turbulent flames are thinner than their corresponding planar laminar flames. The intermittency of the conditional scalar dissipation rate is found to exhibit a unique non-monotonicity of the exponent of the stretched exponential function, conventionally used to describe probability density function tails of such variables. The non-monotonicity is attributed to the detailed chemical structure of hydrogen-air flames where heat release occurs close to the unburnt reactants at near free stream temperatures. [Preview Abstract] |
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