Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session A17: Reacting Flows: DNS |
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Chair: Jacqueline Chen, Sandia National Laboratories Room: D131 |
Sunday, November 20, 2016 8:00AM - 8:13AM |
A17.00001: Direct Numerical Simulation of a Cavity-Stabilized Ethylene/Air Premixed Flame Jacqueline Chen, Aditya Konduri, Hemanth Kolla, Andreas Rauch, Harsha Chelliah Cavity flame holders have been shown to be important for flame stabilization in scramjet combustors. In the present study the stabilization of a lean premixed ethylene/air flame in a rectangular cavity at thermo-chemical conditions relevant to scramjet combustors is simulated using a compressible reacting multi-block direct numerical simulation solver, S3D, incorporating a 22 species ethylene-air reduced chemical model. The fuel is premixed with air to an equivalence ratio of 0.4 and enters the computational domain at Mach numbers between 0.3 and 0.6. An auxiliary inert channel flow simulation is used to provide the turbulent velocity profile at the inlet for the reacting flow simulation. The detailed interaction between intense turbulence, nonequilibrium concentrations of radical species formed in the cavity and mixing with the premixed main stream under density variations due to heat release rate and compressibility effects is quantified. The mechanism for flame stabilization is quantified in terms of relevant non-dimensional parameters, and detailed analysis of the flame and turbulence structure will be presented. [Preview Abstract] |
Sunday, November 20, 2016 8:13AM - 8:26AM |
A17.00002: DNS of a turbulent, self-igniting n-dodecane / air jet Giulio Borghesi, Jacqueline Chen A direct numerical simulation of a turbulent, self-igniting temporal jet between n-dodecane and diluted air at p$=$25 bar has been conducted to clarify certain aspects of diesel engine combustion. The thermodynamics conditions were selected to result in a two-stage ignition event, in which low- and high-temperature chemical reactions play an equally important role during the ignition process. Jet parameters were tuned to yield a target ignition Damkohler number of 0.4, a value representative of conditions found in diesel spray flames. Chemical reactions were described by a 35-species reduced mechanism, including both the low- and high-temperature reaction pathways of n-dodecane. The present work focuses on the influence of low-temperature chemistry on the overall ignition transient. We also study the structure of the flames formed at the end of the autoignition transient. Recent studies on diluted dimethyl ether / air flames at pressure and temperature conditions similar to those investigated in this work revealed the existence of tetra- and penta-brachial flames, and it is of interest to determine whether similar flame structures also exist when diesel-like fuels are used. [Preview Abstract] |
Sunday, November 20, 2016 8:26AM - 8:39AM |
A17.00003: Large scale Direct Numerical Simulation of premixed turbulent jet flames at high Reynolds number Antonio Attili, Stefano Luca, Ermanno Lo Schiavo, Fabrizio Bisetti, Francesco Creta A set of direct numerical simulations of turbulent premixed jet flames at different Reynolds and Karlovitz numbers is presented. The simulations feature finite rate chemistry with 16 species and 73 reactions and up to 22 Billion grid points. The jet consists of a methane/air mixture with equivalence ratio $\phi$~=~0.7 and temperature varying between 500 and 800~K. The temperature and species concentrations in the coflow correspond to the equilibrium state of the burnt mixture. All the simulations are performed at 4 atm. The flame length, normalized by the jet width, decreases significantly as the Reynolds number increases. This is consistent with an increase of the turbulent flame speed due to the increased integral scale of turbulence. This behavior is typical of flames in the thin-reaction zone regime, which are affected by turbulent transport in the preheat layer. Fractal dimension and topology of the flame surface, statistics of temperature gradients, and flame structure are investigated and the dependence of these quantities on the Reynolds number is assessed. [Preview Abstract] |
Sunday, November 20, 2016 8:39AM - 8:52AM |
A17.00004: DNS investigation of differential-diffusion effects on temporarily evolving turbulent diffusion flames Antonio Almagro, Manuel Garcia-Villalba, Oscar Flores, Antonio L Sanchez The peak temperature of nonpremixed flames is known to have a profound effect on kinetically controlled processes with a strong temperature dependence, such as strain-induced extinction and NOx production. Here, the influence of differential diffusion on the flame temperature in diffusion-controlled combustion is investigated by direct numerical simulations of a turbulent diffusion flame in a temporarily evolving mixing layer for non-unity Lewis numbers of the fuel. The problem is formulated in the limit of infinitely fast combustion in terms of Shvab-Zel’dovich conserved scalars, not changed directly by the reactions, obtained through chemistry-free linear combinations of the temperature and reactant mass fractions. A previously developed low-Mach-number code is used in the numerical integrations, which consider values of the thermochemical parameters -- characterizing the exothermicity and stoichiometry of diffusion-controlled combustion -- and fuel Lewis number typical of hydrogen-air and hydrocarbon-air flames. The results of the simulations are used to asses the effect of turbulence and fuel diffusivity on the flame response. [Preview Abstract] |
Sunday, November 20, 2016 8:52AM - 9:05AM |
A17.00005: DNS of High Pressure Supercritical Combustion Shao Teng Chong, Venkatramanan Raman Supercritical flows have always been important to rocket motors, and more recently to aircraft engines and stationary gas turbines. The purpose of the present study is to understand effects of differential diffusion on reacting scalars using supercritical isotropic turbulence. Focus is on fuel and oxidant reacting in the transcritical region where density, heat capacity and transport properties are highly sensitive to variations in temperature and pressure. Reynolds and Damkohler number vary as a result and although it is common to neglect differential diffusion effects if Re is sufficiently large, this large variation in temperature with heat release can accentuate molecular transport differences. Direct numerical simulations (DNS) for one step chemistry reaction between fuel and oxidizer are used to examine the differential diffusion effects. A key issue investigated in this paper is if the flamelet progress variable approach, where the Lewis number is usually assumed to be unity and constant for all species, can be accurately applied to simulate supercritical combustion. [Preview Abstract] |
Sunday, November 20, 2016 9:05AM - 9:18AM |
A17.00006: LES-inspired forcing technique for DNS of turbulent premixed flames Chandru Dhandapani, Guillaume Blanquart Direct numerical simulations (DNS) of high Karlovitz number (Ka) flames have been performed extensively in an inflow/outflow configuration, but in the absence of mean shear. Without a mean shear to sustain turbulence, the turbulent kinetic energy decays in the domain. Hence, a turbulence forcing has been used in previous simulations to emulate the missing shear effects. Rather than using an arbitrary forcing, the current study uses a source term for this turbulence forcing obtained from the results of previously performed large eddy simulations (LES) of a practical turbulent jet flame. The pseudo-shear term used for this turbulence forcing is linear and takes the form $A_{ij}u_j$ in the momentum equation, such that the source term is proportional to the velocity fluctuations. Different forms of the proportionality matrix $A_{ij}$ are considered, including a scalar matrix $A\delta_{ij}$. DNS of high Ka n-heptane air flames are performed with the new forcing and the energy spectrum is calculated. This energy spectrum is combined with that obtained from the LES and compared with results from previously performed experiments of turbulent jet flames. [Preview Abstract] |
Sunday, November 20, 2016 9:18AM - 9:31AM |
A17.00007: Direct Numerical Simulation of Combustion Using Principal Component Analysis Opeoluwa Owoyele, Tarek Echekki We investigate the potential of accelerating chemistry integration during the direct numerical simulation (DNS) of complex fuels based on the transport equations of representative scalars that span the desired composition space using principal component analysis (PCA). The transported principal components (PCs) offer significant potential to reduce the computational cost of DNS through a reduction in the number of transported scalars, as well as the spatial and temporal resolution requirements. The strategy is demonstrated using DNS of a premixed methane-air flame in a 2D vortical flow and is extended to the 3D geometry to further demonstrate the computational efficiency of PC transport. The PCs are derived from \textit{a priori} PCA of a subset of the full thermo-chemical scalars' vector. The PCs' chemical source terms and transport properties are constructed and tabulated in terms of the PCs using artificial neural networks (ANN). Comparison of DNS based on a full thermo-chemical state and DNS based on PC transport based on 6 PCs shows excellent agreement even for species that are not included in the PCA reduction. The transported PCs reproduce some of the salient features of strongly curved and strongly strained flames. The 2D DNS results also show a significant reduction of two orders of magnitude in the computational cost of the simulations, which enables an extension of the PCA approach to 3D DNS under similar computational requirements. [Preview Abstract] |
Sunday, November 20, 2016 9:31AM - 9:44AM |
A17.00008: ABSTRACT WITHDRAWN |
Sunday, November 20, 2016 9:44AM - 9:57AM |
A17.00009: Conditional budgets of second-order statistics in nonpremixed and premixed turbulent combustion Jonathan F. MacArt, Temistocle Grenga, Michael E. Mueller Combustion heat release modifies or introduces a number of new terms to the balance equations for second-order turbulence statistics (turbulent kinetic energy, scalar variance, etc.) compared to incompressible flow. A major modification is a significant increase in viscosity and dissipation in the high-temperature combustion products, but new terms also appear due to density variation and gas expansion (dilatation). Previous scaling analyses have hypothesized that dilatation effects are important in turbulent premixed combustion but are unimportant in turbulent nonpremixed combustion. To explore this hypothesis, a series of DNS calculations have been performed in the low Mach number limit for spatially evolving turbulent planar jet flames of hydrogen and air in both premixed and nonpremixed configurations. Unlike other studies exploring the effects of heat release on turbulence, the turbulence is not forced, and detailed chemical kinetics are used to describe hydrogen-air combustion. Budgets for second-order statistics are computed conditioned on progress variable in the premixed flame and on mixture fraction in the nonpremixed flame in order to locate regions with respect to the flame structure where dilatation effects are strongest. [Preview Abstract] |
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