Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session LU: Reacting Flows II |
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Chair: Lester Su, Johns Hopkins University Room: Hyatt Regency Long Beach Regency A |
Monday, November 22, 2010 3:35PM - 3:48PM |
LU.00001: Large Eddy Simulations Of A Turbulent Auto-Igniting C2H4 Flame DNS Edward Knudsen, Shashank, Heinz Pitsch, Ed Richardson, Jackie Chen Large eddy simulations of a turbulent auto-igniting flame are performed to analyze the interaction of different combustion regimes in a flamelet modeling framework. The case that is considered is a direct numerical simulation (DNS) of a non-premixed jet flame at Re=10,000 with heated co-flow. This DNS was performed by Yoo et al. (Proc. Comb. Inst., 2010) using 1.29 billion cells and a 22 species mechanism. A series of flamelet-type approaches are applied in successive large eddy simulations of the flame to understand the importance and interaction of dissipation and auto-ignition. Simulations are first performed by relying either on purely 0-D auto-ignition chemistry or on purely steady non-premixed flamelet chemistry. These simulations significantly under- and over-predict the lift-off height, respectively. Two approaches are then considered that simultaneously account for these processes: a well known tabulated unsteady flamelet formulation and a multi-regime formulation that combines the limiting steady and auto-ignition solutions according to a regime indicator. Comparisons with the DNS demonstrate that these approaches lead to improved liff-off height predictions. [Preview Abstract] |
Monday, November 22, 2010 3:48PM - 4:01PM |
LU.00002: Spark Ignition: Effects of Fluid Dynamics and Electrode Geometry Sally Bane, Jack Ziegler, Joseph Shepherd The concept of minimum ignition energy (MIE) has traditionally formed the basis for studying ignition hazards of fuels, and standard test methods for determining the MIE use a capacitive spark discharge as the ignition source. Developing the numerical tools necessary to quantitatively predict ignition is a challenging research problem and remains primarily an experimental issue. In this work a two-dimensional model of spark discharge in air and spark ignition was developed using the non-reactive and reactive Navier-Stokes equations. The simulations were performed with three different electrode geometries to investigate the effect of the geometry on the fluid mechanics of the evolving spark kernel and on flame formation. The computational results were compared with high-speed schlieren visualization of spark and ignition kernels. It was found that the electrode geometry had a significant effect on the fluid motion following spark discharge and hence influences the ignition process and the required spark energy. [Preview Abstract] |
Monday, November 22, 2010 4:01PM - 4:14PM |
LU.00003: Critical ignition in rapidly expanding self-similar flows Matei I. Radulescu, Brian McN. Maxwell The generic problem of ignition of a particle undergoing an expansion given by a power law rate of decay behind a decaying shock is addressed in the present study. It is demonstrated, using a one-step Arrhenius irreversible reaction, that a sufficiently strong expansion wave can quench the reaction. The critical conditions for extinction are obtained in closed form in terms of the time scale for the expansion process and the thermo-chemical properties of the gas, yielding a critical Damkohler number, i.e. the ratio of the expansion time scale to the homogeneous ignition time scale, given by ($\gamma $-1)$E_{a}$/\textit{RT}-1/$n$, where $n$ is the power law exponent of the self-similar expansion $\rho \sim t^{-n}$. The critical ignition criteria, which is valid in the asymptotic limit $n(\gamma $-1)($E_{a}$/\textit{RT)=O}(1), was found in excellent agreement with numerical results. The applicability of the results obtained are discussed for ignition in rapidly expanding flows which occur behind decaying shock waves, as encountered in problems of detonation initiation by a Taylor-Sedov blast wave, and reacting jet start-up, and for reactions in steady hypersonic flows around projectiles. [Preview Abstract] |
Monday, November 22, 2010 4:14PM - 4:27PM |
LU.00004: Effects of turbulence on syngas ignition in rapid compression machines Matthias Ihme, Asko Soimakallio Comparisons of ignition delays between predictions and measurements showed considerable differences for high-pressure/low-temperature syngas mixtures. Although effects of reaction-chemistry and large-scale hydrodynamic mixing have been identified as potential sources for these discrepancies, the significance of turbulence and turbulence/chemistry interaction has not be quantified. To address this issue, a theoretical model has been developed in which rapid-distortion theory and a Lagrangian Fokker Planck model have been combined to model turbulence amplification and autoignition in rapid compression machines (RCMs). The model was applied to a realistic RCM-configuration, and parametric studies were performed. From this study, a Damkoehler criterion was derived to quantify the sensitivity of the induction chemistry to turbulence. [Preview Abstract] |
Monday, November 22, 2010 4:27PM - 4:40PM |
LU.00005: Simulation of Electron and Ion Transport in Methane-Air Counterflow Diffusion Flames Sangkyu Choi, Fabrizio Bisetti, Suk Ho Chung The spatial distribution of charged species in a methane-air counterflow diffusion flame is simulated with a detailed ion chemistry. The electric field induced by the distribution of charged species is calculated and compared to that obtained invoking the ambipolar diffusion assumption. The two calculations showed identical profiles for charged species and electric field. The profiles of ion mole fractions show two peaks: one near the maximum temperature and a second peak on the oxidizer side. The major ions near the maximum temperature are electron, C$_{2}$H$_{3}$O+ and H$_{3}$O+. CHO$_{3}$- and H$_{3}$O+ contribute to the second peak. These profiles are quite different from those adopting a simplified three-step mechanism based solely on E-, CHO+ and H$_{3}$O+, which shows only a single peak. Reaction pathway analyses showed that near the flame region, the proton is transferred by the path of CHO+ $\to $ H$_{3}$O+ $\to $ C$_{2}$H$_{3}$O+ $\to $ CHO+ in a circulating manner. In the second peak, CHO$_{3}$- is produced though the pathway of E- $\to $ O- $\to $ OH- $\to $ CHO$_{3}$-. The sensitivity of the charged species profiles to transport properties is investigated, and it is found that the variation of charged species profiles near peak temperature is relatively small, while on the oxidizer side, it is quite sensitive to transport properties. [Preview Abstract] |
Monday, November 22, 2010 4:40PM - 4:53PM |
LU.00006: Direct Numerical Simulation of electrochemical reactions in a turbulent electrolyte Olivier Doche, Frederic Bauer, Sedat Tardu In electrochemical processes, such as industrial electrodeposition, the flow state can influence the mass transfer of the active chemical species in solution. This could lead to significant modifications of reaction kinetics at the electrode and obviously affects the global performance of the system. We aim here to describe via DNS the behavior of a turbulent electrolyte in a channel configuration where electrode are placed at each wall. Since the whole problem is governed by a full multiphysic coupling, we resolve in 3D and at each time step a set of equations constituted by 2 turbulent transport equations -momentum and a passive scalar- completed by the potential distribution resolution. These 3 distinct physics are coupled through the Butler-Volmer boundary condition which acts at the electrode/electrolyte interface and governs the whole electrochemical activity. We present the numerical methodology used in this work and all the quantitative results obtained. We also report significant differences with the literature, mainly on the mass transfer statistics, which tend to confirm that a fully coupled approach is necessary to obtain a reliable description of the physic involved in such electrochemical transformations. [Preview Abstract] |
Monday, November 22, 2010 4:53PM - 5:06PM |
LU.00007: A CFD model for biomass fast pyrolysis in fluidized-bed reactors Qingluan Xue, T.J. Heindel, R.O. Fox A numerical study is conducted to evaluate the performance and optimal operating conditions of fluidized-bed reactors for fast pyrolysis of biomass to bio-oil. A comprehensive CFD model, coupling a pyrolysis kinetic model with a detailed hydrodynamics model, is developed. A lumped kinetic model is applied to describe the pyrolysis of biomass particles. Variable particle porosity is used to account for the evolution of particle physical properties. The kinetic scheme includes primary decomposition and secondary cracking of tar. Biomass is composed of reference components: cellulose, hemicellulose, and lignin. Products are categorized into groups: gaseous, tar vapor, and solid char. The particle kinetic processes and their interaction with the reactive gas phase are modeled with a multi-fluid model derived from the kinetic theory of granular flow. The gas, sand and biomass constitute three continuum phases coupled by the interphase source terms. The model is applied to investigate the effect of operating conditions on the tar yield in a fluidized-bed reactor. The influence of various parameters on tar yield, including operating temperature and others are investigated. Predicted optimal conditions for tar yield and scale-up of the reactor are discussed. [Preview Abstract] |
Monday, November 22, 2010 5:06PM - 5:19PM |
LU.00008: Condensed Phase Combustion in the Presence of Altered Acoustic Disturbances Jeffrey Wegener, Jennifer Smolke, Cristhian Sevilla, Sophonias Teshome, Owen Smith, Ann Karagozian This experimental study focuses on fuel combustion characteristics of liquid droplets and solid spheres during exposure to external acoustic disturbances generated within a closed acoustic waveguide. The study examines combustion during excitation conditions in which the droplet or sphere is situated at or in the vicinity of a pressure node (PN) or a pressure antinode (PAN). During such acoustic excitation, flame orientation is observed to be consistent with the sign of a theoretical acoustic acceleration, analogous to a gravitational acceleration, acting on the burning system. Yet experimentally estimated acoustic accelerations differ quantitatively from that predicted by one theory of the acoustic radiation forces\footnote{Tanabe, et al., {\bf PCI}, 2000}. Altered orientations of the waveguide are used to examine the nature of acoustically generated forces. The present experimental configuration provides a useful test bed for the evaluation of the response of different burning fuels to an acoustically resonant environment, including conditions leading to flame extinction, and can be used to explore a range of condensed phase combustion processes during such acoustic coupling. [Preview Abstract] |
Monday, November 22, 2010 5:19PM - 5:32PM |
LU.00009: LES Subfilter Modeling of Soot-Turbulence Interactions Michael Mueller, Heinz Pitsch The evolution of soot in turbulent reacting flows is driven by the small-scale interactions between turbulence, chemistry, and soot. Due to an infinite Schmidt number, soot is confined to thin structures which are stretched by turbulent eddies. In addition, soot is formed from Polycyclic Aromatic Hydrocarbons which are present only in regions of low scalar dissipation rate resulting in a spatially and temporally intermittent soot field. In this work, soot-turbulence interactions are modeled with a presumed soot subfilter PDF approach. Based on the characteristics of soot fields in turbulent flows, a double delta distribution is proposed with one of the delta peaks fixed at zero soot in the internal phase space. The distribution is validated \textit{a priori} using a recent two-dimensional Direct Numerical Simulation of soot formation and growth in a non-premixed flame subjected to decaying isotropic turbulence. The analysis shows that the proposed soot subfilter PDF leads to substantial improvement in predictions of total intermittency as well as soot-soot correlations in the soot source terms compared to using simply the mean values of the soot scalars (i.e. a single delta distribution). The double delta distribution requires one parameter in addition to the mean values of the soot scalars, and several approaches for specifying the parameter are evaluated with the DNS data. [Preview Abstract] |
Monday, November 22, 2010 5:32PM - 5:45PM |
LU.00010: On the formation and early evolution of soot in turbulent nonpremixed flames F. Bisetti, G. Blanquart, M.E. Mueller, H. Pitsch A direct numerical simulation of soot formation in a turbulent nonpremixed flame has been performed to investigate unsteady hydrodynamic strain effects on soot growth processes and transport immediately following nucleation. For the first time in a DNS, polycyclic aromatic hydrocarbon (PAH) species are included in the chemical kinetics mechanism to describe soot inception. A novel statistical representation of soot aggregates based on the Hybrid Method of Moments (HMOM) is employed. In agreement with previous experimental studies in laminar flames, Damk\"{o}hler number effects are significant, and soot nucleation and growth are locally inhibited by high scalar dissipation rate. Upon formation on the rich side of the flame, soot is displaced relative to curved mixture fraction iso-surfaces due to differential diffusion effects. Soot traveling towards the flame is oxidized, and aggregates displaced away from the flame grow by condensation of PAH species on the surface of soot aggregates. In contrast to previous DNS studies employing simplified models, we find that soot-flame interaction plays a limited role in soot growth. Nucleation and condensation processes occurring in the fuel stream are responsible for the greatest generation of soot mass. [Preview Abstract] |
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