Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session G39: Flames: Pre-Mixed Flames and Flame Instabilities |
Hide Abstracts |
Chair: Hong G. Im, King Abudllah University of Science and Technology, Saudi Arabia Room: Sheraton Back Bay C |
Monday, November 23, 2015 8:00AM - 8:13AM |
G39.00001: Experimental observations of the development and growth of flame instabilities formed during vented deflagrations C. Regis Bauwens, Jeffrey M. Bergthorson, Sergey B. Dorofeev The formation of instabilities on the surface of large expanding flames can significantly increase the rate of flame propagation and heat release. As the rate of heat release is the key parameter that determines the pressures that develop, the formation of these instabilities have a strong role in determining the consequences of accidental explosions. For this work, large-scale experiments of uniform propane-air mixtures in a 64 m$^{3}$ vented enclosure were performed. The formation of hydrodynamic flame instabilities, including the Darrieus-Landau and Rayleigh-Taylor instabilities, as well as strong flame-acoustic interactions, was observed. These instabilities were found to be the primary driver of the pressures that developed and were ultimately responsible for the overall maximum overpressure. [Preview Abstract] |
Monday, November 23, 2015 8:13AM - 8:26AM |
G39.00002: Filtered chemical source term modeling for LES of high Karlovitz number premixed flames Simon Lapointe, Guillaume Blanquart Tabulated chemistry with the transport of a single progress variable is a popular technique for large eddy simulations of premixed turbulent flames. Since the reaction zone thickness is usually smaller than the LES grid size, modeling of the filtered progress variable reaction rate is required. Most models assume that the filtered progress variable reaction rate is a function of the filtered progress variable and its variance where the dependence can be obtained through the probability density function (PDF) of the progress variable. Among the most common approaches, the PDF can be presumed (usually as a $\beta$-PDF) or computed using spatially filtered one dimensional laminar flames (FLF). Models for the filtered source term are studied \textit{a priori} using results from DNS of turbulent $n$-heptane/air premixed flames at varying Karlovitz numbers. Predictions from the optimal estimator and models based on laminar flames using a $\beta$-PDF or a FLF-PDF are compared to the exact filtered source term. For all filter widths and Karlovitz numbers, the optimal estimator yields small errors while $\beta$-PDF and FLF-PDF approaches present larger errors. Sources of differences are discussed. [Preview Abstract] |
Monday, November 23, 2015 8:26AM - 8:39AM |
G39.00003: Bluff-body stabilized flame dynamics of lean premixed syngas combustion Hong G. Im, Yu Jeong Kim, Bok Jik Lee Recently, syngas combustion has been actively investigated for the potential application to integrated gasification combined cycle (IGCC) systems. While lean premixed combustion is attractive for both reduced emission and enhanced efficiency, flame instability becomes often an issue. Bluff-bodies have been adopted as effective flame holders for practical application of premixed flames. In the present study, high-fidelity direct numerical simulations are conducted to investigate the dynamics of lean premixed syngas flames stabilized on a bluff-body, in particular at the near blow-off regime of the flame. A two-dimensional domain of 4 mm height and 20 mm length with a flame holder of a 1 mm-by-1 mm square geometry is used. For a syngas mixture with the equivalence ratio of 0.5 and the CO:H2 ratio of 1, several distinct flame modes are identified as the inflow velocity approaches to the blowoff limit. The sequences of extinction pathway and combustion characteristics are discussed. [Preview Abstract] |
Monday, November 23, 2015 8:39AM - 8:52AM |
G39.00004: Highly enhanced flame propagation by turbulence through differential diffusion Sheng Yang, Abhishek Saha, Fujia Wu, Chung Law Turbulent flame speed is an essential parameter in turbulent combustion. The existence of turbulence significantly enhances the flame speed of a premixture, mainly through the increase in the total flame surface area and modification of the flame structure. As of now, the highest turbulent flame speed reported is around 35 times those of the laminar flames, and there is no consensus if this is the upper limit or even if there exists one. In the present experimental work, we report highly enhanced turbulent flame propagation, with the ratio of turbulent flame speed to laminar flame speed reaching 200. Moreover, we demonstrated that such enhancements occur for extremely weak mixtures, whose adiabatic flame temperatures are lower than 900 K and are commonly believed to be beyond the flammability of sustained one-dimensional laminar flame propagation. We further identified that such a strong enhancement effect occurs for mixtures with either extremely small Lewis number or large mass diffusivity of the deficient reactant and that such flames exhibit different morphology from previously observed turbulent flames, as finger-shape structures are developed on the flame fronts and local extinction and re-ignition are frequently observed. This work demonstrates the extension of flammability limit by turbulence and differential diffusion, enabling sustained flame propagation with extremely low burnt gas temperature (\textless 1000 K), and the highest flame speed enhancement by turbulence so far. [Preview Abstract] |
Monday, November 23, 2015 8:52AM - 9:05AM |
G39.00005: Continuous Diffusion Flames and Flame Streets in Micro-Channels Shikhar Mohan, Moshe Matalon Experiments of non-premixed combustion in micro-channels have shown different modes of burning. Normally, a flame is established along, or near the axis of a channel that spreads the entire mixing layer and separates a region of fuel but no oxidizer from a region with only oxidizer. Often, however, a periodic sequence of extinction and reignition events, termed collectively as ``flame streets", are observed. They constitute a series of diffusion flames, each with a tribrachial leading edge stabilized along the channel. This work focuses on understanding the underlying mechanism responsible for these distinct observations. Numerical simulations were conducted in the thermo-diffusive limit in order to study the effects of confinement and heat loss on non-premixed flames in three-dimensional micro-channels with low aspect ratios. The three dimensionality of the channel was captured qualitatively through a systematic asymptotic analysis that led to a two dimensional problem with an effective parameter representing heat losses in the vertical direction. There exist three key flame regimes: (1) a stable continuous diffusion flame, (2) an unsteady flame, and (3) a stable ``flame street"; the transition between regimes demarcated primarily by Reynolds and Nusselt numbers. [Preview Abstract] |
Monday, November 23, 2015 9:05AM - 9:18AM |
G39.00006: An Improved Flamelet-Based Model for Non-Premixed Supersonic Combustion Zhipeng Lou, Foluso Ladeinde, Wenhai Li The flamelet approach to turbulent reacting flows, though originally developed for essentially incompressible flows, has been used by many authors to simulate supersonic combustion, often without much justification other than that pressure scales in certain ways. In a compressible flow, pressure and temperature vary strongly, meaning that the use of a fixed value of pressure for generating flamelet libraries may be prone to errors in the flamelet modeling of supersonic combustion. We study the influence of static pressure on the flamelet solutions intended for use in modeling supersonic combustion. With various values of static pressure, we found significant differences in the values of the quenching stoichiometric scalar dissipation rate, reaction rate of species and progress variable, heat release rate and the temperature profile. As a result, at high static pressures, the flame is less likely to extinguish and the S-curve shows a steeper angle. We have experimented with the addition of pressure as an independent variable in the flamelet table, toward modeling pressure-sensitive properties and the variable quenching conditions. The effects of this kind of scheme on supersonic combustion will be discussed. [Preview Abstract] |
Monday, November 23, 2015 9:18AM - 9:31AM |
G39.00007: An Investigation of a Hybrid Mixing Model for PDF Simulations of Turbulent Premixed Flames Hua Zhou, Shan Li, Hu Wang, Zhuyin Ren Predictive simulations of turbulent premixed flames over a wide range of Damköhler numbers in the framework of Probability Density Function (PDF) method still remain challenging due to the deficiency in current micro-mixing models. In this work, a hybrid micro-mixing model, valid in both the flamelet regime and broken reaction zone regime, is proposed. A priori testing of this model is first performed by examining the conditional scalar dissipation rate and conditional scalar diffusion in a 3-D direct numerical simulation dataset of a temporally evolving turbulent slot jet flame of lean premixed H$_{\mathrm{2}}$-air in the thin reaction zone regime. Then, this new model is applied to PDF simulations of the Piloted Premixed Jet Burner (PPJB) flames, which are a set of highly shear turbulent premixed flames and feature strong turbulence-chemistry interaction at high Reynolds and Karlovitz numbers. [Preview Abstract] |
Monday, November 23, 2015 9:31AM - 9:44AM |
G39.00008: Large Eddy Simulation of Radiation Effects on Pollutant Emissions in Diluted Turbulent Premixed Flames A. Cody Nunno, Michael E. Mueller Radiation effects are examined in turbulent premixed flames using a detailed Large Eddy Simulation (LES) approach. The approach combines a tabulated premixed flamelet model (Flamelet Generated Manifolds) with an optically thin radiation model. Radiation heat loss is tracked using an enthalpy deficit coordinate. Heat loss in the flamelets is calculated by varying a coefficient on the radiation source term, ranging from zero (adiabatic) to unity (full optically thin heat loss). NOx emissions are modeled with an additional transport equation that is able to capture unsteady effects resulting from slow kinetics. The model is compared against experimental measurements of methane-air piloted turbulent premixed planar jet flames with increasing levels of water dilution that maintain a constant adiabatic flame temperature. The effects of water dilution on global flame structure and NO emissions resulting directly and indirectly from radiation are examined in detail. [Preview Abstract] |
Monday, November 23, 2015 9:44AM - 9:57AM |
G39.00009: Lagrangian Analysis of Premixed Turbulent Flames Clarissa Briner, Peter Hamlington, Alexei Poludnenko Turbulent premixed combustion is a complicated problem that requires understanding of turbulence and chemistry, as well as their interactions. By contrast to the Eulerian approach, Lagrangian analyses track the evolution of chemical species and flow properties for an advecting fluid parcel. This approach permits detailed analysis of chemical reaction rates and validation of chemical reaction models. Lagrangian trajectories also allow changes in chemical species and flow properties to be examined locally and instantaneously through premixed flamelets. In this study, a Lagrangian analysis has been performed on data from direct numerical simulations of premixed H$_2$-air flames for two different turbulence intensities, using a 8-species chemical reaction mechanism. The relative contributions of dynamical budget terms are calculated for both chemical species, including reaction and diffusion terms, as well as vorticity, which depends on baroclinic torque, dilatation, and viscous effects. Scales of motion throughout the flame are also characterized using multi-point correlations. The results reveal complicated dynamics, including non-monotonic behavior of temperature and fuel mass fractions along trajectories, as well as changing scales of motion through the flameout. [Preview Abstract] |
Monday, November 23, 2015 9:57AM - 10:10AM |
G39.00010: An Investigation of Hydrodynamic Instabilities in Wind-Driven Flames Colin Miller, Salman Verma, Arnaud Trouve, Mark Finney, Jason Forthofer, Sara McAllister, Michael Gollner Recent findings on the importance of convective heating by direct flame contact in wildland fire spread have highlighted the importance of fluid dynamics in the flame spread process. Researchers have observed several dominant coherent structures in the three-dimensional flame in both small and large-scale experiments. This experimental study seeks an understanding of the physical mechanisms by which coherent structures are induced by hydrodynamic instabilities. Experimental data is derived from both a nonreactive hot plate and a stationary burner in a well-characterized laminar flow wind tunnel. Streamwise vortices promote upwash and downwash regions of the flow, and scaling analyses of temperature and velocity maps are proposed. Emphasis is placed on elucidating the regimes in which certain instability mechanisms dominate. The relative strength of shear forces and buoyant forces at certain locations in the boundary layer are examined as contributors to behavior analogous to Klebanoff modes, Gortler vortices, Rayleigh-Taylor instabilities, or Tollmien-Schlichting waves. To further supplement experimental results, comparisons to numerical simulations of hot plates will be made. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700