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 M2: Flames: Non-premixed Flames |
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Chair: Fabrizio Bisetti, King Abdullah University of Science and Technology Room: 101 |
Tuesday, November 24, 2015 8:00AM - 8:13AM |
M2.00001: Scaling of velocity and mixture fraction fields in laminar counterflow configurations Fabrizio Bisetti, Gianfranco Scribano Counterflow configurations are widely used to characterize premixed, nonpremixed, and partially premixed laminar flames. We performed a systematic analysis of the velocity and mixture fraction fields in the counterflow configuration and obtained scaling laws, which depend on two suitable nondimensional numbers: (i) the Reynolds number based on the bulk velocity $U$ and half the separation distance between the nozzles $L$, and (ii) the ratio of the separation distance $H=2L$ to the nozzle diameter $D$. Our study combines velocity measurements via Particle Image Velocimetry, detailed two-dimensional simulations including the nozzle geometry, and an exhaustive analysis of the data based on the nondimensional numbers. The flow field is shown to be moderately sensitive to the Reynolds number and strongly affected by the ratio $H/D$. By describing the self-similar behavior of the flow field in counterflow configurations comprehensively, our results provide a systematic explanation of existing burner designs as well as clear guidelines for the design of counterflows for pressurized nonpremixed flames. Finally, questions related to the limitations of one-dimensional models for counterflows are addressed conclusively. [Preview Abstract] |
Tuesday, November 24, 2015 8:13AM - 8:26AM |
M2.00002: A model for the effective/turbulent Lewis numbers in turbulent non-premixed flames Nicholas Burali, Guillaume Blanquart Turbulent mixing has a strong impact on the structure of turbulent premixed and non-premixed flames. Experimental results have highlighted that, with growing turbulence intensities, turbulent transport becomes gradually dominant over molecular mixing. As this occurs, the (average) turbulent flame structure transitions to that of a unity Lewis number unstretched flame. In the current work, this transition is characterized by developing an \textit{a priori }model for the effective/turbulent species Lewis numbers in turbulent non-premixed flames. This model is developed from a Reynolds--averaged Navier--Stokes (RANS) formulation of the species and energy transport equations, and validated using existing experimental and numerical data. The results of this work provide a simple framework to estimate the Lewis numbers to be used in one-dimensional flame calculations for chemistry tabulation. [Preview Abstract] |
Tuesday, November 24, 2015 8:26AM - 8:39AM |
M2.00003: Hydrodynamic and chemical effects of hydrogen dilution on soot evolution in turbulent nonpremixed bluff body ethylene flames Sili Deng, Michael E. Mueller, Qing N. Chan, Nader H. Qamar, Bassam B. Dally, Zeyad T. Alwahabi, Graham J. Nathan A turbulent nonpremixed bluff body ethylene/hydrogen (volume ratio 2:1) flame is studied and compared with the ethylene counterpart [Mueller et al. Combust. Flame, 160, 2013]. Similar to the ethylene buff body flame, a low-strain recirculation zone, a high-strain neck region, and a downstream jet-like region are observed. However, the maximum soot volume fraction in the recirculation zone of the hydrogen diluted case is significantly lower than the ethylene case. Large Eddy Simulation is used to further investigate soot evolution in the recirculation zone and to elucidate the role of hydrogen dilution. Since the central jet Reynolds numbers in both cases are the same (approximately 30,900), the jet velocity of the hydrogen diluted case is higher, resulting in a shorter and leaner recirculation zone. In addition, hydrogen dilution chemically suppresses soot formation due to the reduction of C/H ratio. Consequently, the reduction of the soot volume fraction for the hydrogen diluted ethylene flame is attributed to two major effects: hydrodynamic and chemical effects. [Preview Abstract] |
Tuesday, November 24, 2015 8:39AM - 8:52AM |
M2.00004: In-Situ Analysis of Gradient Trajectories in a Reactive Turbulent Shear Flow Felix Dietzsch, Michael Gauding, Christian Hasse Most understanding of turbulent fine-scale mixing has been gained from conditional statistics. Conditional statistics are examined along gradient trajectories, which constitute a natural, intrinsic coordinate system of the underlaying scalar field. Statistics along gradient trajectories contain information about the temporal mechanism of turbulent mixing and combustion. Analyzing these statistics is an important step to understand the transient behaviour of the interaction between turbulence and chemistry. The tracking of gradient trajectories is very challenging and has to be conducted in-situ in order to capture the smallest time-scales. The analysis is based on a direct numerical simulation of a turbulent diffusion flame exhibiting extinction and reignition. [Preview Abstract] |
Tuesday, November 24, 2015 8:52AM - 9:05AM |
M2.00005: Local Velocity Field Measurements towards Understanding Flame Stabilization of Turbulent Non-premixed Jet Flames in Vitiated Coflow Aravind Ramachandran, Anirudh Reddy Mothe, Venkateswaran Narayanaswamy Turbulent combustion of a non-premixed methane jet issuing into a vitiated coflow is being studied in our lab. Flame luminosity studies demonstrated three dominant characteristic flame motions -- a stable flame base (Mode A), complete blowout (Mode B), and partial blowout followed by re-anchoring of the flame by autoignition kernels (Mode C). The experiments presented in this work focused on Mode A, and were carried out over a range of oxidizer temperatures, oxygen molefractions, and fuel jet Reynolds numbers. Measurements of 2-D velocity fields near the base of the lifted jet flame were obtained using Particle Image Velocimetry (PIV) with the objective to delineate the dominant mechanisms involved in the flame stabilization. Statistical analysis of these instantaneous velocity fields will be presented, which shows non-trivial contributions from autoignition kernels as well as edge flame propagation towards flame stabilization. The effect of vortices and high local strain rates was observed to produce local extinctions and destabilize the flame, indicating their role as precursors to (unstable) Mode B and Mode C motions. [Preview Abstract] |
Tuesday, November 24, 2015 9:05AM - 9:18AM |
M2.00006: Effects of local extinction on mixture fraction and scalar dissipation statistics in turbulent nonpremixed flames Antonio Attili, Fabrizio Bisetti Passive scalar and scalar dissipation statistics are investigated in a set of flames achieving a Taylor's scale Reynolds number in the range $100\le{\rm Re}_{\lambda}\le150$ [Attili {\it et al.} Comb. Flame 161, 2014; Attili {\it et al.} Proc. Comb. Inst. 35, 2015]. The three flames simulated show an increasing level of extinction due to the decrease of the Damk\"ohler number. In the case of negligible extinction, the non-dimensional scalar dissipation is expected to be the same in the three cases. In the present case, the deviations from the aforementioned self-similarity manifests itself as a decrease of the non-dimensional scalar dissipation for increasing level of local extinction, in agreement with recent experiments $[$Karpetis and Barlow Proc. Comb. Inst. 30, 2005; Sutton and Driscoll Combust. Flame 160, 2013$]$. This is caused by the decrease of molecular diffusion due to the lower temperature in the low Damk\"ohler number cases. Probability density functions of the scalar dissipation $\chi$ show rather strong deviations from the log-normal distribution. The left tail of the pdf scales as $\chi^{1/2}$ while the right tail scales as $e^{-c\chi^\alpha}$, in agreement with results for incompressible turbulence [Schumacher {\it et al.} J. Fluid Mech. 531, 2005]. [Preview Abstract] |
Tuesday, November 24, 2015 9:18AM - 9:31AM |
M2.00007: Turbulent non - premixed flames driven by the Richtmyer-Meshkov Instability Hilda Varshochi, Nitesh Attal, Praveen Ramaprabhu We report on Direct Numerical Simulations of shock-induced mixing between fuel (H$_{\mathrm{2}})$ and Oxidizer (O$_{\mathrm{2}})$ streams separated by a sharp interface and driven by the Richtmyer-Meshkov instability (RMI). The resulting non-premixed flame is dominated by vigorous mixing that is a consequence of deposition of baroclinic vorticity at the interface. Such RMI-driven flames, when properly controlled, could play a decisive role in improving the performance of supersonic combustors such as scramjets. While the majority of past research efforts in this area have focused on the shock-bubble flame interaction, our configuration is fundamentally different and involves a planar shock interacting with a planar interface. This allows for the placement of well-defined, precisely controlled initial perturbations on the planar surface. Furthermore, the interface is statistically homogenous in all directions perpendicular to shock traverse, thus rendering the problem amenable to reduced-order 1D modeling of planar-averaged quantities. From detailed, high-resolution DNS [1], we describe flow and flame characteristics of a repeatedly reshocked turbulent RMI flame. We observe that with each reshock event, fresh deposition of vorticity on the already nonlinear interface greatly enhances mixing and combustion. [1] Attal, N., et al. Comput. Fluids 107 (2015): 59-76. [Preview Abstract] |
Tuesday, November 24, 2015 9:31AM - 9:44AM |
M2.00008: Strain rate effects on soot evolution in turbulent nonpremixed flames Jeffry K. Lew, Michael E. Mueller, Saleh Mahmoud, Zeyad T. Alwahabi, Bassam B. Dally, Graham J. Nathan Large Eddy Simulations (LES) of turbulent nonpremixed ethylene/hydrogen/nitrogen (2/2/1 by volume) jet flames are conducted to investigate the effects of global strain rate on soot evolution. The exit strain rate is varied by fixing the Reynolds number as the burner diameter and exit velocity are altered. A detailed integrated LES approach is employed that includes a nonpremixed flamelet model that accounts for heat losses from radiation, a transport equation model to account for unsteadiness in polycyclic aromatic hydrocarbon (PAH) evolution, a detailed soot model based on the Hybrid Method of Moments [Mueller $\textit{et al}$. Combust. Flame 156 (2009)], and a novel presumed subfilter PDF model for soot-turbulence interactions. As the strain rate increases, the maximum soot volume fraction decreases due to the suppression of PAH formation. This trend with increasing strain rate is validated against experimental measurements conducted at The University of Adelaide. [Preview Abstract] |
Tuesday, November 24, 2015 9:44AM - 9:57AM |
M2.00009: Coherent structure dynamics during turbulence-flame interaction Eileen Haffner, Melissa Green, Peter Hamlington, Alexi Poludnenko, Elaine Oran Several studies have been conducted to characterize the turbulence-flame interaction in reacting flows quantitatively. It has been observed that increased turbulence intensity both wrinkles and broadens the flame front throughout the preheat zone and reaction zone. In addition, previous studies showed that interaction with the flame changes the orientation of turbulent structures and and in some cases incites loss of vorticity, but the physical mechanism of this interaction was still unclear. An Eulerian analysis (Q criterion) is preformed to track structures through the flow, and to visualize the vortex transformation as it encounters the flame. This is coupled with the contours of the fuel-mass fraction, density, and pressure throughout the flame brush to provide insight into the physical interaction between turbulent structures and the flame. A complete description of the physical mechanism could provide insight into ways to design engine inlets for efficient mixing in combustion applications. [Preview Abstract] |
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