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 H39: Flames: Premixed Flames |
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Chair: Guillaume Blanquart, California Institute of Technology Room: Sheraton Back Bay C |
Monday, November 23, 2015 10:35AM - 10:48AM |
H39.00001: Anisotropy of small scale turbulence in premixed flames Brock Bobbitt, Guillaume Blanquart The three Kolmogorov hypotheses are fundamental to the description and modeling of turbulence. It is currently unclear if all three hypotheses remain valid within premixed flames, where dramatic changes in density and viscosity occur. The objective of this study is to assess the validity of Kolmogorov's hypothesis of local isotropy within turbulent premixed flames. Anisotropy is investigated by considering the vorticity vector, which is characteristic of the smallest turbulent scales. This study is performed on a series of direct numerical simulations of n-heptane/air flames which spans a wide range of Karlovitz numbers and density ratios. It is found that the vorticity becomes isotropic for sufficiently high Karlovitz numbers, supporting the validity of the hypothesis of local isotropy. For smaller Karlovitz numbers, the extent of small scale anisotropy can vary through the flame and depends on the Reynolds number as well as the local Karlovitz number. These results are explained through preferential orientation of the flame surface and its alignment with vorticity. A correlation is proposed for the magnitude of the anisotropy with statistics of the flame surface orientation. [Preview Abstract] |
Monday, November 23, 2015 10:48AM - 11:01AM |
H39.00002: One Dimensional Modeling of Vorticity in high Karlovitz Number Turbulent Premixed Flames Chandru Dhandapani, Brock Bobbitt, Guillaume Blanquart Turbulent combustion involves the interaction of two complex non-linear phenomena: turbulence and chemistry. The current study focuses on modeling the effects of the flame on the turbulence characteristics, more specifically on the vorticity $\omega$, which is characteristic of the smallest turbulent scales. This is performed through an a-priori analysis of high Karlovitz number turbulent premixed flames. The objective is to derive a one-dimensional model equation for the transport of enstrophy, $\omega^{2} = \omega\cdot\omega$. The terms in the enstrophy transport equation are modeled and scaled by a combination of flow properties. Results from a series of previously performed direct numerical simulations(DNS), spanning a range of Karlovitz number(Ka), Reynolds number(Re) and flame density ratios, are analysed to obtain the coefficients in the one-dimensional differential equation for enstrophy and demonstrate their dependence, or lack thereof, on $Ka$ and $Re$. Lastly, the model equation is solved and the results are compared with the DNS results. [Preview Abstract] |
Monday, November 23, 2015 11:01AM - 11:14AM |
H39.00003: Dynamic Mode Decomposition (DMD) application to premixed Low Swirl Injector flames Paul Palies, Robert Cheng, Dustin Davis, Milos Ilak DMD is implemented and applied to premixed flame image data from the Low Swirl Injector.\footnote{Therkelsen \textit{et al}., Combustion and Flame, Vol 160 (2), 307-321.} The data consists of high speed video flame images at three different equivalence ratios, corresponding to low-amplitude oscillation, transient growth, and high-amplitude oscillation regimes. DMD reveals spectra of growth rates and frequencies with corresponding spatial modes, ranked by mode norm. For the low-amplitude oscillation regime, DMD does not capture any dominant mode shapes or frequencies. For the high-amplitude oscillation case, the frequency of the dominant mode and its harmonics match the frequency recorded by pressure measurement. The spatial mode from DMD is used to extract the propagation velocity of perturbations. In the transient regime, DMD captures the growth rate and frequency of the transient mode. The corresponding DMD spatial mode shows a similar shape to the high oscillation case indicating that the transition to a limit cycle is associated with a convective mode. The underlying mechanism of unsteady heat release is identified as induced by a convected wave along the flame front, whose velocity is confirmed by a separate analysis. [Preview Abstract] |
Monday, November 23, 2015 11:14AM - 11:27AM |
H39.00004: Direct numerical simulations of flow-chemistry interactions in statistically turbulent premixed flames Paul Arias, Harshavardhana Uranakar, Swetaprovo Chaudhuri, Hong Im The effects of Damk\"{o}hler number and Karlovitz number on the flame dynamics of three-dimensional statistically planar turbulent premixed flames are investigated by direct numerical simulation incorporating detailed chemistry and transport for a hydrogen-air mixture. The mean inlet velocity was dynamically adjusted to ensure a stable flame within the computational domain, allowing the investigation of time-averaged quantities of interest. A particular interest was on understanding the effects of turbulence on the displacement speed of the flame relative to the local fluid flow. Results show a linear dependence on the displacement speed as a function of total strain, consistent with earlier work on premixed-laminar flames. Additional analysis on the local flame thickness reveals that the effect of turbulence is twofold: (1) the increase in mixing results in flame thinning due to the enhancement of combustion at early onset of the flame, and (2) for large Reynolds number flows, the penetration of the turbulence far into the preheat zone and into the reaction zone results in localized flame broadening. [Preview Abstract] |
Monday, November 23, 2015 11:27AM - 11:40AM |
H39.00005: Quantifying real-gas effects on a laminar n-dodecane -- air premixed flame Abishek Gopal, Shashank Yellapantula, Johan Larsson With the increasing demand for higher efficiencies in aircraft gas-turbine engines, there has been a progressive march towards high pressure-ratio cycles. Under these conditions, the aviation fuel, Jet A, is injected into the combustor at supercritical pressures. In this work, we study and quantify the effects of transcriticality on a 1D freely propagating laminar n-dodecane -- air premixed flame. The impact of the constitutive state relations arising from the Ideal Gas equation of state(EOS) and Peng-Robinson EOS on flame structure and propagation is presented. The effects of real-gas models of transport properties, such as viscosity on laminar flame speed, are also presented. [Preview Abstract] |
Monday, November 23, 2015 11:40AM - 11:53AM |
H39.00006: The flame anchoring mechanism and associated flow structure in bluff-body stabilized lean premixed flames Dan Michaels, Santosh Shanbhogue, Ahmed Ghoniem We present numerical analysis of a lean premixed flame anchoring on a heat conducting bluff-body. Different mixtures of CH$_{\mathrm{4}}$/H$_{\mathrm{2}}$/air are analyzed in order to systematically vary the burning velocity, adiabatic flame temperature and extinction strain rate. The study was motivated by our experimental measurements in a step combustor which showed that both the recirculation zone length and stability map under acoustically coupled conditions for different fuels and thermodynamic conditions collapse using the extinction strain rate. The model fully resolves unsteady two-dimensional flow with detailed chemistry and species transport, and without artificial flame anchoring boundary conditions. The model includes a low Mach number operator-split projection algorithm, coupled with a block-structured adaptive mesh refinement and an immersed boundary method for the solid body. Calculations reveal that the recirculation zone length correlates with the flame extinction strain rate, consistent with the experimental evidence. It is found that in the vicinity of the bluff body the flame is highly stretched and its leading edge location is controlled by the reactants combustion characteristics under high strain. Moreover, the flame surface location relative to the shear layer influences the vorticity thus impacting the velocity field and the recirculation zone. The study sheds light on the experimentally observed collapse of the combustor dynamics using the reactants extinction strain rate. [Preview Abstract] |
Monday, November 23, 2015 11:53AM - 12:06PM |
H39.00007: Study on Turbulent Premixed Flame Regimes with Ignition Using a Reactor Assisted Turbulent Slot Burner Sang Hee Won, Christopher Reuter, Bret Windom, Yiguang Ju Turbulent premixed flames of n-heptane/air and toluene/air mixtures affected by ignition have been experimentally investigated by using a reactor-assisted turbulent slot (RATS) burner at two burner temperatures, 450 K and 700 K. Turbulent burning velocities (ST) and flame structures have been measured by the simultaneous OH and CH2O planar laser-induced fluorescence (PLIF) imaging at various equivalence ratios and turbulent Reynolds numbers. Three distinct turbulent premixed flame regimes are identified for n-heptane/air mixture; chemical frozen (CF) regime at low temperature, low temperature ignition (LTI) regime, and high temperature ignition (HTI) regime for respectively lean and rich conditions at 700 K. For CF regime, the measured turbulent burning velocities of n-heptane and toluene at 450 K follow a conventional correlation of turbulent intensity (defined as u'/SL). In LTI regime, substantial changes in chemical composition alter the laminar flame speed and transport property, leading to rapid increase of turbulent burning velocity. In HTI regime, it is found that the turbulent premixed flame structure is significantly modified by the appearance of volumetric ignition kernel structures associated with the transition from LTI to HTI. The turbulent premixed flame regime in HTI is no longer represented by the thin reaction zone regime. The measured turbulent burning velocities in HTI regime increase substantially as increasing ignition Damk\H{o}hler number over those in LTI regime. [Preview Abstract] |
Monday, November 23, 2015 12:06PM - 12:19PM |
H39.00008: Analysis of the electric currents in 1D premixed flames under applied voltages Jie Han, Memdouh Belhi, Fabrizio Bisetti, Tiernan Casey, Hong G. Im, Jyh-Yuan Chen Studying electric currents in flames has practical aspects such as the determination of the ionic structure of a flame, the analysis of the flame behavior under an electric field and the use of flame electric properties for combustion diagnostics. This study proposes a simplified model to compute the electric currents in lean-to-stoichiometric 1D premixed flames under applied voltages. The Navier-Stokes equations coupled with transport equations for neutral and charged species along with a Poisson equation for the electric potential are solved. The model reproduces qualitatively the voltage-current characteristic found experimentally. The sensitivity of the electric currents to the applied voltage, equivalence ratio, and pressure is studied and the key parameters affecting the saturation current are determined. Results show that the saturation current is controlled by the amount of charged species created by the chemi-ionization reaction. We found that the recombination rate of electrons with cations and transport coefficients of charged species are the most important parameters affecting the voltage at witch saturation occurs. Analytical formulas for the voltage-current characteristic and the potential of saturation are developed and used to explain the obtained results. [Preview Abstract] |
Monday, November 23, 2015 12:19PM - 12:32PM |
H39.00009: Time-resolved stereoscopic PIV study of flashback in swirl flames at elevated pressures Rakesh Ranjan, Dominik Ebi, Noel Clemens Boundary layer flashback of turbulent premixed swirl flames can pose a major challenge to the operation of stationary gas turbines, especially with hydrogen-rich fuels. To improve our understanding of the physics behind this phenomenon at gas turbine relevant conditions, it is essential to investigate flashback at elevated pressures. With this purpose in mind, flashback experiments with hydrogen/methane-air premixtures are conducted in a model swirl combustor installed in an optically accessible high-pressure combustion facility. We have employed stereoscopic PIV in conjunction with high speed chemiluminiscence imaging to study the upstream propagation of the flame in the premix tube during flashback. Experiments are run at pressures ranging from 1 atm to 5 atm. These time-resolved measurements provide valuable insight into the flame-flow interaction during flashback at elevated pressures. [Preview Abstract] |
Monday, November 23, 2015 12:32PM - 12:45PM |
H39.00010: Vorticity Dynamics in Single and Multiple Swirling Reacting Jets Travis Smith, Michael Aguilar, Benjamin Emerson, David Noble, Tim Lieuwen This presentation describes an analysis of the unsteady flow structures in two multinozzle swirling jet configurations. This work is motivated by the problem of combustion instabilities in premixed flames, a major concern in the development of modern low NOx combustors. The objective is to compare the unsteady flow structures in these two configurations for two separate geometries and determine how certain parameters, primarily distance between jets, influence the flow dynamics. The analysis aims to differentiate between the flow dynamics of single nozzle and triple nozzle configurations. This study looks at how the vorticity in the shear layers of one reacting swirling jet can affect the dynamics of a nearby similar jet. The distance between the swirling jets is found to have an effect on the flow field in determining where swirling jets merge and on the dynamics upstream of the merging location. [Preview Abstract] |
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