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 D26: Reacting Flows: Instability |
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Chair: Peter Schmid, Imperial College - London Room: E146 |
Sunday, November 20, 2016 2:57PM - 3:10PM |
D26.00001: The effect of mean flow swirl on the transfer function of an M-flame Calum Skene, Peter Schmid Direct numerical simulations of the compressible, reactive Navier-Stokes equations are used to probe the influence of mean flow swirl on the frequency response of an axisymmetric M-flame. Using linearized governing equations, coupled with its adjoint analogue, the optimal gain with respect to harmonic forcing is computed using an iterative direct-adjoint looping technique. The discrete adjoint equations are determined by modular automatic differentiation of the linearized code. The direct and adjoint information are further used to provide sensitivity information with respect to the forcing frequency, as well as to changes in the governing parameters (swirl number, Reynolds number, etc.). Special emphasis will be put on the influence of mean flow swirl on amplification and frequency shifts in the flame transfer function (FTF). [Preview Abstract] |
Sunday, November 20, 2016 3:10PM - 3:23PM |
D26.00002: Two-dimensional corrugated flames - a consequence of the Darrieus-Landau instability Advitya Patyal, Moshe Matalon In this study we present for the first time the development of corrugated flame surfaces resulting from gas expansion in a three-dimensional flow as a consequence of the Darrieus-Landau instability. The computations are carried out within the context of the hydrodynamic theory where the flame is treated as a surface of density discontinuity separating burned gas from the fresh combustible mixture, and its movement is tracked via a level-set method with a propagation speed that depends on the local curvature and hydrodynamic strain. To this end, a surface parameterization method is used to accurately capture the velocity jump across the flame and the strain rate along the flame interface. The numerical scheme is shown to accurately recover the exact pole-solutions predicted by the nonlinear Michelson-Sivashinsky equation in the weak gas expansion limit, and corroborates the bifurcation results from linear stability analysis. It is shown that, in accord with experimental observations, the new conformations that evolve beyond the instability threshold have a sharp crest pointing towards the burned gas with ridges along the troughs, and that these structures propagate steadily, nearly 50\% faster than planar flames. [Preview Abstract] |
Sunday, November 20, 2016 3:23PM - 3:36PM |
D26.00003: Numerical simulations of Rayleigh-Taylor instability in non-premixed flames using detailed chemistry Nitesh Attal, Praveen Ramaprabhu The Rayleigh-Taylor (RT) instability occurs at a perturbed interface separating fluids of different densities, when the lighter fluid accelerates the heavier fluid. We examine the occurrence of the RT instability, when the perturbed interface demarcates a light, fuel stream from a heavier air stream at elevated temperatures. The study is conducted using the FLASH code with modifications that include detailed chemistry, temperature-dependent EOS, and diffusive transport. The fuel-air interface is initialized at thermal equilibrium (T$_{\mathrm{fuel}}=$T$_{\mathrm{air}}=$1000K) in a constant background acceleration (g). We vary the density difference across the interface by diluting the H$_{\mathrm{2}}$ fuel stream with inert N$_{\mathrm{2}}$. The non-premixed flame formed across a burning interface alters the underlying density ($\rho )$ stratification, so that an initially RT unstable (stable) interface can be locally stabilized (destabilized). We observe this change in local stability for both single-wavelength and multimode perturbations, and draw some conclusions on the implications of these findings to applications such as ultra-compact combustors. We also make some comparisons of the reacting, non-premixed RT problem with the corresponding inert flow. [Preview Abstract] |
Sunday, November 20, 2016 3:36PM - 3:49PM |
D26.00004: Turbulent non-premixed combustion driven by the Richtmyer-Meshkov instability Hilda Varshochi, Praveen Ramaprabhu, Nitesh Attal We report on 3D high resolution numerical simulations of a non-premixed, reacting Richmyer-Meshkov (RM) instability performed using the FLASH code. In the simulations, a Mach 1.6 shock traverses a diffuse, corrugated material interface separating Hydrogen at 1000 K and Oxygen at 300 K, so that local misalignments between pressure and density gradients induce baroclinic vorticity at the contact line. The vorticity deposition drives the RM instability, which in turn results in combustion and flame formation. We study the evolution of the interface and the flame as the resulting RM instability grows through linear, nonlinear and turbulent stages. We develop a detailed understanding of the effects of heat release and combustion on the underlying flow properties by comparing our results with a baseline non-reacting RM flow. We document the properties of the instability (growth rates, pdfs, spectra) and the flame (scalar dissipation rate, flame surface area, heat release rate) as well as the nature of the coupling between the two. Our findings are relevant to supernovae detonation, knocking in IC engines and scramjet performance, while the underlying flow problem defined here represents a novel canonical framework to understand the broader class of non-premixed turbulent flames. [Preview Abstract] |
Sunday, November 20, 2016 3:49PM - 4:02PM |
D26.00005: Implementation of Thermal Diffusion in Chemistry Tabulation for Unstable Premixed Flames Jason Schlup, Guillaume Blanquart The inclusion of thermal diffusion, by means of multicomponent diffusion transport models, has been shown to affect the results of numerical simulations of thermo-diffusively unstable lean hydrogen flames. However, the multicomponent diffusion model involves costly matrix inversion operations, leading it to be useful in only simplified flame configurations and computational domains. In this work, a mixture-averaged thermal diffusion model is implemented into a tabulated chemistry framework. The resulting reacting flows are compared to one- and two-dimensional detailed chemistry simulations of lean hydrogen-air flames with multicomponent diffusion. The configurations used to validate the mixture-averaged thermal diffusion model with tabulated chemistry include flat and cellular tubular flames. Three-dimensional flames, both laminar and turbulent, are also considered as an application of the mixture-averaged thermal diffusion model using tabulated chemistry. These flames are compared to cases neglecting thermal diffusion and cases using detailed chemistry with the mixture-averaged thermal diffusion model. [Preview Abstract] |
Sunday, November 20, 2016 4:02PM - 4:15PM |
D26.00006: An investigation of streaklike instabilities in laminar boundary layer flames Colin Miller, Mark Finney, Jason Forthofer, Sara McAllister, Michael Gollner Observations of coherent structures in boundary layer flames, particularly wildland fires, motivated an investigation on flame instabilities within a boundary layer. This experimental study examined streaklike structures in a stationary diffusion flame stabilized within a laminar boundary layer. Flame streaks were found to align with pre-existing velocity perturbations, enabling stabilization of these coherent structures. Thermocouple measurements were used to quantify streamwise amplification of flame streaks. Temperature mapping indicated a temperature rise in the flame streaks, while the region in between these streaks, the trough, decreased in temperature. The heat flux to the surface was measured with a total heat flux gauge, and the heat flux below the troughs was found to be higher at all measurement locations. This was likely a function of the flame standoff distance, and indicated that the flame streaks were acting to modify the spanwise distribution of heat flux. Instabilities in boundary layer combustion can have an effect on the spanwise distribution of heat transfer. This finding has significant implications for boundary layer combustion, indicating that instantaneous properties can vary significantly in a three-dimensional flow field. [Preview Abstract] |
Sunday, November 20, 2016 4:15PM - 4:28PM |
D26.00007: Hydrodynamic Stability Analysis of Multi-jet Effects in Swirling Jet Combustors Benjamin Emerson, Tim Lieuwen Many practical combustion devices use multiple swirling jets to stabilize flames. However, much of the understanding of swirling jet dynamics has been generated from experimental and computational studies of single reacting, swirling jets. A smaller body of literature has begun to explore the effects of multi-jet systems and the role of jet-jet interactions on the macro-system dynamics. This work uses local temporal and spatio-temporal stability analyses to isolate the hydrodynamic interactions of multiple reacting, swirling jets, characterized by jet diameter, D, and spacing, L. The results first identify the familiar helical modes in the single jet. Comparison to the multi-jet configuration reveals these same familiar modes simultaneously oscillating in each of the jets. Jet-jet interaction is mostly limited to a spatial synchronization of each jet's oscillations at the jet spacing values analyzed here (L/D$=$3.5). The presence of multiple jets vs a single jet has little influence on the temporal and absolute growth rates. The biggest difference between the single and multi-jet configurations is the presence of nearly degenerate pairs of hydrodynamic modes in the multi-jet case, with one mode dominated by oscillations in the inner jet, and the other in the outer jets. The close similarity between the single and multi-jet hydrodynamics lends insight into experiments from our group (Aguilar, M., Malanoski, M., Adhitya, G., Emerson, B., Acharya, V., Noble, D. and Lieuwen, T., 2015. J. Engr. Gas Turbines and Power, 137(9); Smith T., Emerson B., Chterev, I., Noble D., Lieuwen T., 2016. ASME Paper GT2016-57755). [Preview Abstract] |
Sunday, November 20, 2016 4:28PM - 4:41PM |
D26.00008: Linear stability analysis of Clarke-Riley diffusion flames Daniel Gomez-Lendinez, Wilfried Coenen, Antonio L Sanchez The buoyancy-driven laminar flow associated with the Burke-Schumann diffusion flame developing from the edge of a semi-infinite horizontal fuel surface burning in a quiescent oxidizing atmosphere displays a self-similar structure, first described by Clarke and Riley (Journal of Fluid Mechanics, 74:415--431). Their analysis was performed for unity reactant Lewis numbers, with the viscosity and thermal conductivity taken to be linearly proportional to the temperature. Our work extends this seminal work by considering fuels with non-unity Lewis numbers and gas mixtures with a realistic power-law dependence of the different transport properties. The problem is formulated in terms of chemistry-free, Shvab-Zel'dovich, linear combinations of the temperature and reactant mass fractions, not changed directly by the reactions, as conserved scalars. The resulting self-similar base-flow solution is used in a linear stability analysis to determine the critical value of the boundary-layer thickness---measured by the local Grashof number---at which the flow becomes unstable, leading to the development of G\"ortler-like streamwise vortices. The analysis provides the dependence of the critical Grashof number on the relevant flame parameters. [Preview Abstract] |
Sunday, November 20, 2016 4:41PM - 4:54PM |
D26.00009: Spatio-temporal Linear Stability Analysis of Multiple Reacting Wakes Jacob Kunnumpuram Sebastian, Benjamin Emerson, Tim Lieuwen Hydrodynamic stability of reacting shear flows plays a key role in controlling a variety of combustor behaviors, such as combustion instability, mixing and entrainment, and blowoff. A significant literature exists on the hydrodynamics of single bluff body flows, but not the multi- bluff body flows that are found in applications. The objective of this work was to compare the spatio-temporal stability of multiple reacting wakes and single reacting wakes, within the framework of linear stability theory. Spatio-temporal stability analyses are conducted on model velocity and density profiles, with key parameters being the density ratio across the flame, bluff body spacing, dimensionless shear, and asymmetry parameters (if the two wakes are dissimilar). The introduction of the additional bluff body can exert both a stabilizing and destabilizing effect on the combined two-wake system, depending on the spatial separation between the bluff bodies. Furthermore, while the most rapidly amplified mode of the single wake mode is the sinuous (asymmetric) one, in the two wake system, the most rapidly amplified mode can be either sinuous or varicose (symmetric), also depending on spatial separation. [Preview Abstract] |
Sunday, November 20, 2016 4:54PM - 5:07PM |
D26.00010: Direct numerical computation of linear stability of gaseous detonations Dmitry Kabanov, Aslan Kasimov We develop an algorithm for the computation of linear stability of gaseous detonations that combines the elements of normal-mode analysis and direct simulation. A shock-fitting method is applied to governing equations which are linearized assuming the general time dependence. The computed time series of the shock perturbation is postprocessed to determine the growth rate of instability and neutral boundaries. The method is applied to the reactive Euler equations and its simplified analogs. [Preview Abstract] |
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