Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session L02: Reacting Flows: Theory & Modeling |
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Chair: Alexei Poludnenko, Texas A&M University Room: Georgia World Congress Center B203 |
Monday, November 19, 2018 4:05PM - 4:18PM |
L02.00001: Positive and negative stationary edge flame velocities in non-parallel opposing jets with finite heat release Benjamin Shields, Jonathan Freund, Carlos Pantano We study edge flames as approximate models of extinction, reignition, and flame lifting in turbulent non-premixed combustion. An adaptive resolution finite element method is developed to solve for a laminar edge flame in a spatially varying straining flow generated by non-parallel opposing jets. The variable-density zero-Mach-number Navier-Stokes equations are used to solve for both steadily advancing or retreating edge flames. A homotopy transformation technique is used to continue solutions from constant to variable density flow. Dependence of the edge flames on strain rate, stoichiometry and Lewis numbers will be discussed. Preliminary results suggest retreating edge flames may experience a bifurcation and we will discuss the underlying mechanism. |
Monday, November 19, 2018 4:18PM - 4:31PM |
L02.00002: Study of propagating methane flames in strained mixing layers using 4-step reduced chemistry Prabakaran Rajamanickam, Antonio L Sanchez, Forman A Williams Flames propagating along strained mixing layers exhibit interesting structures depending on the local flow conditions. If the imposed strain rate is small, a tribraichial flame is found with a lean premixed wing on one side and a rich premixed wing on the other, followed by a trailing diffusion flame. When this strain rate is increased, these premixed wings merge with the trailing diffusion flame, forming an edge flame structure. Further increase in the strain rate leads to a retreating edge flame, subsequently extinguishing at a particular strain rate. These aspects of triple-flame propagation and its structure in the canonical counterflow configuration are studied through numerical integration of the conservation equations using a reduced four-step model for the combustion chemistry. In particular, a parametric study of flame propagation speed vs. strain rate is carried out for two different combination of feed-stream mixtures (methane-air and the methane-oxygen) and the results are compared with predictions employing the one-step Arrhenius chemistry. |
Monday, November 19, 2018 4:31PM - 4:44PM |
L02.00003: Acoustic response of near-equilibrium diffusion flames with large activation energies Adam Weiss, Antonio L Sanchez, Forman A Williams This paper examines the interaction of a strained-reacting mixing layer separating two impinging streams with an acoustic pressure wave of characteristic wavelength large compared to the dimensions of the mixing layer. The exothermic reaction, confined to a thin layer within the mixing region, is described with use made of one-step Arrhenius chemistry and its interaction with the pressure wave is examined with both numerical integrations and asymptotic methods for systems with large activation energies. An asymptotic formulation is provided for arbitrary values of the ratio of the acoustic amplitude ε to the inverse of the Zeldovich number β; the acoustic limit βε ≪ 1 is then considered in detail. The results are used to investigate implications towards acoustic instabilities making use of the Rayleigh index whose frequency dependence is examined. Results indicate that finite-rate effects dominate the acoustic pressure response of strained flamelets near extinction. For robust, diffusion-controlled flames unsteady modifications to the outer chemical-equilibrium transport regions produce only moderate effects. |
Monday, November 19, 2018 4:44PM - 4:57PM |
L02.00004: DNS Investigation of Near-Wall Flame Behavior Including Radical Quenching Effect Kosuke Narukawa, Yuki Minamoto, Masayasu Shimura, Tanahashi Mamoru The laminar flame displacement speed changes drastically near a wall, and this dependency could be applied to the experimental measurement of the laminar flame speed. In a previous experimental study, temporal variation of the flame displacement speed of the methane-air premixed combustion propagating towards a wall was measured, and it was concluded that the flame displacement speed just before impinges on the wall corresponds to laminar flame speed approximately. In this study, to investigate the above features more detail, direct numerical simulation (DNS) is performed under several conditions, by considering radical quenching process on the wall. Following results are obtained: (i) displacement speed of the premixed flame propagating perpendicular to a wall have a local minimum; (ii) where the wall temperature equals unburnt gas temperature, the local minimum value almost corresponds to laminar flame speed of a corresponding thermochemical condition regardless of ignition position, equivalence ratio, and fuel types; (iii) these characteristics do not depend on the presence/absence of radical quenching processes. |
Monday, November 19, 2018 4:57PM - 5:10PM |
L02.00005: Numerical investigation and CSP analysis of laminar V-flames of H2-CH4-air mixtures Francisco E. Hernandez Perez, Efstathios-Al. Tingas, Yuriy Shoshin, Jeroen van Oijen, Philip de Goey, Hong G. Im A numerical study of steady, rod-stabilized, V-type, CH4-air and H2-CH4-air premixed laminar flames is conducted, particularly addressing the peculiar stabilization/blow-off of H2-CH4-air flames reported by Shoshin et al. (2013). For the CH4-air flames, either decreasing the inlet equivalence ratio or increasing the mean inflow velocity leads to a larger standoff distance and a lower heat flux to the rod, and below a critical value of the inlet equivalence ratio or above a critical value of the inflow velocity the flame blows off. For the H2-CH4-air flames, decreasing the inlet equivalence ratio has similar effects; however, increasing the inflow velocity reduces the standoff distance and increases the heat flux to the rod. The predicted behaviour of the flames is fully consistent with the experimental observations. Both the CH4-air and H2-CH4-air flames exhibit preferential diffusion effects such as superadiabatic temperatures and local equivalence ratio variations, which are more pronounced for the H2-CH4-air flames. Moreover, algorithmic tools from computational singular perturbation (CSP) are used to investigate the reactive dynamics and identify the dominant physical processes of the flames. |
Monday, November 19, 2018 5:10PM - 5:23PM |
L02.00006: Compressibility effect on volumetric heat loss and its influence on the Darrieus-Landau instability of a planar front of premixed flame Yasuhide Fukumoto, Keigo Wada, Snezhana I Abarzhi The effect of compressibility on the Darrieus-Landau instability (DLI) of a plane front of a premixed flame is investigated in the form of the M^2 expansion for small Mach number M. The method of matched asymptotic expansions is used to derive jump conditions for hydrodynamic variables across the flame front which itself is separated into the preheat and the reaction zonez sandwiched by the former. With this jump conditions across the flame front, we obtain the correction to the growth rate of the DLI to first order in M^2. If the Prandtl number and the heat release are sufficiently large, the compressibility effect can suppress the DLI. Our treatment accounts for the volumetric heat-loss effect, without having to add an ad hoc sink term in the heat-conduction equation. We show that the compressibility raises the temperature in the upstream (unburned) side of the reaction zone and decreases it in the downstream (burned) side and that the maximum value of the temperature is attained at some position located inside the reaction zone. This is peculiar to the compressibility effect, which is brought by pressure variation terms. |
Monday, November 19, 2018 5:23PM - 5:36PM |
L02.00007: Propagating fronts in fluids with backaction Saikat Mukherjee, Mark Paul We numerically study propagating fronts traveling through a long and shallow layer of fluid where the depth is parallel with the direction of gravity. The flow field is determined using a modified form of the Boussinesq equations and the propagating fronts are generated using a reaction-advection-diffusion equation with a nonlinear reaction. We explore the propagating fronts in the presence of backaction where the fronts affect the underlying fluid motion which affects the fronts and so on. We consider the case where the products are lighter than the reactants and where the reaction generates heat. We first study fronts propagating though an initially quiescent flow field. Next, we explore the propagating fronts as they travel through a cellular convective flow field that has been generated by including a constant temperature difference between the bottom and top surfaces of the fluid layer. We quantify the dynamics and geometry of the propagating fronts due to the backaction. We explore the flow field structures near the front and investigate their influence upon the geometry of the front interface and the front velocity. |
Monday, November 19, 2018 5:36PM - 5:49PM |
L02.00008: Computational Modeling of Accidental Fire Spread in Under-Ventilated Compartments Danyal Mohaddes, Matthias Ihme The reliable prediction of fires in under-ventilated compartment environments is critical to the design against and mitigation of accidental engine fires. Of specific concern in aviation gas turbines is the ignition and spread of fires arising in the fan case compartment due to the potential for leakage from pressurized fuel or hydraulic fluid lines and the presence of ignition sources in the form of hot surfaces. This presentation will describe recent efforts toward the analysis and development of modeling capabilities for the prediction of such compartment fires. By considering specific operating environments pertaining to aircraft flight conditions, relevant physical mechanisms will be reviewed, which include liquid fuel injection, evaporation, wall filming, ignition, and flame/wall coupling. A test case will be presented that captures the relevant physical processes, and results will be discussed which consider the ignition and fire-spread arising from the forced ignition of liquid kerosene fuel at low-pressure conditions in an under-ventilated environment. |
Monday, November 19, 2018 5:49PM - 6:02PM |
L02.00009: Compressible and Reactive Navier-Stokes Simulations of a Forced Fire Whirl Xiao Zhang, Joseph Chung, Carolyn Kaplan, Elaine S Oran A fire whirl forms when there is strong coupling of circulation and combustion of reactive material. In this work, we demonstrate the capability of a recently developed numerical model to simulate fire whirls. We consider a square enclosure that is open at the top and closed at the bottom with air forced in at the corners. The bottom wall has a constant flux of heptane in a specified diameter at the center. The convective portion of the compressible and reactive Navier-Stokes equations are solved using the barely implicit correction (BIC) and fourth-order flux corrected transport (FCT) algorithms. All diffusion terms use three-point central for spatial discretization and 2nd order Runge-Kutta time integration. The chemical energy release is modeled using the chemical-diffusive model. The results show Rankine-like vortex near the flame sheet. Buoyancy produces a jet-like vertical velocity profile. Furthermore, by reducing the circulation and fuel flow rate, we observe a state of reactive vortex breakdown. We examine and compare the structure of this state to the fire whirl. |
Monday, November 19, 2018 6:02PM - 6:15PM |
L02.00010: Jetting and Ignition due to Laser-Based Deposition of Energy in a Gas Jonathan M Wang, David A Buchta, Jonathan B Freund Ignition in the interior of a flow can be achieved by deposition of energy by a focused laser. The induced hydrodynamics affect ignition and can result in a jet by which hot gas is convected out of the breakdown region to a distance several times the size of the plasma kernel. High-resolution numerical simulations of a instantaneous deposition of thermal energy in a gas provide a hydrodynamic description of the cause of this jetting phenomenon. Jet strength and direction are sensitive to the initial geometry of the energized region. Changes in curvature of its bounding surface alter the evolving misalignment of density and pressure gradients, leading to early-time production of vorticity. A sufficiently intense energy deposition produces a ring-like vortex that causes involution of the kernel and propagates along the laser axis as a jet of hot gas that is above the autoignition temperature for common mixtures. Recent experiments confirm that this can be a mechanism of ignition. The geometry of the kernel can be modified to magnify mismatch in the signs of vorticity produced and ultimately change the direction of jetting. The dependence of net vorticity production with respect to geometry, energy input, and Reynolds number is also presented. |
Monday, November 19, 2018 6:15PM - 6:28PM |
L02.00011: Abstract Withdrawn
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Monday, November 19, 2018 6:28PM - 6:41PM |
L02.00012: Thermo-mechanics: The response of gases to spatially resolved, transient heat addition David R Kassoy, Adam Norris The non-dimensional Navier-Stokes equations, including a generic heat source in the energy equation , are used to identify four non-dimensional parameters that can be used to characterize the thermo-mechanical response of the gas to energy deposition; μ, the ratio of the characteristic energy deposition time-scale of the defined source to the characteristic acoustic time-scale of the affected volume of gas; HR, the ratio of the characteristic heat release relative to the initial internal energy in the affected volume; ε ,representative of a high activation energy Arrhenius reaction; and finally Kn, the local Knudsen number. The parameters are used to identify at least three remarkably diverse physical responses: Nearly constant volume heat addition, Nearly isobaric heat addition, Linear and non-linear wave propagation driven by heat addition. The first is characterized by the pressure rising with temperature and small changes in density and can include “explosive” energy release. The second is characterized by density reduction with rising temperature. Finally, the third describes the physical conditions necessary for traditional linear and non-linear wave propagation in the affected volume of gas, compatible with traditional thermo-acoustic studies.
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