Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session H06: Reacting Flow Instabilities and Near-Limit Flames |
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Chair: Carlos Pantano, University of Southern California Room: 205 |
Monday, November 25, 2019 8:00AM - 8:13AM |
H06.00001: A Computational Study on Chemical Characteristics of Bluff-body-stabilized Lean Premixed Flames Yu Jeong Kim, Wonsik Song, Francisco E. Hernández Pérez, Bok Jik Lee, Hong G. Im Bluff-body flame holders have been used to achieve stable combustion by recirculating hot product gases behind the holders in premixed reacting flow systems under highly unstable conditions. Despite assisting to improve combustion stability, holders also induce instabilities to flames and flow fields such as vortex shedding and blow-off. In the present study, high-fidelity direct numerical simulations are conducted to investigate flame dynamics behind a bluff-body in lean premixed mixtures and to understand the flame stabilization mechanism, in particular under hydrodynamic instabilities. A three-dimensional rectangular channel with a square-section bluff-body flame holder is selected as configuration. Several distinct flame instabilities are identified as the blow-off limit is approached. The sequence of key physical mechanisms, such as extinction and re-ignition, and combustion and chemical characteristics of the flames are also examined using the combined approach of computational singular perturbation and tangential stretching rate. [Preview Abstract] |
Monday, November 25, 2019 8:13AM - 8:26AM |
H06.00002: Effect of compressibility on laminar flame speed and its influence on the Darrieus-Landau instability of a planar front of premixed flame Yasuhide Fukumoto, Keigo Wada, Snezhana Abarzhi The effect of compressibility on a premixed flame front is investigated by use of the method of $M^2$ expansions for small Mach numbers. We study the inner structure of the reaction layer, by applying the method of matched asymptotic expansions to an overall one-step irreversible chemical reaction expressed by the Arrhenius law. We figure out the structure of freely propagating deflagration wave from the perspective of the temperature distribution and the laminar flame speed. The temperature distribution is greatly influenced by the compressibility effect which originates from the material derivative of the pressure in the source term of the heat-conduction equation. This effect naturally embodies the volumetric heat loss, without having to include any artificial sink term, by decreasing the temperature, with the Mach number, on the burned side of the reaction zone, accompanied by the overshoot of the temperature in the midway of the reaction layer. We then seek the laminar flame speed through calculation of the burning-rate eigenvalue and find that it sensitively drops down by the compressibility effect. This implies that the compressibility acts to reduce the growth rate of the Darrieus-Landau instability. [Preview Abstract] |
Monday, November 25, 2019 8:26AM - 8:39AM |
H06.00003: Gravity effects on the nonlinear dynamics of premixed flames Christophe Almarcha, Basile Radisson, Bruno Denet During their propagation, premixed flames are unstable and undergo a rich dynamics that can be favorably compared to the integration of Michelson-Sivashinsky (MS) equation. Three main ingredients are involved in this nonlinear partial differential equation : the Darrieus-Landau non local instability, the normal propagation of the front and a small scale diffusive stabilization. In order to take into consideration the long range effects of gravity, we study a modified (MS) equation where a linear stabilizing term is added. We demonstrate that this modified equation is capable of reproducing features of experimental 2D flames propagating downwards in a Hele-Shaw burner. In particular, although the additional term is linearly stabilizing, it is reponsible for an increase in the fragmentation of the interface into smaller cells. [Preview Abstract] |
Monday, November 25, 2019 8:39AM - 8:52AM |
H06.00004: Nonlinear stability of a premixed flame subjected to a transverse shear Xiaoyi Lu, Moshe Matalon, Carlos Pantano We present our findings on the hydrodynamic stability behavior of a premixed flame subjected to a transverse shear. The analysis is first carried out in the weak thermal expansion limit resulting in a modified Michelson-Sivashinsky (MS) equation, which describes the evolution of the flame surface. Numerical solutions to the MS equation show that due to the transverse shear, the flame develops a skewed cusp-like flame, that steadily propagates into the unburned gases and simultaneously translates along the transverse direction. Both propagation and translation velocities are shear-dependent. The fully nonlinear evolution of premixed flames with a realistic density jump at higher shear levels is then investigated using the Direct Numerical Simulations approach. [Preview Abstract] |
Monday, November 25, 2019 8:52AM - 9:05AM |
H06.00005: Flow Field Measurements for Instability Mitigation in Lean Direct Injection Joshua Krsek Lean direct injection (LDI) is a method of combustion used in aviation gas turbines which results in reduced emission of carbon monoxide, nitric oxides (NOx), soot, and unburned hydrocarbons. LDI often results in potentially destructive thermoacoustic instabilities, which can lead to combustor structural failure. A ceramic foam insert has been shown to passively mitigate such thermoacoustic instabilities. Further work has shown that this effect is caused at least in part by the porosity of the insert, as fully dense inserts are not universally effective at mitigating these instabilities. This study attempts to develop a fundamental understanding of the combustor flow structure by analyzing particle image velocimetry (PIV) for combustion under the presence of a porous insert, solid insert, and no insert. The results from this study will play a pivotal role in the design of an optimal instability-weakening porous insert for the NASA developed swirler-venturi injector. [Preview Abstract] |
Monday, November 25, 2019 9:05AM - 9:18AM |
H06.00006: Acoustically-Coupled Combustion Dynamics of Laminar Microjet Diffusion Flames Andres Vargas, Jose Guerrero, Ann Karagozian, Hyung Sub Sim The present experiments focus on the response of burning gaseous fuel jets to prescribed transverse acoustic excitation as a means of exploring the coupling of reactive, acoustic, and flow processes relevant to combustion instabilities. Microjets with several alternative configurations and sizes (including single and multiple jets) and different fuels are exposed to transverse standing wave disturbances within an acoustic waveguide for which a range of resonant frequencies and amplitudes of excitation are applied. Temporal flame response to acoustic excitation is studied via OH* chemiluminescence and visible imaging, with quantification of the dynamics via proper orthogonal decomposition (POD), Rayleigh indices, and temporal flame distortion measurements. Characteristic signatures associated with various types of flame response are identified, including weakly oscillatory combustion, full-scale flame lock-in to excitation, and multi-mode flame dynamics preceding flame extinction. Phase space plots of dominant POD mode coefficients produce periodic as well as strange attractor-like shapes for high amplitude forcing. [Preview Abstract] |
Monday, November 25, 2019 9:18AM - 9:31AM |
H06.00007: Theoretical Source Statistics of Thermo-Acoustic Instability within Reacting Flow Steven Miller We develop a statistical tool based on the equations of motion to quantify the various acoustic sources that cause thermo-acoustic instability. The Navier-Stokes equations, energy equation, and mass fraction equations are decomposed into their base, aerodynamic, and acoustic components. The acoustic components are solved for via the method of the vector Green's function. The spectral densities of acoustic fluctuations are then formed. The resultant two-point cross-correlations of the Navier-Stokes operator, energy equation operator, and mass fraction operator on the base and aerodynamic flow-field yield thermo-acoustic statistical sources. Resultant sources are the two-point cross-correlations consisting of terms involving traditional aerodynamic interactions, aerodynamic-combustion interactions, and combustion-combustion interactions. We show that under certain simplifying assumptions, the traditional Rayleigh criterion and combustion acoustic equations are recoverable. We compare the newly proposed theory to traditional Rayleigh criterion and acoustic equations. The benefit of the theory is all thermo-acoustic sources are accounted for in a single two-point cross-correlation model, which is consistent with traditionally accepted theory. [Preview Abstract] |
Monday, November 25, 2019 9:31AM - 9:44AM |
H06.00008: Computational Investigation of Combustion Instability in a Gas Turbine Model Combustor Nicholas Arnold-Medabalimi, Cheng Huang, Karthik Duraisamy Gaseous combustion is expected to see continued use in propulsion and energy applications for the foreseeable future. Lean operating conditions are desired from an emissions standpoint but can lead to thermoacoustic instabilities. Gas Turbine Model Combustors (GTMC) have been investigated in recent years to improve our understanding of the underlying phenomena. A well-examined experimental design is that of Meier et al. investigated both at DLR and the University of Michigan. In this work, we perform reacting Large Eddy Simulations (LES) of this partially premixed swirl stabilized burner at various operating conditions with particular interest in thermoacoustically unstable regimes. Detailed validations are presented with experiments, and time-averaged field characteristics, acoustics, and coherent structures are examined. The impact of turbulent combustion closure models is examined by comparing the performance of finite-rate chemistry and flamelet based methods. [Preview Abstract] |
Monday, November 25, 2019 9:44AM - 9:57AM |
H06.00009: Temporal linear stability analysis of laminar flames on inclined fuel surfaces Raquel Hakes, Wilfried Coenen, Antonio Sanchez, Michael Gollner, Forman Williams Experiments have found substantial structural differences between buoyancy-driven flames developing on the upper and lower surfaces of inclined burning plates. These differences cannot be explained with existing analytical solutions of steady semi-infinite flames, which provide identical descriptions for the top and bottom configurations. We perform a temporal linear stability analysis to investigate the potential role of flame instabilities in the experimentally observed flow differences. The problem is formulated in the limit of infinitely fast reaction, considering the non-unity Lewis number of the fuel vapor. The analysis incorporates nonparallel effects of the base flow and considers separately spanwise traveling waves and G\"ortler-like streamwise vortices. The solution to the stability problem determines the downstream location at which the flow becomes unstable, characterized by a critical value of the local Grashof number, which varies with the plate inclination angle. The results for the underside flame indicate that instabilities emerge further downstream than they do for a topside, in agreement with experimental observations. Increased baroclinic vorticity production is reasoned to be responsible for the augmented instability tendency of topside flames. [Preview Abstract] |
Monday, November 25, 2019 9:57AM - 10:10AM |
H06.00010: Near-limit H$_2$-O$_2$-N$_2$ combustion in nonpremixed counterflow mixing layers Prabakaran Rajamanickam, Jaime Carpio, Antonio L. Sanchez, Paul D. Ronney, Forman A. Williams Numerical computations employing detailed chemistry and experiments in a slot-jet counterflow burner are used to characterize the different combustion modes emerging in mixing layers separating N$_2$-diluted counterflowing planar streams of hydrogen and oxygen. Attention is focused on high degrees of dilution resulting in near-critical flames with peak temperatures close to the crossover temperature. A bifurcation diagram is presented in the mixture-fraction vs. strain-rate plane that identifies six different combustion regimes involving four different flame types, namely, diffusion-flame sheets, propagating/retreating edge flames, broken-flame tubes, and isolated flame tubes. While the isolated flame tube is always stationary, the broken flame tubes can be stationary or can propagate with an oscillatory speed. The observed flame behavior exhibits hysteresis in some parametric regions, where the flow that is established depends on the ignition mechanism. [Preview Abstract] |
Monday, November 25, 2019 10:10AM - 10:23AM |
H06.00011: Quantification of local combustion modes in laminar nonpremixed $n$-dodecane/air coflow flames Chao Xu, Sibendu Som Nonpremixed lifted jet flames in heated air coflow are important to a range of practical propulsion systems, while stabilization mechanisms for complex fuels have not been fully understood. To better understand the structure and stabilization mechanisms, two-dimensional laminar $n$-dodecane jet flames in heated air coflow at 30 atm are simulated with detailed chemical kinetics. To quantify roles of different sub-processes (e.g, chemistry, diffusion, radiation, etc.) and associated local combustion modes, a new flame diagnostic based on chemical explosive mode (CEM) analysis (CEMA) is developed, by projecting local chemical and diffusion source terms to the direction of CEM and quantifying the competition between these terms. Diffusion processes in both normal and tangential directions of the non-premixed flame sheet (defined as the mixture fraction isocontours) are further accounted for. Additional local combustion modes specific to nonpremixed flames (compared to premixed flames) are identified, highlighting different roles of flame-normal and flame-aligned diffusion processes in stabilizing the flames. Effects of radiative loss on local combustion modes and the flame stabilization mechanism, are also discussed based on an optically-thin radiation model. [Preview Abstract] |
Monday, November 25, 2019 10:23AM - 10:36AM |
H06.00012: Numerical study on the stabilization of ball-like reacting fronts at normal gravity Francisco Hernandez Perez, Zhen Zhou, Yuriy Shoshin, Jeroen van Oijen, Philip de Goey, Hong Im A computational investigation on the stabilization and dynamics of lean premixed flames in tubes at near lean-limit conditions is conducted. Hydrogen-methane-air premixed mixtures are considered in tubes with two different diameters (13.5 and 55 mm) and under the influence of normal gravity. As the lean flammability limit is approached, individual (13.5 mm tube) and multiple (55 mm) ball-like flames are formed and stabilized. Through high-fidelity simulations with detailed transport and chemical models as well as the inclusion of heat losses, the stabilization and dynamics of such flames are examined. [Preview Abstract] |
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