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 D5: Combustion I |
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Chair: Javier Urzay, Stanford University Room: 104 |
Sunday, November 22, 2015 2:10PM - 2:23PM |
D5.00001: Influence of equivalence ratio on the mechanism of pressure wave generation during knocking combustion Hiroshi Terashima, Mitsuo Koshi Knocking in spark-assisted engines is known as a severe pressure oscillation mainly caused by hot-spot autoignition in end-gas regions. In this study, knocking combustion of n-heptane/air mixtures modeled in a one-dimensional constant volume reactor is simulated with particular emphasis on the effects of equivalence ratio (0.6 to 2.0) on the mechanism of pressure wave generation. An efficient compressible flow solver with detailed chemical kinetics of n-heptane (373 species and 1071 reactions) is applied. The results demonstrate that the presence of negative temperature coefficient region significantly influence the knocking timing and knocking intensity, i.e., pressure wave amplitude in end-gas regions. The condition with equivalence ratios lower than 1.0 mostly leads to the reduction of the knocking intensity because of slower heat release rates of end-gas autoignition. On the other hand, the results with higher equivalence ratios of 1.2 to 2.0 indicate that a significant peak in the knocking intensity is produced at an equivalence ratio, which varies with initial temperature conditions. The final presentation will address the relationship between the knocking intensity and equivalence ratio with the discussion on detailed physics of pressure wave generation. [Preview Abstract] |
Sunday, November 22, 2015 2:23PM - 2:36PM |
D5.00002: Combustion properties in multi-particulate flows with direct numerical simulation Longhui Zhang, Changfu You Multiphase combustion is widely applied in industries. With high solid concentrations, the influence of particle interactions must be taken into account in the combustion models. Many literatures have developed group combustion models with particles treated as point sources. However, for dense phase flow the particle size is in the same scale with the average particle spacing, and the point source assumption is not accurate enough. This work presents the fully resolved direct numerical simulation results of reacting particulate flows. The particles are considered as finite sized regions in flow fields. Therefore the influence of particle motion and distribution on combustion properties can be obtained. The moving, colliding and burning process of char particles in confined space is calculated. The flow and combustion characteristics under different conditions are observed. The char burning rate is compared to that of fixed char particles with uniform distribution. The result shows that the burning rate decreases when the particle distribution non-uniformity perpendicular to the main flow direction increases. A model of char group combustion rate is developed using non-uniformity coefficients. [Preview Abstract] |
Sunday, November 22, 2015 2:36PM - 2:49PM |
D5.00003: Evaporation and Combustion Characteristics of Multicomponent Fuels Pavan Govindaraju, Alessandro Stagni, Matthias Ihme Current generation fuels are mixtures of hundreds of complicated organic compounds and accurate modeling of their combustion characteristics provides fundamental physical insights which also help in the design of efficient combustors. This however requires accurate simulation of both evaporation and combustion processes, which, in case of such fuels, demands an approach based on calculating properties using only the information of functional groups present in the mixture. The presentation will elaborate on the assumptions and the framework utilized for evaporation and chemical mechanisms. We also present a comparison between various fuels used in the aviation industry as test cases while highlighting on their pros and cons. The focus of the talk will however be on the physical aspects captured using 1D simulations, i.e., preferential evaporation of each species, ignition parameters and emissions while justifying the numerical calculations with experimental data at each stage. Further work involving the coupling of flow with evaporation and combustion can be performed and we briefly discuss why a DNS is necessary to characterize the various combustion regimes. [Preview Abstract] |
Sunday, November 22, 2015 2:49PM - 3:02PM |
D5.00004: LES of Mild Combustion using Pareto-efficient Combustion Adaptation Hao Wu, Michael Evans, Matthias Ihme Moderate or Intense Low-Oxygen Dilution (MILD) combustion is a combustion regime that provides opportunities for improved thermal efficiency and reduced pollutant emissions. In this study, large-eddy simulation is used to investigate the ignition, mixing, and stabilization of a jet flame in this kinetics-controlled combustion regime. The combustion process is modeled by a Pareto-efficient combustion (PEC) formulation that optimally combines reaction-transport and chemistry combustion models. In this approach, a three-stream flamelet/progress variable model is used as a computationally efficient description of equilibrated flame regions, and a finite-rate chemistry representation is employed to accurately represent the ignition behavior and flame stabilization. Through comparisons with experiments and simulations with single-regime combustion models, it will be shown that this Pareto-efficient combustion submodel assignment accurately captures important dynamics in complex turbulent flame configurations. [Preview Abstract] |
Sunday, November 22, 2015 3:02PM - 3:15PM |
D5.00005: LES of combustion dynamics near blowout in a realistic gas-turbine combustor Lucas Esclapez, Medhi B. Nik, Peter C. Ma, Jeff O'Brien, Serena Carbajal, Matthias Ihme Driven by increasingly stringent emission regulations, modern gas turbines operate at lean conditions to reduce combustion chamber temperature and NO$_x$ emissions. However, as the combustor operates closer to the lean blow-out (LBO) limit, flame stabilization mechanisms are weakened, which increases the risk for complete flame blowout. To better understand the LBO-process, large-eddy simulations of the combustion dynamics near blowout are performed in a realistic two-phase flow combustor. An unstructured incompressible Navier-Stokes solver is used in combination with a Lagrangian dispersed phase formulation. Flame dynamics near and at LBO conditions are studied to identify the role of the liquid fuel composition, spray evaporation, and complex flow pattern on the LBO limit. [Preview Abstract] |
Sunday, November 22, 2015 3:15PM - 3:28PM |
D5.00006: A Jet-Stirred Apparatus for Turbulent Combustion Experiments Abbasali Davani, Paul Ronney A novel jet-stirred combustion chamber is designed to study turbulent premixed flames. In the new approach, multiple impinging turbulent jets are used to stir the mixture. It is well known that pair of counterflowing turbulent jets produces nearly a constant intensity (u') along the jet axes. In this study, different numbers of impinging jets in various configurations are used to produce isotropic turbulence intensity. FLUENT simulations have been conducted to assess the viability of the proposed chamber. In order to be able to compare different configurations, three different non dimensional indices are introduces. Mean flow index; Homogeneity index, and Isotropicity index. Using these indices one can compare various chambers including conventional Fan-stirred Reactors. Results show that a concentric inlet/outlet chamber (CAIO) with 8 inlets and 8 outlets with inlet velocity of 20 m/s and initial intensity of 15{\%} produces near zero mean flow and 2.5 m/s turbulence intensity which is much more higher than reported values for Fan-stirred chamber. [Preview Abstract] |
Sunday, November 22, 2015 3:28PM - 3:41PM |
D5.00007: Simulation of High-Pressure Methane Hydrate Combustion Pavel Popov, William Sirignano With its prevalence in ocean floor deposits, methane hydrate has recently attracted considerable attention in the combustion community. We present a new scheme for the simulation of methane hydrate combustion at high, near critical pressures. This process features a combination of solid, liquid and gas phases, wherein the solid methane hydrate melts into a bubbly liquid, which then evaporates into a gas phase; methane-air combustion occurs in the gas phase. In addition to its multiphase nature, this problem features the additional challenge of modelling the gas/liquid phase transition at near-critical pressures. A new computational procedure has been developed to simulate this problem, using a detailed chemical mechanism for the simulation of reaction in the gas phase, and featuring a volume-of-fluid (VOF) approach for the simulation of the liquid phase with gas bubbles -- a low Stokes number is assumed. This procedure is applied to a laminar shear flow methane hydrate combustion problem. Particular attention is directed to the effects on simulation results of the high-pressure equation of state, liquid/gas phase transition modelling, and the bubbly liquid phase modelling. Simulation results are compared to experimental observations. [Preview Abstract] |
Sunday, November 22, 2015 3:41PM - 3:54PM |
D5.00008: Analysis of Fuel Injection and Atomization of a Hybrid Air-Blast Atomizer. Peter Ma, Lucas Esclape, Timo Buschhagen, Sameer Naik, Jay Gore, Robert Lucht, Matthias Ihme Fuel injection and atomization are of direct importance to the design of injector systems in aviation gas turbine engines. Primary and secondary breakup processes have significant influence on the drop-size distribution, fuel deposition, and flame stabilization, thereby directly affecting fuel conversion, combustion stability, and emission formation. The lack of predictive modeling capabilities for the reliable characterization of primary and secondary breakup mechanisms is still one of the main issues in improving injector systems. In this study, an unstructured Volume-of-Fluid method was used in conjunction with a Lagrangian-spray framework to conduct high-fidelity simulations of the breakup and atomization processes in a realistic gas turbine hybrid air blast atomizer. Results for injection with JP-8 aviation fuel are presented and compared to available experimental data. [Preview Abstract] |
Sunday, November 22, 2015 3:54PM - 4:07PM |
D5.00009: Multiscale Interactions and Backscatter in Premixed Combustion Peter Hamlington, Colin Towery, Jeffrey O'Brien, Alexei Poludnenko, Javier Urzay, Matthias Ihme Multiscale interactions and energy transfer between turbulence and flames are fundamental to understanding and modeling premixed turbulent reacting flows. To investigate such flows, direct numerical simulations of statistically planar turbulent premixed flames have been performed, and the dynamics of kinetic energy transfer are examined in both spectral and physical spaces. In the spectral analysis, two-dimensional kinetic energy spectra and triadic interactions are computed through the flame brush. It is found that there is suppression of turbulent small-scale motions in the combustion products, along with backscatter of energy for a range of scales near the thermal laminar flame width. In the physical-space analysis, a differential filter is applied to examine the transfer of kinetic energy between subgrid and resolved scales in the context of large eddy simulations. Subgrid-scale backscatter of kinetic energy driven by combustion is found to prevail over forward scatter throughout the flame brush. The spectral- and physical-space analyses thus both suggest an enhancement of reverse-cascade phenomena in the flame brush, which is possibly driven by accumulation of kinetic energy in the scales where combustion-induced heat release is preferentially deployed. [Preview Abstract] |
Sunday, November 22, 2015 4:07PM - 4:20PM |
D5.00010: A single-fluid multiphase formulation for diffuse-interface modeling of high-pressure liquid-fueled transcritical mixing layers Lluis Jofre, Javier Urzay, Ali Mani, Parviz Moin Liquid propellants are often used in propulsion systems. In subcritical conditions, atomization involves the rupture of the liquid volume through the competition between aerodynamic shearing and surface tension. In contrast, the classic atomization description becomes inadequate at supercritical conditions when the characteristic temperature and pressure of the gas environment are above the corresponding critical values. In that limit, the latent heat of vaporization vanishes and there is virtually no surface tension that prevents rupture of the liquid core and diffusive mixing with the gas environment. In particular, in high-pressure gas turbines the liquid fuel is seldom preheated to supercritical temperatures before injection, and the presence of both subcritical and supercritical conditions in the combustion chamber is warranted. Consideration of the liquid phase is therefore required in addition to the gas phase and the supercritical mixture. A single-fluid multiphase formulation of this problem is presented to investigate mixing and combustion in fuel-air transcritical mixing layers. The formulation makes use of diffuse-interface concepts facilitated by the relatively larger interface thicknesses at these high pressures. [Preview Abstract] |
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