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 L17: Reacting Flows: Modeling |
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Chair: Hong Im, KAUST Room: D131 |
Monday, November 21, 2016 4:30PM - 4:43PM |
L17.00001: Modelling of Turbulent Scalar Fluxes in the Broken Reaction Zones Regime. Hong G. Im, Nilanjan Chakraborty, Markus Klein, Christian Kasten, Paul Arias The LES filtered species transport equation in turbulent reacting flow simulations contains the unclosed turbulent scalar flux that needs to be modelled. It is well known that the statistical behavior of this term and its alignment characteristics with resolved scalar gradient depend on the relative importance of heat release and turbulent velocity fluctuations. Counter-gradient transport has been reported in several earlier studies where the flames under investigation were located either in the corrugated flamelets or thin reaction zones regime of premixed turbulent combustion. Therefore it is useful to understand the statistical behavior of turbulent scalar fluxes if the flame represents the broken reaction zones regime (BRZR). The present analysis aims to provide improved understanding on this subject through an a-priori analysis of a detailed chemistry database consisting of three freely-propagating statistically planar turbulent H2-air premixed flames representing three different regimes of combustion. Results indicate that heat release effects weaken with increasing Karlovitz number, but that counter-gradient transport can still occur for large LES filter size in the BRZR. Furthermore the behaviour of the flux and in particular its sign are different for reactant and product species. [Preview Abstract] |
Monday, November 21, 2016 4:43PM - 4:56PM |
L17.00002: ABSTRACT WITHDRAWN |
Monday, November 21, 2016 4:56PM - 5:09PM |
L17.00003: Flamelet Regime Diagram for Turbulent Combustion Simulations Wai Lee Chan, Matthias Ihme, Hemanth Kolla, Jacqueline Chen The flamelet model has been widely used in numerical combustion investigations, particularly for the closure of large-eddy simulations (LES) of turbulent reacting flows. In most cases, the simulation results demonstrated good agreements with their experimental counterparts. However, a systematic analysis of the flamelet model's applicability, as well as its potential limitations, is seldom conducted, and the model performance is usually based only on \emph{a-posteriori} comparisons. The objective of this work is to derive a metric that can formally quantify the suitability of the flamelet model in different flame configurations. For this purpose, a flamelet regime diagram has been developed and studied in the context of direct numerical simulations (DNS) of a turbulent lifted jet flame. The implementation of the regime diagram in LES has been investigated through explicit filtering of the DNS results. [Preview Abstract] |
Monday, November 21, 2016 5:09PM - 5:22PM |
L17.00004: Modified Flamelet-Based Model for Non-Premixed High Speed Combustion Zhipeng Lou, Foluso Ladeinde, Wenhai Li The influence of static pressure and the use of Troe's model on flamelet solutions in supersonic combustion are studied. With various values of the background static pressure, we have observed significant effects on the flamelet solutions in such quantities as the quenching stoichiometric scalar dissipation rate, reaction rate of species and progress variable, heat release rate, and the temperature profile. In addition, the Troe's model shows opposite effects for low and high pressure conditions. The baseline flamelet table has been constructed with respect to mixture fraction and its stoichiometric scalar dissipation rate, where the information on both the stable and unstable flamelet solutions have been included. We have also experimented with the addition of pressure as an independent variable in the table, toward modeling compressibility and/or pressure-sensitive properties and the variable quenching conditions in real dual-mode scramjet operations. [Preview Abstract] |
Monday, November 21, 2016 5:22PM - 5:35PM |
L17.00005: Examination of flamelet differential molecular diffusion models in oxy-fuel turbulent jet flames Chao Han, Robert Barlow, Haifeng Wang Flamelet modeling of a series of oxy-fuel turbulent jet flames is conducted to examine the model’s capability for predicting the turbulence-chemistry interactions and the effect of differential molecular diffusion. There are two sets of the oxy-fuel jet flames, A series and B series. The A series includes three flames with a fixed Re and different Da, and the B series includes three flames with a fixed Da and different Re. These flames enable us to analyze the scaling of turbulence-chemistry interactions and differential molecular diffusion with respect to Re and Da, respectively. The modeling results are carefully compared with the experimental data for a critical assessment of the model. The model’s capability to reproduce the scaling with respect to Re and Da is also examined in detail. [Preview Abstract] |
Monday, November 21, 2016 5:35PM - 5:48PM |
L17.00006: A consistent transported PDF model for treating differential molecular diffusion Haifeng Wang, Pei Zhang Differential molecular diffusion is a fundamentally significant phenomenon in all multi-component turbulent reacting or non-reacting flows caused by the different rates of molecular diffusion of energy and species concentrations. In the transported probability density function (PDF) method, the differential molecular diffusion can be treated by using a mean drift model developed by McDermott and Pope (Journal of Computational Physics, 226, 947-993, 2007). This model correctly accounts for the differential molecular diffusion in the scalar mean transport and yields a correct DNS limit of the scalar variance production. The model, however, misses the molecular diffusion term in the scalar variance transport equation, which yields an inconsistent prediction of the scalar variance in the transported PDF method. In this work, a new model is introduced to remedy this problem that can yield a consistent scalar variance prediction. The model formulation along with its numerical implementation is discussed, and the model validation is conducted in a turbulent mixing layer problem. [Preview Abstract] |
Monday, November 21, 2016 5:48PM - 6:01PM |
L17.00007: An Investigation of a Hybrid Mixing Timescale Model for PDF Simulations of Turbulent Premixed Flames Hua Zhou, Mike Kuron, Zhuyin Ren, Tianfeng Lu, Jacqueline H. Chen Transported probability density function (TPDF) method features the generality for all combustion regimes, which is attractive for turbulent combustion simulations. However, the modeling of micromixing due to molecular diffusion is still considered to be a primary challenge for TPDF method, especially in turbulent premixed flames. Recently, a hybrid mixing rate model for TPDF simulations of turbulent premixed flames has been proposed, which recovers the correct mixing rates in the limits of flamelet regime and broken reaction zone regime while at the same time aims to properly account for the transition in between. In this work, this model is employed in TPDF simulations of turbulent premixed methane-air slot burner flames. The model performance is assessed by comparing the results from both direct numerical simulation (DNS) and conventional constant mechanical-to-scalar mixing rate model. [Preview Abstract] |
Monday, November 21, 2016 6:01PM - 6:14PM |
L17.00008: An Inadequacy Formulation for an Uncertain Flamelet Model David Sondak, Todd Oliver, Chris Simmons, Robert Moser We report progress on the development of an uncertain flamelet library for use in non-premixed turbulent combustion. A stochastic inadequacy operator is generalized from previous work and is now used to incorporate uncertainties in chemical reaction mechanisms in a flamelet model. The original form of the inadequacy operator was designed to enforce positivity of chemical species concentrations and conservation of species while representing inadequacies in reduced chemical mechanisms. As a first step towards generalization, we are exploring temperature dependent modifications to the inadequacy operator. The temperature dependence helps ensure that the operator is inactive in the absence of chemical reactions and becomes active only after ignition. A Bayesian inverse problem is used to calibrate the stochastic operator on a hydrogen-oxygen zero-dimensional reactor and to infer model parameters, and their uncertainties, from data obtained via a detailed chemical mechanism. The inferred model parameters are then propagated through a laminar, non-premixed, counterflow hydrogen-oxygen flame. Temperature and species profiles at various scalar dissipation rates are compared to those predicted from a five-reaction reduced model and the detailed model. [Preview Abstract] |
Monday, November 21, 2016 6:14PM - 6:27PM |
L17.00009: Towards Model Inadequacy Representations for Flamelet-Based RANS Combustion Simulations Todd Oliver, M.K. Lee, David Sondak, Chris Simmons, Robert Moser Flamelet-based RANS simulations are commonly used in combustion engineering. In such simulations, chemical reactions are represented by a ``flamelet-library'' of laminar diffusion flame solutions generated with some chemical mechanism, and turbulence is represented using typical eddy-viscosity-based RANS closures. Modeling errors are introduced through both of these models as well as their interaction. In this work, we formulate and apply physics-based stochastic model inadequacy representations to capture the effects of possible modeling errors, allowing their impact on quantities of interest to be estimated. Specifically, the uncertainty introduced by inadequacy of the chemical mechanism is represented using a recently developed stochastic operator approach, which is extended to the diffusion flame here, leading to a stochastic diffusion flame library. A Karhunen-Loeve decomposition applied to these random fields enables low-dimensional representation of this uncertainty. A stochastic extension of typical eddy-viscosity-based RANS models is developed to represent inadequacy in the turbulence closures. The full stochastic model is demonstrated on simulations of a planar jet flame. [Preview Abstract] |
Monday, November 21, 2016 6:27PM - 6:40PM |
L17.00010: Reduction of Chemical Models under Uncertainty Habib Najm, Riccardo Malpica Galassi, Mauro Valorani We discuss recent developments for dynamical analysis and reduction of hydrocarbon fuel chemical kinetic models under uncertainty. We rely on computational singular perturbation analysis, allowing for uncertainties in reaction rate parameters. We outline a construction for representation of uncertain reduced chemical models, and estimation of probabilities for inclusion of sets of reactions in the reduced model. We demonstrate the approach in the context of homogeneous ignition of a hydrocarbon fuel-air mixture, illustrating the robustness of the reduced model under parametric uncertainty. [Preview Abstract] |
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