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 A39: CFD: Reactive Flows I |
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Chair: Xinyu Zhao, University of Connecticut Room: Sheraton Back Bay C |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A39.00001: DNS evaluation of Reynolds stress models and Generalized Langevin models using velocity-acceleration correlation Xinyu Zhao, Alexandros Mathioudakis Velocity-acceleration correlation is used to evaluate the pressure-rate-of-strain term for Reynolds-stress based models and the drift coefficient in the generalized Langevin model. The direct numerical simulations (DNS) of a non-premixed temporally-evolving slot jet flame and a premixed temporally-evolving slot jet flame are used. Both flames feature moderate Reynolds numbers, as well as highly anisotropic and inhomogeneous flow environment. Good agreement is achieved between turbulent statistics obtained from velocity-acceleration correlation and those obtained directly from DNS. Different filter sizes are then applied to the DNS database to further test the feasibility of representing pressure-rate-of-strain term and the drift coefficient using velocity-acceleration correlation in experiments or large eddy simulations. Behaviors of turbulent statistics obtained from the premixed flame and those from the nonpremixed flame are analyzed. Finally, the applicability of existing generalized Langevin model coefficients to flame simulations is discussed. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A39.00002: Direct Numerical Simulation of a supersonic reacting jet with thermochemical nonequilibirum Romain Fi\'evet, Stephen Voelkel, Heeseok Koo, Philip Varghese, Venkat Raman In flows that exhibit nonequilibrium of internal energies, the ignition and stabilization of flames can exhibit complex dependencies. Common to shock-containing flows, the lack of equilibrium between vibrational and translational motion of the molecules can significantly alter the initiation of the fuel oxidation process. In this study, direct numerical simulation is used to understand the impact of nonequilibrium on flame stabilization. An important aspect of this work is the determination of chemical reaction rates consistent with such nonequilibrium. For this purpose, quasi-classical trajectory analysis based two-temperature reaction rates have been formulated. The nonequilibrium multi-species mixture is described using species-specific temperature, leading to an enhanced set of momentum, species, and energy equations. A jet-in-crossflow configuration is used to understand the onset of chemical reactions under such nonequilibrium conditions. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A39.00003: Semi-implicit iterative methods for low Mach number turbulent reacting flows Jonathan F. MacArt, Michael E. Mueller A formally second-order accurate Strang splitting approach has been developed and applied to the solution of scalar transport/reaction equations for Direct Numerical Simulation (DNS) of low Mach number turbulent reacting flows. The temporal discretization errors of this scheme are analyzed and compared with a formally first-order accurate Lie splitting approach and variations on a second-order accurate monolithic preconditioned scheme, utilizing two different preconditioners: the full Jacobian of the chemical source term and its diagonal approximation. The effect of chemical mechanism size on the relative performance of these schemes is assessed with a simple one-dimensional unsteady test case. The improved stability of the full Jacobian preconditioner is found to outpace its increased cost per time step compared to the diagonal approximation, and this advantage is found to increase with mechanism size. Likewise, the Strang splitting scheme is demonstrated to achieve better performance than the other approaches due to greater stability at larger time steps, despite greater cost per step. Finally, the schemes are evaluated with a three-dimensional unsteady turbulent planar jet flame, and similar conclusions are found as for the one-dimensional test case for relative performance. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A39.00004: Effects of High-Order Schemes and Turbulence Models on Supersonic Combustion of Liquid Jet in Cross Flow Ken Alabi, Foluso Ladeinde, Wenhai Li In this study, we investigate the effects of various numerical schemes, which are mostly high-order, and different turbulence models, on combustion in liquid jets in supersonic cross flow. The baseline models for two-phase (liquid/gas) flow and evaporation, which include the modeling of the primary and secondary breakup of liquid injectants and the modeling of the evaporation and fate of the spherical particles that result from the breakup models, are based on Sirignano's approach, with Spalding-like closures. The Eulerian-Lagrangian approach is implemented. Several variations of the weighted essentially-non-oscillatory (WENO) schemes are used for spatial discretization, with different turbulence-combustion modeling approaches, including the approaches of laminar flamelets and transported mass fractions. The results show the advantages of high-order (spatial) schemes relative to the second-order approximate factorization procedure of Beam-Warming. Approaches investigated for improving the computational efficiency of the otherwise expensive two-phase flow calculations will be discussed. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A39.00005: LES/FMDF of turbulent jet ignition in a rapid compression machine AbdoulAhad Validi, Harold Schock, Elisa Toulson, Farhad Jaberi Turbulent Jet Ignition (TJI) is an efficient method for initiating and controlling combustion in combustion systems, e.g. internal combustion engines. It enables combustion in ultra-lean mixtures by utilizing hot product turbulent jets emerging from a pre-chamber combustor as the ignition source for the main combustion chamber. Here, we study the TJI-assisted ignition and combustion of lean methane-air mixtures in a Rapid Compression Machine (RCM) for various flow/combustion conditions with the hybrid large eddy simulation/filtered mass density function (LES/FMDF) computational model. In the LES/FMDF model, the filtered form of compressible Navier-Stokes equations are solved with a high-order finite difference scheme for the turbulent velocity, while the FMDF transport equation is solved with a Lagrangian stochastic method to obtain the scalar (species mass fraction and temperature) field. The LES/FMDF data are used to study the physics of TJI and combustion in RCM. The results show the very complex behavior of the reacting flow and the flame structure in the pre-chamber and RCM. [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A39.00006: Computational Efficiency and Accuracy of the Two Forms of the Rate-Controlled Constrained-Equilibrium Method Fatemeh Hadi, Reza Sheikhi In this study, the Rate-Controlled Constrained-Equilibrium (RCCE) method in constraint potential and constraint forms have been investigated in terms of accuracy and numerical performance. The RCCE originates from the observation that chemical systems evolve based on different time scales, dividing reactions into rate-controlling and fast reactions. Each group of rate-controlling reactions imposes a slowly changing constraint on the allowed states of the system. The fast reactions relax the system to the associated constrained-equilibrium state on a time scale shorter than that of constraints. The two RCCE formulations are equivalent mathematically; however, they involve different numerical procedures and thus show different computational performance. In this work, the RCCE method is applied to study methane oxygen combustion in an adiabatic, isobaric stirred reactor. The RCCE results are compared with those obtained by direct integration of detailed chemical kinetics. Both methods are shown to provide very accurate representation of the kinetics. It is also evidenced that while the constraint form involves less numerical stiffness, the constraint potential implementation results in more overall saving in computation time. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A39.00007: Computational Study of Fluid Flow in a Rotational Chemical Vapor Deposition (CVD) Reactor Sun Wong, Yogesh Jaluria In a typical Chemical Vapor Deposition (CVD) reactor, the flow of the reacting gases is one of the most important considerations that must be precisely controlled in order to obtain desired film quality. In general, the fluids enter the reactor chamber, travel over to the heated substrate area, where chemical reactions lead to deposition, and then exit the chamber. However, the flow inside the reactor chamber is not that simple. It would often develop recirculation at various locations inside the reactor due to reactor geometry, flow conditions, buoyancy effects from temperature differences and rotational effects cause by the rotating substrate. This recirculation causes hot spots and affects the overall performance of the reactor. A recirculation fluid packet experiences a longer residence time inside the reactor and, thus, it heats up to higher temperatures causing unwanted chemical reactions and decomposition. It decreases the grow rate and uniformity on the substrate. A mathematical and computational model has been developed to help identify these unwanted hot spots occurring inside the CVD reactor. The model can help identify the user parameters needed to reduce the recirculation effects and better control the flow. Flow rates, pressures, rotational speeds and temperatures can all affect the severity of the recirculation within the reactor. The model can also help assist future designs as the geometry plays a big role in controlling fluid flow. The model and the results obtained are discussed in detail. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A39.00008: LES study of intermittency in soot formation in a model aircraft combustor Heeseok Koo, Venkat Raman, Michael Mueller, Klaus Peter Geigle Intermittent soot formation is one of the modeling challenges that prevent accurate predictions of soot concentration in a turbulent reacting flow. Due to the highly unsteady and irregular sooting behavior, formation of soot is acutely sensitive to the flow and gas phase history. Therefore, we need to accurately capture interactions between soot chemistry, particle dynamics, and turbulent flame as well as the turbulent reacting flow. In this study, large eddy simulation (LES) is used to understand the model sensitivity to the soot prediction. Hybrid method of moment (HMOM) soot model is used that accommodates detailed process of soot particle and soot precursor evolution. Gas phase chemistry uses flamelet progress variable approach with an additional enthalpy dimension to include soot radiation effect. The developed numerical model is tested on the DLR swirl combustor that emulates the rich-quench-lean (RQL) configuration using secondary oxidation air injection. [Preview Abstract] |
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