A Comparative Computational Fluid Dynamics Study on Flamelet-Generated Manifold and Steady Laminar Flamelet Modeling for Turbulent Flames

Abstract

A C o m p a ra tiv e C o m p u ta tio n a l F lu id D y n a m ic s S tudy on F la m e le t-G e n e ra te d M a n ifo ld and S te a d y L a m in a r F la m e le t M o d e lin g fo r T u rb u le n t F la m e s The laminar flamelet model (LFM) (Peters, 1986, "Laminar Diffusion Flamelet Models in Non-Premixed Combustion," Prog. Energy Combust. Sci., 10, pp. 319-339; Peters, "Laminar Flamelet Concepts in Turbulent Combustion," Proc. Combust. Inst., 21, pp. 1231-1250 represents the turbulent flame brush using statistical averaging of laminar flamelets whose structure is not affected by turbulence. The chemical nonequilibrium effects considered in this model are due to local turbulent straining only. In contrast, the flamelet-generated manifold (FGM) (van Oijen and de Goey, 2000, "Modeling of Pre mixed Laminar Flames Using Flamelet-Generated Manifolds," Combust. Sci. Technol., 161, pp. 113-137) model considers that the scalar evolution; the realized trajectories on the thermochemical manifold in a turbulent flame are approximated by the scalar evolu tion similar to that in a laminar flame. This model does not involve any assumption on flame structure. Therefore, it can be successfully used to model ignition, slow chemistry, and quenching effects far away from the equilibrium. In FGM, ID premixed flamelets are solved in reaction-progress space rather than physical space. This helps better solution convergence for the flamelets over the entire mixture fraction range, especially with large kinetic mechanisms at the flammability limits (ansys flu en t 14.5 Theory Guide Help Document, http:llwww.ansys.com). In the present work, a systematic comparative study o f the FGM model with the LFM for four different turbulent diffusion!premixed flames is presented. The first flame considered in this work is methane-air flame with dilution air at the downstream. The second and third flames considered are jet flames in a coaxial flow of hot combustion products from a lean premixed flame called Cabra lifted H2 and CH4 flames (Cabra, et al., 2002, "Simultaneous Laser Raman-Rayleigh-LIF Measurements and Numerical Modeling Results of a Lifted Turbulent H2IN2 Jet Flame in a Vitiated Coflow," Proc. Combust. Inst., 29(2), pp. 1881-1888; Lifted CH4/AirJet Flame in a Viti ated Coflow, http://www.me.berkeley.edu/cal/vcb/data/VCMAData.htmI) where the react ing flow associated with the central jet exhibits similar chemical kinetics, heat transfer, and molecular transport as recirculation burners without the complex recirculating fluid mechanics. The fourth flame considered is a Sandia flame D (Barlow et al., 2005, "Piloted Methane/Air Jet Flames: Scalar Structure and Transport Effects," Combust. Flame, 143, pp. 433-449), a piloted methane-air jet flame. It is observed that the simula tion results predicted by the FGM model are more physical and accurate compared to the LFM in all the flames presented in this work. The autoignition-controlled flame lift off is also captured well in the cases of lifted flames using the FGM model. • Expt --...

Experimental and numerical studies on the premixed syngas swirl flames in a model combustor

“…Tabulated chemistry method such as the Flamelet generated manifold (FGM) is used to reduce the combustion chemistry in simulation study which enables the prediction of intermediate species as well as pollutants [22]. The FGM method considers a multidimensional flame as an ensemble of one dimensional flames similar to a flamelet approach [23]. The modelling of reacting flow using FGM method has been conducted by several groups.…”

“…The study shows that when Lewis number of unity is considered, the FGM method which consists of counterflow diffusion flamelets is able to predict temperature and species concentrations in good agreement with the detailed solution (with a maximum difference ~2%). Nakod et al[23] conducted a systematic comparative study of the FGM model and laminar flamelet method (LFM) for various diffusion/premixed flames. The simulation results predicted by FGM model are more physical and accurate compared to the LFM method for all the flames tested.…”

Injector Design Space Exploration for an Ultra-Low NOx Hydrogen Micromix Combustion System

“…This may involve solving hundreds of molecular species, in a solution time compatible with the engineering design cycle time [23]. Hence, the combustion reaction kinetics for the present work are modelled using the FGM approach for a partially premixed diffusion flamelets [24] [25]. FGM is an industry standard approach used to reduce the computational expenses associated with directly solving the reaction kinetics.…”

“…These fast chemical processes can therefore be decoupled. This reduces the stiffness of the numerical model and therefore reduces the computational costs [24]. The turbulent flame is assumed to be an ensemble of laminar flames, the internal structure of which is not significantly influenced by the flow turbulence.…”

“…The kinetics of the thermal NOx formation rate is much slower than the compared to the energy-releasing reactions, and so most of the thermal NOx is formed only after the complete combustion. Theoretically, the FGM modelling approach can be used to simulate the slow chemistry of NOx formation but the degree of uncertainty associated with it is very high [24]. For the purposes of this analysis, the thermal NOx predictions are estimated, in a post-processing exercise, using the extended Zeldovich mechanism on the frozen reactive flow field predicted by the FGM model.…”