Three-Dimensional Numerical Simulations of Turbulent, Bluff-Body Stabilized, Lean, Premixed Combustion
TL;DR: In this paper, three-dimensional (3D) unsteady numerical simulations of turbulent, bluff-body, stabilized, lean, premixed combustion are reported, and the three dimensionality of the flame is clearly demonstrated through velocity and temperature profiles.
Abstract: Results from three-dimensional (3D) unsteady numerical simulations of turbulent, bluff-body, stabilized, lean, premixed combustion are reported. Shear stress transport (SST) k − ω model has been used for modeling turbulence, while a detailed 43-step mechanism has been used for modeling the methane, air chemistry. Turbulence chemistry interaction has been modeled using the eddy dissipation concept. Calculation has been done with buoyancy effects included to account for the effect of buoyancy on the predicted results. The three dimensionality of the flame is clearly demonstrated through velocity and temperature profiles. The intensity of turbulence is greatly enhanced by the flame, which is located at the shear layer, and thus the role of the flame as a turbulence generator is highlighted. The shear layer separates the cold reactants and the hot products, and is thus highly strained. The spectral analysis of time histories of pressure and velocity reveal that the lowest and highest dominant frequencies corr...
Citations
More filters
TL;DR: In this article, a large eddy simulation of turbulent premixed combustion behind a bluff-body is performed using unstrained flamelet model with a presumed probability density function to calculate filtered reaction rate.
36 citations
Dissertation•
05 Sep 2017
TL;DR: In this paper, a numerical study of turbulent combustion systems was pursued to examine different computational modeling techniques, namely computational fluid dynamics (CFD) and chemical reactor network (CRN) methods.
Abstract: A numerical study of turbulent combustion systems was pursued to examine different computational modeling techniques, namely computational fluid dynamics (CFD) and chemical reactor network (CRN) methods. Both methods have been studied and analyzed as individual techniques as well as a coupled approach to pursue better understandings of the mechanisms and interactions between turbulent flow and mixing, ignition behavior and pollutant formation. A thorough analysis and comparison of both turbulence models and chemistry representation methods was executed and simulations were compared and validated with experimental works. An extensive study of turbulence modeling methods, and the optimization of modeling techniques including turbulence intensity and computational domain size have been conducted. The final CFD model has demonstrated good predictive performance for different turbulent bluff-body flames. The NOx formation and the effects of fuel mixtures indicated that the addition of hydrogen to the fuel and non-flammable diluents like CO2 and H2O contribute to the reduction of NOx. The second part of the study focused on developing chemical models and methods that include the detailed gaseous reaction mechanism of GRI-Mech 3.0 but cost less computational time. A new chemical reactor network has been created based on the CFD results of combustion characteristics and flow fields. The proposed CRN has been validated with the temperature and species emission for different bluff-body flames and has shown the capability of being applied to general bluff-body systems. Specifically, the rate of production of NOx and the sensitivity analysis based on the CRN results helped to summarize the reduced reaction mechanism, which not only provided a promising method to generate representative reactions from hundreds of species and reactions in gaseous mechanism but also presented valuable information of the combustion mechanisms and NOx formation. Finally, the proposed reduced reaction mechanism from the sensitivity analysis was applied to the CFD simulations, which created a fully coupled process between CFD and CRN, and the results from the reduced reaction mechanism have shown good predictions compared with the probability density function method. Computational Study of Turbulent Combustion Systems and Global Reactor Networks Lu Chen GENERAL AUDIENCE ABSTRACT Turbulent combustion has been regarded as one of the most typical occurrences with industrial burners, where turbulent flow is produced by large vortex eddies when fuel and oxidizer mixes. Due to increasing demands for energy and concerns for environmental pollution, it is important to have a comprehensive understanding of turbulent combustion processes. To help provide information related to turbulent combustion, computational modeling can be used to give physical insights of the combustion process. A numerical study of turbulent combustion systems was pursued to examine different computational modeling techniques and to understand the mechanisms in terms of fluid dynamics and chemical kinetics. Computational fluid dynamics (CFD) was used to predict the flow field, including gas velocities, temperatures and fuel characteristics. Another computational technique known as the chemical reactor network (CRN) was used to provide information related to the chemical reactions and pollutant production. A method was developed as part of the study to couple the computational methods to pursue better understandings of the mechanisms and interactions between turbulent flow and mixing, ignition behavior and pollutant formation. Results have been compared with experimental data to optimize the modeling techniques and validate the developed model. The CRN model with the detailed gaseous reaction mechanism from the Gas Research Institute GRI-Mech 3.0 created a reacting network across the combustor with flame chemistry details. By post-processing the CRN results using a sensitivity analysis, the reduced reaction mechanism was summarized, which provided a promising method to generate representative reactions of the system from hundreds of species and reactions that occur in the combustion process. The proposed reduced reaction mechanism was applied to the CFD simulations, which created a fully coupled process between CFD and CRN. The results from the reduced reaction mechanism have shown good predictions compared with the probability density function method, which is a simplified way to model combustion. Pollutant emission such as NOx has also been studied in both CFD and CRN models, in terms of the effects of fuel mixtures, the formation mechanisms and influential factors as well as reactions to the formation process. The work provides guidance for an integrated framework to model and study turbulence and chemical reactions for turbulent combustion systems.
5 citations
Cites background from "Three-Dimensional Numerical Simulat..."
...[43] examined the recirculation zones in 2D axisymmetric and 3D domains and compared profiles along the centerline....
[...]
References
More filters
01 Jan 1977
TL;DR: In this paper, a model for the rate of combustion which takes into account the intermittent appearance of reacting species in turbulent flames is presented, which is applicable to premixed as well as diffusion flames.
Abstract: Principles of mathematical models as tools in engineering and science are discussed in relation to turbulent combustion modeling. A model is presented for the rate of combustion which takes into account the intermittent appearance of reacting species in turbulent flames. This model relates the rate of combustion to the rate of dissipation of eddies and expresses the rate of reaction by the mean concentration of a reacting specie, the turbulent kinetic energy and the rate of dissipation of this energy. The essential features of this model are that it does not call for predictions of fluctuations of reacting species and that it is applicable to premixed as well as diffusion flames. The combustion model is tested on both premixed and diffusion flames with good results. Special attention is given to soot formation and combustion in turbulent flames. Predictions are made for two C 2 H 2 turbulent diffusion flames by incorporating both the above combustion model and the model for the rate of soot formation developed by Tesner et al., as well as previous observations by Magnussen concerning the behavior of soot in turbulent flames. The predicted results are in close agreement with the experimental data. All predictions in the present paper have been made by modeling turbulence by the k -∈ model. Buoyancy is taken into consideration in the momentum equations. Effects of terms containing density fluctuations have not been included.
2,575 citations
"Three-Dimensional Numerical Simulat..." refers result in this paper
...The results obtained from simulations were compared with experimental results of Nandula et al. (1996). Simulations were carried out in an axisymmetric domain of size 0.5D in the radial direction and 4.6D in the axial direction. The results obtained from simulations were compared with experimental data available at x=D locations 0.3 and 1.0. As radiation effects were neglected, the temperature was found to be overpredicted. Although the peak values of OH did not match the experimental values, the trends were captured well. The effect of variable mixing time was also studied. The velocity and concentration of NO were not affected by changes in mixing time, while a 25% variation in the concentration of CO was observed. Minimal changes in results were observed when Gas Research Institute (GRI) mechanism 3 was used instead of mechanism 2.11. In the present study, full 3D unsteady Reynolds averaged Navier–Stokes (URANS) calculation of the turbulent, reacting flow in the lean, premixed combustor studied experimentally by Nandula et al. (1996) has been carried out....
[...]
...The results obtained from simulations were compared with experimental results of Nandula et al. (1996). Simulations were carried out in an axisymmetric domain of size 0....
[...]
TL;DR: In this article, a computational technique based on the in situ adaptive tabulation (ISAT) of the accessed region of the composition space is proposed to control the tabulation errors.
Abstract: A computational technique is described and demonstrated that can decrease by three orders of magnitude the computer time required to treat detailed chemistry in reactive flow calculations. The method is based on the in situ adaptive tabulation (ISAT) of the accessed region of the composition space - the adaptation being to control the tabulation errors. Test calculations are performed for non-premixed methane - air combustion in a statistically-homogeneous turbulent reactor, using a kinetic mechanism with 16 species and 41 reactions. The results show excellent control of the tabulation errors with respect to a specified error tolerance; and a speed-up factor of about 1000 is obtained compared to the direct approach of numerically integrating the reaction equations. In the context of PDF methods, the ISAT technique makes feasible the use of detailed kinetic mechanisms in calculations of turbulent combustion. The technique can also be used with reduced mechanisms, and in other approaches for calculating rea...
1,067 citations
"Three-Dimensional Numerical Simulat..." refers methods in this paper
...The stiff solver in FLUENT uses the in situ adaptive tabulation (ISAT) procedure proposed by Pope (1997) to calculate the source terms in the species transport equation....
[...]
01 Jan 1971
TL;DR: In this paper, a method for solving the partial parabolic differential equation of turbulent flame spread has been developed, which is applied to the spread of flame behind a baffle in a plane-walled duct.
Abstract: A calculation procedure has been developed for solving the partial parabolic differential equation of turbulent flame spread. This procedure has been applied to the spread of flame behind a baffle in a plane-walled duct, with two distinct models for the kinetics of the reaction. In the first model, the time-mean reaction rate is related to the time-mean concentrations and temperature at the point in question by a bimolecular Arrhenius expression. In the second model, the local reaction rate is taken to depend also on the rate of break-up of the eddies by fits the experiemntal data better than the first; the eddy-break-up term appears to be essential if the dominance of hydrodynamic processes is to be correctly simulated. A third model of turbulent combustion is also described. It involves the calculation of the magnitude of the fluctuating concentrations, and correctly predicts themain features of turbulent diffusion flames. One of its implications is a finite reaction-zone thickness, even through there is no chemical-kinetic resistance.
686 citations
"Three-Dimensional Numerical Simulat..." refers background in this paper
...Furthermore, investigations on premixed turbulent flames were more concerned about thermoacoustic instabilities (Armitage et al., 2006; Birbaud et al., 2007; Borghesi et al., 2006; Spalding, 1970) than on turbulence heat release interaction (Dowling, 1999; Giacomazzi et al....
[...]
...Furthermore, investigations on premixed turbulent flames were more concerned about thermoacoustic instabilities (Armitage et al., 2006; Birbaud et al., 2007; Borghesi et al., 2006; Spalding, 1970) than on turbulence heat release interaction (Dowling, 1999; Giacomazzi et al., 2004)....
[...]
TL;DR: In this article, a premixed ducted flame, burning in the wake of a bluff-body flame-holder, is considered and a kinematic model of the response of the flame to flow disturbances is developed.
Abstract: A premixed ducted flame, burning in the wake of a bluff-body flame-holder, is considered. For such a flame, interaction between acoustic waves and unsteady combustion can lead to self-excited oscillations. The concept of a time-invariant turbulent flame speed is used to develop a kinematic model of the response of the flame to flow disturbances. Variations in the oncoming flow velocity at the flame-holder drive perturbations in the flame initiation surface and hence in the instantaneous rate of heat release. For linear fluctuations, the transfer function between heat release and velocity can be determined analytically from the model and is in good agreement with experiment across a wide frequency range. For nonlinear fluctuations, the model reproduces the flame surface distortions seen in schlieren films.Coupling this kinematic flame model with an analysis of the acoustic waves generated in the duct by the unsteady combustion enables the time evolution of disturbances to be calculated. Self-excited oscillations occur above a critical fuel–air ratio. The frequency and amplitude of the resulting limit cycles are in satisfactory agreement with experiment. Flow reversal is predicted to occur during part of the limit-cycle oscillation and the flame then moves upstream of the flame-holder, just as in experimental visualizations. The main nonlinearity is identified in the rate of heat release, which essentially ‘saturates’ once the amplitude of the velocity fluctuation exceeds its mean. We show that, for this type of nonlinearity, describing function analysis can be used to give a good estimate of the limit-cycle frequency and amplitude from a quasi-nonlinear theory.
394 citations
"Three-Dimensional Numerical Simulat..." refers background in this paper
..., 2006; Spalding, 1970) than on turbulence heat release interaction (Dowling, 1999; Giacomazzi et al., 2004)....
[...]
...Furthermore, investigations on premixed turbulent flames were more concerned about thermoacoustic instabilities (Armitage et al., 2006; Birbaud et al., 2007; Borghesi et al., 2006; Spalding, 1970) than on turbulence heat release interaction (Dowling, 1999; Giacomazzi et al., 2004)....
[...]
01 Jan 1998
TL;DR: In this article, a new flame-wrinkling large-eddy simulation (LES) model using conditional filtering is proposed, which represents an alternative approach to the traditional flame-surface density based models in that the flame distribution is represented by a flamewrinkle density function and the effects of flame stretch and curvature are handled through a modeled transport equation for the perturbed laminar flame speed.
Abstract: The necessity for turbulent combustion modeling in the large-eddy simulation (LES) of premixed turbulent combustion is evident from the computational cost and the complexity of handling flame kinetics reaction mechanisms directly. In this paper, a new flame-wrinkling LES combustion model using conditional filtering is proposed. The model represents an alternative approach to the traditional flame-surface density based models in that the flame distribution is represented by a flame-wrinkle density function and that the effects of flame stretch and curvature are handled through a modeled transport equation for the perturbed laminar flame speed. For the purpose of validating the LES combustion model, LESs of isothermal and reacting shear layers formed at a rearward-facing step are carried out, and the results are compared with experimental data. For the isothermal case, the agreement between LES and the experimental data is excellent. For the reacting case, the evolution and topology of coherent structures is examined, and direct comparisons are made with time-averaged profiles of velocity and its fluctuations. temperature, and reaction products. Good agreement is obtained, to a large extent due to accurate modeling of the flame-wrinkle density but also to the novel treatment of the strain-rate effects on the laminar flame speed of the lean propane-air mixture.
277 citations
"Three-Dimensional Numerical Simulat..." refers background in this paper
...It is clear from Figure 2 that the recirculation zone for the nonreacting flow is much smaller than that of a reacting flow, due to thermal expansion of the vortex core (Fureby and Löfström, 1994; Weller et al., 1998)....
[...]
...It is clear from Figure 2 that the recirculation zone for the nonreacting flow is much smaller than that of a reacting flow, due to thermal expansion of the vortex core (Fureby and Löfström, 1994; Weller et al., 1998)....
[...]