Eulerian particle flamelet modeling of a bluff-body CH4/H2 flame (original) (raw)
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Large-eddy simulation of a bluff-body stabilized nonpremixed flame
Combustion and Flame, 2006
Large-eddy simulations have been performed for a turbulent nonpremixed bluff-body stabilized CH 4 :H 2 (50:50 vol.) flame at a Reynolds number of 15,800. The corresponding isothermal flow has also been computed. The Sydney bluff-body burner under consideration has been investigated experimentally by Masri and co-workers, who obtained velocity and scalar statistics. The focus of the current work is on flow and mixing effects with the thermochemistry evaluated using a steady-state laminar flamelet approach. The incompressible (low-Mach-number) governing equations for mass, momentum, and mixture-fraction have been solved on a structured cylindrical grid and resolution effects investigated using up to 3.643 × 10 6 nodes. The corresponding nonreactive case was resolved by 5.76 × 10 5 nodes, resulting in a resolution of more than 80% of the turbulence kinetic energy. The reacting case yields a resolution in excess of 75% on the finest grid-arguably sufficient to permit conclusions regarding the accuracy of submodels. Comparisons with experimental data show that for high resolutions comparatively good agreement is obtained for the flow field and for species other than nitric oxide. However, resolution effects are important and results obtained with 4.51 × 10 5 nodes show that a resolution of less than 70% of the turbulent kinetic energy is insufficient in the context of the Smagorinsky subgrid model combined with the dynamic procedure of Germano. The latter result is consistent with the analysis of Pope. (A. Kempf). reactive flows the situation is less clear with chemical reaction and heat release occurring in the fine (unresolved) scales. The influence of persistent nongradient transport is also a particular problem in the context of premixed or partially premixed combustion. Many of the challenges associated with the inclusion of thermochemistry related terms are shared by the approaches of LES and RANS. Accordingly, many LES techniques for combusting flows are typically developed on the basis of already existing theories for turbulent reacting flows. A discussion of early 0010-2180/$ -see front matter
Laminar flamelet model prediction of NOx formation in a turbulent bluff-body combustor
Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy, 2008
A bluff-body combustor, with recirculation zone and simple boundary conditions, is ideal as a compromise for an industrial combustor for validating combustion models. This combustor, however, has proved to be very challenging to the combustion modellers in a number of previous studies. In the present study, an improved prediction has been reported through better representation of turbulence effect by Reynolds stress transport model and extended upstream computational domain. Thermo-chemical properties of the flame have been represented by a laminar flamelet model. A comparison among reduced chemical kinetic mechanism of Peters and detailed mechanisms of GRI 2.11, GRI 3.0, and San Diego has been studied under the laminar flamelet modelling framework. Computed results have been compared against the well-known experimental data of Sydney University bluff-body CH4/H2 flame. Results show that the laminar flamelet model yields very good agreement with measurements for temperature and majo...
Journal of Naval Architecture and Marine Engineering, 2009
 In this work simulation of a turbulent H2/N2 jet diffusion flame with flamelet modelling has been presented. The favre-averaged mixture fraction has been employed to model the combustion. Favre-averaged scalar quantities have been calculated from flamelet libraries by making use of a presumed Probability Density Function (PDF) method. The predicted flame temperature profiles and chemical species concentrations are compared with TNF experimental data obtained from Sandia/California. Predictions considering the unity Lewis number flamelet are found to be in good agreement with temperature and chemical species measurements. Predicted NO results also compared and shown with other species. This study shows that the combustion simulation using unity Lewis number flamelets are effective for predicting the flow, temperature and chemical kinetics of H2/N2 diffusion flame. To account for fluctuations of mixture fraction, its distribution is presumed to have the shape of a beta-function. Key...
Flow Turbulence and Combustion, 2009
A transported probability density function (PDF) approach closed at the joint scalar level was used to model a bluff body stabilised turbulent diffusion flame (HM2) investigated experimentally by Masri and co-workers. The current effort extends a previous study of HM1 (Re = 15,800) to a flame with a higher degree of local extinction (Re = 23,900). The impact of an algebraic model that accounts for local Damköhler number effects on the time-scale ratio of scalar to mechanical turbulence is also evaluated along with the impact of improved thermochemistry. The computations have been performed using a hybrid Monte Carlo/finite volume algorithm and a systematically reduced H/C/N/O mechanism featuring 300 reactions, 20 solved and 28 steady-state species. The joint scalar PDF equations were solved using moving particles in a Lagrangian framework and the velocity field was closed at the second moment level. The redistribution terms were modelled using the Generalized Langevin model of Haworth and Pope. Results show that scalar fields are reproduced with encouraging accuracy and that the revised time scale model improves agreement with experimental data. A high sensitivity to the NO chemistry was observed and encouraging agreement was obtained for the first two moments following adoption of updated reaction rates proposed in an earlier study.
NO formation in flameless combustion: comparison of different modeling approaches
The prediction of NO emissions from industrial burners represents a key goal of Computational Fluid Dynamics (CFD) aided design. Simplified NO formation mechanisms are usually desirable, to reduce the computational effort related to the numerical simulations; however, they must be able to capture the NO trends with acceptable accuracy. Simplified mechanisms for the thermal and prompt NO formation routes are generally available within the existing commercial CFD packages and they provide acceptable NO predictions at relatively high temperatures. However, when operating at lower temperatures and with hydrogen, other mechanisms can be relevant, such as those involving N 2 O and NNH intermediates. This can become particularly relevant in non traditional combustion regimes, such as flameless combustion, characterized by operating temperature far below the levels observed in traditional burners. The present work shows a numerical and experimental investigation of a flameless combustion burner operating with methane-hydrogen mixtures with a H 2 content up to 50% by wt. The work is focused on the requirements of the CFD model for the accurate prediction of the NO emissions from the burner. In particular, the influence of the combustion model and kinetic mechanism on the temperature fields on which the NO prediction is based is thoroughly discussed, together with the simplified NO formation paths to be included in the model. The approach based on the direct coupling of simplified NO mechanisms to the CFD calculation is compared to a different methodology, based on the post-processing of the CFD results with detailed kinetic mechanism for the gasphase combustion and pollutants formation. A validation methodology is also implemented to quantitatively assess the degree of agreement between the numerical results and the experiments and to guide the selection of the modeling parameters required for predicting NO emissions accurately.
NO formation in flameless combustion: comparison of dierent modeling approaches
The prediction of NO emissions from industrial burners represents a key goal of Computational Fluid Dynamics (CFD) aided design. Simplified NO formation mechanisms are usually desirable, to reduce the computational effort related to the numerical simulations; however, they must be able to capture the NO trends with acceptable accuracy. Simplified mechanisms for the thermal and prompt NO formation routes are generally available within the existing commercial CFD packages and they provide acceptable NO predictions at relatively high temperatures. However, when operating at lower temperatures and with hydrogen, other mechanisms can be relevant, such as those involving N 2 O and NNH intermediates. This can become particularly relevant in non traditional combustion regimes, such as flameless combustion, characterized by operating temperature far below the levels observed in traditional burners. The present work shows a numerical and experimental investigation of a flameless combustion burner operating with methane-hydrogen mixtures with a H 2 content up to 50% by wt. The work is focused on the requirements of the CFD model for the accurate prediction of the NO emissions from the burner. In particular, the influence of the combustion model and kinetic mechanism on the temperature fields on which the NO prediction is based is thoroughly discussed, together with the simplified NO formation paths to be included in the model. The approach based on the direct coupling of simplified NO mechanisms to the CFD calculation is compared to a different methodology, based on the post-processing of the CFD results with detailed kinetic mechanism for the gasphase combustion and pollutants formation. A validation methodology is also implemented to quantitatively assess the degree of agreement between the numerical results and the experiments and to guide the selection of the modeling parameters required for predicting NO emissions accurately.
Combustion and Flame, 2005
Joint probability density function (PDF) calculations are reported of the bluff-body stabilized flames (HM1, HM2, and HM3) and the results are compared with the available experimental data. The calculations are based on the modeled transport equation for the joint PDF of velocity, turbulence frequency, and composition (species mass fractions and enthalpy) using the interaction by exchange with the mean and Euclidean minimum spanning tree mixing models. The methane chemistry is described by a 19-species augmented reduced mechanism, and is implemented using in situ adaptive tabulation. The numerical accuracy of the calculations is carefully studied, and the associated errors are quantified. For flame HM1 (which has the least local extinction), there is generally good agreement between calculations and measurements, although (for all flames) the quality of the agreement deteriorates at downstream locations. The calculations correctly show essentially inert mixing in the shear layer between the recirculation zone and the coflow in flame HM1, but not in flames HM2 and HM3. In general, the calculations of flames HM2 and HM3 are not in good agreement with the experimental data and do not exhibit the observed local extinction. This deficiency is attributed to the inaccurate calculations of the mean mixture fraction in the recirculation zone (for flames HM2 and HM3). The sensitivity of the calculation to the mixing model constant is investigated, and the mean scalar dissipation is reported.
Thermal Science and Engineering Progress, 2018
The present work reports on the numerical investigation of NOx in three turbulent piloted diffusion flames of different levels of extinction. The study involves two-dimensional axisymmetric modeling of combustion in these flames with fairly detailed chemistry, i.e. GRI 3.0 mechanism. The main focus of the study is to analyze the effects of the two different combustion model approaches, such as infinitely fast chemistry based unsteady flamelet and finite rate chemistry based EDC, in predicting the NOx formation in three piloted methane jet flames (Sandia D, E, and F). The EDC approach is able to predict the passive scalar quantities but shows over-prediction in the reactive scalar quantities and NO prediction, while the unsteady flamelet modeling is found to be essential in predicting the accurate formation of slow kinetic species like NOx. The inability of flamelet and EDC approach in capturing localized flame extinction is observed, which lead to an over-prediction of NOx at larger downstream locations. Further, the dominance of NOx formation pathways is investigated in all three flames.