Turbulent Lifted Hydrogen Jet Flame in a Vitiated (original) (raw)
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Flow, Turbulence and Combustion, 2018
A new methodology for modeling and simulation of reactive flows is reported in which a 3D formulation of the Linear Eddy Model (LEM3D) is used as a post-processing tool for an initial RANS simulation. In this hybrid approach, LEM3D complements RANS with unsteadiness and small-scale resolution in a computationally efficient manner. To demonstrate the RANS-LEM3D model, the hybrid model is applied to a lifted turbulent N 2-diluted hydrogen jet flame in a vitiated co-flow of hot products from lean H 2 /air combustion. In the present modeling approach, mean-flow information from RANS provides model input to LEM3D, which returns the scalar statistics needed for more accurate mixing and reaction calculations. Flame lift-off heights and flame structure are investigated
Combustion Science and Technology, 2019
A novel model, the hybrid RANS-LEM3D model, is applied to a lifted turbulent N 2 diluted hydrogen jet flame in a vitiated co-flow of hot products from lean H 2 /air combustion. In the present modeling approach, meanflow information from RANS provides model input to LEM3D, which returns the scalar statistics needed for more accurate mixing and reaction calculations. The dependence of lift-off heights and flame structure on iteration schemes and model parameters are investigated in detail, along with other characteristics not available from RANS alone, such as the instantaneous and detailed species profiles and small-scale mixing. Two different iteration procedures, a breadth-first search and a checkerboard algorithm, as well as parameters of the model framework are examined and tested for sensitivity towards the results. The impact of heat release and thermal expansion is demonstrated and evaluated in detail. It is shown for the current application that LEM3D provides additional details compared to the RANS simulation with a low computational cost in comparison to a conventional DNS simulation.
Large Eddy Simulation of a H 2 /N 2 Lifted Flame in a Vitiated Co-Flow
Combustion Science and Technology, 2008
A lifted turbulent H 2 =N 2 flame in a vitiated co-flow is studied using Large Eddy Simulation together with a closure based on perfectly stirred reactors. A part of the closure, chemical look-up tables, are generated to close the filtered temperature equations and to compute local radical concentrations throughout the computational domain. The approach has been used to simulate a lifted turbulent flame. The results have been found to be insensitive to the combustion model employed and to the grid resolution. However, the results are very sensitive to the temperature of the co-flow stream and this result is well in line with previous findings. The numerical predictions were further compared to detailed experimental data obtained by Cabra et al. (2002). The agreement between the two sets of data is very good, indicating that the present approach predicts successfully the combustion process including the OH mass fractions. Finally, the LES data were used to study the flame dynamics and stabilization mechanisms.
Direct Numerical Simulation of Turbulent Lifted Hydrogen/Air Jet Flame in Heated Coflow
Direct numerical simulation of the near field of a three-dimensional spatially developing turbulent slot-burner lifted jet flame in heated coflow is performed with a detailed hydrogen-air mechanism and mixture averaged transport properties at a jet Reynolds number of 11,000 with over 900 million grid points. The results show that auto-ignition in a fuel-lean mixture immediately upstream of the flame base is the main source of stabilization of the lifted jet flame and that HO2 radical plays an important role in initiating and facilitating autoignition in both fuel-rich and fuel-lean mixtures. A Damköhler number analysis and intermediate species behavior near the leading edge of the lifted flame clearly show that autoignition occurs at the flame base. The flame index shows that both lean premixed and nonpremixed flame modes exist at the flame base, followed downstream by a prevailing premixed flame mode, and even further downstream, by the emergence of both rich premixed and nonpremixed flame modes. The DNS of the near field precludes the transition to a nonpremixed flame mode anticipated in the far-field of the jet. In addition to auto-ignition, vorticity generation due to baroclinic torque near the flame base assists in stabilizing the flame base by reducing the incoming local flow velocity, and thereby providing an environment enabling auto-ignition to proceed.
Validation of Combustion Models for Lifted Hydrogen Flame
E3S Web of Conferences
Within a Reynolds Averaged Numerical Simulation (RANS) approach for turbulence modelling, a computational investigation of a turbulent lifted H2/N2 flame is presented. Various turbulent combustion models are considered including the Eddy Dissipation Model (EDM), the Eddy Dissipation Concept (EDC), and the composition Probability Density Function transport model (PDF) in combination with different detailed and global reaction mechanisms. Turbulence is modelled using the Standard k-ɛ model, which has proven to offer a good accuracy, based on a preceding validation study for an isothermal H2/N2 jet. Results are compared with the published measurements for a lifted H2/N2 flame, and the relative performance ofthe turbulent combustion models are assessed. It is observed that the prediction quality can vary largely depending on the reaction mechanism and the turbulent combustion model. The best and quite satisfactory agreement with experiments is provided by two detailed reaction mechanism...
A linear eddy sub-grid model for turbulent reacting flows: Application to hydrogen-AIR combustion
Symposium (International) on Combustion, 1992
A new sub-grid mixing model for use in large eddy simulations of turhulent combustion is presented and applied to a hydrogen-air diffusion flame. The sub-grid model is based on Kerstein's Linear Eddy Model (Comb. Sci. Tech. 60, 391, 1988), which reduces the description of the scalar field to a locally one-dimensional representation. The formulation involves performing separate linear eddy calculations in each cell to describe the small-scale scalar mixing and reaction process. Convective transport across grid surfaces is accomplished by "'splicing" events by which linear eddy elements are copied to and fi'om neighboring grid cells based on the grid-resolved velocity field.
Numerical modeling of hydrogen diffusion jet flame: the role of the combustion model
The prediction of turbulent reactive flows largely depends on the proper choice of a combustion model. The closure of the highly non linear production term in the species conservation equation is one of the most challenging aspects when modeling turbulent combustion with RANS/FANS approaches. The main value of the species reaction rates cannot be obtained by simply substituting the mean variables into the Arrhenius equations, and the effects of turbulence on temperature and species concentration fields must be taken into account. Several approaches have been proposed in the literature to provide a proper modelling of mean reaction rates. The Eddy Dissipation Model is based on the infinitely fast chemistry hypothesis and assumes that the reaction rate is controlled by turbulent mixing . A generalized formulation of the Eddy Dissipation Model, i.e. EDM/FR, has been proposed in order to take into account finite rate chemistry effects, however, this model is not suited to treat detailed combustion mechanisms and fails with a number of reactions larger than three or four. The Eddy Dissipation Concept (EDC) is an extension of EDM which allows detailed kinetics to be included in the calculations . The model assumes that chemical reactions occur within the smallest turbulent structures, called fine structures. These are treated as perfectly stirred reactors (PSR) which exchange mass with the surrounding fluid. The overall reaction rate in each PSR is controlled by chemical kinetics. The properties of the fine structures are derived from a step-wise energy cascade model and expressed with quantities related to the main flow, such as the turbulent kinetic energy, k, and the turbulent dissipation rate, _. Two formulations of EDC have been developed, for fast and finite rate chemistry respectively [2]. Both combustion models described above are based on the reaction rate approach. A different approach is based on the primitive variable and originates a group of combustion models. In its simplest formulation, this approach allows the decoupling of chemistry and flow field calculation. The chemical reaction rates are computed off-line and stored in lookup tables, accessible by the flow solver. The instantaneous variables are expressed in terms of mixture fraction and enthalpy and, then, integrated over an assumed _-PDF probability density function to get the mean scalar variables. In the Flamelet approach, the flame is modelled as an ensemble of thin and steady laminar flames, strained by the turbulent motion. According to this model, two scalar equations, for the mixture fraction and its variance, have to be solved independently of the number of species involved in the chemical mechanism. The influence of the outer flow field on the inner reaction zone is described by the scalar dissipation rate. This represents a reciprocal residence time within the flame sheets which is increased by stretch effects of turbulence motion and reduced by diffusion. At a critical value of the dissipation rate the flame shows a threshold behaviour and extinguishes. The mean values of scalar properties are computed from the instantaneous values which are integrated over a probability density function with a presumed _-PDF shape. The integration is not carried out during the CFD calculation but is part of the flamelet library generation, in order to reduce the computational cost. The flamelet libraries are created with a mixture fraction ranging between zero and unity and for discrete
A COMPARATIVE STUDY OF TURBULENCE MODELLING IN HYDROGEN-AIR NONPREMIXED TURBULENT FLAMES
Combustion Science and Technology, 2006
The goal of this paper is to investigate the predictive capability of two turbulence models which are the k-ε model and the Reynolds Stress Model (RSM) within flamelet approach. A co-flow axisymetric turbulent non-premixed hydrogen flame investigated experimentally by and is used as a test case. The chemical mechanism of Yetter's and al. (1991) is adopted for the generation of the flamelet library. It consists of 10 chemical species and 21 reactions. The comparisons with experimental data demonstrate that predictions based on the Reynolds stress turbulence model are slightly superior to those obtained using the k-ε model. Overall, profile predictions of axial velocity, turbulent kinetic energy, mixture fraction, flame temperature and major species are in reasonable agreement with data and compare favourably with the results of earlier investigations.
Large-eddy simulation of a lifted methane jet flame in a vitiated coflow
Combustion and Flame, 2008
The impact of burned gases on flame stabilization is analyzed under the conditions of a laboratory jet flame in vitiated coflow. In this experiment, mass flow rate, temperature, and the exact chemical composition of hot products mixed with air sent toward the turbulent flame base are fully determined. Autoignition and partially premixed flame propagation are investigated for these operating conditions from simulations of prototype combustion problems using fully detailed chemistry. Using available instantaneous species and temperature measurements, a priori tests are then performed to estimate the prediction capabilities of chemistry tabulations built from these archetypal reacting flows. The links between autoignition and premixed flamelet tables are discussed, along with their controlling parameters. Using these results, large-eddy simulation of the turbulent diluted jet flame is performed, a new closure for the scalar dissipation rate of reactive species is discussed, and numerical predictions are successfully compared with experiments.
Numerical Investigation of Turbulent Hydrogen-Methane-Nitrogen Non-Premixed Jet Flame
Journal of Energy Technologies and Policy, 2016
In this work, the numerical investigation of the two-dimensional axisymmetric turbulent diffusion flame of a composite fuel was performed by using a computational fluid dynamics code to predict flame structure. The composite fuel was an H 2 /CH 4 /N 2 gas mixture. The amount of H 2 and N 2 in the fuel mixture varies under constant volumetric fuel flow rate. Fluent, which solves the governing and reaction equations using the finite volume method, was used as the computational fluid dynamics program. The non-premixed model was used for computation of the combustion. The standard k-ε model was used for modeling the turbulent flow. The interaction of the chemistry and turbulence was accounted for by the program with the probability density function model. This model was validated against the experimental data taken from literature. In general, the numerical results of the temperature, velocity, and CO 2 concentration distributions were in satisfactory agreement with the experimental results. The numerical results showed that adding H 2 to the fuel mixture decreases the flame length and generally increases the maximum temperature of the flame. On the other hand, adding N 2 to the mixture decreases both the flame length and maximum flame temperature. The flame length corresponds to the axial position of the peak flame temperature.