Top PDF Progress in Numerical Modeling of Non-Premixed Combustion

Progress in Numerical Modeling of Non-Premixed Combustion

Progress in Numerical Modeling of Non-Premixed Combustion

using results obtained with Mesh 3 and 4. Results obtained using Mesh 5 is not included since Mesh 5 uses the exact same grid as Mesh 4 in these shear layers. The Favre-averaged mean velocity is shown to focus on the development of the Kelvin-Helmholtz instability and the shear layer. The mean and root mean square of mixture fraction highlight the mixing of fuel and air in the turbulent shear layer. These three quantities are critical in reproducing any turbulent flames. For all these quantities, Fig. 6.6 shows that both meshes provide virtually the same results. Using an even finer grid than Mesh 4 in these shear layers will burden the simulation with additional computational cost, and is expected to have negligible effects on the fluid mechanics and primary combustion characteristics (mixture fraction). Overall, Mesh 4 provides sufficient grid resolution to characterize the shear layers without introducing too much computational overload.
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Eddy dissipation model for modeling of turbulent non-premixed combustion with radiation effect using openfoam

Eddy dissipation model for modeling of turbulent non-premixed combustion with radiation effect using openfoam

Although the origin of fire making is a mystery, it played a huge role in the progress of our civilization development. The fire was used directly for heating, cooking and as defense weapon against the wild animals. It was just a matter of time that the idea of controlling the fire appeared. Combustion is the controlled version of the fire. Controlling the fire means how to start or stop a fire and how to control precisely the temperature. Combustion can be considered as an efficient controlled fire.
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Comparative study of non-premixed and partially-premixed combustion simulations in a realistic Tay model combustor

Comparative study of non-premixed and partially-premixed combustion simulations in a realistic Tay model combustor

Keywords: CFD; Combustion Modelling; Gas Turbine Combustor; Non-Premixed Flame; Partially-Premixed Flame Abstract A comparative study of two combustion models based on non-premixed assumption and partially premixed assumptions using the overall models of Zimont Turbulent Flame Speed Closure Method (ZTFSC) and Extended Coherent Flamelet Method (ECFM) are conducted through Reynolds stress turbulence modelling of Tay model gas turbine combustor for the first time. The Tay model combustor retains all essential features of a realistic gas turbine combustor. It is seen that the non-premixed combustion model fails to predict the combustion completely due to an incorrect assumption of diffusion flame scenario invoking infinitely fast chemistry in complicated flow environments while the two partially premixed combustion models accurately predict the flame pattern in the primary region of the combustor. The ZTFSC model outperformed the ECFM model by producing a better temperature agreement with the experimental result. The latter model predicts lower temperature due to the underestimation of reaction progress. Additionally, a cross-comparison of the present RSM prediction invoking ZTFSC model with LES prediction reported in the literature is conducted. The former produces more accurate species concentration and flame pattern than the latter. This is mainly due to the incorrect assumption of non- premixed combustion used in LES prediction reported in the literature. It is interesting to find that when non- premixed combustion model is used for both RSM and LES predictions, the LES predicts higher temperature closer to the injection nozzle of combustor than the RSM model, though the flame shape in both cases is incorrect.
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Comparative study of non-premixed and partially-premixed combustion simulations in a realistic Tay model combustor

Comparative study of non-premixed and partially-premixed combustion simulations in a realistic Tay model combustor

Keywords: CFD; Combustion Modelling; Gas Turbine Combustor; Non-Premixed Flame; Partially-Premixed Flame Abstract A comparative study of two combustion models based on non-premixed assumption and partially premixed assumptions using the overall models of Zimont Turbulent Flame Speed Closure Method (ZTFSC) and Extended Coherent Flamelet Method (ECFM) are conducted through Reynolds stress turbulence modelling of Tay model gas turbine combustor for the first time. The Tay model combustor retains all essential features of a realistic gas turbine combustor. It is seen that the non-premixed combustion model fails to predict the combustion completely due to an incorrect assumption of diffusion flame scenario invoking infinitely fast chemistry in complicated flow environments while the two partially premixed combustion models accurately predict the flame pattern in the primary region of the combustor. The ZTFSC model outperformed the ECFM model by producing a better temperature agreement with the experimental result. The latter model predicts lower temperature due to the underestimation of reaction progress. Additionally, a cross-comparison of the present RSM prediction invoking ZTFSC model with LES prediction reported in the literature is conducted. The former produces more accurate species concentration and flame pattern than the latter. This is mainly due to the incorrect assumption of non- premixed combustion used in LES prediction reported in the literature. It is interesting to find that when non- premixed combustion model is used for both RSM and LES predictions, the LES predicts higher temperature closer to the injection nozzle of combustor than the RSM model, though the flame shape in both cases is incorrect.
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Direct Quadrature Conditional Moment Closure for Turbulent Non-Premixed Combustion

Direct Quadrature Conditional Moment Closure for Turbulent Non-Premixed Combustion

complexity of the detailed model with two moments of the particle dynamics model. Louloudi also shows that when more than first two moments are used, the prediction of soot didn’t improve significantly [95]. Frenklach’s method of moments is certainly a progress however it still uses some arbitrary modelling constants in the soot surface growth model. This raises question concerning the generality and usefulness of the detailed model. Overall, there still exist great uncertainty about the fundamental aspect of soot formation and oxidation modelling in the literature. At present, both semi-empirical and detailed models are not satisfactorily capable to cover a wider range of fuels and flame conditions. Thus, further research and studies are essential to develop both semi-empirical and detailed soot models.
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Unsteady flamelet / progress variable approach for non-premixed turbulent lifted flames

Unsteady flamelet / progress variable approach for non-premixed turbulent lifted flames

in-order to attain lower emissions. Experimentalists in the research of diffusion combustion claim to observe the flame lift-off, re-ignition and extinction phenomena more often. Regulations on emissions from gas turbines also pressurize the research in combustion to develop models which predict close to modern day gas turbine combustors. Pollutants such as NO x and CO form the prime target for all the combustion models. The combustion models that have been in extensive use for numerical modeling of non-premixed turbulent flames are laminar flamelet model [1], conditional moment closure [2] and joint PDF model [3]. Laminar flamelet model is most widely used for modeling aspects of all practical combustors. Flamelet model is considered as a turbulent diffusion flame as an ensemble of laminar diffusion flamelets subjected to stretch in the turbulent flow. Thermo-chemical state of any flamelet is given as a function of scalar dissipation rate and mixture fraction prior to turbulent calculations.
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Numerical Investigations of Transport and Chemistry Modeling for Lean Premixed Hydrogen Combustion

Numerical Investigations of Transport and Chemistry Modeling for Lean Premixed Hydrogen Combustion

ABSTRACT The use of hydrogen as a fuel for power generation applications has been suggested as an additive to, or replacement of, hydrocarbon fuels. The safety of hydrogen combustion has also received recent attention due to nuclear power plant disas- ters and the rise of hydrogen refuelling stations. In these uses and scenarios, lean hydrogen–air flames are prone to thermo-diffusive instabilities which can be danger- ous to equipment and personnel. These instabilities are heavily influenced by two mechanisms: transport properties (e.g., diffusion) and chemical species production rates. This thesis investigates lean premixed hydrogen combustion using direct nu- merical simulations. A wide range of flame configurations are considered, spanning one-dimensional steady configurations to three-dimensional unsteady laminar and turbulent flames with high curvature. In particular, the two controlling mechanisms of thermo-diffusive instabilities are carefully investigated.
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Numerical Simulation and Analysis of Dual Fuel (Diesel + Ethanol) Combustion Engine and Comparison between Non Premixed and Premixed Combustion

Numerical Simulation and Analysis of Dual Fuel (Diesel + Ethanol) Combustion Engine and Comparison between Non Premixed and Premixed Combustion

Vishal Batti 1 , Vardan Singh Nayak 2 1 M.Tech Scholar Mechanical Egg. Dept. VIST BHOPAL 2 Asst. Professor Mechanical Engg. Dept. VIST BHOPAL Abstract: Alternative fuels have been getting more attention as concerns escalate over exhaust pollutant emissions produced by internal combustion engines, higher fuel costs, and the depletion of crude oil. Various solutions have been proposed, including utilizing alternative fuels as a dedicated fuel in spark ignited engines, diesel pilot ignition engines, gas turbines, and dual fuel and bi-fuel engines. Among these applications, one of the most promising options is the diesel derivative dual fuel engine with Alternate fuel as the supplement fuel. In present study we are using Ethanol as alternate fuel with Diesel to investigate the Dual fuel model with non-premixed & premixed combustion and compare on the basic of combustion efficiency and pollutant emissions rate like carbonic oxides and nitric oxides. Ethanol is taking as an Alternate fuel which is cheaper in cost and easily available as compare to the conventional fuels. CFD Results shows a excellent flow phenomenon which is stable in nature and due to this the accuracy of the simulation results are higher for layer formation system in combustion. The pollutant emissions (Carbonic oxides) are decreasing in non premixed combustion as compare to the premixed combustion that shows the complete combustion rate is increased. NOx emissions are also decreasing in Non-premixed dual fuel (Diesel + Ethanol) model as compare to premixed combustion. In second part of the study we are using Chemkin (Chemical kinetic) mechanism for evaluating the NOx pollutant which is responsible for thermal NOx . CFD Simulation results in Table no. 2 are clearly shows that mass fraction of NO, NO2 and N2O is
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Large eddy simulation of premixed and non-premixed combustion

Large eddy simulation of premixed and non-premixed combustion

Swirl flames exhibits complex flow features in terms of various recirculation zones and these features are important in flame stabilization. Fig. 4 shows the LES predicted mean flow pattern with stream traces of axial velocity plotted on temperature contours. Numerical results correctly predict two bluff body recirculation zones. These two counter rotating vortex zones lead to a high temperature region above the bluff body. Detailed results are presented for velocity flow field, temperature, mixture fraction and species mass fractions and compared with respective experimental data. Comparison of predicted axial and swirl velocity components compared with the experiments at various axial locations are shown in Fig. 5 and 6. It can be seen that LES results agree well with the experimental data indicating that overall flow features in this complex swirl flow situation have been predicted well by the LES based combustion model. LES resolves the axial velocity component very well at all locations except at one downstream location z/D=2.5. This location corresponds to the axial vortex breakdown region of this swirl flame and therefore flow is highly unstable. Because of this highly unstable nature current LES technique does not completely capture the exact flow and flame properties and this could well be a result of the deficiencies of the steady laminar flamelet concept which does not include transient, extinction and re-ignition effects. In Fig 6 the correct development of the swirl velocity pattern at radial distance of r/R = {1.0-1.2} at the initial three axial locations are captured well with both combustion models (NAFM and AFM).
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Mixing and non-premixed combustion at supercritical pressures

Mixing and non-premixed combustion at supercritical pressures

species non-reacting real gas model, has been implemented in the highly scalable spectral element code nek5000 [ 15 ]. Real gas mixing has been investigated using DNS and state of the art ther- modynamic and transport properties. Transcritical and supercritical temporal jets, with thermodynamic conditions perfectly matching the cryogenic nitrogen injection experiments at supercritical pressures [ 16 ], have been simulated. To the best of the author’s knowledge these conditions have been simulated directly for the first time in this thesis. The pseudo-boiling phenomenon, occurring in transcritical flows, has been demonstrated to significantly influence the jet devel- opment. A liquid-like jet break-up and a mitigating effect on the development of shear layer instabilities have been observed. Moreover its presence is limited in a narrow spatial region suggest particular care in the sub-grid modeling for LES and RANS approaches. A statistical approach based on a standard presumed sub-grid pdf of temperature has been discussed and a priori tested on DNS data.
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Towards an Unsteady/Flamelet Progress Variable method for non-premixed turbulent combustion at supercritical pressures

Towards an Unsteady/Flamelet Progress Variable method for non-premixed turbulent combustion at supercritical pressures

a Department of Mechanical and Aerospace Engineering, Sapienza University, Rome, Italy Abstract Combustion devices operating at elevated pressures, such as liquid rocket engines (LRE), are usually characterized by supercritical thermodynamic conditions. Propellants injected into the combustion cham- ber experience real fluid e ffects on both their mixing and combustion. Transition through super-criticality implies abrupt variations in thermochemical properties which, together with chemical reactions and high turbulent levels introduce spatial and temporal scales that make these processes impractical to be simulated directly. Reynolds-Averaged Navier-Stokes (RANS) and Large Eddies Simulation (LES) equipped with suitable turbulent combustion modeling are therefore mandatory to attempt numerical simulation on real- istic length scales. In the present work, the building blocks for extending the unsteady /flamelet progress variable approach for turbulent combustion modeling to supercritical non-premixed turbulent flames are presented. Such approach requires a large number of unsteady supercritical laminar flamelet solutions at supercritical pressures, usually referred as flame structures, to be preliminarily established by solving the flamelet equations with suitable real fluid thermodynamics. Given such unsteady flame structures, flamelet libraries can then be generated for all thermochemical quantities. The explicit dependence on flamelet time is usually eliminated using mixture fraction, reaction progress parameter, and maximum scalar dissipation rate as independent flamelet parameters. Real fluid thermodynamics used for such unsteady supercritical laminar flamelet solutions, is taken into account by means of a computationally efficient cubic equation of state. In order to have a better handling of real gas mixtures, the real gas equation of state is written in a comprehensive three-parameter fashion. A priori analysis at supercritical pressures of transient flame structures is performed in order to study how solutions populate the flamelet state space which is usually characterized by the S-shape diagram representing a collection of steady solutions. High-pressure condi- tions ranging from 60 to 300 bar are chosen as representative of a methane /liquid-oxygen rocket engine operating conditions.
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Autoignition and flame propagation in non-premixed MILD combustion

Autoignition and flame propagation in non-premixed MILD combustion

a Department of Engineering, University of Cambridge, Trumpington Street, Cambridge, CB2 1PZ, United Kingdom Abstract Direct Numerical Simulation (DNS) data of Moderate or Intense Low-oxygen Dilution (MILD) combustion are analysed to gather insights on autoignition and flame propagation in MILD combustion. Unlike in conventional combus- tion, the chemical reactions occur over a large portion of the computational domain. The presence of ignition and flame propagation and their coexis- tence are studied through spatial and statistical analyses of the convective, diffusive and chemical effects in the species transport equations. Autoigni- tion is observed in regions with lean mixtures because of their low ignition delay times and these events propagate into richer mixtures either as a flame or ignition. This is found to be highly dependent on the mixture fraction length scale, ` Z , and autoignition is favoured when ` Z is small.
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Tabulated Combustion Model Development For Non-Premixed Flames.

Tabulated Combustion Model Development For Non-Premixed Flames.

RNG turbulence model is used in conjunction with a grid-converged discrete phase model for the liquid phase. The minimum number of flamelets required is determined to suf- ficiently represent the large variation of stoichiometric scalar dissipation rates in the domain. Different forms of the presumed scalar probability density functions (PDFs) were also examined. The modeling results are then compared with the experimental data at different ambient temperatures, ambient O 2 concentrations, ambient densities, and injection pressures. The effects of different chemical kinetic mechanisms (103-species and 106-species skeletal mechanisms) are also studied to further understand the performance of the model. Overall, the RIF model is observed to capture the measured ignition delay and flame lift-off length very well, especially under certain conditions characterized by low ambient temperatures, densities, and oxygen concentrations. The need for initializing multiple flamelets is highlighted in order to obtain simulation results devoid of model- ing artifacts. Overall, the efficacy of using an advanced turbulent combustion model is demonstrated. The same modeling framework is then applied towards modeling a single cylinder diesel engine. Parametric variations show exactly the same type of trends that were observed with the Spray A cases and the simulation results match well with the reported experimental data.
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CH4/NOx Reduced Mechanisms Used for Modeling Premixed Combustion

CH4/NOx Reduced Mechanisms Used for Modeling Premixed Combustion

This study has identify useful reduced mechanisms that can be used in computational fluid dynamics (CFD) simulation of the flow field, combustion and emissions of gas turbine engine combustors. Reduced mechanisms lessen computa- tional cost and possess the ability to accurately predict the overall flame structure, including gas temperature and spe- cies as CH 4 , CO and NO x . The S-STEP algorithm which based on computational singular perturbation method (CSP) is performed for reduced the detailed mechanism GRI-3.0. This algorithm required as input: the detailed mechanism, a numerical solution of the problem and the desired number of steps in the reduced mechanism. In this work, we present a 10-Step reduced mechanism obtained through S-STEP algorithm. The rate of each reaction in the reduced mechanism depends on all species, steady-state and non-steady state. The former are calculated from the solution of a system of steady-state algebraic relations with the point relaxation algorithm. Based on premixed code calculations, The numeric results which were obtained for 1 atm  Pressure  30 atm and 1.4    0.6 on the basis of the ten steps global mecha- nism, were compared with those computed on the basis of the detailed mechanism GRI-3.0. The 10-step reduced mechanism predicts with accuracy the similar results obtained by the full GRI-3.0 mechanism for both NO x and CH 4
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Modeling, control, and implementation of enhanced premixed combustion in diesel engines

Modeling, control, and implementation of enhanced premixed combustion in diesel engines

This online database contains the full-text of PhD dissertations and Masters’ theses of University of Windsor students from 1954 forward. These documents are made available for personal study and research purposes only, in accordance with the Canadian Copyright Act and the Creative Commons license—CC BY-NC-ND (Attribution, Non-Commercial, No Derivative Works). Under this license, works must always be attributed to the copyright holder (original author), cannot be used for any commercial purposes, and may not be altered. Any other use would require the permission of the copyright holder. Students may inquire about withdrawing their dissertation and/or thesis from this database. For additional inquiries, please contact the repository administrator via email
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Tracking and analysis of flamelet structures in turbulent non-premixed combustion

Tracking and analysis of flamelet structures in turbulent non-premixed combustion

Direct numerical simulations are unrivaled in the level of detail, but this comes at the expense of extremely high computational costs. While even moderately sized DNSs have roughly O (10 8 ) grid points and must be run on hundreds of processors to have an acceptable response time, today’s high-fidelity simulations require nearly seven billion grid points and are run on more than 100,000 computing cores [6]. As a result, the amount of data generated by such simulations can be overwhelmingly huge and typically lies in the range of O (10 0 ) TB to O (10 2 ) TB for a single simulation [7], which in turn can only be postprocessed by O (10 3 ) computing cores. Due to the resolution requirements of DNSs and available computational resources, they are often limited to fairly low Reynolds numbers, which makes them impractical to simulate laboratory-scale combustion devices. Also, the preferably short response times are difficult to achieve using DNSs. Rather, direct numerical simulations are a tool of academic interest and often applied in benchmark cases focusing on single aspects of a turbulent reactive flow, e.g. turbulence chemistry interaction. The insights gained from these simulations are then often used to develop and validate closure models for LES or RANS approaches, which, by design, do not resolve all scales.
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ODT Based Closure Model for Non-Premixed Combustion LES.

ODT Based Closure Model for Non-Premixed Combustion LES.

choice implementations; however, tabulation methods still have their place in future developments. Stand-alone methods offer a high degree of runtime simplicity with a smaller up-front investment, and they have the ability to interchange reaction mechanisms with little changes to the integrated model. Much of the new research required for coupled solutions involves the coupling mechanism between LES and ODT and improved efficiencies in the implementation. The interchange of information between these two solutions is paramount to the effectiveness of the method. Variables in the LES space require increased resolution, while information obtained from ODT requires reduced resolution. The difference introduces unnecessary error which may affect the outcome. In contrast, within the tabulation space, the biggest concern is finding an effective and efficient mechanism to represent the statistical nature of the solution. These methods either encapsulate the statistics and the spatial information within the same constructs, or they separate the statistics and combine the model via an integration step. Accurately representing the statistical nature is essential to predicting unresolved quantities within highly turbulent reactive flows. With the many layers of complex information, modeling the statistics can be quite difficult otherwise.
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State-of-the-art in premixed combustion modeling using flamelet generated manifolds

State-of-the-art in premixed combustion modeling using flamelet generated manifolds

This is done by investigating the impact of strong stretch, curvature and preferential diffusion effects on the flame dynamics as described by the local mass burning rate. This so-called strong stretch theory is derived and analyzed in Part I, as well as multiple simplifications of it, to compare the strong stretch theory with existing stretch theories. The results compare well with numerical results for flames with thin reaction layers, but described by multiple-species transport and chemistry. This opens the way to use the generalized flamelet model as a firm basis for applying FGM in strongly stretched laminar and turbulent flames in Part II. The complete FGM model is derived first and the use of FGM in practice is reviewed. The FGM model is then validated by studying effects of flame stretch, heat loss, and changes in elements, as well as NO formation. The application to direct numerical simulations of turbulent flames is subsequently studied and validated using the strong stretch theory. It is shown that the generalized flamelet model still holds even in case of strong stretch and curvature effects, at least as long as the re- action layer is dominated by reaction and diffusion phenomena and not perturbed too much by stretch related perturbations. The FGM model then still performs very well with a low number of control vari- ables. Turbulent flames with strong preferential diffusion effects can also be modeled efficiently with an FGM model using a single additional control variable for the changes in element mass fractions and en- thalpy. Finally FGM is applied to the modeling of turbulent flames using LES and RANS flow solvers. For these cases, the flame front structure is not resolved anymore and unresolved terms need to be modeled.
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Numerical simulation of partially premixed combustion using a flame surface density approach

Numerical simulation of partially premixed combustion using a flame surface density approach

Keywords: Partially premixed turbulent combustion, Flame Surface Density, Mixture fraction, DISI engine combustion, OpenFOAM . R Abstract. Partially premixed combustion is characterized by a variable equivalence ratio of the mixture in space and time, and where there are both lean and rich mixture zones. Thus the reaction evolves along with a turbulent mixture process, which modifies the composition of reactants and products. In this situation a so-called triple flame could be encountered, in which a rich and a lean premixed flame front as well as a diffusion flame are present. The diffusion flame develops behind the premixed flame front due to turbulent mixing in the hot combustion products. This kind of combustion could be found in Direct Injection Spark Ignition (DISI) engines when they are operated in the stratified charge mode. The model considered in this work assumes a simplified one-step irreversible chemical reaction in which fuel and oxidant react together in stoichiometric proportions giving products with the composition corresponding to a complete combustion. A transport equation is solved for the oxidant and fuel, from which the amount of products and the combustion progress are computed, while the turbulence is modeled with RANS (Reynolds-Average Navier-Stokes). The reaction rate is assumed in the model as proportional to the product of the Flame Surface Density (FSD) by the local laminar flame speed. Aside from the state and composition of the mixture, the local laminar flame speed is afected by the turbulent mixing process. This mixing process is taken into account by means of the classical β-PDF (Probability Density Function), which is a function of the mixture fraction and its variance. A transport equation is solved for both, the mixture fraction and its variance, and the FSD is computed through a transport equation where several models are available for the source terms. The model is implemented in the open-source toolkit OpenFOAM . Computational results are obtained for partially premixed combustions inside R constant-volume vessels with several initial configurations, which are compared with numerical results available in the literature.
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Dynamic Subgrid Scale Combustion Modeling for Large Eddy Simulation of Premixed Flames

Dynamic Subgrid Scale Combustion Modeling for Large Eddy Simulation of Premixed Flames

Figure 5.26: Contour plots of the instantaneous dynamic wrinkling model quantities. a: test filtered progress variable source term c ω ˙ ˜ c [kg/m 3 s]; b: source term from test filtered progress variable and mixture fraction ˙ ω c ˆ ˜ [kg/m 3 s] for the test case 4b. The profiles of time averaged temperature and the corresponding resolved rms, at 50 : 400 [mm] nozzle downstream positions are depicted in Fig.5.27, Fig.5.28 for the cases 4a, 4b and 4c (See Tab.5.4). In this test cases the dynamic version of the power-law wrinkling model is used in which the power-law factor β is averaged over whole domain and has the same value for all grid cells. The dynamic calculation of the power-law factor β is updated every 5 time step in these investigated cases. The results of time averaged temperature for all three investigated cases are in very good agreement with each other and also with experiment. The flame height is in good agreement with experiment which is consequence of correct prediction of turbulent flame speed. The comparison of the results of the test cases 4a and 4b with different thickening factors shows the same behavior despite different thickening. The results of test cases with smaller thickening factor shows a slightly improvement in flame shape.
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