Download or read book Large Eddy Simulation of Premixed Combustion Using Artificial Flame Thickening Coupled with Tabulated Chemistry written by Guido Künne and published by . This book was released on 2012 with total page 282 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Large Eddy Simulations of Premixed Turbulent Flame Dynamics written by Gaurav Kewlani and published by . This book was released on 2014 with total page 300 pages. Available in PDF, EPUB and Kindle. Book excerpt: High efficiency, low emissions and stable operation over a wide range of conditions are some of the key requirements of modem-day combustors. To achieve these objectives, lean premixed flames are generally preferred as they achieve efficient and clean combustion. A drawback of lean premixed combustion, however, is that the flames are more prone to dynamics. The unsteady release of sensible heat and flow dilatation in combustion processes create pressure fluctuations which, particularly in premixed flames, can couple with the acoustics of the combustion system. This acoustic coupling creates a feedback loop with the heat release that can lead to severe thermoacoustic instabilities that can damage the combustor. Understanding these dynamics, predicting their onset and proposing passive and active control strategies are critical to large-scale implementation. For the numerical study of such systems, large eddy simulation (LES) techniques with appropriate combustion models and reaction mechanisms are highly appropriate. These approaches balance the computational complexity and predictive accuracy. This work, therefore, aims to explore the applicability of these methods to the study of premixed wake stabilized flames. Specifically, finite rate chemistry LES models that can effectively capture the interaction between different turbulent scales and the combustion fronts have been implemented, and applied for the analysis of premixed turbulent flame dynamics in laboratory-scale combustor configurations. Firstly, the artificial flame thickening approach, along with an appropriate reduced chemistry mechanism, is utilized for modeling turbulence-combustion interactions at small scales. A novel dynamic formulation is proposed that explicitly incorporates the influence of strain on flame wrinkling by solving a transport equation for the latter rather than using local-equilibrium-based algebraic models. Additionally, a multiple-step combustion chemistry mechanism is used for the simulations. Secondly, the presumed-PDF approach, coupled with the flamelet generated manifold (FGM) technique, is also implemented for modeling turbulence-combustion interactions. The proposed formulation explicitly incorporates the influence of strain via the scalar dissipation rate and can result in more accurate predictions especially for highly unsteady flame configurations. Specifically, the dissipation rate is incorporated as an additional coordinate to presume the PDF and strained flamelets are utilized to generate the chemistry databases. These LES solvers have been developed and applied for the analysis of reacting flows in several combustor configurations, i.e. triangular bluff body in a rectangular channel, backward facing step configuration, axi-symmetric bluff body in cylindrical chamber, and cylindrical sudden expansion with swirl, and their performance has been be validated against experimental observations. Subsequently, the impact of the equivalence ratio variation on flame-flow dynamics is studied for the swirl configuration using the experimental PIV data as well as the numerical LES code, following which dynamic mode decomposition of the flow field is performed. It is observed that increasing the equivalence ratio can appreciably influence the dominant flow features in the wake region, including the size and shape of the recirculation zone(s), as well as the flame dynamics. Specifically, varying the heat loading results in altering the dominant flame stabilization mechanism, thereby causing transitions across distinct- flame configurations, while also modifying the inner recirculation zone topology significantly. Additionally, the LES framework has also been applied to gain an insight into the combustion dynamics phenomena for the backward-facing step configuration. Apart from evaluating the influence of equivalence ratio on the combustion process for stable flames, the flame-flow interactions in acoustically forced scenarios are also analyzed using LES and dynamic mode decomposition (DMD). Specifically, numerical simulations are performed corresponding to a selfexcited combustion instability configuration as observed in the experiments, and it is observed that LES is able to suitably capture the flame dynamics. These insights highlight the effect of heat release variation on flame-flow interactions in wall-confined combustor configurations, which can significantly impact combustion stability in acoustically-coupled systems. The fidelity of the solvers in predicting the system response to variation in heat loading and to acoustic forcing suggests that the LES framework can be suitably applied for the analysis of flame dynamics as well as to understand the fundamental mechanisms responsible for combustion instability. KEY WORDS - large eddy simulation, LES, wake stabilized flame, turbulent premixed combustion, combustion modeling, artificially thickened flame model, triangular bluff body, backward facing step combustor, presumed-PDF model, flamelet generated manifold, axi-symmetric bluff body, cylindrical swirl combustor, particle image velocimetry, dynamic mode decomposition, combustion instability, forced response.

Download or read book Large Eddy Simulation of Premixed and Non premixed Combustion in a Stagnation Point Reverse Flow Combustor written by Satish Undapalli and published by . This book was released on 2008 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: A new combustor, referred to as Stagnation Point Reverse Flow (SPRF) combustor has been developed at Georgia Tech to meet increasingly stringent emission regulations. The combustor incorporates a novel design to meet the conflicting requirements of low pollution and high efficiency in both premixed and non-premixed modes. The objective of this thesis is to perform Large Eddy Simulations (LES) on this lab-scale combustor and explain the underlying physics. To achieve this, numerical simulations are performed in both the premixed and non-premixed combustion modes. The velocity field, species field, entrainment characteristics, flame structure, emissions and mixing characteristics are then analyzed. Simulations have been carried out first for a non-reactive case and the flow features in the combustor are analyzed. Next, the simulations have been extended for the premixed reactive case by employing different sub-grid scale combustion chemistry closures - Eddy Break Up (EBU), Artificially Thickened Flame (TF) and Linear Eddy Mixing (LEM) models. Only LEMLES which is an advanced scalar approach is able to accurately predict both the velocity and species field in the combustor. The results from LEM with LES (LEMLES) using a reduced chemical mechanism have been analyzed in the premixed mode. The results showed that mass entrainment occurs along the shear layer in the combustor. The entrained mass carried products into the reactant stream and provided preheating. The product entrainment enhances the reaction rates and stabilizes the flame even at very lean conditions. These products are shown to enter into the flame through local extinction zones present on the flame surface. The flame structure is further analyzed and the combustion mode is found to be primarily in thin reaction zones. The emissions in the combustor are studied using simple global mechanisms for NOx. Computations show extremely low NOx values comparable to the measured emissions. These low emissions are shown to be primarily due to the low temperatures in the combustor. LEMLES computations are also performed with detailed chemistry to capture more accurately the flame structure. The flame in the detailed chemistry case is more sensitive to strain effects and show more extinction zones very near to the injector. LEMLES approach is also used to resolve the combustion mode in the non-premixed case. The studies indicate that mixing of fuel and air close to the injector controls the combustion process. The predictions in the near field are shown to be very sensitive to the inflow conditions. Analysis shows that fuel and air mixing occurs to lean proportions in the combustor before any burning takes place. The flame structure in the non-premixed mode is very similar to the premixed mode. Along with fuel-air mixing, the products also mix with the reactants and provide the preheating effects to stabilize the flame in the downstream region of the combustor.

Download or read book Large Eddy Simulation of Premixed and Partially Premixed Combustion written by Ionuţ Porumbel and published by . This book was released on 2006 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Large Eddy Simulation (LES) of bluff body stabilized premixed and partially premixed combustion close to the flammability limit is carried out in this thesis. The LES algorithm has no ad-hoc adjustable model parameters and is able to respond automatically to variations in the inflow conditions. Algorithm validation is achieved by comparison with reactive and non-reactive experimental data. In the reactive flow, two scalar closure models, Eddy Break-Up (EBULES) and Linear Eddy Mixing (LEMLES), are used and compared. Over important regions, the flame lies in the Broken Reaction Zone regime. Here, the EBU model assumptions fail. The flame thickness predicted by LEMLES is smaller and the flame is faster to respond to turbulent fluctuations, resulting in a more significant wrinkling of the flame surface. As a result, LEMLES captures better the subtle effects of the flame-turbulence interaction. Three premixed (equivalence ratio = 0.6, 0.65, and 0.75) cases are simulated. For the leaner case, the flame temperature is lower, the heat release is reduced and vorticity is stronger. As a result, the flame in this case is found to be unstable. In the rich case, the flame temperature is higher, and the spreading rate of the wake is increased due to the higher amount of heat release Partially premixed combustion is simulated for cases where the transverse profile of the inflow equivalence ratio is variable. The simulations show that for mixtures leaner in the core the vortical pattern tends towards anti-symmetry and the heat release decreases, resulting also in instability of the flame. For mixtures richer in the core, the flame displays sinusoidal flapping resulting in larger wake spreading. More accurate predictions of flame stability will require the use of detailed chemistry, raising the computational cost of the simulation. To address this issue, a novel algorithm for training Artificial Neural Networks (ANN) for prediction of the chemical source terms has been implemented and tested. Compared to earlier methods, the main advantages of the ANN method are in CPU time and disk space and memory reduction.

Download or read book Large Eddy Simulation of Turbulent Combustion written by Heinz Pitsch and published by . This book was released on 2006 with total page 32 pages. Available in PDF, EPUB and Kindle. Book excerpt: In the first part of this work, new models for describing sub-grid quantities in reactive LES settings were developed. These models included a new model for the sub-filter variance of a conserved scalar, a new method of filtering the G-equation, a resolution-sensitive description of the turbulent burning velocity, and a flamelet formulation valid near premixed fronts. The models were shown to offer improved predictive capability through application to experimental flames. In the second part, a new method to automatically generate skeletal kinetic mechanisms for surrogate fuels, using the directed relation graph method with error propagation, was developed. These mechanisms are guaranteed to match results obtained using detailed chemistry within a user-defined accuracy for any specified target. They can be combined together to produce adequate chemical models for surrogate fuels. A library containing skeletal mechanisms of various accuracies and domains of applicability was assembled.

Download or read book Large eddy Simulation of Premixed Turbulent Combustion Using Flame Surface Density Approach written by Wen Lin and published by . This book was released on 2010 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Advanced Turbulent Combustion Physics and Applications written by N. Swaminathan and published by Cambridge University Press. This book was released on 2022-01-06 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Explore a thorough and up to date overview of the current knowledge, developments and outstanding challenges in turbulent combustion and application. The balance among various renewable and combustion technologies are surveyed, and numerical and experimental tools are discussed along with recent advances. Covers combustion of gaseous, liquid and solid fuels and subsonic and supersonic flows. This detailed insight into the turbulence-combustion coupling with turbulence and other physical aspects, shared by a number of the world leading experts in the field, makes this an excellent reference for graduate students, researchers and practitioners in the field.

Download or read book Theoretical and Numerical Combustion written by Thierry Poinsot and published by R.T. Edwards, Inc.. This book was released on 2005 with total page 544 pages. Available in PDF, EPUB and Kindle. Book excerpt: Introducing numerical techniques for combustion, this textbook describes both laminar and turbulent flames, addresses the problem of flame-wall interaction, and presents a series of theoretical tools used to study the coupling phenomena between combustion and acoustics. The second edition incorporates recent advances in unsteady simulation methods,

Download or read book Large Eddy Simulation of Premixed Combustion Using Flamelets written by Ivan Langella and published by . This book was released on 2016 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Flow and Combustion in Advanced Gas Turbine Combustors written by Johannes Janicka and published by Springer Science & Business Media. This book was released on 2012-10-29 with total page 495 pages. Available in PDF, EPUB and Kindle. Book excerpt: With regard to both the environmental sustainability and operating efficiency demands, modern combustion research has to face two main objectives, the optimization of combustion efficiency and the reduction of pollutants. This book reports on the combustion research activities carried out within the Collaborative Research Center (SFB) 568 “Flow and Combustion in Future Gas Turbine Combustion Chambers” funded by the German Research Foundation (DFG). This aimed at designing a completely integrated modeling and numerical simulation of the occurring very complex, coupled and interacting physico-chemical processes, such as turbulent heat and mass transport, single or multi-phase flows phenomena, chemical reactions/combustion and radiation, able to support the development of advanced gas turbine chamber concepts

Download or read book Combustion Noise written by Anna Schwarz and published by Springer Science & Business Media. This book was released on 2009-06-17 with total page 304 pages. Available in PDF, EPUB and Kindle. Book excerpt: November, 2008 Anna Schwarz, Johannes Janicka In the last thirty years noise emission has developed into a topic of increasing importance to society and economy. In ?elds such as air, road and rail traf?c, the control of noise emissions and development of associated noise-reduction techno- gies is a central requirement for social acceptance and economical competitiveness. The noise emission of combustion systems is a major part of the task of noise - duction. The following aspects motivate research: • Modern combustion chambers in technical combustion systems with low pol- tion exhausts are 5 - 8 dB louder compared to their predecessors. In the ope- tional state the noise pressure levels achieved can even be 10-15 dB louder. • High capacity torches in the chemical industry are usually placed at ground level because of the reasons of noise emissions instead of being placed at a height suitable for safety and security. • For airplanes the combustion emissions become a more and more important topic. The combustion instability and noise issues are one major obstacle for the introduction of green technologies as lean fuel combustion and premixed burners in aero-engines. The direct and indirect contribution of combustion noise to the overall core noise is still under discussion. However, it is clear that the core noise besides the fan tone will become an important noise source in future aero-engine designs. To further reduce the jet noise, geared ultra high bypass ratio fans are driven by only a few highly loaded turbine stages.

Download or read book High order Finite volume CENO Scheme for Large Eddy Simulation of Premixed Flames written by Luiz Tobaldini Tobaldini Neto and published by . This book was released on 2019 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: A novel, parallel, high-order, central essentially non-oscillatory (CENO), cell-centered, finite-volume scheme is developed and applied to large-eddy simulation (LES) of turbulent premixed flames. The high-order CENO finite-volume scheme is applied to the solution of the Favre-filtered Navier-Stokes equations governing turbulent flows of a fully compressible reactive mixture on a three-dimensional, multi-block, body-fitted, computational mesh consisting of hexahedral volume elements. The CENO method uses a hybrid reconstruction approach based on a fixed central stencil. The discretization of the inviscid fluxes combines an unlimited high-order least-squares reconstruction technique based on the optimal central stencil with a monotonicity preserving, limited, linear, reconstruction algorithm. Switching in the hybrid procedure is determined by a smoothness indicator such that the unlimited high-order reconstruction is retained for smooth solution content that is fully resolved and reverts to the limited lower-order scheme, enforcing solution monotonicity, for regions with abrupt variations (i.e., discontinuities and under-resolved regions). The high-order viscous fluxes are computed to the same order of accuracy as the hyperbolic fluxes based on a high-order accurate cell interface gradient derived from the unlimited, cell-centered, reconstruction. The proposed cell-centered finite-volume scheme is formulated for three-dimensional multi-block mesh consisting of generic hexahedral cells and applied to LES of premixed flames. For the reactive flows of interest here, a flamelet-based subfilter-scale (SFS) model is used to describe the unresolved influences of interaction between the turbulence and combustion. This SFS combustion model is based on a presumed conditional moment (PCM) approach in conjunction with flame prolongation of intrinsic low-dimensional manifold (FPI) tabulated chemistry. Numerical results are discussed for a laboratory-scale lean premixed methane-air Bunsen-type flame. The performance of the proposed high-order scheme for turbulent reactive flows is analysed by a systematic mesh refinement study using different spatial orders of accuracy.

Download or read book Large Eddy Simulation of Premixed and Partially Premixed Flames written by Max Staufer and published by . This book was released on 2009 with total page 124 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Large Eddy Simulations of Turbulent Combustion Including Radiative Heat Transfer written by Rogério Goncalves Dos santos and published by . This book was released on 2008 with total page 125 pages. Available in PDF, EPUB and Kindle. Book excerpt: The combustion is one of the principal ways to produced energy used nowadays, it is also a complex phenomenon, where the turbulent flow, chemical reactions, different phases and different heat transfer phenomena can interact. Better understanding of these interactions is essential to improve the actual combustion system and to developed the new ones. The goal of this thesis is to study the interaction of the turbulent combustion with the thermal radiation by the use of three-dimensional numerical simulation. For that, using a computational tool named CORBA, a code for the combustion Large Eddy Simulation (LES) was coupled with a radiative heat transfer code. This technique allows the exchange of information between the two codes without big changes in their structure, then it is possible to take advantages of the different characteristic time from each phenomenon in a high performance parallel computational environment. In a first time, two-dimensional simulation of a turbulent propane/air premixed flame stabilized downstream a triangular flame holder has been realised. After the changing of the twodimensional radiation code for another three-dimensional one, the same configuration was simulated in 3D. A mesh with more than 4.7 millions cells for the combustion code (AVBP) and more than 3.3 millions cells for the radiation code (DOMASIUM) are used. Results show a changing in the temperature and species fields, as well as in the flame dynamics when the thermal radiation was taken into account, with a minor intensity in the three-dimensional simulations. This method, also, shows that it is possible to perform 3D complex simulations in a industrial acceptable time.

Download or read book Chemical Modeling for Large Eddy Simulation of Turbulent Combustion written by and published by . This book was released on 2009 with total page 41 pages. Available in PDF, EPUB and Kindle. Book excerpt: In the present project the focus was on developing advanced combustion models for large-eddy simulations (LES) and to develop automatic chemistry reduction techniques and reduced chemical mechanisms for JP-8 surrogate fuels. The aim of the combustion LES modeling part was to advance the models for non-premixed and premixed combustion towards a generalized combustion model that covers all combustion regimes. Towards this end, for the premixed regime, a dynamic model for the turbulent burning velocity was developed, which eliminates adjustable coefficients from the premixed combustion model, and a flame structure model was presented, which considers local broadening of the flame preheat zone. Further, based on asymptotic arguments, a formalism to identify the correct combustion regime was developed, which will be an important element in a future generalized combustion-regime independent combustion model. In the second part of the project, several advancements led to a fully automatic chemistry reduction method. New developments include a refined DRGEP method for species and reaction elimination, a chemical lumping procedure, and an automatic procedure for selecting steady state species. Further, several potential surrogate fuel components have been included in the component library, and a reduced JP-8 surrogate mechanism was constructed and tested with experimental data.

Download or read book Large Eddy Simulations with a Tabulated Conditional Moment Closure Model for Turbulent Premixed Combustion with Heatloss written by Carlos Alberto Velez Busto and published by . This book was released on 2015 with total page 124 pages. Available in PDF, EPUB and Kindle. Book excerpt: The DLR data sets provide both unity (E.g.Methane) and non-unity (E.g. Hydrogen) Lewis number fuels, allowing for the proposed numerical model to be validated against both unity and non-unity Lewis # flames. Velocity, temperature and major/minor species are compared to the experimental data. Once validated, this model is intended to be useful for designing lean premixed combustors for gas turbines which operate primarily in the corrugated premixed combustion regime (38), where chemical and turbulent time scales are of the same order requiring adequate models for their interaction. LES results with heat loss match the experimental data better than the adiabatic Reynolds Averaged Navier Stokes (RANS/URANS) solution (39) and is able to better resolve the transient features of the flame with an increase in run time of only 50%, when compared to URANS.

Download or read book Dominant Interscale Dynamics in Premixed Turbulent Combustion for Application to Large Eddy Simulation written by Yash Girish Shah and published by . This book was released on 2021 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The large-eddy simulation (LES) approach has become an important tool for engineering design and optimization of combustion devices. In a properly designed LES, the larger-scale flow variations are resolved by the effective numerical grid, which incorporates the implicit/explicit filtering introduced by the spatial filtering from the grid, artificial diffusion from numerical schemes, and modeled terms. The dynamical consequences of the remaining subfilter-scale (SFS) fluctuations below the effective grid to the evolution of grid-resolved scales (RS) motions in dynamically relevant physical variables are modeled. The nonlinearities in the dynamical evolution of resolved-scale variables that result from the inherent nonlinear coupling between the resolved and subfilter scales are often not sufficiently well captured in turbulent combustion, as the intermediate species that participate in the heat release process are dominantly only at chemical length and time scales well within the subfilter scales. In this research work, we explore new LES modeling strategies that represent the RS-SFS interactions in the evolution of RS primary variables (momentum, energy and species concentrations) more directly by approximating the SFS content that contributes dominantly to the dynamical evolution of RS quantities using simple mathematical forms. To achieve this, we first obtain a highly resolved three-dimensional Direct Numerical Simulation (DNS) dataset of flame-turbulence interactions that capture the essential RS-SFS interactions in primary variables over a wide range of scales. To systematically analyze the RS-SFS interscale couplings for LES, we then obtain the scale content as the Fourier space representation of the inherently inhomogeneous DNS dataset by applying a discontinuity removal procedure that removes the unphysical gradients naturally introduced at the boundaries of the computational domain by the periodic extension that occurs with the Fourier transform over a bounded domain. The Fourier space representation of primary variables in turbulence-flame dynamical interactions are systematically analyzed. Variables that display high gradients across the flame front over length scales comparable to or larger than flame scales are found to have higher Fourier variance contributions at wavenumbers below flame scale wavenumber, while Fourier variance contributions from variables that are localized only within the flame are found to be distributed to higher wavenumbers in Fourier space. Using this Fourier space representation, we systematically determine the scales of the energy-dominant flow variations in momentum and enthalpy that are resolved by LES and objectively identify the RS and SFS fluctuations for other primary variables. Variables that display frontal variations surrounding the flame are found to be dominantly resolved while those variables that are localized only within the flame are found to be dominantly subfilter scale. These differences in Fourier variance distributions are shown to have implications to the extent of RS-SFS interactions between these variables. We take advantage of the mathematical property of the Fourier spectral description that allows the nonlinearities from the advective transport and chemical reaction rates in the dynamical system to be expressed as elemental sums over triadic interactions involving three wavevectors and quadrad interactions involving four wavevectors between the RS and SFS fluctuations. Using this elemental representation, the SFS content that contributes dominantly to the dynamical evolution of resolved-scales advective nonlinearity is identified by applying the triads downselection procedure [75]. We find that RS-SFS interactions involving SFS content from significantly larger scales compared to the smallest resolvable scales in the DNS are required to adequately estimate the resolved-scale advective nonlinearity in LES. These dynamically dominant SFS for RS advective nonlinearity span over a broader range of wavenumbers for dominantly SFS variables compared to variables that are dominantly resolved-scale. To study the dominant RS-SFS interactions in the chemical nonlinearity, a new two-stage downselection procedure is developed in this work, which expresses the quadrad interactions between the reaction rate constant and the species concentrations to the resolved chemical reaction rates first into triadic interactions between the chemical reaction rate constant and the product of species concentrations. The product of species concentrations is then expressed as a triadic sum over interactions between individual species concentrations and the corresponding dynamically dominant SFS is extracted from both stages using triad downselect procedure for second-order nonlinearities. The dynamically dominant SFS resulting from this procedure is found to be considerably reduced from the full SFS and is shown to be effective in adequately approximating the chemical reaction rates at resolved scales through RS-SFS interactions. The structure underlying the distribution of these dynamically dominant SFS fluctuations in species concentrations are identified for key species in representative reactions in regions where the incorporation of the SFS content is impactful to the estimation of chemical reaction rates in LES. The dynamically dominant SFS species structure is found in two groupings: ``single-banded'' structure characterized by one distinct peak, and ``double-banded'' structure characterized by two peaks of opposite signs. Species that are produced and consumed within the flame are observed to have single-banded structure and species displaying a frontal behavior are observed to have double-banded structure in their dynamically dominant SFS concentrations on average. The local structure of the dynamically dominant SFS species concentrations surrounding the flame is impacted by neighboring flame-flame interactions as well as by variations in flame curvature. The impacts of the flame-flame interactions are strong when the dynamically dominant SFS species structure has ``large'' length scales with concentration peaks significantly displaced from the flame front. Finally, mathematical forms to approximate the mean single and double banded structure in the dynamically dominant SFS concentrations are proposed for application within a structure-based SFS modeling strategy which directly embeds the interaction between the modeled dominant SFS content and the RS evolution within existing LES frameworks. This research lays the groundwork for future LES model developments that utilize this strategy for improving LES predictions of resolved-scale dynamics.