Download or read book Effects of D C Electric Fields and Acoustic Excitation on the Lean Limit Stability of Bluff body Stabilized Conical Turbulent Premixed Flames written by Alper Ata and published by . This book was released on 2003 with total page 218 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Stability of Conical Bluff body Stabilized Premixed Flames with Upstream Mixture Gradients written by Matthew William Andel and published by . This book was released on 2008 with total page 194 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 Experimental Investigation Into Thermo Acoustic Instability in Pre Mixed Pre Vaporized Bluff Body Stabilized Flames written by Jeffrey Ross Monfort and published by . This book was released on 2015 with total page 277 pages. Available in PDF, EPUB and Kindle. Book excerpt: An experimental campaign was undertaken to investigate the thermo-acoustic properties of a bluff-body stabilized flame in an atmospheric pressure facility at the Air Force Research Laboratory. Of particular interest were the possible interactions between the acoustic properties of the test rig, the vortex shedding due to the presence of the bluff-body, and the unsteady heat release within the chamber. An analysis of the vortex shedding modes due to the bluff-body and the acoustic modes indicated that there are regions in the operating envelope where the two mode types share similar frequencies given an operating condition, creating a scenario where feedback might be possible. Further investigation into the fluctuating velocity components in the wake of the bluff-body indicated that the Strouhal number is not single-valued, and that vortices of varying sizes, and accompanying characteristic frequencies, are shed from a single bluff-body.With previous research indicating that lean blow-off is preceded by local extinctions within the reaction zone, and blow-off being closely related to the ratio of chemical and fluidic time scales, an experiment was conducted to determine whether or not flames undergoing thermo-acoustic instability also exhibit regions of decreased residence time. This experiment concluded that the regions of acoustically-coupled flames which undergo large-scale oscillations do, in fact, correlate with decreased residence time. This conclusion links both lean static stability and near-stoichiometric dynamic stability to simple time scales prescribed by vortex behavior in the wake of a bluff-body.An investigation was conducted which utilized simultaneous high-speed particle image velocimetry (PIV), planar laser-induced fluorescence (PLIF) and pressure measurements in the near-wake region of a bluff-body stabilized flame. In addition to the simultaneous measurements listed, high-speed broadband chemiluminescence was also collected. The 2-D nature of these measurements led to their analysis through the utilization of proper orthogonal decomposition (POD). The decomposition of the highly-complicated data sets allowed the dominant features to be extracted. These dominant features, in an acoustically-coupled flame, show remarkable symmetry that is not readily apparent in uncoupled flames. Further analysis of an objective measure of the flame symmetry as a function of equivalence ratio indicated that the fluctuations in the axial component of velocity best correlate with overall sound pressure level. This correlation indicates that the feedback interactions are based around the longitudinal acoustic modes of the combustion chamber.
Download or read book Advances in Turbulent Combustion Dynamics Simulations in Bluff Body Stabilized Flames written by Jonathan Michael Tovar and published by . This book was released on 2015 with total page 89 pages. Available in PDF, EPUB and Kindle. Book excerpt: This work examines the three main aspects of bluff-body stabilized flames: stationary combustion, lean blow-out, and thermo-acoustic instabilities. For the cases of stationary combustion and lean blow-out, an improved version of the Linear Eddy Model approach is used, while in the case of thermo-acoustic instabilities, the effect of boundary conditions on the predictions are studied. The improved version couples the Linear Eddy Model with the full-set of resolved scale Large Eddy Simulation equations for continuity, momentum, energy, and species transport. In traditional implementations the species equations are generally solved using a Lagrangian method which has some significant limitations. The novelty in this work is that the Eulerian species concentration equations are solved at the resolved scale and the Linear Eddy Model is strictly used to close the species production term. In this work, the improved Linear Eddy Model approach is applied to predict the flame properties inside the Volvo rig and it is shown to over-predict the flame temperature and normalized velocity when compared to experimental data using a premixed single step global propane reaction with an equivalence ratio of 0.65. The model is also applied to predict lean blow-out and is shown to predict a stable flame at an equivalence ratio of 0.5 when experiments achieve flame extinction at an equivalence ratio of 0.55. The improved Linear Eddy Model is, however, shown to be closer to experimental data than a comparable reactive flow simulation that uses laminar closure of the species source terms. The thermo-acoustic analysis is performed on a combustor rig designed at the Air Force Research Laboratory. The analysis is performed using a premixed single step global methane reaction for laminar reactive flow and shows that imposing a non-physical boundary condition at the rig exhaust will result in the suppression of acoustic content inside the domain and can alter the temperature contours in non-physical ways. It can be concluded from this work that it is important to include the proper exhaust configuration for reacting thermo-acoustic calculations so that non-physical boundary conditions do not compromise the solution.
Download or read book Mt Rose Missionary Baptist Church written by and published by . This book was released on with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Chemistry and Turbulence Effects in Bluff body Stabilized Flames written by J. C. Pan and published by . This book was released on 1992 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book The Effect of Bluff Body in Premixed Flame Stabilization written by Mazlan Abdul Wahid and published by . This book was released on 2008 with total page 82 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Effects of Diluents and High Potential Electric Fields on Flame Stabilization and Liftoff written by James David Kribs and published by . This book was released on 2013 with total page 162 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Effect of Harmonic Forcing on Turbulent Flame Properties written by Sai Kumar Thumuluru and published by . This book was released on 2010 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Lean premixed combustors are highly susceptible to combustion instabilities, caused by the coupling between heat release fluctuations and combustor acoustics. In order to predict the conditions under which these instabilities occur and their limit cycle amplitudes, understanding of the amplitude dependent response of the flame to acoustic excitation is required. Extensive maps of the flame response were obtained as a function of perturbation amplitude, frequency, and flow velocity. These maps illustrated substantial nonlinearity in the perturbation velocity - heat release relationship, with complex topological dependencies that illustrate folds and kinks when plotted in frequency-amplitude-heat release space. A detailed analysis of phase locked OH PLIF images of acoustically excited swirl flames was used to identify the key controlling physical processes and qualitatively discuss their characteristics. The results illustrate that the flame response is not controlled by any single physical process but rather by several simultaneously occurring processes which are potentially competing, and whose relative significance depends upon forcing frequency, amplitude of excitation, and flame stabilization dynamics. An in-depth study on the effect of acoustic forcing on the turbulent flame properties was conducted in a turbulent Bunsen flame using PIV measurements. The results showed that the flame brush thickness and the local consumption speed were modulated in the presence of acoustic forcing. These results will not only be a useful input to help improve combustion dynamics predictions but will also help serve as validation data for models.
Download or read book Flame Extinction Dynamics of Lean Premixed Bluff body Stabilized Flames written by Marissa Kelley MacDonald and published by . This book was released on 2014 with total page 218 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Characterization of Turbulent Flame vortex Dynamics for Bluff Body Stabilized Flames written by Cal Rising and published by . This book was released on 2019 with total page 44 pages. Available in PDF, EPUB and Kindle. Book excerpt: Modern propulsion systems primarily operate under highly turbulent conditions in order to promote greater efficiency through an increase in mixing. The focus of this thesis is to identify the turbulent flame-vortex interaction to provide insights into the turbulent combustion process. This work is accomplished through the use of turbulent ramjet-style combustor which is stabilized through use of a bluff-body. The facility is equipped with a custom turbulence generator to modulate the incoming turbulence levels to allow flames across various regimes to be analyzed. High-speed particle image velocimetry (PIV) and CH* chemiluminescence diagnostics are implemented to resolve the flow field and flame position. The flame-vortex interaction can be described by the vorticity transport which has four terms; vortex stretching, baroclinic torque, dilatation, and viscous diffusion. The vorticity mechanisms are calculated through the implementation of a Lagrangian tracking scheme, which allows for the individual mechanisms to be decomposed along the path of individual tracks. The mechanisms are compared across different turbulence levels to determine the effects of turbulence on the vorticity mechanisms. The mechanisms are calculated along the flame front as well to determine the individual effects of the vorticity mechanisms on the evolving structure of the turbulent premixed flame. The flame front curvature is also compared across the various turbulence conditions. The results confirm that as the flame-front experiences increased turbulence levels the combustion induced mechanisms of baroclinic torque and dilation decrease, while vortex stretching increases. This is a result of the turbulent energy exchange becoming the controlling factor within the flow-field. In addition, increased flame curvature is experience by the flame front due to increased local baroclinicity and turbulent energy exchange.
Download or read book Blowoff Behavior of Bluff Body Stabilized Flames in Vitiated and Partially Premixed Flows written by Steven G. Tuttle and published by . This book was released on 2010 with total page 358 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Experimental Investigation of the Dynamics and Blowoff Characteristics of Bluff body Stabilized 2D V Shaped Turbulent Premixed Flames with Different Gaseous Hydrocarbon Fuels written by Rishi Roy and published by . This book was released on 2018 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book The Effect of a Transverse Electric Field on Bluff Body Flame Extinction Limits written by Thomas A. Parker and published by . This book was released on with total page 128 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Effects of Combustion Chamber Blockage on Bluff Body Flame Stabilization written by James Roger Foster and published by . This book was released on 1956 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Investigation of Blowoff Mechanism and Forced Response of Bluff Body Stabilized Turbulent Premixed Flames written by Swetaprovo Chaudhuri and published by . This book was released on 2010 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt:
