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• W89877964 abstract "The flashback type known as combustion induced vortex breakdown of a modern premixed gas turbine combustion chamber was simulated numerically. The uRANS simulation technique was applied to capture the instantaneous behavior of the unisotropic flow field. An extended formulation of a flame surface density model was developed and utilized for combustion modeling. In addition, the dependency of the laminar flamelets under turbulent flow conditions was accounted for based on the local strain of the flamesheet and the Markstein number. The results of this work demonstrate the importance of fluid dynamics with respect to the stability behavior of premixed combustion systems. ∗ Corresponding author: Torsten.Voigt@vbt.uni-karlsruhe.de Proceedings of the European Combustion Meeting 2009 Introduction Premixed combustion systems are of peculiar interest for technical applications because of their low emissions compared to diffusion flame systems. However, a major drawback of premixed combustion systems is the potential instability behavior of flame flashback. Up to date, flashback is mainly known to occur in three different forms: in the boundary layer due to low downstream velocities in the core flow due to turbulent increased burning velocities in the core flow due to aerodynamically or rather combustion instabilities Therefore, industrially utilized premixed combustion systems are commonly designed with a high axial downstream velocity to prevent the aforementioned flashback varieties to occur and to make the system safer. Despite this precaution, a different flashback mode was detected recently [1] in which the flame propagated upstream paradoxically just near the rotational axis where the highest downstream velocities occurred. The flow field of this system does not only exhibit a strong axial but also a pronounced tangential velocity as it is typical of modern gas turbine combustion chambers where the flame is stabilized aerodynamically. This detected kind of instability is known as combustion induced vortex breakdown (CIVB) because the flashback is induced by the combustion but driven by the aerodynamics. The present work deals with fully three dimensional uRANS simulations of the combustion induced vortex breakdown phenomenon of the experimentally investigated gas turbine combustion chamber [1] under several thermodynamic conditions. A flame surface density model was applied for the numerical simulation which determines the altering extent of local flame surface by the use of a transport equation. The instantaneously changing extent of flame surface is triggered by the flow field and by the flame itself. These effects of flame formation and destruction were simulated with the flame surface density model by taking into account laminar and turbulent stretch effects and furthermore flame merging and annihilation. Chemistry effects were considered by the eigenvalue of laminar flames, the burning velocity. Thereby, two different approaches were applied, one without considering stretch effects on flame speed and one approach that accounts for the effects of flow inhomogeneity on flame behaviour. The straining effects were captured by preliminary simulations of laminar stationary counter flow flames using detailed chemical reaction mechanisms. Thus, tabulated values of the laminar flame speed in dependency on strain rate, preheat temperature and air equivalence ratio were provided. Additionally, a new approach [2] for flame speed on strain rate was implemented into the flame surface density model to account for the non-steady-state effect of turbulent motion. Specific Objectives The flashback mechanisms of combustion induced vortex breakdown are a current field of research since the driving processes are not completely known yet. Therefore, one of the main objectives of this work is the accurate simulation of CIVB to give a more detailed insight into the accompanied physical fundamentals than experiments can. Furthermore, the capabilities and limits of the applied combustion model in the context of uRANS simulations were to be assessed. In this context, an extended approach of a flame surface density model was applied due to the presumed strong interaction between flow field and flame. Thus, this work considers the physics of the combustion induced vortex breakdown and elucidates the capability of a simulation technique that is of great interest for both, science and industry. Theory and governing equations This paragraph describes the equations used for the fully three dimensional uRANS simulations of flame flashback. The section is subdivided into three parts: the description of (i) the simulation of the flow field, (ii) the combustion model and (iii) the preliminary flamelet calculations. (i) Simulation of the flow field: A Reynolds stress turbulence model solving a transport equation for each of the six independent turbulent stresses τij was utilized for closure assumptions of the balance equations of momentum. The turbulent length scale was determined by means of solving the transport equation for the turbulent frequency ω near the walls and a balance equation for the dissipation of the turbulent kinetic energy e elsewhere in the flow field. It has been shown that the use of a more simplified closure assumption for the turbulent stresses (e.g. the standard k,e-model) leads to a rapid transport of momentum towards greater radii and hence to a unification of the velocity profiles. This in turn contradicts the real velocity distribution and suppresses the CIVB phenomenon. (ii) Description of the combustion model: The combustion model is based on an irreversible, infinitely fast one-step chemical reaction mechanism for methane-air. This procedure reduces the determination of the mean, statistically distributed reaction progress variable to the solution of only one progress variable. This favre averaged progress variable c~ can be" @default.
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• W89877964 date "2009-01-01" @default.
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• W89877964 title "Numerical Investigation of Combustion Induced Flame Flashback in a Premixed Combustion System" @default.
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