Sukruth Somappa
(Advisor: Prof. Tim Lieuwen]
will defend a doctoral thesis entitled,
Dynamics of finite thickness and harmonically forced turbulent premixed flames
On
Wednesday, July 5 at 12:30 p.m.
Montgomery Knight Building, Room 317
[270 Ferst Dr, Atlanta, GA 30332]
Summary
Turbulent combustion investigates the interactions of broadband fluid dynamic disturbances and combustion. It plays an important role in the design and operation of modern combustion devices. This thesis considers two aspects of turbulent premixed combustion: first, the interaction of broadband turbulence and narrowband acoustic/hydrodynamic disturbances and next, the effect of Arrhenius kinetics on finite thickness turbulent flames.
In modern combustion devices, turbulent flames are inherently subject to superposition of broadband turbulent flow fluctuations and narrowband coherent disturbances. These coherent disturbances can be due to underlying instabilities in shear flows or acoustic background in confined environments. Although a large body of work exists on turbulent flame dynamics, they have not been explored in the presence of harmonic disturbances. In particular, first part of this thesis considers experimental investigation of V-shaped turbulent premixed flame anchored by an oscillating flame holder, in a novel-experimental facility. Time resolved Mie scattering images are used to simultaneously track the flame position and to characterize velocity fields using particle image velocimetry for varying turbulent and convective wrinkles. Ensemble-averaged dynamics are explored by averaging flow fields at a particular phase with respect to the forcing cycle. In such environments, results show that tangential strain rate and flame speed exhibit coherent modulation in addition to time-averaged and stochastic variation. In addition, the modulation in both tangential strain and flame speed are strongly correlated with ensemble-averaged curvature. Strain-curvature correlation in particular, appears to be much stronger than the weak correlation observed in flames subject to broadband turbulence alone. A simplified semi-empirical analysis captures the variation in strain-curvature correlation suggesting that modification of approaching reactant flow by the flame through gas expansion could result in such a correlation.
Furthermore, ensemble-averaged flame speed results show a strong correlation with negative curvature values but a weak correlation with positive curvature values. Ensemble-averaged flame brush width (quantified by turbulence intensity) in relation to the convective wrinkle wavelength shows a strong influence on the ensemble-averaged flame front topology. In particular, negative and positively curved regions of the flame become more symmetric with the increase in turbulent brush width with respect to the convective wavelength. In addition, non-dimensional turbulent Markstein numbers (quantifying the sensitivity of flame speed-curvature correlation) scale with turbulence intensity and coherent wavelength such that, it exhibits a weak dependence on mean axial velocity. These scalings provide for a more accurate reduced-order model for spatio-temporal dynamics of turbulent flames in the presence of harmonic perturbations. For instance, flame describing functions can be computed without resorting to time-consuming computational simulations using ensemble-averaged G-equation approach.
Lastly, the second part of this thesis considers a more phenomenological question of why turbulent flame speed increases with turbulence intensity - whether the flame speed is controlled by global averaged properties or space-time local features. Concave reaction rate profiles in Favre-averaged progress variable equation are amenable to travelling wave solutions with the flame speed dependent only on the leading edge. These are termed as "pulled fronts". However, Arrhenius kinetics combined with finite flamelet thickness implies that no real flame has a concave reaction rate profile but exhibits a small deviation from the concave shape. The effect of such a deviation is analyzed using numerical and analytical techniques in this work. Results show a strong correction to the flame speed introduced by Arrhenius deviation of the reaction rate from the concave shape. This correction follows inverse logarithmic form. Further, in the asymptotic limit of high Zeldovich number, the correction can be predicted (up to leading order) using a variational principle. The results suggest that even with the deviation to the reaction rate profile due to Arrhenius kinetics, flame speed (up to the leading order) is still controlled by a small region close to the leading edge.
Committee
- Dr. Tim Lieuwen – School of Aerospace Engineering (advisor)
- Dr. Jechiel Jagoda – School of Aerospace Engineering
- Dr. Adam Steinberg – School of Aerospace Engineering
- Dr. Ellen Mazumdar – School of Mechanical Engineering
- Dr. Ari Glezer – School of Mechanical Engineering