Nicholas Magina

Bio: Nicholas Magina is an academic researcher from Georgia Institute of Technology. The author has contributed to research in topics: Strouhal number & Premixed flame. The author has an hindex of 5, co-authored 6 publications receiving 78 citations.

Response of non-premixed flames to bulk flow perturbations

Abstract: This paper describes the dynamics of non-premixed flames responding to bulk velocity fluctuations, and compares the dynamics of the flame sheet position and spatially integrated heat release to that of a premixed flame. The space–time dynamics of the non-premixed flame sheet in the fast chemistry limit is described by the stoichiometric mixture fraction surface, extracted from the solution of the -equation. This procedure has some analogies to premixed flames, where the premixed flame sheet location is extracted from the G = 0 surface of the solution of the G-equation. A key difference between the premixed and non-premixed flame dynamics, however, is the fact that the non-premixed flame sheet dynamics are a function of the disturbance field everywhere, and not just at the reaction sheet, as in the premixed flame problem. A second key difference is that the non-premixed flame does not propagate and so flame wrinkles are convected downstream at the axial flow velocity, while wrinkles in premixed flames convect downstream at a vector sum of the flame speed and axial velocity. With the exception of the flame wrinkle propagation speed, however, we show that that the solutions for the space–time dynamics of the premixed and non-premixed reaction sheets in high velocity axial flows are quite similar. In contrast, there are important differences in their spatially integrated unsteady heat release dynamics. Premixed flame heat release fluctuations are dominated by area fluctuations, while non-premixed flames are dominated by mass burning rate fluctuations. At low Strouhal numbers, the resultant sensitivity of both flames to flow disturbances is the same, but the non-premixed flame response rolls off slower with frequency. Hence, this analysis suggests that non-premixed flames are more sensitive to flow perturbations than premixed flames at O(1) Strouhal numbers.

Propagation, dissipation, and dispersion of disturbances on harmonically forced, non-premixed flames

Abstract: This paper describes the dynamics of non-premixed flames responding to bulk velocity fluctuations, and compares the dynamics of the flame sheet position and spatially integrated heat release to a similarly excited premixed flame. Bulk axial or transverse excitation, in either case, leads to the excitation of wrinkles on the flame that propagate axially. Inclusion of axial diffusion in the non-premixed case, and burning velocity stretch sensitivity in the premixed case, cause wrinkle dissipation and dispersion. There are important differences in spatially integrated unsteady heat release dynamics between premixed and non-premixed flames. For general Strouhal numbers, mass burning rate fluctuations are the dominant contributor to non-premixed flame heat release fluctuations, while area fluctuations are the dominant contributor to premixed flame heat release fluctuations. Moreover, the heat release response of non-premixed flames rolls off much slower with frequency, O(St−1/2) compared to O(St−1) for premixed flames, and, hence, are more sensitive to flow perturbations than premixed flames at high Strouhal numbers. The asymptotic tendencies of the non-premixed flame, however, are largely controlled by the near burner exit region with high transverse gradients and, thus, are expected to be quite sensitive to burner exit details and finite chemistry effects.

Adjoint Methods as Design Tools in Thermoacoustics

Abstract: © 2019 by ASME. In a thermoacoustic system, such as a flame in a combustor, heat release oscillations couple with acoustic pressure oscillations. If the heat release is sufficiently in phase with the pressure, these oscillations can grow, sometimes with catastrophic consequences. Thermoacoustic instabilities are still one of the most challenging problems faced by gas turbine and rocket motor manufacturers. Thermoacoustic systems are characterized by many parameters to which the stability may be extremely sensitive. However, often only few oscillation modes are unstable. Existing techniques examine how a change in one parameter affects all (calculated) oscillation modes, whether unstable or not. Adjoint techniques turn this around: They accurately and cheaply compute how each oscillation mode is affected by changes in all parameters. In a system with a million parameters, they calculate gradients a million times faster than finite difference methods. This review paper provides: (i) the methodology and theory of stability and adjoint analysis in thermoacoustics, which is characterized by degenerate and nondegenerate nonlinear eigenvalue problems; (ii) physical insight in the thermoacoustic spectrum, and its exceptional points; (iii) practical applications of adjoint sensitivity analysis to passive control of existing oscillations, and prevention of oscillations with ad hoc design modifications; (iv) accurate and efficient algorithms to perform uncertainty quantification of the stability calculations; (v) adjoint-based methods for optimization to suppress instabilities by placing acoustic dampers, and prevent instabilities by design modifications in the combustor's geometry; (vi) a methodology to gain physical insight in the stability mechanisms of thermoacoustic instability (intrinsic sensitivity); and (vii) in nonlinear periodic oscillations, the prediction of the amplitude of limit cycles with weakly nonlinear analysis, and the theoretical framework to calculate the sensitivity to design parameters of limit cycles with adjoint Floquet analysis. To show the robustness and versatility of adjoint methods, examples of applications are provided for different acoustic and flame models, both in longitudinal and annular combustors, with deterministic and probabilistic approaches. The successful application of adjoint sensitivity analysis to thermoacoustics opens up new possibilities for physical understanding, control and optimization to design safer, quieter, and cleaner aero-engines. The versatile methods proposed can be applied to other multiphysical and multiscale problems, such as fluid-structure interaction, with virtually no conceptual modification.

Linear Response of Laminar Premixed Flames to Flow Oscillations: Unsteady Stretch Effects

Abstract: �c and � � s ,forcurvatureand hydrodynamic strain, respectively Unsteady curvature effects on the flame surface area become significant when j� � c jSt 2 2 � O� 1� and are responsible for the experimentally observed reduction in the flame front wrinkle size in the flow direction [referred to as “filtering” by Bourehla and Baillot (Bourehla, A, and Baillot, F, “Appearance and Stability of a Laminar Conical Premixed Flame Subjected to an Acoustic Perturbation,” Combustion and Flame,

Global modes, receptivity, and sensitivity analysis of diffusion flames coupled with duct acoustics

Abstract: © 2014 Cambridge University Press. In this theoretical and numerical paper, we derive the adjoint equations for a thermo-acoustic system consisting of an infinite-rate chemistry diffusion flame coupled with duct acoustics. We then calculate the thermo-acoustic system's linear global modes (i.e. The frequency/growth rate of oscillations, together with their mode shapes), and the global modes' receptivity to species injection, sensitivity to base-state perturbations and structural sensitivity to advective-velocity perturbations. Some of these could be found by finite difference calculations but the adjoint analysis is computationally much cheaper. We then compare these with the Rayleigh index. The receptivity analysis shows the regions of the flame where open-loop injection of fuel or oxidizer will have the greatest influence on the thermo-acoustic oscillation. We find that the flame is most receptive at its tip. The base-state sensitivity analysis shows the influence of each parameter on the frequency/growth rate. We find that perturbations to the stoichiometric mixture fraction, the fuel slot width and the heat-release parameter have most influence, while perturbations to the Peclet number have the least influence for most of the operating points considered. These sensitivities oscillate, e.g. positive perturbations to the fuel slot width either stabilizes or destabilizes the system, depending on the operating point. This analysis reveals that, as expected from a simple model, the phase delay between velocity and heat-release fluctuations is the key parameter in determining the sensitivities. It also reveals that this thermo-acoustic system is exceedingly sensitive to changes in the base state. The structural-sensitivity analysis shows the influence of perturbations to the advective flame velocity. The regions of highest sensitivity are around the stoichiometric line close to the inlet, showing where velocity models need to be most accurate. This analysis can be extended to more accurate models and is a promising new tool for the analysis and control of thermo-acoustic oscillations.

Dynamics and Diagnostics of Flame-Acoustic Interactions

Abstract: Flame-acoustic interactions are witnessed in the context of combustion instability in gas turbine combustors and other propulsion devices, such as rockets, besides confined combustion systems in general, such as furnaces and heaters. The confinement causes acoustic standing wave modes that interact with the flame to cause fluctuations in all quantities to grow in amplitude. This review categorizes the different canonical flame-holding geometries that mostly involve flow recirculation zones for flame stabilization, which are inherently unstable and feed into the flame-acoustic interaction cycle. The receptivity of the nonreacting shear layer to prevalent acoustic forcing in terms of development of coherent structures and instability of different hydrodynamic modes in the recirculation are detailed. The case of reacting flow instabilities involves several mechanisms of flame-acoustic coupling, such as vortex combustion; vortex-wall interactions; vortex-vortex interactions; flame area fluctuations an...